Series and parallel circuits: the potential difference across each component is the same

The idea

This statement applies to components connected in parallel. In a parallel circuit, each branch is connected directly across the same two points of the supply, so every branch receives the full supply potential difference. If a 6 V battery feeds two lamps in parallel, the potential difference across each lamp is 6 V, even if the lamps have different resistances. This differs from a series circuit, where the supply potential difference is shared between components. The rule holds because all parallel branches share the same pair of connection nodes, so the electrical 'push' on each branch is identical. A voltmeter connected across any branch reads the same value as across the supply. This explains why household appliances, wired in parallel, each operate at the full mains potential difference and can be switched independently.

What earns the marks

• State that the statement describes components connected in parallel, where branches share the same two connection points.
• Explain that each parallel branch is connected directly across the supply, so each branch has the same potential difference as the supply.
• Compare with series circuits, where the supply potential difference is shared between components rather than being the same across each one.
• Apply the rule to a numerical example, such as a 6 V supply with two parallel lamps, giving 6 V across each lamp.
• Recognise that components in parallel may have different resistances and therefore different currents, while their potential differences remain equal.
• Describe how a voltmeter placed across any parallel branch gives the same reading as one placed across the supply.

Common mistakes and how to put them right

• Thinking potential difference is shared equally between parallel branches.

  Put it right by stressing that parallel branches each receive the full supply potential difference, while current is shared.

• Confusing the parallel rule with the series rule.

  Put it right by contrasting them, since in series the potential differences add to the supply value, whereas in parallel they are equal to it.

• Assuming equal potential difference means equal current.

  Put it right by explaining that current also depends on resistance, so a lower-resistance branch takes a larger current.

Exam tips

• Sketch the circuit and label the potential difference across each branch before writing your answer, so your reasoning is visible to the examiner.
• Use the phrase 'connected across the same two points' to justify why the potential differences are equal.
• If a question gives component values, calculate branch currents separately to show that equal potential difference does not mean equal current.