Electromagnetic induction and generators – IGCSE Physics exam questions with mark scheme

1Grade 4–55 marks

A coil of wire is connected to a sensitive ammeter. When the north pole of a bar magnet is pushed into the coil, the needle of the ammeter moves to the right.

State what is seen on the ammeter when:

(a) the magnet is held still inside the coil [1]

(b) the magnet is pulled out of the coil [1]

(d) Explain why a current is produced when the magnet moves. [2]

(a) no reading / needle reads zero(1)
(b) needle moves to the left / reading in the opposite direction(1)
(c) needle moves further to the right / larger reading(1)
(d) the coil (wire) cuts the magnetic field lines / the magnetic field through the coil changes(1)
(d) this induces a voltage (e.m.f.) in the coil and, as the circuit is complete, a current flows; allow 'a current is induced (in the coil)'(1)
Answer: (a) zero (b) left (c) larger deflection to the right (d) changing field induces a voltage in the complete circuit
Examiner tip: No relative movement means no induced voltage: a stationary magnet, however strong, gives zero reading.

2Grade 6–76 marks

A coil is rotated at a steady rate in a magnetic field. The graph shows how the voltage induced across the coil varies with time.

(a) Use the graph to calculate the frequency of the alternating voltage. [2]

(b) The coil is now rotated twice as fast. Describe how the graph would change. [2]

(a) period = 40 ms / 0.040 s read from graph; allow 2 cycles in 80 ms(1)
(a) frequency = 1 ÷ 0.040 = 25 Hz(1)
(b) peak voltage increases / peaks are taller; allow doubles / 12 V(1)
(b) period halves to 20 ms / frequency doubles to 50 Hz / twice as many cycles in the same time; allow 'waves closer together'(1)
(c) each side of the coil moves up through the field then down (moves in the opposite direction) every half turn; allow the (magnetic) field through the coil changes from increasing to decreasing (and vice versa) every half turn(1)
(c) so the direction in which the field lines are cut reverses, reversing the induced voltage (current); allow so the direction of the induced voltage reverses(1)
Answer: (a) 25 Hz (b) peaks at 12 V, period 20 ms (c) each side of the coil reverses its direction of motion through the field every half turn
Examiner tip: Many students only say the waves get closer together; you also need to say the peaks get taller, because faster cutting induces a bigger voltage.

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