Grade 8
Physical Science
Oak Meadow
Coursebook
Oak Meadow, Inc.
Post Office Box 1346
Brattleboro, Vermont 05302-1346
oakmeadow.com
Item #b085010
Grade
8
Contents
Introduction:.................................................... ix
General Guidelines.............................................................x
A Note About Materials................................................... xi
Keep Your Work in a Safe Place....................................... xi
Final Note.......................................................................... xii
Lessons
Lesson 1: A Brief History of Physical Science ................ 1
The Flow of Discovery
Studying Science
Lesson 2: Measuring....................................................... 13
Taking Measurements
Measuring Systems
Converting Between U.S. Customary and Metric Systems
Measuring Linear Distance
Measuring Area
Measuring the Volume of Solids
Measuring the Volume of Liquids
Lesson 3: Scientific Method........................................... 25
Variable and Constant Factors
Controlled Versus Uncontrolled Environment
The Scientific Method
Using the Scientific Method
Introduction to Mass and Matter
Matter, Molecules and Atoms
iii
Contents
Grade 8 Physical Science
Lesson 4: Energy.............................................................. 39
Types of Energy
Potential Energy and Kinetic Energy
Energy Can Change Form
Used Energy and Heat Energy
Lesson 5: Thermodynamics and Conservation
of Energy...................................................................... 49
The Laws of Thermodynamics
First law of Thermodynamics and Conservation of Energy
Pendulums and Conservation of Energy
Lesson 6: Force................................................................ 59
Different Kinds of Force
Force and Motion
Resultants
Lesson 7: Force of Gravity.............................................. 69
Newton’s Law of Gravity
Mass, Weight, and Gravity
Units of Weight and Mass
Center of Gravity
Lesson 8: The Laws of Motion....................................... 83
Friction
Minimizing Friction
Projectiles
Lesson 9: More Motion.................................................. 93
Velocity
Acceleration
Newton’s Second Law of Motion
Newton’s Third Law of Motion
Lesson 10: Work and Power........................................ 103
Work
Power
Unit of Measurement for Power
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Contents
Lesson 11: Machines..................................................... 113
Inclined Planes and Mechanical Advantage
Ramps
Wedges
Levers
The Laws of Thermodynamics and Machines
Friction, Gravity and Machines
Lesson 12: More Machines.......................................... 127
Wheels and Axles
Gears
Pulleys
Lesson 13: Waves as Moving Energy........................... 137
Wave Parts, Period and Frequency
Wave Velocity
Transverse and Longitudinal Waves
Wave Interference and Reflection
Waves You Can’t See
Lesson 14: Sound.......................................................... 149
How Does Sound Travel?
Loudness
Pitch
The Speed of Sound
Lesson 15: More Sound................................................ 161
Transmission, Absorption, and Reflection of Sound Waves
Acoustics Inside Buildings
Noise Pollution
Lesson 16: Light............................................................. 173
Light Waves
Illumination
Reflection
Lesson 17: Opaque Materials and Shadows.............. 181
Transparent Materials and Refraction
Light Technology
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Contents
Grade 8 Physical Science
Lesson 18: Color............................................................ 191
Visible Spectrum
Refraction and Dispersion of Light
Blue Skies and Red Sunsets
The Electromagnetic Spectrum
Lesson 19: Lenses.......................................................... 205
Converging and Diverging Light Rays
How Eyes Work
Creating Images
Wearing Glasses
Perspective
Cameras
Lesson 20: Electricity.................................................... 217
What causes Electricity
Laws of Electrical Change
Static Electricity
Current Electricity
Conductors and Insulators
Circuits and Switches
Lesson 21: Batteries...................................................... 231
Wet Cells
Dry cells
Lesson 22: Electric Circuits and Measuring
Electricity................................................................... 237
Series Circuits
Parallel Circuits
Measuring Electricity
Lesson 23: Resistance and Ohm’s Law....................... 247
Ohm’s Law
Resistors and Circuits
Resistors and Series Circuits
Resistors and Parallel Circuits
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Contents
Lesson 24: Home Electricity......................................... 257
Overload
Short Circuits
Electric Shocks
Electric meters
Lesson 25: Magnetism.................................................. 267
Magnets and Poles
Magnetism and Atoms
Creating Magnets
Magnetic Fields
The Earth’s Magnetism
Lesson 26: Magnetism and Electricity......................... 279
Making a Magnet with Electricity
Making Electricity from Magnetism
Direct Current and Alternating Current
Lesson 27: Matter......................................................... 291
Element Names and Symbols
Atomic Mass
Electron Shells
Lesson 28: Mixtures, Compounds and Molecules..... 305
Molecules
Oxidation
Photosynthesis
Lesson 29: Solutions..................................................... 313
Types of Solutions
Solubility and Concentration
Lesson 30: Heat, Temperature and Pressure.............. 323
Heat On the Move
Thermal Expansion and Contraction
Temperature
Pressure
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Contents
Grade 8 Physical Science
Lesson 31: Aerodynamics and Flight........................... 341
Aerodynamic Forces
Bernoulli’s Principle and Lift
Airplanes
Supersonic Aircraft
Lesson 32: Modern Machines...................................... 353
Microwaves
Television
Satellites
Compact Discs and Long-Playing Records
Lesson 33: Cars............................................................. 367
Hydraulics
Hybrid Cars
Lesson 34: Energy Use in Our World.......................... 375
Fossil Fuels
Hydroelectric Power
Electricity from Steam
Geothermal Energy
Wind Power
Biomass
Solar Energy
Nuclear Energy
Lesson 35: Energy Problems........................................ 395
Energy and Food
Chemicals
Pollutants
Air Pollution
The Greenhouse Effect
Acid Rain
Organic Compounds and Ozone Depletion
Thermal Pollution
Lesson 36: Final Review................................................ 415
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3
Scientific Method
Science describes what we know about our world. We learn about the
world by observing what is happening all around us. We observe through
all our senses: we watch, we listen, we feel, we smell, we taste, and we use
our intuition. Then we reach conclusions about what it all means. This is
how we make sense out of the world.
Observing and exploring nature and the workings of our earth is largely
a matter of being receptive to what lies all around us. This does not take
special training; look at any small child and you’ll see that he/she observes
many things that many of us don’t notice. As we said in lesson 1, our
species has survived because we pay attention to novel events. Careful
observation is the basis of scientific inquiry.
In this lesson, you will learn about the classic “scientific method”. This
is an organized way of testing observed phenomena, useful in science
courses and in certain research applications. However, it is not the only
way that scientific progress is made! Scientists observe the world like
children do: exploring every corner, every new thing. It is observation and
questioning that is scientific inquiry, and this can come about in many
different ways. Sometimes you cannot create experiments around the
observed phenomena. If a shower of meteors falls to the earth, how can
you devise an experiment to test that they are meteors? You can’t recreate
it, but you can observe carefully, and it can open your eyes to new
possibilities and new things to observe. This is the way science works.
We need to be constantly aware. In fact, most scientific discoveries happen
completely by surprise. However, even the surprises aren’t surprising,
because scientists expect them!
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Grade 8 Physical Science
Scientific
Method
(continued)
We are all scientists. We ask questions, we guess what the answer will be,
we watch to see what happens. Our minds record the results and then we
decide what the results mean. We take this knowledge and use it throughout
our lives as we decide what to do and how to do it. In the scientific method,
observations are made about the world, and then experiments are
conducted to explain the observation. How the experiment is designed and
then conducted is important, because only then can we get an accurate
explanation for the observation. If the experiment is not controlled, then
it will not give us a reliable explanation. We will next look at the different
things which can make an experiment controlled or uncontrolled, and
therefore more or less reliable.
Variable and Constant Factors
When we make observations about the
world, it is important to understand
what possible variable factors there may be
in what we are observing. Factors is a term
that describes all the possible parts of an
observation or experiment. A variable factor
is something that can be varied or changed.
Factors that do not change are called constant (or determinate) factors.
Let’s say that using the observation above
about the ice cream, you decide to figure
out why the ice cream is soft sometimes and
really hard other times. You have thought
about it and come up with four of the
variable factors listed above: temperature,
placement, type of ice cream, and length of
time in freezer. To determine which variable is causing the ice cream
to be hard or soft at different times, you decide to conduct a series of
experiments to explain your observation.
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Examples of Variable Factors
Let’s suppose that we have made the observation that
sometimes the ice cream in the freezer is really hard and
sometimes it is a little soft. What are some of the variable
factors that could explain this?
Lesson 3
Scientific
Method
(continued)
• The temperature of the freezer
• The placement of the ice cream in the freezer
• The type of ice cream
• How long the ice cream has been in the freezer
• How many times the door has been opened
• How much ice cream is lef the container
Ice Cream Experiment #1
Let’s say you first decide that you think the most important factor is the
placement in the freezer. In order to test this, you put some ice cream in
a certain spot in the freezer and then after a while you go and test it for
hardness. It seems pretty hard. The next day, when you go to test the ice
cream again, you realize that someone ate it all, and there is another kind
of ice cream right in the same spot. Since it is in the same place in the
freezer, you do another hardness test. It is pretty soft. Uh-oh!
When you think about why the ice cream was soft the second time, you
come up with several possible reasons:
a. The ice cream was a different kind, so that might be why it was
soft the second time
b. Maybe the ice cream had not been in the freezer for very long.
Maybe it was just put in there after sitting in the car on the
way home from the grocery store.
c. Maybe the temperature changed in the freezer.
Your hardness tests of the ice cream didn’t really prove anything because
you still don’t really know why the ice cream is soft sometimes and hard
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Lesson 3
Grade 8 Physical Science
Scientific
Method
(continued)
at other times. After you did the experiment, you don’t know if it has to do
with placement in the freezer any more than you did before. The problem
was that there were too many variable factors in your original experiment.
This is an example of an uncontrolled experiment - there was not enough
control over the variables to find an explanation for the observation. If you
really want to find out what causes the ice cream to be harder or softer at
different times, you will need to limit the variables.
This brings us to an important rule about experiments: only one variable
factor allowed in each experiment! The only way you can figure out why
something is happening is to limit the variable factors to one. Each
experiment should only have one variable factor.
Limiting Variables
Scientists often simplify the world in order to study just one
or two things. This is known as limiting the variables. Think
of it like this: if you had an allergic reaction to something
that you ate one day, you would probably not be able, at
first, to figure out which food it was that gave you the allergic
reaction. Each food is a variable. The way to figure it out
would be to make a list of all the foods that you ate on that
day (a list of all the variables), and then each day eat only one
of them at a time. In this way you isolate each food (each
variable) until you figure out which one is making you react.
This is the process of limiting the variables.
How would this work with the ice cream hardness question? Let’s redesign
the experiment to make all the factors constant except one; the variable
factor will be the placement of the ice cream in the freezer.
Variables and Constants
• Only one variable factor in each experiment.
• All other factors should be constant.
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Grade 8 Physical Science
Ice Cream Experiment #2
This time you need to make sure that all other factors stay constant (stay
the same) throughout the experiment. Now you will have to decide what
constants you will need to have in your experiment to make sure only one
factor is variable. Let’s say you come up with these constant factors:
Lesson 3
Scientific
Method
(continued)
a. The temperature of the freezer. You discuss with your family
that no one is to touch the freezer control for a couple of days
while you conduct the experiment.
b. The type and amount of ice cream. You buy three containers of
the same ice cream, all in the same size container , and you ask
that no one in your family eat any of it, or move it, for the next
several days. (Why three containers? Keep reading!)
c. How long the ice cream has been in the freezer. You place each
of the three ice cream containers in the freezer at the same time
and you make a note of the time you put them into the freezer.
You decide that the variable factor you will test is the location of the ice
cream in the freezer. You are going to vary this factor by placing three
identical containers of the same type of ice cream in three different places
in your freezer. You then conduct the experiment by checking the hardness
in each of the three containers on a set schedule - every six hours, for
example — and you write your results down each time. As you do the
experiment, you are careful not to change the location of any of the three
containers.
Now let’s look at your results. If the results were that the ice cream in one
of the containers was soft and the ice cream in the other two containers
was hard, then the placement of the ice cream in the freezer affects the
hardness of the ice cream! If the results were that the ice cream in all of
the containers was equally hard or soft in all locations, then the placement
of the ice cream in the freezer is not the variable that affects the hardness
and softness of the ice cream. You will have to design another experiment
that has a different factor as a variable, and where the placement of the
ice cream in the freezer is a constant.
As we have seen, the experiment must be controlled so as not to have too
many variable factors.
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Lesson 3
Grade 8 Physical Science
Scientific
Method
(continued)
1. Take some time to make an observation around your home.
Perhaps you notice that your cat naps in different places at different
times of day. Or maybe you see that the temperature on one side of
your house generally feels colder than on the other. Then make a list
of variable factors that you might consider if you were to design an
experiment. After each variable you list, explain how you might
control that variable to make it a constant in your experiment.
Controlled Versus Uncontrolled Environments
The environment in which an experiment is conducted has an effect on
the outcome of a scientific experiment. It is important to control the
environment (the variables), or you will not get an accurate explanation
for your observation or question.
A controlled environment is an environment where there is only one, or
at most, a few variable factors. Most scientists, when they are working
to explain an observation they have made, strive to design and conduct
experiments in a controlled environment and to limit the variable factors
to as few as possible. An example of a controlled environment is a science
laboratory where the scientist can control the temperature, the humidity,
and the materials that are used.
An uncontrolled environment is an environment where there are many
variable factors. Some kinds of observations cannot be reduced to
experiments that can be conducted in a laboratory with controlled
variable factors. For example, when dealing with a global environmental
issue such as the effect of ozone depletion (which we will discuss more
thoroughly in Lesson 35), it is impossible to create a controlled environment
to examine this problem. Since ozone depletion occurs on a very large
scale - the Earth - it cannot be made to fit into a laboratory. The best that
can be done is to study certain pieces of it.
Sometimes it is impossible to isolate variables. Other times the variables
work together, and isolating them doesn’t give you an accurate assessment.
This has been the case when studying the human body. Scientists have
isolated different organs and studied them individually and made conclusions,
only to find later that each organ is quite connected to the whole body/
mind system. They interact with the system in many complex ways, and
controlled systematic study of each separately isn’t quite so simple!
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Grade 8 Physical Science
The Scientific Method
The scientific method is one procedure that all scientists use in trying to
understand the earth and all that occurs. You will use this method throughout
the rest of this course and many times throughout your life. The scientific
method is a series of steps that ask, “Why does something happen?” “Can
I figure out why it happened?” “Did I figure out what happened?” and “What
did I learn from this?” The steps of the scientific method are as follows:
Lesson 3
Scientific
Method
(continued)
1. Observation: Identify a problem or a question. This is called the
observation.
2. Hypothesis: Make a guess about the answer to the question, based
on what you know already. This is called the hypothesis, otherwise
known as an “educated guess.”
3. Experiment: Figure out an experiment to test your hypothesis. Try to
control the experiment or procedure in order to have as few variable
factors as possible. Describe your experiment and the specific steps.
Do the experiment. List all the variables that you can figure out.
4. Results: Describe what happened when you did your experiment.
What happened is called the results. Sometimes results can be
presented in a chart or graph form.
5. Conclusion: Review your original question (Step 1) and your hypothesis
(Step 2).
Compare your hypothesis with what actually happened (Step 4).
• Did what you think would happen actually happen?
• Did something unexpected happen?
• Describe the variables and which ones may have impacted your
results.
• Consider possible explanations for what happened in your
experiment.
• Try to come up with an explanation for your results. This is
called the conclusion. The ultimate goal of experimenting is to
find scientific truths, or principles, that are true in any situation.
This is called a theory, and theories are formed after much
experimentation with consistent results.
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Lesson 3
Grade 8 Physical Science
Scientific
Method
(continued)
Remember: Control the Variables
It is often difficult to create a completely controlled experiment, but
scientists do their best to control as many factors as possible. It is always
important to remember that your observations and/or experiment may
have variable factors that are affecting your results, so in your role as a
scientist you need to control as many factors as you can.
You have now been introduced to the scientific method. Learn the steps
well, as you will use the scientific method in almost every lesson in this
course, and in all your future science courses.
Before we go on, however, let’s review some related concepts. In lesson 1,
you learned that scientific observations must be measurable, repeatable,
and that we strive for objective analysis. Remember to apply these principles
when using the scientific method. Whenever you use the scientific method
for a controlled experiment, it should be written clearly such that somebody
else can read your experiment and repeat exactly what you did. You need
to document your method precisely! This allows other scientists to verify
your results, and it is how scientific theories are proven.
Scientific Experiments Should Be Repeatable By Other
Scientists
A repeatable experiment doesn’t mean that the same results will be obtained
again! If the experiment is repeatable, it means you’ve documented your
method very well, and others can try it. If the results are repeatable, then
we have learned a new scientific truth, and a theory can be formed!
