Magnetism and Electromagnetism
5 marks · NormalA coil of wire is connected to a sensitive ammeter. A bar magnet is brought quickly towards the coil and then away from it.
State and explain what the ammeter shows as the magnet moves towards the coil.
Describe what happens if the magnet is then held stationary inside the coil.
Name the effect shown in this experiment and explain why the current changes direction when the magnet is pulled back out of the coil.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) The ammeter shows a deflection in one direction, as current is induced; the movement of the magnet causes magnetic field lines to cut through the coil.
(b) The current drops to zero; no current is induced when the magnet is stationary.
(c) The effect is electromagnetic induction. Pulling the magnet out reverses the direction of cutting field lines, so the induced current flows in the opposite direction.
Show mark scheme and worked solution
Marking points
B6a (a) · 2 marks · States current is induced as magnet moves (1) and explains due to cutting field lines (1).
B6b (b) · 1 mark · Explains current is zero when magnet is stationary inside coil (1).
B6c (c) · 2 marks · Names electromagnetic induction (1) and explains change in field direction reverses current (1).
Worked solution
a) When the magnet moves towards the coil, the ammeter shows a current (deflection) in one direction because a voltage is induced as the magnetic field through the coil changes.
b) If the magnet is held still inside the coil, the ammeter returns to zero. There is no changing field, so no induced current.
c) The effect is called electromagnetic induction. When the magnet is pulled out, the magnetic field through the coil decreases in the opposite sense, so the current is induced in the opposite direction.