http://www.bbc.co.uk/print/schools/gcsebitesize/physics/
Changing the current in a circuit
Changing the number of batteries
If you increase the voltage across a component, more current will flow through the component.
Changing the resistance
If you increase the number of bulbs in a series circuit, less current will flow. The lamps resist the current, so if you
put more lamps into the circuit there is more resistance
You could do the same with a variable resistor
As the resistance of the variable resistor decreases the current in the circuit increases until the bulb burns out.
The link between voltage, current and resistance (Ohm's Law)
If you know two of these, you can work out the last one using this formula:
V=IxR
V = The voltage across a component
I = The current going through a component
R = The resistance of a component
The other ways of writing Ohm's Law are:
R = V/I or I = V/R
The symbol for voltage is V. It is measured in units of Volts V
The symbol for current is I. It is measured in units of Amps A
The symbol for resistance is R. It is measured in units of Ohms
Remember
The formula V = IR will not be given to you on any examination paper - you have to learn it.
Try this calculation
Question 1
Bicycles with battery operated lights often have different size bulbs for the front and rear lights. The filament in the
front lamp has a resistance of 3.4 Ohms It takes a current of 0.7A. What voltage does it work at?
1. 0.205V
2. 2.38V
3. 4.86V
4. 6V
The Answer
The answer is 2.38V. If you didn't get the correct answer make sure you have used the formula correctly.
Resistance in parallel circuits
Total resistance gets less when components are in parallel. When components are in parallel there are two (or more)
routes for the electric current. This makes it easy for more current to flow out of the battery.
Components that do not obey Ohm's Law
Some devices work in slightly different ways.
Thermistor
Thermistors have a lower resistance at higher temperatures. They let more current flow through them at higher
temperatures.
LDR Light Dependent Resistor
Light dependent resistors have lower resistance when there is more light.
Diode
A diode lets the current flow one way only, in the direction of the arrow. This means that it has a low resistance
when the current is flowing in the direction of the arrow, but a very high resistance when the current tries to flow the
other way.
Series and parallel circuits
 Series circuits - the current goes in one loop only - battery - lamp(s) - battery
 Parallel circuits - the current splits into more than one direction
Here are some examples of series and parallel circuits. See if you know which is which before checking the answers.
What happens inside the wires
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There are charges in the wires even when no current is flowing.
If the switch is open the charges can't move.
When the switch is closed the battery pushes the charges round the wires
All the charges move at the same time
Using an ammeter
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An ammeter measures electric current
Ammeters go in series - make a gap in the circuit and fit the meter into the gap.
Current in a parallel circuit
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
The current splits into different branches, then combines again before it goes back into the battery.
When the current splits,the current in each branch after the split adds up to the same as the current just
before the split.
How to charge and discharge objects
When you rub insulators they pick up an electric charge. This only works for insulated objects - conductors let the
charge flow to earth. Discharge objects by holding them briefly in a flame. The flame contains ionised particles
which will neutralise the charge on the object.
Where charge comes from
When you use a cloth to rub an insulator such as the balloon or the plastic ruler, electrons are rubbed off one onto
the other. If there are too many electrons, the object is negative. If there are too few electrons, the object is positive.
Types of charge
There are only two types of charge, so we call them positive and negative. If you bring two insulators close to each
other, what will happen?
If charges are the same they repel.
If charges are different they attract.
If one is charged and the other is not then they attract.
Electric charge and current
An electric current is a movement of charge, so if the charge can move off the insulator it makes a current. If the
current flows through you, you can sometimes feel the electric shock.
Examples of charge producing a current
If you rub your feet on some types of carpet you will build up a charge. When you touch something else the charge
flows off your body and you feel the shock.
Car and lorry tyres are made of rubber, so they are insulating. They build up a charge as they move along the road.
Some cars give you a shock when you get out of them as the charge goes through you down to the ground. Petrol
tankers have to be earthed before they deliver petrol at a garage - to make sure that any electric charge cannot form a
spark.
