If the conductor is part of a complete circuit, a current is induced in…

The idea

An induced potential difference is a push; a current only flows if there is somewhere for the charge to go. Connect the ends of the moving conductor into a complete loop, through a sensitive ammeter, a lamp or a resistor, and the induced potential difference drives a current round it. Break the loop and the potential difference is still there, and a voltmeter will still read it, but the ammeter reads zero. That distinction earns marks, because questions ask what is induced and the right answer depends on whether the circuit is closed. It also explains a familiar observation: a dynamo is easy to spin with nothing plugged into it and much harder once a lamp is connected, because only then is there an induced current to oppose the movement.

What earns the marks

• The conductor being part of a complete circuit.
• The induced potential difference driving an induced current round that circuit.
• Stating that an open circuit gives a potential difference but no current.
• A suitable way of detecting the current, such as a sensitive ammeter or a lamp.

Common mistakes and how to put them right

• Using induced current and induced potential difference as though they meant the same thing.

• Claiming a current flows when the ends of the conductor are not joined.

• Reading a diagram with a break in the circuit and still answering that a current is induced.

• Saying the magnet supplies the current, when the source is the induced potential difference.

Exam tips

• Check the diagram for a break in the circuit before deciding whether to write current or potential difference.
• If the question says a lamp lights, you can safely write about an induced current.
• Name the meter you would use; a sensitive ammeter or a galvanometer scores where a meter may not.