# Many students do not recognise that the potential difference across each resistor in a simple parallel circuit is the potential difference across the battery

Electricity and Magnetism

Misconception Many students approach questions about simple parallel circuits by working out the effective resistance of the load, rather than by recognising that the potential difference across each resistor is the potential difference across the battery.

### