Generators & Transformers

Generators & Transformers

You already know a current makes a magnetic field. The reverse is just as powerful: move a magnet near a coil and you generate electricity. That one idea runs power stations, microphones and the whole National Grid. It also explains why a transformer, plugged into a battery, does absolutely nothing. (Higher tier.)

Inducing a potential difference

When the magnetic field through a coil changes, a potential difference is induced across the coil. Complete the circuit and an induced current flows. You can change the field by moving a magnet in or out, or by moving a wire through a field. Three things make the induced p.d. bigger: • moving faster • using a stronger magnet • using more turns on the coil And one word carries all the weight: changing. A field that is not changing induces nothing at all, however strong it is.

Watch the meter

A magnet is pushed into a coil, held still inside it for a few seconds, then pulled back out. A sensitive meter is connected across the coil. What does the meter show?

• A deflection one way as it goes in, nothing at all while it is held still, then a deflection the other way as it comes out
• A steady deflection the whole time the magnet is anywhere near the coil
• A deflection the same way on the way in and on the way out, since the same magnet is involved
• Nothing at any point, because there is no battery in the circuit

Alternator or dynamo?

Spin a coil in a magnetic field and you have a generator. The only difference between the two kinds is how the spinning coil is connected to the outside world:

Which generator made this?

A generator is connected to an oscilloscope. The trace rises to a peak, falls back through zero, dips to a trough below the line, and repeats. Which kind of generator is it, and which part is responsible?

• An alternator, because the trace goes below zero, meaning the current reverses. Its slip rings never swap the connections over
• A dynamo, because the trace is a repeating wave
• Either: the trace shape depends on how fast the coil spins rather than on the type
• Neither: a generator cannot produce a current that reverses direction

Inside a microphone

A moving-coil microphone is one of the neatest applications of the generator effect, and it is literally a loudspeaker running backwards. Sound waves strike a thin diaphragm with a coil attached to it, sitting in a magnetic field. The sound makes the diaphragm and coil vibrate, the field through the coil therefore keeps changing, and a varying potential difference is induced whose pattern copies the pressure pattern of the sound wave. A loudspeaker does the same thing in reverse: feed a varying current into a coil in a field and the coil moves, pushing the air. Wire a loudspeaker to a sensitive amplifier and shout at it, and it genuinely works as a poor microphone.

Label the transformer

Here is a transformer. Drag each label onto the correct part.

How a transformer works

Five things to have straight. The third is asked about far more often than its one line on a diagram suggests:

Why not a battery?

A student connects a transformer's primary coil to a battery instead of the mains and finds that the secondary produces nothing, apart from a brief flicker at the instant of connection. Why?

• A battery gives a steady current, so the field in the core is steady, and only a CHANGING field induces a p.d. The flicker happens as the field builds up at the moment of switching on
• A battery is too weak, and mains voltage is needed to reach the secondary coil
• The battery is connected the wrong way round, so the induced p.d. cancels out
• Direct current cannot pass through an iron core

Find the secondary voltage

A transformer has 100 turns on its primary coil and 300 turns on its secondary. The primary p.d. is 12 V. What is the secondary p.d., in volts?

Find the secondary current

An ideal transformer has a primary p.d. of 200 V and a primary current of 3 A. Its secondary p.d. is 50 V. What is the secondary current, in amps?

Step up or step down?

Which are true?

Select ALL THREE statements that are TRUE.

• A transformer does nothing useful with a steady direct current, because an unchanging field induces no potential difference
• A step-up transformer raises the voltage and lowers the current, so the power it passes on is unchanged
• Holding a magnet still inside a coil induces nothing at all, however strong the magnet is
• A transformer increases the power available, which is why the National Grid uses them
• A dynamo produces alternating current
• The iron core carries current from the primary coil to the secondary

Grid summary

A magnetic field that is _____ through a coil induces a potential difference, which is the generator effect. An alternator uses slip rings and gives a.c., while a dynamo uses a split-ring commutator and gives d.c. A transformer uses two coils on an iron _____, which carries the changing magnetic field between them, and it works only on _____. Because power in equals power out, a step-up transformer raises the voltage and lowers the _____, which is what makes long-distance transmission efficient.