Keep in mind that there is a difference between controlled and uncontrolled
experiments. If your experiment is not controlled, it will not have the feature
of objective analysis, nor will it be repeatable. It is very important to always
consider what variables there are in your experiment. Try to limit the number
of variables so you can figure out what you are actually measuring.
Memorize the Scientific Method
It is very important to learn the scientific method, as you will
use it in each lesson of this course. Whenever you are asked
for observations, conclusions, or a hypothesis, refer to the
format presented here when you prepare your answer.
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Grade 8 Physical Science
Using the Scientific Method
Let’s look at an example of the scientific method in action. Pretend that
you are washing the dishes in the sink one day, and you notice something
about them. This is how the scientific method would be used to make a
conclusion about your observation:
Lesson 3
Scientific
Method
(continued)
Scientific Method Experiment #1:
1. Observation: You have noticed that some objects sink when put in
water, and that others float. You decide to test several items to see
if you can figure why certain things sink and others don’t.
2. Hypothesis: There are several variables that you need to identify, so
that you can test one of them at a time. Some variables that might
affect whether an object sinks or floats are shape, size, weight, and
density. You decide to test density (which is mass per unit volume).
You need to state your hypothesis quite specifically: “Objects that are
the same shape and size, but different densities, will act differently
in water. Objects that are less dense will float, and the more dense
objects will sink. Wood will float and clay will sink.”
3. Experiment: Now you need to clearly document your method,
identifying how you will control each variable: “I will take a small
block of wood and a lump of clay. I will form the clay to be the exact
shape and size as the block of wood. I will put each of them in a sink
with water in it and ob- serve whether they sink or float. Both are
exposed to the exact same conditions in the room and the water.
The only difference is the material they are made of.”
4. Results: Write your results in detail: “The block of wood floated and
the clay block sank.”
5. Conclusion: First review your original observation (that some objects
sink and others float), and your hypothesis. Your results indicate that
what you predicted did actually happen. But what is your conclusion?
Basically, all you can conclude from this is that wood floats and clay sinks.
You would like to make the theory that objects that are less dense will
float and those that are more dense will sink. As you think about it more,
though, you wonder whether this is always true. “Less dense” and “more
dense” are vague terms. Less dense than what? What about ships that sail
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Lesson 3
Grade 8 Physical Science
Scientific
Method
(continued)
on the ocean? They are metal and quite dense, but they don’t sink. Will clay
always sink, no matter what shape it’s in? There are many more questions
raised by this experiment than answers obtained! This is the way science works!
Your experiment is an important start. Information was learned, and now
further testing can be done. You see that you need to clarify your hypothesis
even more, perhaps adding that those objects that are more dense than
water will sink, and those less dense than water will float. But that still
raises the question about the ships that float. Uh-oh, maybe there is more
than one variable that determines whether an object will float! There could
be variables that you haven’t thought of yet.
It’s important to remain inquisitive and keep questioning. You need to ask
yourself if your conclusion is always true. Consider all the variables you’ve
come up with, any experience you might have with any of them, and raise a
new question to test. You conclude that further experimentation is needed.
2. Now it’s your turn:
Scientific Method Experiment #2
You are to design an experiment that
tests whether the shape of an object has
an effect on whether it sinks or floats.
We’ll get you started with the observation:
Ask yourself:
• Is this always true?
• Consider the variables
Observation: Light things float and heavy things sink. But some heavy
things, such as ships, also float. Why is that?
Now you design the rest of the experiment to test this. You can use clay
as your heavy object since it is easy to change the shape of. Write your
hypothesis and how you will conduct the experiment. Clearly state the
variables involved and how you will keep all but the shape constant. Do
the experiment, write your results, and form a conclusion based on your
hypothesis and results. Finally, write what other questions might come up,
and ideas you have on further testing the variables that affect whether an
object floats or sinks. Is there more than one variable involved, and might
they work together?
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Lesson 3
Introduction to Mass and Matter
When you look around you, almost everything
The Scientific Method you see is matter. What is matter? It is the stuff
1. Observation
all around you! Matter is anything with mass that
2. Hypothesis
3. Experiment
4. Results
5. Conclusion
Scientific
Method
(continued)
takes up space. For example, when you pick up a
pencil, you can feel that it is made up of matter.
This matter takes up space. You are matter, your
desk is matter, your dog is matter, even the air
around you is matter. All of this matter has mass,
and the mass is what we measure.
So what is mass? In lesson 2, you were introduced to the concept of mass
and how it is similar, but different, from weight. When scientists measure
the amount of something, they use the term mass instead of weight. Mass
is the actual quantity of matter an object contains, whereas weight is the
measure of heaviness. Weight has to do with gravity, while mass doesn’t.
You have the same amount of mass whether you are on earth, in outer
space, or on the moon. But your weight in each of those places is quite
different, because of the difference in gravity. Here on earth, weight and
mass will be the same, which is why we can convert kilograms to lbs. That
conversion factor (2.2lb/1kg) wouldn’t be the same on the moon! You will
learn more about the difference between mass and weight later. Just know
that when something feels heavy to you, you are feeling its weight, which
is a combination of the amount of mass in the object and the effect of
gravity on that mass.
3. Are you matter? Remember, matter has mass and takes up space.
Design an experiment to prove that you are matter. The experiment
must demonstrate that you have mass and that you take up space.
It must also be measurable; your experiment should provide
measurements of your mass and space. You do not have to carry
out this experiment; just explain it clearly.
Matter, Molecules and Atoms
4. Write down three examples of matter in liquid phase and three examples of matter in solid phase. Write down two examples of matter
in the gas phase.
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Lesson 3
Grade 8 Physical Science
Scientific
Method
(continued)
Now that we have reviewed matter, and seen its different phases, let’s
look more closely at the different substances that make up matter. Most
substances are made up of several other substances. Water is made up of
substances called hydrogen and oxygen. Salt is made up of sodium and
chlorine. Water can be broken down into hydrogen and oxygen; salt can
be broken down into chlorine and sodium. Hydrogen, oxygen, chlorine
and sodium are called elements. Elements are the building blocks of all
matter, and cannot be broken down further.
ELEMENTS
• Cannot be broken down
• Building blocks of all matter
Various elements join together
in different combinations to
make all matter.
But what makes each element
different from one another?
As we have seen, elements are the basic buildings blocks of all matter. But
each element is made of a different kind of atom. For example, the element
“hydrogen” is made of a hydrogen atom, and the element “oxygen” is
made of oxygen atoms. Likewise, the elements “sodium” and “chlorine”
are made of a sodium atom and a chlorine atom, respectively. Atoms are
very small. Millions and millions and millions of atoms could fit on the
head of a pin.
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Grade 8 Physical Science
Lesson 3
Atoms of different types join together to make
matter. Hydrogen and oxygen join together
to make water. In the case of water, two
hydrogen atoms join with one oxygen atom.
When they join together, they make a molecule
of water. A molecule is made of several atoms
of different types joined together to make
to make something like water, salt, skin, hair
and air. Billions of molecules of water make
up a cup of water that you drink.
Scientific
Method
(continued)
Water molecule
When scientists draw a picture of how the
different atoms join together to form a molecule, they use illustrations like
the ones here. As you can see, symbols are used to indicate the different
kind of atom, such as the symbol “H” to indicate a hydrogen atom, and the
symbol “O” to indicate an oxygen atom. The next image shows how the
atoms making up the molecule for salt are symbolized.
As you have seen, all matter is made of molecules, and how the molecules
are formed will create different types of matter. Molecules are made up of
atoms of different elements. But what are atoms made of? Atoms are made
up of atomic particles. There are several kinds
of particles which make an atom. These
particles are called protons, electrons, and
neutrons. The protons and neutrons make
up the core, or inner part, of the atom. The
core of an atom is called its nucleus. Electrons
move around the outside of the atom.
Electrons can move from atom to atom.
In Lessons 20-27 you will learn more about
atomic particles and how moving electrons
create electricity.
Salt molecule
5. Define the following terms:
Matter
Solid
Liquid
Gas
Element
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Lesson 3
Grade 8 Physical Science
Notes
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4
Energy
Go outside and look around you. You can see and hear activity all around
you. The wind rustles the leaves on the trees. You feel the heat of the sun.
You see cars and bicycles move by. People are moving, talking, working
and playing. All of these things require energy for them to happen. We
use energy to keep ourselves alive. We use it to work and to make our
work easier. Energy runs all living things; energy runs us and our machines.
