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How do current and potential difference behave in series and parallel circuits?

Circuit symbols and how to build circuits, the rules for current and potential difference in series and parallel circuits, and how total resistance changes when components are added in series or in parallel.

A focused answer to OCR Gateway GCSE Physics A topic P3 on circuits, covering standard circuit symbols, the rules for current and potential difference in series and parallel circuits, and how the total resistance changes when components are added in series or in parallel.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. Building circuits and symbols
  3. Series circuits
  4. Parallel circuits
  5. How resistance changes
  6. Try this

What this topic is asking

OCR wants you to recognise circuit symbols, build series and parallel circuits, and apply the rules for current, potential difference and resistance in each. This is topic P3.2 of the OCR Gateway Physics A (J249) specification.

Building circuits and symbols

You must know the standard circuit symbols: cell and battery, switch, lamp, resistor, variable resistor, ammeter (measures current, connected in series), voltmeter (measures potential difference, connected in parallel across a component), diode, LED, thermistor and LDR. A complete circuit needs an unbroken conducting path from one terminal of the supply to the other.

An ammeter goes in series with the component because the current through them must be the same; a voltmeter goes in parallel across the component because it measures the potential difference between two points.

Series circuits

Because there is only one path, a break anywhere (such as one lamp failing) stops the current everywhere. Sharing the potential difference means that with identical components, each gets an equal share of the supply voltage.

Parallel circuits

Parallel wiring has practical advantages, which is why homes use it: each device gets the full mains voltage, each can be switched on or off independently, and if one device fails the others keep working because their paths are still complete.

How resistance changes

Adding resistors in series always increases the total resistance (more obstacles in one path). Adding resistors in parallel always decreases the total resistance, because there are now more paths for the current to flow through, like opening extra lanes on a road. This is why a parallel combination draws more current from the supply than either resistor alone.

Try this

Q1. Three identical resistors are connected in series to a 9 V9\,\text{V} battery. State the potential difference across each resistor. [1 mark]

  • Cue. 93=3 V\dfrac{9}{3} = 3\,\text{V} across each (the voltage is shared equally).

Q2. State how a voltmeter should be connected to measure the potential difference across a lamp. [1 mark]

  • Cue. In parallel, across the lamp.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20194 marksIn a series circuit, two identical lamps are connected to a 6 V6\,\text{V} battery. State the potential difference across each lamp, and describe what happens to the current and the brightness if a third identical lamp is added in series.
Show worked answer →

A P3 question on series-circuit rules worth four marks. In a series circuit the potential difference is shared between the components, so each of the two identical lamps has 62=3 V\dfrac{6}{2} = 3\,\text{V} across it (1 mark). Adding a third identical lamp in series increases the total resistance of the circuit (1 mark). A higher resistance means a smaller current flows (the same current everywhere in series) (1 mark), so all the lamps become dimmer, and each now has 63=2 V\dfrac{6}{3} = 2\,\text{V} across it (1 mark). Markers reward the shared potential difference, increased resistance, reduced current, and dimmer lamps.

OCR 20213 marksDescribe two advantages of connecting household lamps in parallel rather than in series.
Show worked answer →

A P3 question worth three marks. In parallel, each lamp gets the full potential difference of the supply, so each lamp is at full brightness (1 mark). Each lamp can be switched on and off independently without affecting the others (1 mark). If one lamp breaks, the others stay on, because there is still a complete path through the other branches, whereas in series a single break stops them all (1 mark). Markers reward any two clear advantages, such as full voltage to each, independent switching, and the others staying on if one fails.

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