CIE iGCSE Co-Ordinated Science P4.5.6 The transformer Exam Style Questions Paper 4

(a)(i) A step-up transformer consists of a soft iron core with a primary coil having fewer turns than the secondary coil.
The soft iron core provides a path for the magnetic field, and the turns ratio determines the voltage transformation. In a step-up transformer, the secondary coil has more turns, which increases the voltage.

(a)(ii) \( P = I^2R = 500^2 \times 0.60 = 250,000 \times 0.60 = 150,000 \text{ W} \)
Power loss in a transmission wire is calculated using \( P = I^2R \). The higher the current, the greater the energy lost as heat in the wires. Stepping up the voltage reduces the current and thus reduces power loss.

(b) Direct current (d.c.) flows in one direction only, while alternating current (a.c.) reverses direction periodically.
In d.c., the electric charge flows consistently from positive to negative. In a.c., the direction of charge flow changes repeatedly, typically in a sinusoidal pattern.

(c)(i) Slip rings maintain a continuous electrical connection to each side of the rotating coil, preventing the wires from twisting or tangling.
As the coil rotates, the slip rings rotate with it while the carbon brushes remain stationary, ensuring a constant electrical connection to the external circuit without wires becoming twisted.

(c)(ii) The graph should be a sine wave with positive and negative e.m.f. values, showing constant amplitude and a constant period.
As the coil rotates at constant speed, the e.m.f. varies sinusoidally: maximum positive, zero, maximum negative, and back to zero in one complete rotation. The amplitude and period remain constant.

(c)(iii) The amplitude of the e.m.f. doubles, and the period halves (the frequency doubles).
Doubling the rotation speed means the coil cuts through the magnetic field lines twice as fast, generating double the e.m.f. It also completes each rotation in half the time, so the period is halved.