Air currents moving past each other in clouds can build up charges. Sometimes there is enough to form long sparks we call it lightning.
The units of charge
Charge, Q, is measured in coulombs, C.
Electric current is the movement of charge.
Current in a wire
 Current is moving charge.
 The charged particles that can move in a metal are electrons.
 The current in a wire is made of moving electrons.
Current in liquids
Ionic liquids can conduct electricity.
 Current is moving charge.
 The charged particles that can move in an ionic liquid are ions.
 The current in an ionic liquid is made of moving ions.
The link between current, charge and time
If you know two of these, you can work out the last one using this formula:
current = charge/time
I = Q/t
I = the current going through a component
Q = charge
t = time
The other way of writing this is:
Q=It
The symbol for current is I, it is measured in units of amperes or amps (A)
The symbol for charge is Q, it is measured in units of coulombs (C)
The symbol for time is t, it is measured in units of seconds (s)
If symbols and units have always confused you, remember that temperature works in just the same way. The symbol
for temperature is T, it is measured in units of degrees °C.
Facts about energy efficiency
Here are some facts you need to know about energy efficiency.
It's important to measure how much useful energy we can get out of, for example, a battery-powered car. It helps us
work out how far we can expect the car to travel.
We use the term efficiency, which means the fraction of energy that is usefully transferred.
We use an equation to work out the efficiency of something:
Efficiency = useful energy output/total energy input
You can also think of it in terms of power:
Efficiency = useful power output/total power input
This will give you a decimal number between 0 and 1, but efficiency looks better as a percentage. Multiply the
efficiency by 100 to get this.
Your syllabus might use slightly different words in the efficiency equation. Don't worry, they all mean the same
thing.
There are two equations that you will need:
Kinetic energy = 1/2 x mass x [velocity]2
Ek= 1/2 m v2
and
Change in gravitational potential energy = weight x change in height
EP = W h or EP = m g h
- where m is the mass, h is the change in height and g is the gravitational field strength, which is about 10m/s2 (if
you're on the Earth's surface!).
tips
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Kinetic energy is proportional to mass. But it's proportional to the square of the velocity. This means that
for instance, doubling an object's velocity has much more effect on the kinetic energy than doubling its
mass.
Don't confuse weight(in N) and mass (in kg).
Calculations at higher level will be harder than the calculations for foundation level. For instance, you
might have to transpose (change the subject of) a formula at higher level. You should be also be familiar
with the equations to do with work and power. Your teacher will know what your exam board expects
http://science.howstuffworks.com/atom.htm
It has been said that during the 20th century, man harnessed the "power of the atom." We made atomic bombs and
generated electricity by nuclear power. We even split the atom into smaller pieces called subatomic particles.
The idea of the atom was first devised by Democritus in 530 B.C. In 1808, an English school teacher and scientist
named John Dalton proposed the modern atomic theory. Modern atomic theory simply states the following:
 Every element is made of atoms - piles of paper clips.
 All atoms of any element are the same - all the paper clips in the pile are the same size and color.
 Atoms of different elements are different (size, properties) - like different sizes and colors of paper
clips.
 Atoms of different elements can combine to form compounds - you can link different sizes and colors of
paper clips together to make new structures.
 In chemical reactions, atoms are not made, destroyed, or changed - no new paper clips appear, no paper
clips get lost and no paper clips change from one size/color to another.
 In any compound, the numbers and kinds of atoms remain the same - the total number and types of
paper clips that you start with are the same as when you finish.
Dalton's atomic theory formed the groundwork of chemistry at that time. Dalton envisioned atoms as tiny spheres
with hooks on them. With these hooks, one atom could combine with another in definite proportions. But some
elements could combine to make different compounds (e.g., hydrogen + oxygen could make water or hydrogen
peroxide). So, he could not say anything about the numbers of each atom in the molecules of specific substances.
Did water have one oxygen with one hydrogen or one oxygen with two hydrogens? This point was resolved when
chemists figured out how to weigh atoms
In many materials, the electrons are tightly bound to the atoms. Wood, glass, plastic, ceramic, air, cotton ... These
are all examples of materials in which electrons stick with their atoms. Because the electrons don't move, these
materials cannot conduct electricity very well, if at all. These materials are electrical insulators.
