On a dry day in summer, people who have been jumping on a trampoline sometimes get a small electric chock as they get off.
On a stormy night there may be lightning flashes in the sky followed by the loud claps of thunder.
If you place your hand near a television screen, you may feel the hairs on the back of your hand being attracted towards the screen.
These are just a few examples of electric charges. If an electric charge passes through your body, you may feel an electric shock. A large electric charge may injure you.
Making electric charges
You can make electric charges with the right materials and when the environment is relatively dry. All you need to do is rub certain materials together.
If you rub a plastic ruler (or any other plastic object) on a piece of woollen jumper, you will find that the plastic ruler will attract the hairs on the back of your hand. The ruler must have an electric charge. Indeed on a dry day if you place your finger near the plastic ruler, you may hear lots of crackles as the electric charge jumps to your finger.
Atoms and charges
All things, even the plastic ruler and the woollen cloth, are composed of very, very small particles called atoms.
An atom has a spherical shape like a ball. It is a bit fuzzy like a cotton wool ball. However, deep inside the atom is a very, very small solid lump called the nucleus.
The fuzzy outer layer of the atom is made from a cloud of even smaller particles called electrons.
Each electron in the electron cloud has an electric charge. We give this charge a name. It is a negative charge.
The nucleus in each atom also has an electric charge. It is a positive charge.
You do not usually get an electric shock when you touch atoms because the positive electric charge in the nucleus of each atom usually balances the effect of the negative charge in the electron cloud. We see or feel the effects of electric charges only when the positive and negative charges in a material are unbalanced. We do not make electric charges, we only separate the positive from the negative.
Shifting electric charges
Some of the outer electrons in some atoms are held less strongly to the atom than the outer electrons in some other atoms.
For instance, the outer electrons of the atoms in wool are held more loosely than the outer electrons of the atoms in a plastic ruler.
When you rub the atoms in wool against the atoms in plastic, some of the outer electrons from the atoms in the wool get rubbed off, and they stick to the atoms in the plastic ruler.
The surface of the plastic ruler gets coated with the electrons rubbed off the atoms in the wool. The same effect is seen when you use a plastic comb on your hair.
Because the plastic comb now has all these extra electrons stuck to it, it has an excess of negative electric charge. You can detect this overall negative charge on the atoms by the way the comb attracts the hairs on your hand or the way it attracts small pieces of paper.
If a rod made of Perspex is rubbed on a cotton cloth, an electric charge is also produced on the rod.
However, in this case, the outer electrons of the atoms in the Perspex rod are held more loosely than the outer electrons of the atoms in the cotton cloth.
When these materials are rubbed together, it is more likely that some of the electrons in the Perspex rod will be rubbed off by the atoms in the cotton cloth.
This means the Perspex will not have enough electrons to balance the positive charge of the nucleus. Therefore, the Perspex rod will have an overall positive electric charge.
How do charges affect each other?
If a negatively charged plastic ruler is brought near to a positively charged Perspex rod, they gently attract each other.
If a negatively charged ruler is brought near to another negatively charged ruler, they gently repel each other.
Similarly if a positively charged rod is brought near to another positively charged rod, they gently repel each other.
The charge law
The above effects can be summarised in a law: Opposite charges attract. Like charges repel.
What is electricity?
When you feel an electric shock as you get off a trampoline or after your have been walking across a new clean dry carpet, you are feeling billions of electrons passing through your body.
If you get an electric shock from a power cord, you may have even more electrons passing through your body.
The electricity that flows through a power cord is just a flow of negatively charged electrons. The speed at which they flow is very, very slow, but there are so many billions of them that they can still have a huge effect.
The flow of electrons
Copper atoms, like most other metals atoms, have very loosely held outer electrons. They are so loose that they seem to make a sea of electrons. They can easily move around copper atoms inside the wire. The flow of electric charge, such as the motion of these loosely held electons in a copper wire, is called an electric current.
If a length of copper wire is connected between the top of a battery and the bottom of the battery, the outer electrons in the wire can be made to flow from one end of the wire to the other.
The crowd of negative electrons produced at the base of the battery repel the loose electrons in the copper wire connected to this end of the battery, because they all have the same electric charge.
The positive charge at the top of the battery tends to attract the loose electrons in the copper wire, because they have opposite charges.
However, no electrons in the copper wire can move through the wire unless they all move. This can only happen when both ends of the wire are in contact with the terminals of the battery simultaneously. Only then an electric current can flow through the wire.
If the wire is broken in the middle, all the electrons must stop and no electric current can flow.
Types of circuits
A continuous, uninterrupted path through which charges can move is called an electric circuit. There are two main types of electrical circuits. They are called series and parallel.
When several light globes are connected in a 'chain' they are said to be connected in series.
In a series circuit the same number of electrons must be flowing through each lamp in turn. If a million electrons each second are flowing through the first lamp, then there must be a million electrons flowing through the other lamps each second.
A series circuit has some limitations. If you turn off one of the lamps in series, all the other lamps will be turned off. This is because if you stop the current flowing through one lamp, the current flowing through all the lamps must stop.
