1. How transformers work
A current in a solenoid causes a magnetic field and a changing magnetic field can induce a p.d. This is the principle behind a transformer, demonstrated with an ancient transformer from the former U.S.S.R., showing the primary and secondary coils linked by a laminated iron core.
2. The Transformers Equations
For a 100% efficient transformer the power in is equal to the power out. There are then a number of equations for both step-up and step-down transformers linking the current, potential difference and number of turns.