How does an ideal transformer change AC voltage?

A transformer can produce voltage in a second coil without a conducting wire joining the two coils. The connection is magnetic: a changing magnetic flux links both windings. This makes transformers useful for changing alternating voltages while preserving an energy-transfer relationship between input and output.

The input winding is called the primary, and the output winding is the secondary. In the ideal picture, both windings link the same changing flux through the core. Faraday’s law relates the voltage induced around a turn to how rapidly that linked flux changes. More turns in series add more induced voltage because each experiences the same flux change.

The example has 100 primary turns and 200 secondary turns. Therefore the secondary voltage is twice the primary voltage: 12 volts becomes 24 volts. These are RMS values for the stated sinusoidal AC operation, allowing a useful effective-voltage comparison. The small drawn loops are symbols for the labeled turn counts rather than literally 100 or 200 visible wires.

A doubled voltage does not create extra power. With no losses and the compatible resistive load assumed here, input power equals output power. The stated 12-volt, 2-ampere input transfers 24 watts. At 24 volts, the secondary therefore supplies 1 ampere. In symbols, VpIp = VsIs, so the current ratio goes opposite to the voltage ratio.

The connected load helps determine how much current flows. The turns ratio alone does not command a fixed 1-ampere output under every condition. When a load draws secondary current, its magnetic effect is accompanied by a corresponding primary response, allowing energy transfer from the source. The given currents describe one loaded operating point.

Changing flux is essential. A comparison with settled, unchanging flux has no continuing induced secondary voltage. Switching can briefly change flux, so that statement is not a claim that every moment involving direct current lacks induction. Nor does it describe safely applying constant voltage indefinitely to a real winding.

Real transformers lose energy through heating and other effects, and their cores and insulation have operating limits. The ideal account isolates the useful relationship: a common changing flux gives equal induced voltage per turn, while the coupled current response keeps the voltage change consistent with energy conservation.