Electricity and Magnetism Practice Test

Transformers use electromagnetic induction to change the voltage of alternating current. Two coils wrapped around a magnetic core transfer the energy: when the current in the primary coil changes, it makes the magnetic flux in the core vary, which induces a voltage in the secondary coil. The size of the output voltage depends on the turns ratio between the coils: more turns on the secondary give a higher voltage (step-up), fewer turns give a lower voltage (step-down). In an ideal transformer, power is conserved, so the output power roughly equals the input power (VsIs ≈ VpIp), with current adjusting to keep the power balance.

This only works with alternating current because a changing current is needed to produce a changing magnetic flux that induces the secondary voltage. A steady DC current would quickly produce a constant flux and then no further induction. The other devices don’t perform this voltage transformation: a generator creates electrical energy from mechanical energy, a solenoid is simply a coil that can act as an inductor or electromagnet, and an electric motor converts electrical energy into mechanical energy rather than transforming voltage levels.