GCSE Science/Generating electricity
On the last page we looked at induction. That is how the motor effect acting on electrons in wires causes them to move and create a current. On this page we will look at some practical uses of induction and how electricity is made in power stations.
Look at the diagram of an electric motor. As the coil rotates it cuts the field lines in a downwards direction then an upwards direction. This means that the electricity produced alternates. The current flows one way then the other. The frequency depends on the speed of rotation.
Q1) A motor is rotated manually to produce a current. An oscilloscope is used to investigate this current and produces a trace like the one shown. The time is on the x axis and voltage is on the y axis.
The motor is speeded up. Which one of the following shows the new trace?
In the section above you learned that a dynamo produces a.c. If you've ever looked closely at a school power pack you will probably have noticed that it has a.c. and d.c. outputs. D.c. stands for direct current. The current flows in one direction only and (apart from when it is switched on or off) doesn't change in value. A.c. constantly changes. An a.c. curve is typically that of a sine wave. The current switches back and forth and the value of the current is constantly changing.(This fact is important when we come to study transformers.)
Some devices work equally well with either a.c. or d.c. Others must have the correct type or they won't work. For example think about a child's train on a track.
If a.c. electricity is used the train will not go anywhere! This is because the current is 'telling' the motor to go forwards, go backwards, no, go forwards and so on. Anything with a small electric motor in it has to use d.c. (Special electric motors which use a.c. do exist. However, the ones you'll come across in school or in any toy application will almost certainly be d.c. motors.)
On the other hand a bulb has no problem working on a.c. This is because a bulb converts the energy of the electric current to heat and light. It makes no difference which way the current flows, the energy is still there. So bulbs, electric fires, cookers and so on do not care if they use a.c. or d.c.
Transformers (which we will study in the next section) will only work with a.c. This is because a transformer needs to have a constantly varying current, and d.c. only varies when it is switched on or off.
- Q2) Name a source of a.c. Name a source of d.c.
- Q3) Name a device that only works with d.c. Name a device which will work with a.c. or d.c.
- Q4) A hair dryer contains a heating element and a fan. Will it work best with a.c. or d.c.? Explain your answer.
- Q5) An engineer has an idea about the toy train. He thinks, "If I pass the a.c. through a certain useful electric component, I can get rid of the backward direction current and keep the forward direction current. Like this: