Current, resistance and potential difference: resistance, R, in ohms, Ω

The idea

Resistance measures how strongly a component opposes the flow of electric charge. Resistance is defined by R = V ÷ I, where V is potential difference in volts and I is current in amperes. The unit of resistance is the ohm, symbol Ω; one ohm allows one ampere when one volt is applied. In a series circuit, adding resistors increases total resistance, so current falls for the same supply potential difference. In parallel, adding branches decreases total resistance because there are more paths. Resistance changes with temperature in filament lamps and with light intensity in LDRs, so R = V ÷ I gives the resistance at that particular operating point. When calculating, substitute values carefully.

For example: A 12 V supply driving 3 A gives R = 12 ÷ 3 = 4 Ω.

What earns the marks

• States that resistance is the opposition to current and is measured in ohms or Ω.
• Uses the relationship R = V ÷ I correctly, with V in volts and I in amperes, to calculate resistance.
• Rearranges the relationship to find current or potential difference when resistance is known.
• Explains that for a fixed resistor at constant temperature, resistance remains approximately constant as current changes.
• Describes how adding resistors in series increases total resistance while adding parallel branches decreases it.
• Recognises that resistance of filament lamps and other non-ohmic components changes with temperature or other physical conditions.

Common mistakes and how to put them right

• Confusing the unit of resistance with the volt or amp.

  Put it right: Resistance is measured in ohms (or Ω), potential difference in volts (V), and current in amps (A).

• Dividing current by potential difference instead of potential difference by current.

  Put it right: R = V ÷ I, so 6 V across 2 A gives 3 Ω, not 0.33 Ω.

• Assuming resistance always stays the same for every component.

  Put it right: Filament lamps and thermistors change resistance as conditions change, so R = V ÷ I applies at each operating point.

Exam tips

• Write the equation, substitute values with units, then give the answer with the correct unit, for example 4 Ω or 4 ohms.
• Check rearrangements by substituting simple numbers back into V = I × R before using them in the exam.
• When a graph of current against potential difference is curved, describe resistance as changing rather than constant.