If an electrical conductor "cuts" through a magnetic field, a potential difference is induced across the ends of the conductor.
If a magnet is moved into a coil of wire, a potential difference is induced across the ends of the coil.
The transformer works as follows:
An alternating current flowing through the primary coil induces a changing magnetic field in the iron core
This magnetic field is also induced in the secondary coil
This induces an alternating potential difference across the ends of the secondary coil.
In a step-up transformer, the potential difference across the secondary coil is greater than that across the primary coil.
In a step-down transformer, the potential difference across the secondary coil is less than that across the primary coil.
The equation for calculating the potential difference across the primary or secondary coils of a transformer is:
Vp = potential difference across the primary coil (volts, V)
Ip = potential difference across the secondary coil (volts, V)
np = number of turns on the primary coil
ns = number of turns on the secondary coil
If a transformer is assumed to be 100% efficient, the electrical power output equals the electrical power input.
The equation for calculating the electrical power output is:
Vp = potential difference across the primary coil (volts, V)
Ip = current in the primary coil (amperes, amps, A)
Vs = potential difference across the secondary coil (volts, V)
Is = current in the secondary coil (amperes, amps, A)
Switch mode transformers are much lighter and smaller than traditional transformers and operate at a high frequency (usually from around 50 kHz to 200 kHz). They are used as mobile phone chargers. They do not used much power when they are switched on if no load is applied.