But most metals have electrons that can detach from their atoms and move around. These are called free electrons.
Gold, silver, copper, aluminum, iron, etc., all have free electrons. The loose electrons make it easy for electricity to
flow through these materials, so they are known as electrical conductors. They conduct electricity. The moving
electrons transmit electrical energy from one point to another.
Generators, Volts and Amps
Electricity needs a conductor in order to move. There also has to be something to make the electricity flow from one
point to another through the conductor. One way to get electricity flowing is to use a generator. A generator uses a
magnet to get electrons moving.
There is a definite link between electricity and magnetism. If you allow electrons to move through a wire, they will
create a magnetic field around the wire. Similarly, if you move a magnet near a wire, the magnetic field will cause
electrons in the wire to move.
A generator is a simple device that moves a magnet near a wire to create a steady flow of electrons.
One simple way to think about a generator is to imagine it acting like a pump pushing water along. Instead of
pushing water, however, a generator uses a magnet to push electrons along. This is a slight over-simplification, but it
is nonetheless a very useful analogy.
There are two things that a water pump can do with water:
1. A water pump moves a certain number of water molecules.
2. A water pump applies a certain amount of pressure to the water molecules.
In the same way, the magnet in a generator can:
1. push a certain number of electrons
along
2. apply a certain amount of "pressure" to
the electrons
In an electrical circuit, the number of electrons
that are moving is called the amperage or the
current, and it is measured in amps. The
"pressure" pushing the electrons along is called
the voltage and is measured in volts. So you
might hear someone say, "If you spin this
generator at 1,000 rpm, it can produce 1 amp at
6 volts." One amp is the number of electrons
moving (1 amp physically means that 6.24 x
1018 electrons move through a wire every
second), and the voltage is the amount of
pressure behind those electrons.

Circuits
Whether you are using a battery, a fuel cell or a solar cell to produce
electricity, there are three things that are always the same:

The source of electricity will have two terminals: a positive terminal
and a negative terminal.
 The source of electricity (whether it is a generator, battery, etc.) will
want to push electrons out of its negative terminal at a certain voltage.
For example, a AA battery typically wants to push electrons out at 1.5
volts.
 The electrons will need to flow from the negative terminal to the
positive terminal through a copper wire or some other conductor. When
there is a path that goes from the negative to the positive terminal, you
have a circuit, and electrons can flow through the wire.
 You can attach a load of any type (a light bulb, a motor, a TV, etc.) in the middle of the circuit. The source
of electricity will power the load, and the load will do its thing (create light, spin a shaft, generate moving
pictures, etc.).
Electrical circuits can get quite complex. But at the simplest level, you
always have the source of electricity (a battery, etc.), a load (a light
bulb, motor, etc.), and two wires to carry electricity between the battery
and the load. Electrons move from the source, through the load and back
to the source.
Moving electrons have energy. As the electrons move from one point to
another, they can do work. In an incandescent light bulb, for example,
the energy of the electrons is used to create heat, and the heat in turn
creates light. In an electric motor, the energy in the electrons creates a
magnetic field, and this field can interact with other magnets (through
magnetic attraction and repulsion) to create motion. Each electrical
appliance harnesses the energy of electrons in some way to create a
useful side effect.
Direct Current vs. Alternating Current
Batteries, fuel cells and solar cells all produce something called direct
current (DC). The positive and negative terminals of a battery are
always, respectively, positive and negative. Current always flows in the same direction between those two terminals.
The power that comes from a power plant, on the other hand, is called alternating current (AC). The direction of
the current reverses, or alternates, 60 times per second (in the U.S.) or 50 times per second (in Europe, for example).
The power that is available at a wall socket in the United States is 120-volt, 60-cycle AC power.
