Energy transfers in everyday appliances: E = Q V

The idea

E = Q V is the symbolic form of the relationship between energy transferred, charge flow and potential difference. E is energy in joules (J), Q is charge flow in coulombs (C), and V is potential difference in volts (V). Because one volt is one joule per coulomb, multiplying charge by potential difference gives energy. For example, if 20 C flows through a 230 V mains appliance, E = 20 × 230 = 4600 J. The equation can be rearranged to Q = E ÷ V or V = E ÷ Q. In everyday appliances, the potential difference is usually fixed by the mains supply, so the energy transferred depends on how much charge flows. This links to power, since power is energy transferred per second and current is charge flow per second.

What earns the marks

• Identify each symbol correctly: E is energy transferred in joules, Q is charge flow in coulombs and V is potential difference in volts.
• Substitute values into E = Q V and calculate the energy transferred.
• Rearrange E = Q V to make Q or V the subject and carry out the calculation.
• Explain that one volt is one joule per coulomb, linking the symbolic equation to its meaning.
• Apply the equation to a mains appliance where V is about 230 V and Q depends on current and time.

Common mistakes and how to put them right

• Treating V as energy rather than potential difference.

  Put it right: V is measured in volts and represents energy per unit charge.

• Using current in amperes directly in place of Q.

  Put it right: Charge flow must be in coulombs, so use Q = I t when necessary.

• Forgetting that E is in joules and giving an answer in volts or coulombs.

  Put it right: Match the unit to the quantity.

Exam tips

• Write the full equation E = Q V before substituting numbers to show your method clearly.
• Convert time to seconds and use Q = I t if the question gives current and time rather than charge.
• Use the rearranged forms Q = E ÷ V and V = E ÷ Q confidently, checking by substitution.