Electric Current
Reading Guide
Bellringer
New Vocabulary
Section Focus Transparencies
also are available on the
Interactive Chalkboard CD-ROM.
L2
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Section Focus
Transparency
Describe how voltage difference
causes current to flow.
Explain how batteries produce a
voltage difference in a circuit.
List the factors that affect an
object’s electrical resistance.
Define Ohm’s law.
Review Vocabulary
pressure: amount of force exerted
per unit area
current
•• electric
voltage difference
•• circuit
resistance
• Ohm’s law
Go with the Flow
Do you see how the water flows down the cliff? Water takes the path
of least resistance—it flows where it’s easiest for water to go. Electric
currents flow and experience resistance, too.
Current and Voltage Difference
When a spark jumps between your hand and a metal doorknob, electric charges move quickly from one place to another.
The net movement of electric charges in a single direction is an
electric current. In a metal wire, or any material, electrons are in
constant motion in all directions. As a result, there is no net movement of electrons in one direction. However, when an electric
current flows in the wire, electrons continue their random movement, but they also drift in the direction that the current flows.
Electric current is measured in amperes. One ampere is equal to
6,250 million billion electrons flowing past a point every second.
Figure 11 Electric forces in
a material cause electric current to flow, just as forces in
the water cause water to flow.
Copyright © Glencoe/McGraw-Hill, a division of the McGraw-Hill Companies, Inc.
You control electric current every
time you change the volume on a
TV, stereo, or CD player.
Water
flow
1. Which do you think has more energy, the waterfall in the picture
or Niagara Falls which are higher and have move water flowing
over? Explain your answer.
2. How do people use the energy in water currents?
What is electric current?
L2
Electricity
Tie to Prior Knowledge
Battery Trouble Ask students if
they have ever tried to play a CD
in a portable CD player, only to
find that the batteries were dead.
Ask students what they know
about what happens when a battery dies. Explain that when it
dies, the battery stops producing
enough electric current to operate the CD player. Tell students
that in this section, they will
learn what an electric current is
and how it is produced. L2 LS
High pressure
Low pressure
The force that causes water to flow
is related to a pressure difference.
Charge
flow
High voltage
Low voltage
The force that causes a current to flow
is related to a voltage difference.
200
Voltage Difference The movement of an electron in an electric current is similar to a ball bouncing down a flight of stairs.
Even though the ball changes direction when it strikes a stair, the
net motion of the ball is downward. The downward motion of
the ball is caused by the force of gravity. When a current flows,
the net movement of electric charges is caused by an electric
force acting on the charges.
In some ways, the electric force that causes charges to flow is
similar to the force acting on the water in a pipe. Water flows
from higher pressure to lower pressure, as shown in Figure 11. In
a similar way, electric charge flows from higher voltage to lower
voltage. A voltage difference is related to the force that causes
electric charges to flow. Voltage difference is measured in volts.
CHAPTER 7 Electricity
Logical-Mathematical
Section 2 Resource Manager
Answer the net movement of electric
charge in a single direction
200
CHAPTER 7 Electricity
Chapter FAST FILE Resources
Transparency Activity, p.43
Directed Reading for Content Mastery,
pp. 19
MiniLAB, p. 4
Enrichment, p. 29
Lab Activity, pp. 9–12
Reinforcement, p. 26
Lab Worksheet, pp. 5–6
Home and Community Involvement, p. 25
Mathematics Skill Activities, p. 3
Figure 12 Water or electric current will flow continually only through a closed loop. If any part of the
loop is broken or disconnected, the flow stops.
Water
tank
Lightbulb
Water
wheel
Current Students might believe
that positive charges flow
through wires. Early researchers labeled the direction of
current flow from the positive
terminal to the negative terminal of a battery. This established the convention. After
the discovery of the electron,
researchers realized that the
electrons actually move from
the negative terminal to the
positive terminal. Today, we
talk
about
conventional
current, which is the direction
of the hypothetical flow of
positive charges. However, it is
negatively-charged electrons
that flow in a wire. It is important for students to learn these
distinctions and to realize that
positive charges do not flow
through the wire.
Electron
flow
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Battery
Pump
A pump provides the pressure difference that keeps water flowing.
A battery provides the voltage difference
that keeps electric current flowing.
Electric Circuits A way to have flowing water perform work
is shown in Figure 12. Water flows out of the tank and falls on a
paddle wheel, causing it to rotate. A pump then provides a pressure difference that lifts the water back up into the tank. The
constant flow of water would stop if the pump stopped working.