We live in a sea of energy; energy is all around us.
Energy is the capacity for movement and change. It produces changes in
matter. You get energy from the sun and from the food you eat that stores
the sun’s energy. In fact, most of the energy on earth comes from this one
source — the sun. Your body uses energy every time it does anything. Energy
is needed to make anything move, even the smallest cell. And whenever
anything moves, energy is used.
Most of this course is about energy
and the different forms that it takes.
This lesson is an overview of the types
of energy that we will be studying in
more detail throughout the course.
Energy
• Runs all living things
• Is everywhere
Types of Energy
There are many different types of energy. All of them concern some type
of motion. Everything has at least one type of energy and many things
have several different types of energy. We will discuss some of the most
common ones in turn.
Thermal or heat energy is the energy in moving molecules. All things
contain some heat energy. Rub your hand on your arm and it will become
warm. Adding heat energy to anything makes its molecules move faster.
When you boil water, the water molecules move faster; they move so fast
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Lesson 4
Grade 8 Physical Science
Energy
(continued)
that some molecules begin to leave the container as the water boils away
and evaporates. Heat can turn a solid into a liquid and a liquid into a gas.
With each of these transitions, the molecules are able to move about more
and more freely.
Light energy comes from the sun to Earth in the form of light waves. We
cannot see these waves, but they are very much like ocean waves. (We will
learn more about light waves in Lessons 16 and 17.) Light waves travel in a
straight direction which is described as a ray of light. Anything that gives
off light has light energy. Plants grow by using light energy. Photography is
an excellent example of the ability of light to cause change. Light can form
an image on photographic film by changing the state of the silver coating
on the film.
Electrons are one of the types of atomic particles we looked at in Lesson
3. Electrical energy is the energy that is in moving electrons. Light bulbs,
radios, and appliances use this type of energy. Electrical energy can turn
a motor, and it can transfer your ideas onto a magnetized tape in a tape
recorder or onto a magnetized disc in a computer. It can send your voice
thousands of miles through a telephone system. You will learn more about
electrical energy in future lessons.
Chemical energy is energy that is stored in chemicals. It is released in
chemical reactions or whenever two or more chemicals interact. Chemical
energy heats your home when you burn coal, wood, gas or oil. It is in batteries and changes to electrical energy when the battery is used. Chemical
energy is what your body runs on when you digest food.
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Lesson 4
Some Types of Energy
Energy
Heat or Thermal
Mechanical or Motion
Light
Gravitational Electrical
Sound Chemical
Atomic
(continued)
Mechanical energy is often defined as “the ability to do work.” It is the
energy that is in moving things, or things that have the potential to move
if they weren’t being held back by something. Wind, falling rocks, and
moving water all have mechanical energy. So do all machines that move,
and so do you, when you are running across a field or swimming in a pool.
The rock that is about to fall (but isn’t yet) also has mechanical energy,
just as you do when you are standing on a diving board. You will learn
more about mechanical energy—and the related ideas of work, power,
and force—in Lessons 6 through 10
Gravitational energy is a type of mechanical energy. Gravity is the force
of attraction between two objects and it exists between any two objects
in the universe. The Earth’s huge size makes it easy for you to feel its
gravitational energy but there is also a gravitational force between you
and everything around you—it’s just too small for you to feel it. You will
learn more about gravity and gravitational energy in Lesson 7.
Sound energy is energy caused by vibrating objects. The object vibrating
causes the air to vibrate, and the sound wave travels through the air to
our ears. Have you ever felt the house shake from a really loud thunder
crack? That is sound waves causing
the house to vibrate. This is sound
energy. So is the music you hear from
your CD, and the sound of a kettle of
hot water whistling on the stove. We
will discuss more about sound energy
and sound waves in Lessons 14 and 15.
Atomic or nuclear energy is the energy
that is stored in the nucleus (nucleus
is another word for center or core)
of an atom. The sun produces light
and heat from atomic energy. The
destructive power of atomic energy
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Energy
(continued)
is easy to see in pictures of Hiroshima and Nagasaki after the explosion of
the atomic bombs in World War II. We will learn more about atomic energy
in Lessons 34 and 35.
1. Sit quietly alone in a room for a while. Listen and watch carefully.
After a time, you will start to hear and see signs of energy around you
— perhaps a family member walks by in the hall, or a bicyclist goes by
on the street. Perhaps you can feel air coming through a vent, or see a
curtain moving in the breeze. Write down the signs of energy you find,
and describe which type of energy it is.
2. For each of the eight types of energy just discussed, write down
an example of how or where this energy type occurs. Describe how
your example shows that type of energy.
3. For the following story, list the types of energy present. Your answers
should include at least one of each of the eight energy types you have
learned about:
Pat and her friend Kevin rode to the park on their bicycles (
a
).
The sun was shining brightly ( b ) and by the time they got there,
they were hot and tired (
c ). They were also hungry, so they
pulled out two sandwiches and ate them ( d
); soon they felt
much better. They sat on the swings for a while, swinging back and
forth ( e ) and talking ( f ). After awhile, they decided to
listen to some music on their portable radio (
g ) but soon
realized that their batteries were low ( h ), so they rode home
and listened to the stereo (
i
) at Pat’s house.
Potential Energy and Kinetic Energy
All types of energy can be divided into two states: kinetic energy and
potential energy. The word kinetic means moving. Kinetic energy is energy
of motion. When you are bouncing a ball, it has kinetic energy because
it is moving. When we refer to kinetic energy, we are usually referring to
energies where visible movement occurs. But even sound and electrical
energy involve movement (of atoms and molecules, and electrons,
respectively), so they could technically fall under this category.
What about when you aren’t bouncing the ball and it just sits on a shelf
in your room? Then the ball has potential energy—stored up energy that
is waiting to be released. If the ball got a chance, it would roll off the shelf
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and fall to the floor. We can say that the ball has gravitational potential
energy (which is actually a type of mechanical energy that you just read
about). Potential energy is stored energy. Energy does not have to show
itself in order to exist; it exists even when you can’t see it.
Lesson 4
Energy
(continued)
Potential energy often exists as a result of the position of an object.
Gravitational potential energy is everywhere. Birds have it when they are
in the air or in trees. Trees have it; if disturbed they fall down. You have
it as you hold yourself up. When you are tired, what do you do? You lie
down to lower your potential energy!
Potential energy is energy that is just waiting to happen!
Though gravitational potential energy is quite common, you can see
potential energy in other places. There is electrical potential energy; we
call it voltage. It is stored electrical energy. When it is released it’s not an
object that moves, but an electrical charge (electrons). Electrical potential
energy is also found in batteries. This energy is released when you turn on
the portable CD player or the flashlight. There is also chemical potential
energy stored in molecules. This can be released by a chemical reaction.
When you open a jack-in-the-box, the
potential energy of the coiled spring
inside is released. If you stretch a rubber
band, it has potential energy until it is
released and snaps back to its normal
position. This is called elastic potential
energy, which is a form of mechanical
energy.
Another example of potential energy is a
pile driver. (A “pile” is another word for
a big post, such as the piles which hold
up a pier.) A pile driver is a machine
with a huge weight on the end of a
heavy metal cord. The huge weight is
raised up high and then dropped. As
it falls, it goes faster and faster and by
the time it hits its target, it has enough
energy to drive a huge post into the
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Energy
(continued)
ground. It acquired that
energy because as it was raised
against gravity, the amount of
gravitational potential energy
increased. The potential energy
was then changed into kinetic
energy (energy of motion) when
it was released.
Any system wants to get to the
lowest potential energy possible.
That’s why things fall down! If
you leave the lights of your car
on, the electrical energy will
flow from the battery, eventually
getting to the point where the
battery has no electrical
potential energy! In other
words, you have a dead battery.
If you wind up a spring loaded
toy, it will release and unwind at
the first possible chance. When you think about whether something has
potential energy, think about whether that thing will move on its own if
whatever is holding it in place is removed. A kitchen appliance that plugs
into a wall doesn’t have potential energy of its own, as it needs to have
energy added to it to run. (OK, it does have some gravitational potential
energy because it’s sitting on the shelf!). A ball sitting on the ground
doesn’t have potential energy unless it’s at the top of a hill.
4. Think of and write down two different examples of potential energy
(energy waiting to happen). The examples should be different from
the ones listed in this lesson describing potential energy. Then describe
what can happen to create kinetic energy in each of your examples.