The big advantage that alternating current provides for the power grid is the fact that it is relatively easy to change
the voltage of the power, using a device called a transformer. By using very high voltages for transmitting power
long distances, power companies can save a lot of money. Here's how that works.
Let's say that you have a power plant that can produce 1 million watts of power. One way to transmit that power
would be to send 1 million amps at 1 volt. Another way to transmit it would be to send 1 amp at 1 million volts.
Sending 1 amp requires only a thin wire, and not much of the power is lost to heat during transmission. Sending 1
million amps would require a huge wire.
So power companies convert alternating current to very high voltages for transmission (e.g. 1 million volts), then
drop it back down to lower voltages for distribution (e.g. 1,000 volts), and finally down to 120 volts inside the house
for safety. It is a lot harder to kill someone with 120 volts than with 1 million volts (and most electrical deaths are
prevented altogether today using GFCI outlets).
Ground
When the subject of electricty comes up, you will often hear about electrical grounding, or just ground. For
example, an electrical generator will say, "Be sure to attach to an earth ground before using," or an appliance might
warn, "Do not use without an appropriate ground."
It turns out that the power company uses the earth as one of the wires in the power system. The earth is a pretty good
conductor, and it is huge, so it makes a good return path for electrons. "Ground" in the power-distribution grid is
literally "the ground" that's all around you when you are walking outside. It is the dirt, rocks, groundwater, etc., of
the earth.
The power-distribution system connects into the ground many times. For example, in this photo you can see that one
of the wires is labeled as a ground wire:
In the photo below, the bare wire coming down the side of the pole connects the aerial ground wire directly to
ground:
Every utility pole on the planet has a bare wire like this. If you ever watch the power company install a new pole,
you will see that the end of that bare wire is stapled in a coil to the base of the pole. That coil is in direct contact
with the earth once the pole is installed, and is buried 6 to 10 feet (1.8 to 3 m) underground. It is a good, solid
ground connection. If you examine a pole carefully, you will see that the ground wire running between poles (and
often the guy wires) are attached to this direct connection to ground.
Similarly, near the power meter in your house or apartment there is a 6-foot (2-meter) long copper rod driven into
the ground. The ground plugs and all the neutral plugs of every outlet in your house connect to this rod. (See "How
Power Distribution Grids Work" for details.)
Electricity can be used in so many different ways. Check out the links on the next page to explore dozens of
different applications.
Physics terminology
You've seen a lot of terminology thrown around -- words such as mass, force, torque, work, power and energy.
What do these words really mean, and are they interchangeable?
In this article, we will help to bring all of this terminology together, give some examples of when each is used and
even try a few calculations along the way to get the hang of it.
Throughout this article, we will refer to different types of units. In most of the world, the International System of
Units (SI - from the French Le Systčme International d'Unités), also referred to as the metric system, is accepted as
the standard set of units. This system contains most of the metric units you are used to, like meters and kilograms,
but also includes units for many other physical and engineering properties. Even the United States has officially
adopted the SI system of units, but English Engineering Units (like pounds and feet) are still in everyday use.
Before we jump into explaining these terms, we need to start with some basics. We'll start with mass, and work our
way up to energy.
What is Mass?
Generally, mass is defined as the measure of how much matter an object or body
Common Units
contains -- the total number of subatomic particles (electrons, protons and neutrons) in
of Mass
the object. If you multiply your mass by the pull of Earth's gravity, you get your weight. SI:
So if your body weight is fluctuating, because of eating or exercising, it is actually the
Gram (g)
number of atoms that is changing.
1 g = 0.001 kg
It is important to understand that mass is independent of your position in space. Your
Kilogram (kg)
body's mass on the moon is the same as its mass on Earth, because the number of atoms 1 kg = 2.2 lbm
is the same. The Earth's gravitational pull, on the other hand, decreases as you move
1 kg = 0.0685 slug
farther away from the Earth. Therefore, you can lose weight by changing your
English:
elevation, but your mass remains the same. You can also lose weight by living on the
Pound mass (lbm)
moon, but again, your mass is the same.