The flow of water also would stop if one of the pipes broke.
Then water no longer could flow in a closed loop, and the paddle wheel would stop rotating.
Figure 12 also shows an electric current doing work by lighting a lightbulb. Just as the water current stops flowing if there is
no longer a closed loop to flow through, the electric current
stops if there is no longer a closed path to follow. A closed path
that electric current follows is a circuit. If the circuit in Figure
12 is broken by removing the battery, or the light bulb, or one of
the wires, current will not flow.
Quick Demo
Simple Cell
Materials small glass or plastic
Batteries
In order to keep water flowing continually in the water circuit in Figure 12, a pump is used to provide a pressure difference. In a similar way, to keep an electric current continually
flowing in the electric circuit in Figure 12, a voltage difference
needs to be maintained in the circuit. A battery can provide the
voltage difference that is needed to keep current flowing in a circuit. Current flows as long as there is a closed path that connects
one battery terminal to the other battery terminal.
SECTION 2 Electric Current
201
Battery Makeup A 9-volt battery is actually
composed of six small 1.5-volt batteries connected in series. A 12-volt car battery is a
series of six 2-volt batteries.
container about three-fourths full
of acid solution (lemon juice,
tomato juice, or vinegar), strip of
copper, strip of zinc or silver, two
wires with alligator clips, sensitive voltmeter (0-1 VDC)
Estimated Time five minutes
Procedure Clip one end of a wire
to one metal strip and the other
end to the voltmeter. Repeat for
the second metal strip and wire.
Place the metal strips in the acid
solution, taking care that the
strips do not touch each other.
Have a student read the voltage
difference that the cell produces
between the two electrodes.
SECTION 2 Electric Current
201
Purpose Students discover the
effect of adding an additional
battery to a circuit. COOP LEARN
L2 Kinesthetic, Visual-Spatial, LS
Logical-Mathematical
Materials D-cell batteries (2),
3-volt bulbs and sockets (3),
insulated wire
Safety Precautions Caution students to leave the bulbs burning
only long enough to make observations, and to not touch a
hot bulb.
Troubleshooting Connecting
more than two batteries per
lamp in series will shorten the
bulb’s life or cause it to burn
out.
Analysis
1. 1.5 volts, 3.0 volts
2. As the voltage difference increases
across the lamp, the charge flowing through it increases.
Investigating
Battery Addition
Procedure
1. Make a circuit by using
wire to link two bulbs
and one D-cell battery
in a loop. Observe the
brightness of the bulbs.
2. Assemble a new circuit by
linking two bulbs and two
D-cell batteries in a loop.
Observe the brightness of
the bulbs.
Analysis
1. What is the voltage difference of each D cell? Add
them together to find the
total voltage difference for
the circuit you tested in
step 2.
2. Assuming that a brighter
bulb indicates a greater
current, what can you
conclude about the relationship between the voltage
difference and current?
Dry-Cell Batteries You probably are most familiar with drycell batteries. A cell consists of two electrodes surrounded by a
material called an electrolyte. The electrolyte enables charges to
move from one electrode to the other. Look at the dry cell shown
in Figure 13. One electrode is the carbon rod, and the other is the
zinc container. The electrolyte is a moist paste containing several
chemicals. The cell is called a dry cell because the electrolyte is a
moist paste, and not a liquid solution.
When the two terminals of a dry-cell battery
are connected in a circuit, such as in a flashlight, a reaction involving zinc and several chemicals in the paste
occurs. Electrons are transferred between some of the compounds in this chemical reaction. As a result, the carbon rod
becomes positive, forming the positive () terminal. Electrons
accumulate on the zinc, making it the negative () terminal.
The voltage difference between these two terminals causes
current to flow through a closed circuit. You make a battery
when you connect two or more cells together to produce a
higher voltage difference.
Wet-Cell Batteries Another commonly used type of battery
is the wet-cell battery. A wet cell, like the one shown in Figure 13,
contains two connected plates made of different metals or
metallic compounds in a conducting solution. A wet-cell battery
contains several wet cells connected together.
Positive terminal
Assessment
Performance Have students design and label several possible
arrangements showing how to
light a bulb at different levels of
brightness. Have them demonstrate each method. Use Performance Assessment in the
Science Classroom, p. 127.
L2
Negative terminal
Positive terminal
Plastic insulator
Figure 13 Chemical
reactions in batteries produce a voltage difference
between the positive and
negative terminals.