Also if one lamp in the series circuit 'blows' (ie stops working) all the other lamps will stop working since the blown globe will halt the flow of electricity in the circuit.
Series circuits have been used in some Christmas tree decorations where long chains of lights are needed.
However, series circuits are not suitable for most uses in houses. Image how it would be if to turn on one light, you had to turn on all the lights and if one light stopped working, they would all stop.
When several lamps are connected like the steps on a ladder they are said to be connected in parallel.
In a parallel circuit the flow of electrons coming out of the battery divides up so some of the electrons flow through each lamp.
If the electrons are prevented from flowing through one of the lamps, they can still flow through the other lamps. Each lamp can be turned on and off independently.
Parallel circuits are the most useful circuits used in houses. The use of parallel circuits means that lights and appliances can be turned on and off independently in each room.
The amount of electricity flowing through a circuit can be measured on a device called an ammeter.
It measures an electric current in amps (short for amperes, A).
A flow of 1 A is just over 6,000,000,000,000,000,000 (6 × 1018) electrons per second.
An average household light globe uses about 0.3 A. An electric oven, however, may use over 5 A.
Voltage is like the force that causes the electrons to flow through a circuit.
A 1.5 volt (V) battery can apply enough force to make the electricity flow through a small light globe. However, a 1.5 V battery does not provide enough force to make the electricity flow through a large household lamp.
Most household lamps use 250 V electricity. This voltage can supply a much larger force to make the current flow through the lamp.
Household appliances such as stoves and television sets also use 250 V.
The power stations transfer electricity using even higher voltage, often several hundred thousand volts.
The higher the voltage the more dangerous the electricity can be if it is not handled safely.
The amount of current flowing through a circuit depends upon how easy it is for the electricity to flow. If it is very easy, then a lot of electricity will flow. If it is very difficult, then much less electricity will flow.
The difficulty with which electricity flows through a circuit is called the resistance of the circuit. If it is very difficult for electricity to flow through a circuit, then the circuit is said to have a high resistance.
A torch globe has a fairly high resistance. This is why electricity uses up energy in trying to pass through a globe. If there are two light globes in a chain (in series) in a circuit, then the resistance is doubled. The electricity would have to use twice as much energy to get through. Without that extra energy the two globes will be duller than if only one globe was in the circuit.
To make the two globes in series work at full brightness, then two 1.5 V cells would be needed in series to provide the voltage needed to supply enough energy to the electricity.
Resistance is measured in units called ohms (). The greater the resistance the greater the number of ohms.
Some metals have more resistance than other wires. For example, tungsten metal has a greater resistance than copper wire. Silver has even less resistance than copper.
Non-metals such as plastic and rubber have extremely high resistances. This is why they are called insulators and are used as protection against electric shocks.
The resistance of a wire is also affected by the size of the wire. A very thin wire has a higher resistance than a very fat wire. It is more difficult for the electrons to crowd through a narrow wire.
A longer wire also has more resistance than a shorter wire. It takes more electrical energy to make the electrons travel through a long wire than through a shorter length of the same wire.
Electrical gadgets use electrical energy by converting it into other forms of energy. For example, a stove uses electrical energy by converting it into heat energy. A light globe uses electrical energy by converting it into light energy and heat energy. An electric blender converts electrical energy into motion energy.
Some household gadgets are more powerful than others. For example, an electric drill is more powerful than an electric shaver. An electric oven is more powerful than an electric light.
The power of a gadget tells you how fast that gadget uses the energy supplied by the electricity. Powerful gadgets use energy at a faster rate.
The power of a gadget is measured in watts (W). For example a 100 W light globe uses energy at a faster rate than a 60 W light globe. An electric oven might have a power of several thousand watts.
Measuring electrical energy on your power bill
When you pay your electricity bill you are only paying for the energy supplied to your house by the electricity company.
You do not really buy electricity. The electric current only carries the energy to your house.
The energy company uses the unit of kilowatt hour to measure how much electrical energy you have used.
One kilowatt hour is the energy used by a 100W light globe in ten hours.
Why is electricity so useful?
Electricity is very useful because it can be used to transfer and distribute large amount of energy efficiently.
The power station uses coal or oil to generate energy. However, to get this energy to each house and factory, electricity is used. The electricity transfers the energy through the power lines from the power station to each consumer.
In each house the energy carried by the electricity is transferred to the various appliances (eg toaster, radio, television and oven) where it is used (eg to cook and entertain).
Electricity is also useful because it can carry information. The flow of electricity can be made to start and stop very rapidly and precisely.
If the starting and stopping can be regulated in an orderly manner, the electric current can be used transfer digital information along the wire.
This digital information can be used by a DVD and television set to produce a picture on the screen, or by a computer to represent the words and images on a CD-ROM. The music from a CD is also produced by controlling the stopping and starting of the electrical current in the CD player.
Electricity is by far the most useful means of transferring energy.
© Commonwealth of Australia, 2003