How do transformers change voltage?

1. Electricity is made at a power station and travels a long way on tall power lines. On the trip its push, called voltage, is changed more than once. How?
2. This is a transformer: two coils of wire wound around a ring of iron. One coil brings electricity in and the other sends it out, but the two wires never touch.
3. The electricity in the first coil flows back and forth. A coil carrying electricity is a magnet, so the iron ring fills with a magnet field that keeps growing, shrinking and flipping over.
4. The changing magnet field passes through the second coil and pushes the electrons in its wire. Electricity flows there too, and the lamp lights, with no wire joining the coils.
5. Wind more loops on the second coil than the first and the voltage steps up. Wind fewer and it steps down. The number of loops is the secret.
6. At the power station a transformer steps the voltage up for the long trip. Near your street another one steps it down again, so it is right for your home.

Tap each part of the transformer to find out what it does.

• Input coil. The input coil carries the electricity coming in. As that electricity flows back and forth, the coil becomes a magnet that keeps flipping.
• Iron core. The iron ring is called the core. Magnet fields pass through iron very easily, so the core guides the field from the input coil around to the output coil.
• Magnet field. The magnet field is invisible, so we have drawn it as dots. It loops around the core, and it only works because it keeps changing.
• Output coil. The changing field pushes electrons in the output coil, so electricity comes out here. More loops than the input coil means a higher voltage. Fewer loops means a lower one.
• Thin iron sheets. The core is not one solid block. It is a stack of thin iron sheets, each with a thin coat between. That stops electricity from swirling around inside the iron and wasting energy as heat.
• Screw terminals. The ends of each coil are fixed under screws. One pair joins the supply and the other pair joins whatever the transformer is powering.

Drag the gold knobs to wind loops on or off each coil. Compare the voltage going in with the voltage coming out.

Experiment: What happens to the voltage coming out when the output coil has more loops than the input coil?

1. Give both coils the same number of loops. Look at the two meters.
2. Wind more loops onto the output coil. What do the meter and the lamp do?
3. Now make the output coil the one with fewer loops.

What we found: More loops on the output coil than the input coil steps the voltage up. Fewer loops steps it down. Equal loops give the same voltage out as in.

Turn on JavaScript to try it yourself.

Each gadget needs its own voltage. Wind the coils to make it from the 12 volt supply, then press Switch on.

Electricity and magnets are close cousins. When electricity flows through a coil of wire, the coil becomes a magnet. And when a magnet's field changes near a coil of wire, it pushes electricity through the wire. A transformer uses both tricks, one after the other.

The electricity from a wall socket flows back and forth many times each second. In the input coil it makes a magnet field that keeps flipping. The iron core carries that changing field around to the output coil, where it pushes electrons and makes electricity again.

Each loop of the output coil gets the same little push, and the pushes add up. So more loops than the input coil gives a higher voltage, and fewer loops gives a lower one. A phone charger steps the voltage a long way down. A power station steps it a long way up.

Transformers cannot make extra energy. A higher voltage always comes with a smaller current. And the bike gear comparison only goes so far: gears turn, but nothing in a transformer moves except the electricity and the invisible field.

Key words

Coil

Wire wound round and round in many loops.

Alternating current (AC)

Electricity that flows back and forth, many times each second.

Induction

Making electricity in a wire with a changing magnet field.

Step up and step down

Step up means the voltage comes out higher. Step down means it comes out lower.

Turns ratio

How the number of loops on one coil compares with the other.

Check your thinking

1. What joins the two coils of a transformer together?

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   A changing magnet field in the iron core. The coils never touch. The changing magnet field in the core carries the energy across.

2. The output coil has fewer loops than the input coil. What comes out?

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   A lower voltage. Fewer loops pick up less push from the field, so the voltage steps down.

3. A transformer has 200 primary turns and 50 secondary turns. The input is 120 volts. What is the output?

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   30 volts. The turns ratio is 50 ÷ 200 = one quarter, so the output is a quarter of 120 volts: 30 volts.

4. Why can't a battery on its own run a transformer?

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   Its electricity is steady, so the magnet field does not change. Only a changing magnet field pushes electrons in the second coil. Steady electricity makes a steady field.

5. Why is the voltage stepped up before electricity travels a long way?

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   So less energy is wasted as heat in the long wires. A high voltage needs only a small current to carry the same power, and a small current wastes less heat in the wires.

6. Why is the core built from thin iron sheets instead of one block?

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   To stop electricity swirling in the iron and wasting energy as heat. The thin coat between the sheets blocks swirling currents, so less energy turns into heat.