Recall and use the equation: energy transferred (joule, J) = charge moved…

The idea

The equation energy transferred = charge moved × potential difference links three quantities: energy in joules, charge in coulombs and potential difference in volts. To use it, write the values with their units, substitute into E = Q × V and calculate. If 4 C of charge moves through a potential difference of 6 V, the energy transferred is 4 × 6 = 24 J. Rearranging gives charge as energy divided by potential difference, and potential difference as energy divided by charge. Always check that the answer carries the correct unit: joules for energy, coulombs for charge and volts for potential difference. Converting millicoulombs to coulombs before substituting keeps the arithmetic consistent.

What earns the marks

• Recalls the equation that energy transferred equals charge moved multiplied by potential difference.
• Substitutes charge in coulombs and potential difference in volts to calculate energy in joules.
• Rearranges the equation to find charge moved when energy and potential difference are known.
• Rearranges the equation to find potential difference when energy and charge are known.
• Converts charge given in millicoulombs to coulombs before calculating.

Common mistakes and how to put them right

• Adds charge and potential difference instead of multiplying them.

  Put it right: Multiply the two quantities to obtain energy.

• Uses energy divided by charge when asked for energy.

  Put it right: Multiply charge by potential difference for energy transferred.

• Leaves charge in millicoulombs and reports energy a thousand times too small.

  Put it right: Convert to coulombs first.

Exam tips

• Write the equation, then the substituted values, then the answer with its unit.
• Rearrange symbolically before substituting numbers to reduce arithmetic slips.
• Estimate the answer first so an implausible power of ten is spotted.