Recall and use the equation: potential difference (volt, V) = current…

The idea

This equation links three quantities in a circuit. Potential difference (p.d.), measured in volts (V), is the energy transferred per unit charge between two points. Current, in amperes (A), is the rate of flow of charge. Resistance, in ohms (Ω), measures how hard it is for current to flow. The relationship is V = I × R, so a p.d. of 6 V across a 3 Ω resistor carries a current of 2 A, because I = V ÷ R = 6 ÷ 3 = 2. Rearranged, I = V ÷ R and R = V ÷ I. Always substitute values in base units, keep the unit with the answer, and check that a larger resistance at fixed p.d. gives a smaller current. Recall means stating the equation and its units; use means selecting the correct rearrangement and calculating.

What earns the marks

• States the equation as potential difference = current × resistance, with units volt (V), ampere (A) and ohm (Ω).
• Rearranges the equation correctly to I = V ÷ R or R = V ÷ I before substituting values.
• Substitutes values in base units and calculates the answer, for example 6 V ÷ 3 Ω = 2 A.
• Gives the answer with the correct unit and a sensible magnitude for the circuit described.

Common mistakes and how to put them right

• Adds current and resistance instead of multiplying them.

  Put it right: Multiply I by R to obtain V.

• Divides resistance by current when finding p.d.

  Put it right: Use V = I × R so the larger quantity is the product.

• Omits the unit or writes the wrong unit with the answer.

  Put it right: Attach V, A or Ω to match the quantity calculated.

Exam tips

• Write the equation first, then rearrange before substituting numbers to avoid arithmetic slips.
• Convert any milliamps or kilohms to amperes or ohms before calculating.
• Check the answer by asking whether a bigger resistance should give a smaller current at fixed p.d.