Explain the construction and working of a transformer. Derive the relation between the voltages and the numbers of turns in its two coils, and state two ways of reducing energy losses.

1. 1 · State the principle. Mutual induction: a changing current in one coil induces an emf in a coil linked to it.
2. 2 · Draw the transformer. [Laminated soft iron core; primary N_p joined to AC; secondary N_s joined to a load; flux arrows round the core; all labelled]
3. 3 · Describe the construction. Primary and secondary coils of insulated copper wire on a common laminated soft iron core, insulated from each other.
4. 4 · Explain the working. AC in the primary makes an alternating flux in the core; it links the secondary and induces an emf there.
5. 5 · Derive the ratio. E_p = −N_p dφ/dt and E_s = −N_s dφ/dt, so V_s/V_p = N_s/N_p; with no loss, V_pI_p = V_sI_s, so I_p/I_s = N_s/N_p.
6. 6 · Reduce the losses. Laminate the core (eddy currents), use soft iron (hysteresis), use thick copper wire (copper loss) and wind the coils together (flux leakage).
7. 7 · Link to life. Pole and sub-station transformers step the line voltage down to 230 V for homes.
8. 8 · Close in one line. So, a transformer trades voltage for current, keeping the power almost the same.

Scoring tip: State the ideal-transformer assumptions in one line before you divide the two emfs.