Current, resistance and potential difference: Current, potential difference or resistance can be calculated using the equation:

The idea

The relationship between current, potential difference and resistance is summarised by V = IR, where V is potential difference in volts (V), I is current in amperes (A) and R is resistance in ohms (Ω). Rearranging gives I = V ÷ R and R = V ÷ I. To use the equation, identify the two known quantities, substitute values with units, then rearrange and calculate. For example, a 12 V supply driving 3 A through a resistor gives 12 = 3 × R, so R = 12 ÷ 3 = 4 Ω. This equation can be used to calculate the resistance of any component at a given potential difference and current, including non-ohmic components like filament lamps whose resistance changes.

What earns the marks

• Recalls and writes the equation V = IR with correct symbols and units.
• Substitutes numerical values into the equation before rearranging to find current or resistance.
• Calculates the unknown quantity accurately after correct substitution.
• States the correct unit for the answer: V for potential difference, A for current and Ω for resistance.
• Applies the equation to calculate the resistance of both ohmic and non-ohmic components at a specific potential difference and current.

Common mistakes and how to put them right

• Mixing up the rearrangements, for example using R = I ÷ V.

  Put it right by remembering that resistance is potential difference divided by current.

• Forgetting to convert units, such as using mA directly in V = IR.

  Put it right by converting milliamperes to amperes by dividing by 1000 before substituting.

• Omitting the unit or writing the wrong unit with the final answer.

  Put it right by checking that potential difference is in V, current in A and resistance in Ω.

Exam tips

• Write the equation and substitute the known numbers before rearranging, as this often makes the maths easier and secures substitution marks.
• Check that all values are in the correct units before calculating, converting mA to A or kΩ to Ω where needed.
• Give your final answer with its unit and, where appropriate, to a sensible number of significant figures.