Describe that when a closed circuit includes a source of potential…

The idea

A circuit carries a current only when two conditions hold together: the path is closed (an unbroken conducting loop) and the loop contains a source of potential difference, such as a cell, battery or power supply. The source drives charge around the loop; break the loop, for example by opening a switch or removing a lamp, and the current stops because charge can no longer flow all the way round. A closed loop with no source, such as a plain wire ring, has no current because nothing pushes the charge. So the presence of a source and a closed path are both needed, and either one missing means no current.

What earns the marks

• States that a current flows when the circuit is closed and contains a source of potential difference.
• Identifies that breaking the circuit, for example by opening a switch, stops the current.
• Explains that a closed loop without a source of potential difference carries no current.
• Recognises that the source of potential difference drives charge around the closed circuit.

Common mistakes and how to put them right

• Assumes a closed circuit alone is enough for a current.

  Put it right: Add that a source of potential difference must also be present.

• Believes a current continues through an open switch.

  Put it right: State that an open path breaks the loop and stops the current.

• Thinks the source pushes current only when the circuit is open.

  Put it right by linking current to a closed loop containing the source.

Exam tips

• Read the options for both conditions: closed circuit and a source of potential difference present.
• Reject any option that implies current flows with a broken loop.
• Check that the chosen option names the source as the thing driving the current.