Recall and apply: energy transferred (J) = charge (C) × potential…

The idea

This equation links three circuit quantities. Potential difference, measured in volts (V), is the energy transferred per unit charge, so 1 V means 1 J of energy is transferred for each coulomb of charge passing. Multiplying charge in coulombs (C) by potential difference in volts (V) therefore gives energy transferred in joules (J): E = Q × V. For example, a charge of 4 C moving through a potential difference of 3 V transfers 4 × 3 = 12 J. Rearranged, Q = E ÷ V and V = E ÷ Q. In a multiple-choice question, check the units of each option and confirm which quantity is being asked for before substituting numbers.

What earns the marks

• Selects the option that multiplies charge in coulombs by potential difference in volts to give energy in joules.
• Applies E = Q × V with the correct substitution, for example 4 C × 3 V = 12 J.
• Rearranges the equation correctly when energy and one other quantity are given, for example Q = E ÷ V.
• Rejects options that confuse the equation with power = potential difference × current or that use incorrect units.

Common mistakes and how to put them right

• Multiplies current by potential difference when the question asks for energy transferred.

  Put it right: Use charge × potential difference instead.

• Uses the wrong unit for energy, such as watts.

  Put it right: Energy transferred is measured in joules.

• Adds charge and potential difference rather than multiplying them.

  Put it right: The relationship is a product.

Exam tips

• Write the equation from memory first, then substitute the values with their units.
• Check the unit demanded by the question so you pick the option in joules, not watts.
• Rearrange before substituting when the unknown is charge or potential difference.