Put a current-carrying wire square across a magnetic field and the field pushes on it — the motor effect, and the reason anything electric spins. The push is $F = BIL$ (read aloud: F equals B I L): $F$ is the push on the wire in newtons, $B$ the magnetic field strength, $I$ the current through the wire in amperes, and $L$ the length of wire actually sitting in the field, in metres. $B$ is measured in teslas — a one-tesla field pushes one newton on each metre of wire carrying one ampere. Every wire in this lesson sits square across its field, so the angle factor is 1 and stays out of the way.

The transformer is the other magnetic worker: two coils sharing one iron core, trading voltage by turns: $\dfrac{V_s}{V_p} = \dfrac{N_s}{N_p}$ (read aloud: V-s over V-p equals N-s over N-p). Here $N$ counts the turns of wire wound on a coil and $V$ is that coil's voltage; the subscript p means the primary — the coil fed by the supply — and s means the secondary, the coil that feeds whatever comes next. The instinct that beats the trap: MORE turns sit on the side with MORE voltage, always.