If an electrical conductor moves relative to a magnetic field or if there…

The idea

There are two ways to get a potential difference for nothing: move a conductor across a magnetic field, or hold the conductor still and change the field around it. Both amount to the same thing, the conductor cutting magnetic field lines. Push a wire down between the poles of a magnet and a potential difference appears across its ends; hold it still and the reading drops to zero, because nothing is being cut. Move the wire along the field lines rather than across them and again nothing is induced. A coil carrying an alternating current does the second job with nothing moving at all: its field grows, collapses and reverses, and any conductor sitting in that changing field has a potential difference induced across it. The word induced matters, because nothing is connected to the conductor.

What earns the marks

• The conductor and the magnetic field moving relative to one another, or for the field around the conductor changing.
• The conductor cutting magnetic field lines.
• A potential difference being induced across the ends of the conductor.
• Stating that with no relative movement and no change of field, nothing is induced.

Common mistakes and how to put them right

• Saying a current is induced when the circuit is open, since only a potential difference appears until the circuit is complete.

• Moving the wire along the field lines and still expecting a reading, when no field lines are being cut.

• Describing a steady magnetic field near a stationary coil as inducing a potential difference.

• Writing that the magnet gives the wire charge, rather than that a potential difference is induced.

Exam tips

• Look for the word change; if nothing about the field or the position changes, nothing is induced.
• State clearly which is moving, the conductor or the magnet, because only the relative motion matters.
• Use the phrase a potential difference is induced across the ends, which is the wording the mark scheme prints.