1 lbm = 0.4536 kg
Mass is important for calculating how quickly things accelerate when we apply a force Slug (slug)
to them. What determines how fast a car can accelerate? You probably know that your 1 slug = 14.5939 kg
car accelerates slower if it has five adults in it than if it has just one. We'll explore this
relationship between mass, force and acceleration in a little more detail after we talk about force.
What is Force?
One type of force that everyone is familiar with is weight. This is the amount of force
Common Units
that the Earth exerts on you. There are two interesting things about this force:
of Force
 It pulls you down, or, more exactly, toward the center of the Earth.
SI:
 It is proportional to your mass. If you have more mass, the Earth exerts a
newton (N)
greater force on you.
1 N = 0.225 lb
When you step on a bathroom scale, you exert a force on the scale. The force you apply English:
to the scale compresses a spring, which moves the needle. When you throw a baseball, Pound (lb)
you apply a force to the ball, which makes it speed up. An airplane engine creates a
1 lb = 4.448 N
force, which pushes the plane through the air. A car's tires exert a force on the ground,
which pushes the car along.
Force causes acceleration. If you apply a force to a toy car (for example, by pushing on it with your hand), it will
start to move. This may sound simple, but it is a very important fact. The movement of the car is governed by Isaac
Newton's Second Law, which forms the foundation for classical mechanics. Newton's Second Law states that the
acceleration (a) of an object is directly proportional to the force (F) applied, and inversely proportional to the
object's mass (m). That is, the more force you apply to an object, the greater the rate of acceleration; and the more
mass the object has, the lower the rate of acceleration. Newton's Second Law is usually summarized in equation
form:
a = F/m, or F = ma
To honor Newton's achievement, the standard unit of force in the SI system was named the newton. One newton (N)
of force is enough to accelerate 1 kilogram (kg) of mass at a rate of 1 meter per second per second (m/s 2). In fact,
this is really how force and mass are defined. A kilogram is the amount of weight at which 1 N of force will
accelerate at a rate of 1 m/s2. In English units, a slug is the amount of mass that 1 pound of force will accelerate at 1
ft/s2, and a pound mass is the amount of mass that 1 lb of force will accelerate at 32 feet/s 2.
The Earth exerts enough force to accelerate objects that are dropped at a rate of 9.8 m/s 2, or 32 feet/s2. This
gravitational force is often referred to as g in equations. If you drop something off a cliff, for each second it falls it
will speed up by 9.8 m/s. So, if it falls for five seconds, it will reach a speed of 49 m/s. This is a pretty fast rate of
acceleration. If a car accelerated this quickly, it would reach 60 miles per hour (97 kph) in less than three seconds!
What is Work?
The work we are talking about here is work in the physics sense. Not home work, or chores, or your job or any other
type of work. It is good old mechanical work.
Work is simply the application of a force over a distance, with one catch -- the distance only counts if it is in the
direction of the force you apply. Lifting a weight from the ground and putting it on a shelf is a good example of
work. The force is equal to the weight of the object, and the distance is equal to the height of the shelf. If the weight
were in another room, and you had to pick it up and walk across the room before you put it on the shelf, you didn't
do any more work than if the weight were sitting on the ground directly beneath the shelf. It may have felt like you
did more work, but while you were walking with the weight you moved horizontally, while the force from the
weight was vertical.
Your car also does work. When it is moving, it has to apply a force to counter the forces of friction and aerodynamic
drag. If it drives up a hill, it does the same kind of work that you do when lifting a weight. When it drives back
down the hill, however, it gets back the work it did. The hill helps the car drive down.
Work is energy that has been used. When you do work, you use energy. But sometimes the energy you use can be
recovered. When the car drives up the hill, the work it does to get to the top helps it get back down. Work and
energy are closely related. The units of work are the same as the units of energy, which we will discuss later.
What is Power?
Power is a measure of how quickly work can be done. Using a lever, you may be able to
Common Units
generate 200 ft-lb of torque. But could you spin that lever 3,000 times per minute? That
of Power
is exactly what your car engine does.