Identify when these
chemical reactions occur.
Moist paste
Carbon rod
Zinc container
Negative terminal
Dry cell
Caption Answer
202
Lead dioxide
plate
In this wet cell, chemical reactions
transfer electrons from the lead plates
to the lead dioxide plates.
CHAPTER 7 Electricity
Figure 13 Each cell of a battery has an electrolyte (a
chemical that conducts charge) and two electrodes
(terminals). One electrode provides electrons to
the electrolyte. The other electrode takes electrons
from the electrolyte. In dry cells the electrolyte is a
CHAPTER 7 Electricity
Battery solution
Wet cell
In this dry cell, chemical reactions in
the moist paste transfer electrons to
the zinc container.
Figure 13 The chemical reactions
occur when the battery is connected in a
circuit.
202
Partition
Lead plate
LS Kinesthetic, Visual-Spatial
paste, and in wet cells it is a liquid. What are the
electrodes in this figure? In the dry cell, they are the
carbon rod and the zinc container. In the wet cell, they are
the lead dioxide and lead plates. L2 LS Visual-Spatial
Lead-Acid Batteries Most car batteries are
lead-acid batteries, like the wet-cell battery
shown in Figure 13. A lead-acid battery contains
a series of six wet cells made up of lead and lead
dioxide plates in a sulfuric acid solution. The
chemical reaction in each cell provides a voltage
difference of about 2 V, giving a total voltage difference of 12 V. As a car is driven, the alternator
recharges the battery by sending current through
the battery in the opposite direction to reverse
the chemical reaction.
A voltage difference is provided at electrical
outlets, such as a wall socket. This voltage difference usually is higher than the voltage difference
provided by batteries. Most types of household
devices are designed to use the voltage difference
supplied by a wall socket. In the United States, the
voltage difference across the two holes in a wall
socket is usually 120 V. Some wall sockets supply
240 V, which is required by appliances such as
electric ranges and electric clothes dryers.
Voltage Students sometimes
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think that voltage is the same at
different places in a circuit and
it exists only when current
flows. See page 192F for teaching strategies that address this
misconception.
Use an Analogy
Water Flow You can describe
resistance by building on the analogy of charge flow being similar
to water flow. Suppose a jug of
water has a large hole in one side
and a small hole in the other side.
More water will flow out of the
large hole. Similarly, a wire with a
large diameter has less resistance
and therefore carries greater current than a smaller-diameter wire.
Figure 14 As electrons
Resistance
Flashlights use dry-cell batteries to provide the electric current that lights a lightbulb. What makes a lightbulb glow? Look
at the lightbulb in Figure 14. Part of the circuit through the bulb
is a thin wire called a filament. As the electrons flow through the
filament, they bump into the metal atoms that make up the filament. In these collisions, some of the electrical energy of the
electrons is converted into thermal energy. Eventually, the metal
filament becomes hot enough to glow, producing radiant energy
that can light up a dark room.
move through the filament in
a lightbulb, they bump into
metal atoms. Due to the collisions, the metal heats up and
starts to glow.
Describe the energy conversions that occur in a lightbulb
filament.
Discussion
Making Heat Should the heating
elements on a stove top be made
of high-resistance or low-resistance material? high-resistance L2
Resisting the Flow of Current Electric current loses energy
LS Logical-Mathematical
as it moves through the filament because the filament resists the
flow of electrons. Resistance is the tendency for a material to
oppose the flow of electrons, changing electrical energy into thermal energy and light. With the exception of some substances that
become superconductors at low temperatures, all materials have
some electrical resistance. Electrical conductors have much less
resistance than insulators. Resistance is measured in ohms (Ω).
Copper is an excellent conductor and has low resistance to
the flow of electrons. Copper is used in household wiring
because only a small amount of electrical energy is converted to
thermal energy as current flows in copper wires.
Caption Answer
Figure 14 Electrical energy is converted into thermal energy and radiant
energy.
SECTION 2 Electric Current
203
Ray Ellis/Photo Researchers
Math In a car battery, the following chemical
reaction occurs: Pb PbO2 H2SO4 → PbSO4
H2O. Use coefficients to balance this equation.
Pb PbO2 2H2SO4- → 2PbSO4 2H2O
SECTION 2 Electric Current
203
Temperature, Length, and Thickness The electric resistance of most materials usually increases as the temperature of the
material increases. The resistance of an object such as a wire also
depends on the length and diameter of the wire. The resistance
of a wire, or any conductor, increases as the wire becomes longer.
