Energy transfers in everyday appliances: energy transferred = charge flow × potential difference

The idea

This relationship links the energy a component transfers to the charge passing through it and the potential difference across it. Charge flow Q is measured in coulombs (C), potential difference V in volts (V), and energy E in joules (J). One volt means one joule of energy is transferred per coulomb of charge, so E = Q V. For example, a 12 V lamp with 5 C passing through transfers E = 5 × 12 = 60 J. Rearranged, Q = E ÷ V and V = E ÷ Q. In everyday appliances, the mains potential difference is about 230 V, so a larger charge flow transfers more energy. This explains why appliances with higher power ratings transfer energy faster: power is energy per second, and current is charge per second.

What earns the marks

• State the equation as energy transferred = charge flow × potential difference, with E in joules, Q in coulombs and V in volts.
• Substitute numerical values correctly into E = Q V, keeping units consistent before calculating.
• Rearrange the equation to find charge flow (Q = E ÷ V) or potential difference (V = E ÷ Q) when required.
• Interpret a result as the energy transferred by the component, linking it to everyday appliances such as lamps, heaters and motors.
• Use the idea that one volt equals one joule per coulomb to explain the equation in words.

Common mistakes and how to put them right

• Multiplying charge flow by potential difference but forgetting to include units in the answer.

  Put it right: Write the unit, for example 60 J.

• Confusing charge flow in coulombs with current in amperes.

  Put it right: Current is charge per second, so use Q = I t if current and time are given.

• Rearranging incorrectly, for example writing Q = V ÷ E.

  Put it right: Check by substituting simple numbers back into E = Q V.

Exam tips

• Write the equation, then substitute values with units before calculating to reduce errors.
• If time and current are given instead of charge, first calculate Q = I t, then use E = Q V.
• Check that the final unit matches the quantity asked for: joules for energy, coulombs for charge, volts for potential difference.