Explain why, if two resistors are in series, the net resistance is…

The idea

In a series circuit the same current passes through each component in turn, so the charges must push through both resistors one after another. Each resistor adds to the opposition, so the net (total) resistance is the sum: R = R₁ + R₂, which is larger than either alone. In a parallel circuit the current splits between two separate branches, giving the charges more than one path. The total current for a given p.d. is larger, so the net resistance is smaller than either branch alone; for two resistors 1/R = 1/R₁ + 1/R₂. Explaining means giving the reason in terms of current paths and opposition to flow, not just quoting the formulae.

What earns the marks

• States that in series the same current passes through both resistors in turn, so each adds opposition and the net resistance is the sum.
• States that in parallel the current splits into separate branches, giving more than one path for charge.
• Explains that more paths allow a larger total current for the same p.d., so the net resistance falls below either branch value.
• Uses the relationship R = R₁ + R₂ for series or 1/R = 1/R₁ + 1/R₂ for two parallel resistors to support the reasoning.

Common mistakes and how to put them right

• Claims parallel resistors add like series resistors.

  Put it right: Recognise that parallel branches give extra paths and reduce net resistance.

• Says the net parallel resistance equals the smaller resistor.

  Put it right: State that it is less than either branch value.

• Describes series resistors as sharing the p.d. as the reason resistance rises.

  Put it right: Explain instead that the same current passes through each in turn and each adds opposition.

Exam tips

• Answer 'why' by referring to current paths and opposition to flow, then support with the formula.
• Use a simple numerical example, such as two 4 Ω resistors giving 8 Ω in series and 2 Ω in parallel, to check your reasoning.
• Keep the words 'series' and 'parallel' attached to the correct description to avoid mixing the two cases.