The resistance also increases as the wire becomes thinner.
In a 60 watt lightbulb, the filament is a piece of tungsten wire
made into a short coil a few cm long. The uncoiled wire is about
2 m long and only about 0.25 mm thick. Even though tungsten
metal is a good conductor, by making the wire thin and long, the
resistance of the filament is made large enough to cause the bulb
to glow.
Answer The resistance increases as the
length of the wire increases and as its
thickness decreases.
Static Danger When it comes
to electricity and our bodies, it
often is true that high voltages
are dangerous and low voltages are safer. Students frequently are not aware of a
common situation where this
is not true. In many static electricity situations, such as walking across a carpet, sparks of
1,500 volts or higher are generated. No damage occurs to
our bodies because there is little actual current flow.
How does changing the length and thickness of
a wire affect its resistance?
The Current in a Simple Circuit
Figure 15 The amount of
current flowing through a circuit
is related to the amount of
resistance in the circuit.
A simple electric circuit contains a source of voltage difference, such as a battery, a device, such as lightbulb, that has resistance, and conductors that connect the device to the battery
terminals. When the wires are connected to the battery terminals, current flows in the closed path. An example of a simple
circuit is shown in Figure 15.
The voltage difference, current, and resistance in a circuit are
related. If the voltage difference doesn’t change, decreasing the
resistance increases the current in the circuit, as shown in Figure 15.
Also, if the resistance doesn’t change, increasing the voltage difference increases the current.
Electric Charges How are
current, voltage, and resistance
related?
When the clips on the graphite rod are farther apart,
the resistance of the rod in the circuit is larger. As a
result, less current flows in the circuit and the lightbulb is dim.
204
When the clips on the graphite rod are closer together,
the resistance of the rod in the circuit is less. As a result,
more current flows in the circuit and the lightbulb is
brighter.
CHAPTER 7 Electricity
Thomas Veneklasen
High-Temperature Conductivity Conductivity
depends on temperature. A higher temperature causes more movement of atoms and
generally results in lower conductivity.
However, the increased movement of atoms
204
CHAPTER 7 Electricity
in some substances, such as carbon and semiconductors, frees electrons so that higher
temperatures for these substances means
higher conductivity.
Ohm’s Law The relationship between voltage difference, current and resistance in a circuit is known as Ohm’s law. According
to Ohm’s law, the current in a circuit equals the voltage difference divided by the resistance. If I stands for electric current,
Ohm’s law can be written as the following equation.
Ohm’s Law
voltage difference (in volts)
current (in amperes) ——––––––––––————
resistance (in ohms)
V
R
I Ohm’s law provides a way to measure the resistance of objects
and materials. First the equation above is written as:
Current and the Human
Body When an electric
shock occurs, an electric
current moves through
some part of the body.
The damage caused by an
electric shock depends on
how large the current is.
Research the effects of current on the human body.
Make a table showing the
effects on the body at different amounts of current.
Current and the Human Body
Currents of about 0.5 mA can cause a
slight shock to a person. Higher currents
cause increasing degrees of pain and loss
of muscle control. Currents higher than
about 150 mA will likely result in death.
V
I
R Check for Understanding
An object is connected to a source of voltage difference and
the current flowing in the circuit is measured. The object’s
resistance then equals the voltage difference divided by the
measured current.
Visual-Logical Without students
seeing, reverse one battery in a
flashlight. Show students that
the flashlight does not work.
Ask them to suggest possible
reasons why an electric circuit
may not work. L2 LS Logical-
Summary
Self Check
Current and Voltage Difference
Electric current is the net movement of electric charge in a single direction.
A voltage difference is related to the force
that causes charges to flow.
A circuit is a closed, conducting path.
Batteries
Chemical reactions in a battery produce a
voltage difference between the positive and
negative battery terminals.
Two commonly used types of batteries are
dry-cell batteries and wet-cell batteries.
Resistance and Ohm’s Law
Resistance is the tendency of a material to
oppose the flow of electrons.
Ohm’s law relates the current, I, resistance, R,
and voltage difference, V, in a circuit:
V
I __
R
1. Compare and contrast a current traveling through a
circuit with a static discharge.
2. Explain how a carbon-zinc dry cell produces a voltage
difference between the positive and negative terminals.
3. Identify two ways to increase the current in a simple
circuit.
4. Compare and contrast the flow of water in a pipe and
the flow of electrons in a wire.
5. Think Critically Explain how the resistance of a
lightbulb filament changes after the light has been
turned on.
