Work is done when charge flows in a circuit.

The idea

Work done means energy transferred by a force, and in a circuit the source does work pushing charge round the loop while the charge does work as it passes through each component. The bookkeeping is E = Q V, where the potential difference is the energy transferred per coulomb: a 1 V potential difference means 1 joule is transferred for every coulomb that passes. Moving 2 C through a 6 V lamp therefore transfers 2 × 6 = 12 J. Because charge flow is Q = I t, the equations combine: E = Q V = I t V = P t, which is why the circuit energy and power equations always agree. Work is only done while charge is actually moving, which is why an appliance transfers nothing while its switch is open.

What earns the marks

• Selecting E = Q V, energy transferred equals charge flow multiplied by potential difference.
• Using Q = I t to find the charge when the current and time are given.
• Correct substitution into the chosen equation.
• The energy transferred with the unit joules, J.

Common mistakes and how to put them right

• Treating potential difference as an energy rather than as energy per coulomb.

• Putting the time into the wrong equation, for example writing E = Q V t.

• Leaving the current in the final answer because Q = I t was never applied.

• Saying no work is done in a resistor because nothing visibly moves.

Exam tips

• If the question gives current and time but the equation needs charge, do Q = I t on its own line first.
• Define a volt as a joule per coulomb when asked to explain what potential difference means.
• Check that both routes agree, since E = Q V and E = P t must give the same number for the same situation.