5. Take a rubber band, and stretch it between the thumb and index
finger of one hand. Hold it there for as long as you can. At what state
is the energy in the rubber band? As your hand gets tired, what state of
energy is it fighting against? Release the rubber band from your finger.
At what state is the energy in the rubber band as it is released?
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6. Choose one of the following:
a. Think of a person standing still at the end of a springboard,
getting ready to make a dive into a pool. When the diver is ready,
he will make several jumps on the springboard, and then dive into
the water. Describe each step of his dive, identifying the points
where the amount of potential and kinetic energy change.
Lesson 4
Energy
(continued)
b. Think of a rollercoaster standing still as passengers board. Then
it starts up, climbing to a high point before beginning its first
descent. As it goes along the track it goes up and down, around
turns, and perhaps around loops several times. Describe an
imaginary roller coaster ride, identifying the points where the
amount of potential and kinetic energy change.
Energy Can Change Form
Think about what happens when you strike a match. You are holding a match
in one hand and the match box with a striker on it in the other. You, the
match, the box and all their components have potential energy. You strike the
match on the box, changing your potential energy into mechanical energy
as you move. The chemical in the match head sparks (chemical energy) and
the match head explodes into flame (heat energy) and makes a “whooshing”
sound (sound energy). The chemical energy in the burning match continually
changes into heat and light. You blow the match out (mechanical energy
again) and the match gives off smoke (chemical energy) as it cools.
This is just one example of how a simple action can produce many energy
changes. There are many examples of changing forms of energy all around
us all of the time. Our muscles are continually changing the chemical energy
that we derive from the food we eat into mechanical energy as we move. A
CD player turns electrical energy into mechanical energy (motor that plays
the CD) and sound energy (the music that you hear) and heat energy (heat
is released from the back of the CD player).
Take some time to consider how energy can change form. For example,
think about chemical energy. The chemical energy in food changes form in
your body to give you the energy to move (mechanical energy). What energy
was changed in order to give energy to the food? The sun’s atomic and
light energy was transformed for food to grow. The food stored the sun’s
energy and released it into your body in the form of chemical energy.
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Energy
Used Energy and Heat Energy
(continued)
When energy is used, there is one thing that always happens: heat energy
is produced. No matter which type of energy is used, heat is produced. To
put it another way, whenever energy changes form, heat is produced. To
understand this, let’s look at some examples.
Here is an example of used mechanical energy making heat. Have you ever
bent a thin piece of metal back and forth to break it in half? What happens
to the metal as you bend it back and forth? The mechanical energy that
you are supplying is transferred to the stress point, which becomes warm
until it snaps. (The snap is sound energy.) You can feel the heat from the
broken metal. The heat is slowly released into the air until the metal cools.
When electrical energy is used, it also releases heat. Electrical energy is
used to operate a CD player, stereo, television, or video tape player. If you
ever looked at the back of any of these appliances you would see a grill
covering a vent through which heat is released. When you buy a new
appliance and set it up, the instructions will tell you to place the appliance
away from the wall with enough room for the heat in the back to escape.
Heat is produced whenever you use electrical energy to operate one of
these appliances.
Using chemical energy also releases heat. If you are running, your muscles
use a lot of chemical energy through the food you eat to keep you moving.
The activity of running warms your body. You actually radiate much of this
heat out and away from you, warming the air around you (even though
you may not notice that you’re doing this). As your body uses chemical
energy, heat is released. Have you ever been in a room with a lot of people
dancing or playing an active game? The room warms up with all of the
heat being given off from the moving bodies as they use chemical energy.
Whenever energy changes form, some of it is always changed into heat
energy and released. Scientists and engineers try very hard to minimize
this loss of heat because it is considered “wasted” energy. Imagine that
you like to eat ice cream, but you only like it when it is frozen. You can’t
stand to eat melted ice cream, but every time you eat a bowl of frozen ice
cream some of it melts. Some of the ice cream is wasted because it changes
into a form that you won’t eat. Has the wasted ice cream disappeared?
No, it has just changed into a form that is not useful to you.
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Even when a machine is supposed to produce heat - like a toaster oven —
some of the electrical energy that could be put into making the inside of
the toaster oven hot escapes to the air around it. In every use or change
of energy, some is “lost” as heat.
Lesson 4
Energy
(continued)
When energy is used, heat energy is always produced.
This problem of escaping heat energy is termed by engineers as a problem
of efficiency. If all of the electrical energy were used by a CD player in order
to produce music, the CD player would be considered 100% efficient. This
is not possible however, as some energy is always converted to heat energy
and wasted. Some machines are more efficient than others in using the
energy put into them. A highly efficient machine is one that uses most of the
energy that is put into it and releases very little as heat energy. Efficiency is
an important factor to consider when you are deciding which model of an
appliance to buy. When an appliance is labeled as “energy efficient,” what
it is referring to is how efficient the appliance is at converting the electrical
energy or fuel it runs on into the work the appliance is designed to do, while
eleasing a minimal amount of heat.
Why can’t we have 100% efficient machines? Why is heat energy always
released when energy is used? Does energy disappear? Is energy created?
In the next Lesson, we will learn about the Laws of Thermodynamics and
get the answers to these questions.
7. Examine some appliances around your house. Find where the heat
is released. Write down what appliances you looked at and what you
found.
8.Define the following terms:
Energy
Kinetic energy Potential energy Efficiency
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Notes
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23
Resistance and
Ohm’s Law
We have described how
electric current moves
through a medium, and that
conductors, such as metal,
are media that let electric
current move through them
easily. Other materials,
called insulators, prevent
the movement of electricity.
Resistance is the measure
of how easy or hard it is
for an electric current to
flow through a particular
medium. Conductors have
low resistance while
insulators have high resistance.
Wires can be made of different metals. We have already described
how aluminum and copper can be good conductors as both have low
resistance (although copper is used predominantly in home construction
now due to the fire danger associated with aging aluminum wiring).
However, sometimes metals with a high resistance are used for special
purposes. Nichrome is a metal that is also a conductor, but it has higher
resistance than aluminum or copper. When a large electrical current goes
through nichrome, it gets very hot. Toasters and electric heaters use wire
coils made of nichrome or a metal with similar properties inside them
in order to create heat. Tungsten, which is used as the filament in many
light bulbs, is another metal that is a conductor but has a high resistance.
When current passes through a tungsten wire, it gets so hot that it glows
white-hot.
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Grade 8 Physical Science
Resistance and
Ohm’s Law
(continued)
The amount of resistance a wire has depends on four things: the length
of the wire, how thick it is, what it is made of, and its temperature. The
longer a wire is, the greater its resistance, as the electric current has to
pass through more media. The thinner the wire, the greater its resistance
as well, as it is harder for the electrical current to move along. As we have
seen, the composition of the wire (what it is made of), also determines
resistance. And the warmer a wire is, the greater its resistance.
Let’s look at an analogy. Picture a large elevated tank of water. The height
of the tank determines the potential energy of the water, similar to the
electric potential (voltage) in a circuit. If we want to have water flow out
of the tank (current), we need to have a hose or pipe for it to flow through.
The amount of water flow we get depends a lot on the size of the hose,
as well as on the height of the tank. It is the same with electric current.
A higher voltage can allow a higher current. Also, a larger “pipe” has less
resistance, and a greater current can result. In this way, as we are about to
learn, the current depends on the voltage and the resistance of the material.
Now think about that light bulb again. As you learned in the last lesson, the
tungsten filament is surrounded by a special gas that keeps it from burning
up too quickly. Nevertheless, little by little the tungsten filament evaporates.
As this happens, the wire gets thinner and thinner. This causes the resistance
to be even greater. A greater resistance will cause more heating up, and the
hotter it gets, the more the resistance increases! Gradually, the filament
wears out. Have you noticed that a light bulb usually burns out right when
it is turned on? It glows really bright and then — pow! A cool light bulb
has less resistance, which allows more current to flow. This is more than
the thin filament can take, and when it heats up, the resistance increases
until — pow — out it goes. All this happens in a second!
Superconductors
Some materials are called superconductors because they lose all of their
resistance at low temperatures. Mercury is a good conductor at ordinary
temperatures. It becomes a superconductor at 270 degrees below
zero Celsius. If scientists could find a way to make a superconductor
at normal temperatures, the cost of moving electricity from a power
plant to your house would decrease dramatically!
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Lesson 23
1.Which wire has greater resistance: a light plugged into an extension
cord or a light plugged directly into a wall?
