A transformer is two coils of wire wrapped around the same iron ring, but the two coils never actually touch each other. When electricity flows in the first coil, it makes the iron ring magnetic, and that changing magnetism makes electricity appear in the second coil. The trick is in the number of loops. If the output coil has twice as many loops as the input coil, you get twice the voltage out. Half as many loops, half the voltage. But you never get something for free: when the voltage goes up, the current goes down by the same amount, because the energy coming out has to match the energy going in. Slide the two coil sliders in the simulator and watch the voltage climb up or drop down.
Most people think a transformer gives you extra power for free when it raises the voltage. In fact energy is conserved, so when the voltage goes up the current drops by the same amount.
What's actually happening
The strange thing about a transformer is that the two sides are not connected. There is no wire running from input to output — just two separate coils sharing an iron core, with a gap of insulation between them. Energy crosses that gap as magnetism. Current in the first coil magnetises the iron; because the current is alternating, that magnetism is constantly changing; and a changing magnetic field through the second coil pushes a voltage across it. Michael Faraday found this link between changing magnetism and electricity in 1831, and the whole power grid runs on it.
What sets the voltage is embarrassingly simple: you count the loops. Each loop of the second coil catches the same changing magnetism, and their voltages add up. So if the output coil has twice the loops of the input coil, it produces twice the voltage; ten times the loops, ten times the voltage. Want to step voltage down instead? Give the output fewer loops. A single ratio of two numbers decides whether the box is a step-up or a step-down transformer.
But the catch is iron-clad: energy in must equal energy out. Power is voltage times current, so if a transformer doubles the voltage, it must halve the current to keep the product the same. This trade is the reason the grid exists in the form it does. Power stations step voltage up to hundreds of thousands of volts for the journey across the country, because high voltage means low current, and low current means the wires barely lose any energy heating up. Near your home, substation transformers step it all the way back down to the safe 120 or 240 volts in your sockets. The same trade is why a phone charger is warm: inside is a tiny transformer turning mains voltage down to about 5 volts.
A transformer trades voltage for current at the ratio of its loops, which is why the grid ships power at huge voltages and steps it back down for your home.
- 1Wrap 50 turns of thin insulated wire around one side of an iron core (a nail bundle or a toroid), then wind 100 turns of a second, separate wire around the other side.
- 2Feed a low-voltage AC signal into the 50-turn coil and measure the voltage across the 100-turn coil with a multimeter.
- 3You will read roughly double the input voltage — the turns ratio, made real. Swap which coil is the input and it halves instead.
Common questions
Because energy is conserved: power in must equal power out. Power is voltage times current, so doubling the voltage must halve the current to keep the product the same.
High voltage means low current, and low current means the wires barely lose energy heating up. Power stations step voltage up for the long journey, and substations step it back down to safe levels near your home.
It needs a changing magnetic flux to induce a voltage in the second coil. A steady DC current produces no changing flux, so it cannot transfer energy across the gap.