SI:
The SI unit for power is the watt. A watt breaks down into other units that we have
Watts (W)
already talked about. One watt is equal to 1 Newton-meter per second (Nm/s). You can 1000 W = 1 kW
multiply the amount of torque in Newton-meters by the rotational speed in order to find Kilowatt (kW)
the power in watts. Another way to look at power is as a unit of speed (m/s) combined
1 kW = 1.341 hp
with a unit of force (N). If you were pushing on something with a force of 1 N, and it
English
moved at a speed of 1 m/s, your power output would be 1 watt.
Horsepower (hp)
An interesting way to figure out how much power you can output is to see how quickly 1 hp = 0.746 kW
you can run up a flight of stairs.
1. Measure the height of a set of stairs that takes you up about three stories.
2. Time yourself while you run up the stairs as quickly as possible.
3. Divide the height of the stairs by the time it took you to ascend them. This will give you your speed.
For instance, if it took you 15 seconds to run up 10 meters, then your speed was 0.66 m/s (only your speed in the
vertical direction is important). Now you need to figure out how much force you exerted over those 10 meters, and
since the only thing you hauled up the stairs was yourself, this force is equal to your weight. To get the amount of
power you output, multiply your weight by your speed.
Power (W) = (height of stairs (m) / Time to climb (s) ) * weight (N)
Power (hp) = [(height of stairs (ft) / Time to climb (s) ) * weight (lb)] / 550
What is Energy?
Energy is the final chapter in our terminology saga. We'll need everything we've
Common Units
learned up to this point to explain energy.
of Energy
If power is like the strength of a weightlifter, energy is like his endurance. Energy is
a measure of how long we can sustain the output of power, or how much work we
SI:
can do. Power is the rate at which we do the work. One common unit of energy is
Newton meter (Nm)
the kilowatt-hour (kWh). You learned in the last section that a kW is a unit of
1 Nm = 1 J
power. If we are using one kW of power, a kWh of energy will last one hour. If we Joule (J)
use 10 kW of power, we will use up the kWh in just six minutes.
1 J = 0.239 cal
There are two kinds of energy: potential and kinetic.
Calorie (cal)
Potential Energy
1 cal = 4.184 J
Potential energy is waiting to be converted into power. Gasoline in a fuel tank,
Watt hours (Wh)
food in your stomach, a compressed spring, and a weight hanging from a tree are all 1 Wh = 3,600 J
examples of potential energy.
Kilowatt hours (kWh)
The human body is a type of energy-conversion device. It converts food into power, 1 kWh = 1,000 Wh
which can be used to do work. A car engine converts gasoline into power, which
1 kWh = 3,600,000 J
can also be used to do work. A pendulum clock is a device that uses the energy
1 kWh = 3,412 BTU
stored in hanging weights to do work.
English:
When you lift an object higher, it gains potential energy. The higher you lift it, and Foot - pound (ft lb)
the heavier it is, the more energy it gains. For example, if you lift a bowling ball 1
1 ft lb = 1.356 Nm
inch, and drop it on the roof of your car, it won't do much damage (please, don't try British Thermal Unit (BTU)
this). But if you lift the ball 100 feet and drop it on your car, it will put a huge dent 1 BTU = 1,055 J
in the roof. The same ball dropped from a greater height has much more energy. So, 1 BTU = 0.0002931 kWh
by increasing the height of an object, you increase its potential energy.
Let's go back to our experiment in which we ran up the stairs and found out how much power we used. There is
another way to look at how we calculated our power: We calculated how much potential energy our body gained
when we raised it up to a certain height. This amount of energy was the work we did by running up the stairs (force
* distance, or our weight * the height of the stairs). We then calculated how long it took to do this work, and that's
how we found out the power. Remember that power is the rate at which we do work.
The formula to calculate the potential energy (PE) you gain when you increase your height is:
PE = Force * Distance
In this case, the force is equal to your weight, which is your mass (m) * the acceleration of gravity (g), and the
distance is equal to your height (h) change. So the formula can be written:
PE = mgh