Resistance and
Ohm’s Law
2.Compare the thickness and the length of the cords for several
appliances in your house, including the cords for small appliances
such as table lamps and coffee maker, larger appliances like TV’s
and microwaves, and big appliances such as refrigerators, freezers,
and air conditioning units. List the cords in order of thickness, from
thickest to thinnest, and note the length. Which appliances need the
most electricity to operate? How does the length and thickness of the
cord compare with the demand for electricity each appliance has?
(continued)
3.Now compare these household cords to the cord bringing electricity
into your house (if you can see it) and the overhead power lines
in your neighborhood. If the power lines in your neighborhood are
buried, try to find an area to observe where they are above ground.
Answer the following questions. Are the power lines thicker than the
cords used for the appliances? Why or why not? Are the electrical
wires carrying electricity around your neighborhood underground or
above-ground? What are the advantages and disadvantages of each
of these methods?
4.If you were in charge of designing a wire to carry electricity across
your state or province, which of the following properties would be
most important for your wire to have? Should it be thick or thin,
buried underground or installed out in the sun? What material would
you choose and why?
Ohm’s Law
Ohm:
The unit of measurement for resistance.
How is resistance measured? Remember, electric potential is measured in
units of volts (physicists use the symbol “V” for this). The amount of electric
current is measured in units of amperes (symbol “I”). Resistance is measured
in units of ohms (symbol “R”). The definition of an ohm is the resistance at
which one volt of electric potential allows one ampere of current to flow.
An example of an ohm value is a flashlight bulb. It has the resistance of
1 ohm, meaning that one ampere of current flows through it at one volt.
A 60-watt light bulb has the resistance of about 200 ohms.
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Grade 8 Physical Science
Resistance and
Ohm’s Law
(continued)
George Ohm (1787–1854)
German Physicist, Mathematician
George Ohm was a professor
of mathematics at Jesuits
College in Cologne (Germany)
for ten years. In 1827, Ohm
wrote a pamphlet outlining
his discovery of the law later
named after him. Ohm’s
law states that the current flowing through a conductor
is directly proportional to the voltage, and inversely
proportional to the resistance. This was a major statement
with far reaching implications. His work had a great impact
on the theory and applications of current electricity. Sadly,
it was coldly received and he was so deeply hurt he resigned
his teaching position. It wasn’t until 1841 that his work
began to be recognized. At that time he was awarded the
prestigious Copley Medal of the Royal Society of London.
Ohm’s Law:
Electromotive Force = Resistance x Electric current
The voltage, amperage (number of amperes) and resistance are related
to each other by a rule known as Ohm’s Law. Ohm’s Law states that the
current (I) in a circuit depends on the difference in electric potential
across the circuit (V), and the resistance of the material (R). Specifically,
Ohm’s Law states that the current in a circuit is equal to the voltage
difference divided by the resistance. This is how it looks:
Amperes (I) = Volts (V) ÷ Ohms (R)
We can use Ohm’s Law to calculate the current in a wire when the resistance and the voltage are known. By inverting the equation, we can also
find the voltage or the resistance if the other two elements are known.
Other ways of writing this equation are as follows:
V = I x R or R = V ÷ I
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Lesson 23
For example, if a 4-ohm wire is connected to a 12-volt battery, what would
the current be? We know the value for V is 12, and the value for R is 4.
I=V÷R
I = 12 volts ÷ 4 ohms
I = 3 amperes
Resistance and
Ohm’s Law
(continued)
Ohm’s Law:
V=RxI
I=V÷R
R=V÷I
5. Assume your toaster has a resistance of 10 ohms, and it is plugged
into your house electricity of 120 volts. What is the current in the wire
when your toaster is plugged in and on? Show your calculation.
Resistors and Circuits
In the last lesson you learned about two types
of circuits: a series circuit and a parallel circuit.
To review, a series circuit is one in which there
is only one path for the electricity to follow
and a parallel circuit is one in which there are
several different paths for the electricity to
follow. As you have also learned, you can
design a series circuit by connecting a wire
from one dry cell terminal to a light socket,
and then connect another wire from the
light socket to the other dry cell terminal.
The electricity would then flow from the dry cell, through the light socket
and back to the battery, lighting up the bulb. The light bulb uses the
electrical energy carried in the electric current in order to operate.
A resistor is what scientists call any object, such as an appliance or
machine, that is connected in a closed circuit. A resistor uses the
electrical current running through the circuit to operate. In the series
circuit described above, the light bulb is an example of a resistor.
Every time you turn on a lamp or appliance in your home, it operates
because it is a resistor.
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Resistance and
Ohm’s Law
(continued)
Resistors and Series Circuits
When a number of resistors are connected so that
the entire current flows through one resistor after
the other, the result is a series circuit. This is true
in the previous example of Christmas lights. Each
bulb on the string of lights is a resistor. The electrical
current flows through each bulb, one after the other.
The total resistance on a series circuit can be determined. When several
resistors are linked together in a series circuit, the resistance (in ohms) of
each resistor is added together with all of the other resistors. Together they
equal the total resistance of the circuit. For example, the ohms of each
bulb in a string of fairy lights is added together to find the total resistance
of the entire string of lights.
In the last lesson you learned that in a series circuit, the current remains
the same everywhere in the circuit. When you add more resistance to the
circuit by adding resistors, and the total voltage (determined by the power
source) stays the same, then by Ohm’s Law, the current will decrease. So if
you want to determine the amperage of a series circuit, you must first add
up all the resistances in the circuit.
In a series circuit, the voltage will drop as it goes through each resistor.
The more resistors you have, the less voltage difference there will be across
each one. All the individual voltage drops added together will add up to
the total voltage of the circuit, which is determined by the power source.
Let’s look at an example. Let’s say you have a series circuit connected to a
6-volt battery. You add to the circuit, one at a time, a fan with resistance
of 3 ohms, a light bulb with resistance of 1 ohm, and an electric clock
with resistance of 2 ohms.
First you attach the clock which has R = 2 ohms. If you used an ammeter
to measure the current flowing through the wire, what would you expect
the measurement of the current (in amperes) to be?
I=V÷R
I = 6 volts ÷ 2 ohms
I = 3 amperes
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Lesson 23
Next you attach the light bulb which has R = 1 ohm, and the fan which
has R =3 ohms. Again you use an ammeter to measure the current flowing
through the wire. Remember that no matter where you attach the ammeter,
the current will be the same. What would you expect the measurement of
the current to be? To calculate, you need to add up each resistor to find
the total resistance in the circuit.
Resistance and
Ohm’s Law
(continued)
I=V÷R
I = 6 volts ÷ 2 ohm (clock) + 1 ohm (light bulb) + 3 ohm (fan)
I = 6 volts ÷ 6 ohms
I = 1 ampere
The reason the current is lower when you put all three objects on the
circuit than when you only had the clock on the circuit is because of the
greater resistance in the circuit. According to the formula, if the voltage
stays the same, and the resistance increases, the current has to decrease!
So what about the voltage? As the electric current leaves the negative
terminal of the battery, it is at its greatest potential. The potential (voltage)
drops after each resistor, but the total voltage change is 6 volts. It is at
its lowest potential as it reaches the positive terminal of the battery. The
chemical activity in the battery then raises the moving charge back to a
high potential as it moves into the circuit again.
What would happen if we were to increase the voltage? Let’s say we had
our three resistors, and a total resistance of 6 ohms. We want our current
to be higher than 1 ampere, so we have to increase the voltage of the circuit.
We switch to a 12 volt battery. In reality, we have to be careful that each
of our resistors can handle the increased voltage and current! Now our
formula looks like this:
I = 12 volts ÷ 6 ohms
I = 2 amperes
IN A SERIES CIRCUIT:
The voltage drops across each resistor in the circuit. The current
remains the same anywhere in the circuit. The individual resistances
add up to the total resistance of the circuit.
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Grade 8 Physical Science
Resistance and
Ohm’s Law
(continued)
3 8
Resistors and Parallel Circuits
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Grade 8 Physical Science
Lesson 23
Can you see that this total resistance is less than the resistance of each of
the individual resistors? Compare the total resistance in the series circuit
to the total resistance in the parallel circuit.
Resistance and
Ohm’s Law
(continued)
Series circuit: R = 6 ohms
Parallel circuit: R = 0.545 ohms
Since there is a lower resistance in a parallel circuit when more resistors
are added, according to Ohm’s Law, the total current will increase.
Okay, so what about the voltage? In a parallel circuit, the voltage is the
same anywhere in the circuit. This is quite different from a series circuit!
