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

The idea

Learners explain, without calculation, why combining resistors changes the net resistance. In series, the same current passes through each resistor in turn, so each adds an extra opposition to the flow; the net resistance equals the sum and is larger than either resistor. In parallel, the current splits between alternative branches, giving more paths for charge to flow; the net resistance is smaller than the smallest branch resistance. A useful check is that two equal resistors in parallel give half the resistance of one, while in series they give double. The explanation is qualitative: describe the effect of added opposition or extra paths rather than deriving a formula.

What earns the marks

• States that resistors in series each add opposition so the net resistance is the sum of the individual values.
• Explains that the same current passes through each series resistor, so the total opposition increases.
• States that resistors in parallel provide additional paths for current, lowering the net resistance.
• Explains that the net resistance in parallel is less than the smallest individual resistance.
• Uses the idea of current splitting between branches to justify the reduced net resistance.

Common mistakes and how to put them right

• Adds parallel resistances directly as if in series.

  Put it right: Explain that extra branches lower the net resistance instead.

• Claims parallel net resistance lies between the two values.

  Put it right: State that it is smaller than the smallest.

• Says series resistors share the current equally.

  Put it right: Note that the same current flows through each in series.

• Gives a numerical formula when a qualitative explanation is required.

  Put it right: Describe the effect of extra opposition or paths.

Exam tips

• Contrast the two arrangements in one sentence: series adds opposition, parallel adds paths.
• Anchor the explanation to current behaviour, since current is the same in series and splits in parallel.
• Use the 'smaller than the smallest' check for parallel combinations.
• Keep the answer qualitative unless the question supplies values.