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•
gpscience.com/self_check_quiz
Mathematical,Visual-Spatial
6. Calculate the voltage difference in a circuit with a
resistance of 25 Ω if the current in the circuit is 0.5 A.
7. Calculate Resistance A current of 0.5 A flows in a
60-W lightbulb when the voltage difference between
the ends of the filament is 120 V. What is the resistance of the filament?
SECTION 2 Electric Current
Reteach
Ohm’s Law Use the diagram below
to help students remember how
to rearrange Ohm’s law. When
the desired variable is covered,
the other two variables are in
proper mathematical order. L2
LS Visual-Spatial
V
I
R
205
Process Show the students a flash-
1. Both are a movement of electrons
from the negative to the positive
charge. A circuit has a continuous current provided by a voltage source. A
static discharge is a very rapid, noncontinuous transfer of charge.
2. A chemical reaction causes a negative
charge on the zinc container and a
positive charge on the carbon rod.
3. Increasing the voltage difference or
decreasing the resistance will increase
the current.
4. Water flow is the movement of molecules, not just electrons. The larger
the diameter of pipe or wire, the
greater the flow of water or electrons.
5. A lightbulb filament heats up as
an electic current flows through it,
causing an increase in the wire’s
resistance. According to Ohm’s Law,
the highter resistance results in a
lower current.
6. 12.5 volts
7. 240 ohms
light and ask them to draw its
electric circuit. Have them calculate the current that flows if the
resistance of the bulb is 20 ohms.
Circuit should show battery, wires, bulb,
and possibly a switch. The current will
depend on voltage of batteries. L2 P
LS Visual-Spatial, LogicalMathematical
SECTION 2 Electric Current
205
Identifying ConductJrs
and Insulators
Real-World
Question
The resistance of an insulator is so large that
only a small current flows when it is connected
in a circuit. As a result, a lightbulb connected in
a circuit with an insulator usually will not glow.
In this lab, you will use the brightness of a
lightbulb to identify conductors and insulators.
Purpose Students will compare
the conductivity of various materials. L2 LS Kinesthetic, Visual-
Spatial
Process collect data, make and
use tables, record observations,
draw conclusions
Real-World Question
What materials are conductors and what materials are insulators?
Time Required 40 minutes
Goals
Procedure
■ Identify conductors and insulators.
■ Describe the common characteristics
Teaching Strategy Have a col-
of conductors and insulators.
lection of items ready for
students to test, but let students
offer additional items to be
tested.
Materials
battery
flashlight bulb
Troubleshooting Make sure
bulb holder
insulated wire
Safety Precautions
all connections in the circuit
have good contact.
Procedure
1. Set up an incomplete circuit as pictured in
Conclude and
Apply
the photograph.
lightbulb have something in common?
3. Do all or most of the materials that don’t light
the lightbulb have something in common?
4. Explain why one material may allow the
lightbulb to light and another prevent the
lightbulb from lighting.
5. Predict what other materials will allow the
2. Touch the free bare ends of the wires to various
objects around the room. Test at least 12 items.
3. Copy the table below. In your table, record
which materials make the lightbulb glow
and which don’t.
1. Answers will vary. Students should
realize that metallic items will conduct
electricity.
2. They are metallic.
3. They are nonmetallic.
4. Materials that allow the lightbulb to
glow have metallic bonding. Materials
that do not allow the lightbulb to
glow are nonmetallic.
5. Metallic materials will conduct electricity; bad connections will prevent
the lightbulb from lighting.
6. Answers will vary.
Conclude and Apply
1. Is there a pattern to your data?
2. Do all or most of the materials that light the
lightbulb to light and what will prevent the
lightbulb from lighting.
6. Classify all the materials you have tested as
conductors or insulators.
Material Tested with Lightbulb Circuit
Lightbulb Glows
Results will vary
206
Lightbulb Doesn’t Glow
Results will vary
Compare your conclusions with those of
other students in your class. For more help,
refer to the Science Skill Handbook.
CHAPTER 7 Electricity
Geoff Butler
Process Silicon is an element that is nonconductive. Have students make posters illustrating how
scientists treat silicon to alter its conductivity and
how the altered silicon is used. Use Performance
Assessment in the Science Classroom, p. 145
L2
206
CHAPTER 7 Electricity
LS Visual-Spatial, Logical-Mathematical
Students should discuss why their conclusions did
or did not agree. They can cite references to support
their arguments. L3 LS Linguistic, LogicalMathematical