IN A PARALLEL CIRCUIT:
The voltage is the same anywhere in the circuit. The current differs
throughout the circuit, and the total current is equal to the sum of
the currents through each path. The total resistance decreases as you
add more resistors to the
6. Which kind of circuit has less resistance, a parallel circuit or a series
circuit? Would you say this makes it more or less energy efficient?
7. Answer the following questions, showing your calculations if needed:
a. You have a 10 volt parallel circuit, with 2 resistors on it. What is
the voltage across the first resistor? Across the second?
b. You have the same two resistors on a 10 volt series circuit. Will
the voltage going into the second resistor be more, less, or the
same as that going into the first resistor? Exact numbers aren’t
needed!
c. You have a series circuit with a current of 6 amps and three
resistors on it, with resistances of 10 ohms, 5 ohms, and 6 ohms,
respectively. What is the voltage of this circuit? Show your
calculation.
8. Define the following terms:
resistance
superconductor
ohm
resistor
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Lesson 23
Grade 8 Physical Science
Notes
256
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Grade 8
24 Home Electricity
The examples of electric circuits that we have looked at
so far mostly involved small amounts of electricity in
appliances. There are electrical circuits in your house,
too. Your house is part of a larger circuit that is
connected to the power supply of a power company.
The amount of electricity flowing through your home at
any moment depends on the number of appliances that
are all working at the same time. The total amount of
electricity that flows through your house is determined
by the amount of current being used (the sum of thecurrent being used by
each appliance) and the voltage of your home circuit. Within each home,
there are usually smaller circuits which are wired to the larger circuit
running into the home. These smaller circuits control certain areas of the
home. For instance, there may be a circuit for the upstairs bedrooms,
another for the kitchen, and another for the outdoor lights.
When there are a lot of appliances turned on at the same time in your
house, a lot of electric current flows through the wires of your house.
When large amounts of current pass through a wire, the wire heats up.
If it gets too hot, it can cause materials nearby to heat up and catch fire.
It is therefore very important that wires in your house carry only as much
electrical current as they can safely carry without getting overheated.
Overload
Suppose that you are using a 150-watt light bulb in a bathroom that has
an electrical outlet on the same circuit. It is winter time and you are heating
the room with a 3600-watt space heater plugged into the outlet. You plug
your 1800-watt hair dryer into the outlet and begin to dry your hair and
all of a sudden everything turns off. You are left in the dark with a wet
head and cold feet. This is an example of what is called overload.
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Grade 8 Physical Science
Home
Electricity
(continued)
Let’s figure out what happened to better understand overload. (Take a
moment to review amperes, volts, and watts from Lesson 22 if you need to.)
Remember that there is a relationship between amperes, volts, and watts
that can help us. The rule tells us that amperes equals watts divided by volts,
or I = W ÷ V.
We know that the circuit in your home is a 120-volt circuit, which is standard
in homes in the U.S. We also know the wattage of each appliance. Now let’s
determine how much current (amperes) is being drawn by each appliance.
The light draws 150 watts on a 120-volt circuit.
I = 150 watts ÷ 120 volts
I = 1.25 amperes
The light uses 1.25 amperes of electricity.
The heater draws 3600 watts on a 120-volt circuit.
I = 3600 watts ÷ 120 volts
I = 30 amperes
The hair dryer draws 1800 watts on a 12-volt circuit.
I = 1800 watts ÷ 120 volts
I = 15 amperes
When you turned on all three appliances, the total usage was 46.25
amperes at the same time. Since these appliances were all being used in
one small room (the bathroom), they were probably all wired to a single,
smaller circuit within your home. But why did they all go out?
To prevent wires from overheating, safety devices are installed in homes to
limit the amount of electricity that can flow through the wires. These devices
are called fuses and circuit breakers. In order to understand the overload
problem you experienced in the bathroom, you need to learn about these.
As you now know, electric power is brought into your house in one large
power line from the street. Inside your house it is divided up into several
different circuits. Most houses have about 5 to 15 circuits. Most house
circuits are wired for 120-volts. 240-volt circuits are used for particular
circuits that run major appliances like electric clothes dryers, water heaters,
well pumps, ceramic kilns, or electric stoves that require large amounts of
electricity.
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All houses have either a fuse
box or a circuit breaker box,
but usually not both. Fuses
are inserted into the electric
wiring of your house. They
are located in the fuse box,
which is located at the point
where the power entering
your house is divided up into
the different circuits. A fuse
is made of a strip of wire
that has high resistance but
melts at a relatively low
temperature. The fuse is
placed somewhere in the circuit and if the current gets too high, the wire in the fuse melts and
immediately opens the circuit, stopping the flow of electricity. The circuit
is “blown” and electricity can no longer complete the circuit.
Lesson 24
Home
Electricity
(continued)
A circuit breaker is a similar type of safety device that is part of the electric
wiring of your house. Circuit breakers are located in a circuit breaker box
that is usually placed somewhere convenient in your house, such as on a
wall just inside an attached garage, also at the point where the main line
is divided up into different circuits. A circuit breaker works much the same
way as a fuse. A circuit breaker is a switch with a gap in it. Because of the
gap, no current flows through the switch under normal loads. The heat of
a large electrical overload (caused by too many appliances operating at
the same time) causes a bimetallic (two
metal) strip within the circuit breaker
to bend. When it bends, the metal strip
becomes disconnected from the circuit.
Electricity will arc, or jump, across the gap
and activate the switch which immediately
opens the circuit. When the metal strip
cools sufficiently to be safely connected,
the strip returns to its normal position.
While a fuse must be replaced if it is
blown, the circuit breaker only has to be
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Lesson 24
Grade 8 Physical Science
Home
Electricity
(continued)
reset and moved back into the “on” position after the overloading situation
has been located and corrected. You must manually push the circuit breaker
back to allow the electricity to flow again.
The difference between circuit breakers and fuses is that while a fuse has to
be replaced when it is “blown,” a circuit breaker continues to function over
and over again. A circuit breaker can also be manually switched to open a
circuit so that you can make electrical repairs without the danger of electricity
flowing through the circuit (and you!). Most homes nowadays are set up
with circuit breaker boxes instead of fuse boxes. Some individual appliances
might have their own fuse for additional protection for that appliance.
1.Locate the fuse box or circuit breaker box in your own home or
building. Ask your parents where it is located, and examine it closely.
a. Make a sketch which includes all the fuses or circuit breakers
in the box. Usually they will be labeled (such as “Living Room”
or “Refrigerator” or “Central Air” or “Bedr oom”). Copy these
labels. If the fuses or circuit breakers are numbered instead, there
is usu- ally a list nearby to tell you what number goes with what
room or section of your home.
b. On each fuse or circuit breaker there will be a number like 10, 15,
20 or 40. Make a note of this number . This is the number of
am- peres allowed for that circuit.
c. Add up the total amper es allowed for your house. This is the
total of all the circuits for your home.
d. What is the ampere limit for your bedroom circuit? For your
bathroom circuit?
e. Make a list of everything that is plugged into the outlets of one room
or section of your home (one circuit). Examine each item carefully
and determine how many amperes each item uses. Calculate
the amper es using the formula if necessary. Compare the total
amperes used with the total available amperes on that circuit.
If the wires in a circuit carry too much current, it is possible for the wires
in the circuit to build up an excess amount of heat. If the wires become hot
enough, the insulation around the wire can burn off and then the exposed
wire may ignite whatever is touching it. This is how electrical fires start.
Fuses and circuit breakers prevent dangerous overloading of a circuit by
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Grade 8 Physical Science
building in an intentional weak spot in the circuit. The weak spot is the
fuse or circuit breaker.
2.Now let’s return to the question of why everything went off when
the light, a heater, and a hair dryer were all running in the bathroom
at the same time. After all, the light usually stays on just fine whenever
you use the bathroom. Why would it go off now? Answer this question
assuming the circuit breaker or fuse for the bathroom has a 40-ampere
capacity. Use the terms “circuit,” and “overload.” Then think of at
least two solutions to prevent this from happening again.
Lesson 24
Home
Electricity
(continued)
3. Assume that you have plugged into your bedroom outlets a 3600watt heater, a 300-watt tape recorder, and two 150-watt lamps.
You turn them all on and then you plug in a hair dryer (1600 watts)
and a vacuum cleaner (200 watts) and you dance around the room
vacuuming and drying your hair with your music blasting! Would you
blow the fuse or trip the circuit breaker to your bedroom or not?
Use the ampere rating for your room that you found in Assignment 1.
Calculate the total amperes used, showing your work.
Short Circuits
Sometimes you may find that a fuse or circuit breaker may blow even though
the flow of electric current does not exceed the fuse or circuit breaker rating
(the number of amperes that the circuit is designed to carry). This is usually
caused by a short circuit.
In order to understand short circuits you must remember that all wires going
into appliances are actually made up of two wires — one wire to bring the
electricity into the appliance (into the resistor) and one to carry it back out.
Look closely at a wire on a small
appliance or lamp in your home.
You will see that it has a ditch down
the middle. This ditch is where the
insulator separates out the two
wires. In an electric circuit, electricity
always moves in a circle, from the
electric energy source through a
wire to a resistor (appliance) and
then back through the second wire
to the electrical source again.
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Lesson 24
Grade 8 Physical Science
Home
Electricity
(continued)
A short circuit is caused when the insulation
around one of the two wires is worn or
frayed, allowing the two wires to touch.
When this happens there is no resistance
between them. The path then taken by the current is shortened; instead of
going through the resistor, the current goes from one wire into the other
wire, and back to the energy source. This is why it is called a short circuit.
Think of it as heading out to the library, only to decide to turn around and
go home again, never getting to your destination. Your trip was shortened.
Short circuits are a problem because of the lack of resistance between the
wires and because the current never reaches a resistor. According to Ohm’s
Law (V= R x I) large amounts of electrical current may flow through the circuit.
The voltage (V) of your house is probably 120 volts; if the resistance (R) is
very small, then the current (I) must get very big for the equation to stay
balanced. The resistors on a circuit are important to keep the current from
getting too high.
Here is an example. Let’s see what happens to your hair dryer if it short
circuits. Suppose the cord to your hair dryer was frayed and the dryer
short-circuited. Assuming your hair dryer uses 1800 watts, let’s first look
at the amount of current flowing through the cord before it was frayed.
Remember, amperes = watts ÷ volts.
I = 1800 watts ÷ 120 volts
I = 15 amperes
The flow of current into the hair dryer is 15 amperes. But we need to know
the resistance of the hair dryer, as the resistance of the circuit is what
changed when the cord frayed and short circuited. Remember Ohm’s Law:
V = R x I (or) R = V ÷ I
R = 120 volts ÷ 15 amperes
R = 8 ohms
Now what happens when the cord frays, causing the short circuit? The
wire that brings electricity into the hair dryer is now touching the wire that
takes electricity out of the hair dryer. The flow of current never reaches the
hair dryer because it turns around and flows back through the circuit before
it ever reaches the hair dryer.
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Grade 8 Physical Science
How much current is now flowing through the wire? Ohm’s Law states
that I = V ÷ R. Even though the current is not flowing through a resistor,
there is still some resistance in the wire. Let’s say the resistance of the
circuit is now 1 ohm. The voltage of your house is 120 volts. 120/1 = 120
amperes of current flowing through the wire! Even if there was 2 ohms of
resistance still, the current would be 60 amps, enough to trip the circuit
breaker, opening the circuit.
Lesson 24
Home
Electricity
(continued)
Large amounts of electricity flowing through a broken or frayed wire can
make it very hot, which may result in a fire. Before the wires get hot, the
surge of electricity through the circuit will cause either a fuse to blow or
a circuit breaker to switch and open the circuit. Any short circuit should
be fixed immediately to reduce the potential threat of heat from the short
causing a fire. In addition, broken or worn wires should be replaced.
4.In the United States, household electricity usually has 120 volts of EMF.
Homes in Great Britain and much of Europe have 240 volts of EMF,
often written 220-230 volts. Choose a small appliance you use
regularly, such as a hair dryer or toaster. How do you think it would
work in Britain? Use Ohm’s Law to explain your answer. (I = V ÷ R)
(Hint: the resistance of your hair dryer would be the same, regardless
of where you operate it.)
Electric Shocks
Electrical current can cause shocks. It is current rather than voltage that
causes electrical shocks, although the voltage will help determine the amount
of shock because higher voltages allow higher current. When electrical
current takes a short path through a person, that person is in the middle
of a short circuit. Shocks commonly occur when a person touches a frayed
cord or other exposed electrical conductor. This is why it is important to
keep babies and children away from electrical outlets. The electrical current
will prefer to take a path through a person to the ground, rather than
around a circuit.
How much of a shock do you get? The flow of current
running through a person’s body will depend upon
the applied voltage and the resistance of the person’s
body. Ohm’s Law (I = V ÷ R) determines the amount
of electric shock someone is feeling.
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Lesson 24
Grade 8 Physical Science
Home
Electricity
(continued)
If your body is dry, it will have more resistance. Remember that water is a
very good conductor, and the human body is comprised mostly of water.
However, the outer skin has a lower percentage of water than does inner
tissue. When skin is dry, the resistance of the body is relatively high, and
the current coming from 10 volts may hardly be felt.
If the skin is wet, the body’s resistance drops about one thousand times!
Ten volts can give a strong shock and 120 volts may be fatal.
Many appliances are grounded. That means that a wire is connected from
the appliance to a ground. All the wiring in your house should also be
grounded with a ground. What a ground does is take excess electric current
to prevent it from going where it can cause danger. Many household cords
now have three wires running through them, one for the current going to
the resistor, another for the current going back to the energy source, and
a third to act as a ground should there be a short circuit. More and more
household outlets are now installed to take these three-pronged cords.
Every building also has a ground built into it as a safety device. Usually it is
either a wire or a pipe that leads from the house down into the ground (the
actual earth). The wire to the ground has less electrical resistance than a
person’s body does. In the event of a short circuit, or if you touch a “hot”
wire, any current flowing through the appliance will take the path of least
resistance and flow through the appliance ground to the house ground wire
into the ground outside. This prevents a shock from going through you!
5. We’ve all seen small birds sitting on power lines, and squirrels
running along them, and none of them get an electric shock.
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It seems the high voltage in the lines should kill them! Many large
birds, such as herons and raptors (hawks and eagles) are electrocuted
often by power lines. It is a big problem, and has reduced populations
of some rare birds. Do some research, and answer why power lines kill
large birds but do no harm to small birds that sit on them. Remember,
it is current, not voltage, that causes electric shock.
Lesson 24
Home
Electricity
(continued)
Electric Meters
Electric companies keep track
of how much electricity your
home uses, and use this
information to send your family
a bill to pay for the power you
used. In order to keep track of
your household use, the electric
company will install an electric
meter on the outside of your
home or building in a place
where it can easily be reached
in order to read it every billing
cycle, usually about once a month. Often the electric meter will be close
to where your circuit breaker or fuse box is located. It is usually covered
with a glass cover.
There are two types of meters. One is like the odometer of a car and you
just read the numbers. Some newer digital meters can be read from a
distance by the electric company, so the meter reader can read it while
driving down the road. Older meters have four or five circular dials lined
up in a row, each numbered 0 to 9. They are read in a special way.
The unit of measurement for the use of electricity is kilowatt hours.
On an electric meter, the number that shows is kilowatt hours. Electric
current is doing work every time you turn something on in your home.
That work is being supplied by the electric company. However, the time
it takes to do that work depends on the amount of power it takes to use
that appliance. Power is measured in watts. A kilowatt hour (kWh) is the
number of 1,000-watt units of power that is used in an hour. For example,
if you have a 100 watt light bulb burning for 10 hours, you have used
1000 watt-hours, or 1 kWh, of power.
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Lesson 24
Grade 8 Physical Science
Home
Electricity
(continued)
6. Locate the electric meter for your house. Write down the number it
shows and note the time and date. Come back in a day or two and
write the new number in your notebook and note the time. If it is an
older meter, it will have four or five circular dials lined up in a row,
each numbered 0 to 9. Begin reading at the far left dial. If a number
is between two numbers, record the lower number, and move onto
the next one to the right. If the dial’s pointer is exactly on a number,
check the next dial to the right — if the pointer on the next dial has
passed 0, record the number indicated by the first pointer. If the next
pointer has not passed 0, record the next lower number on the first
dial. Read all the dials in this way, from left to right.
a. How many kilowatt hours did your family use in the amount of
time that passed between your readings? Divide that number by
the number of hours that passed between readings and write down
the average use per hour of your family. How many watts per hour
did your family use? (Hint: convert from kilowatts to watts.)
b. Now ask your parents for a copy of last month’ s electric bill. How
many kilowatt hours did your family use in the last billing cycle?
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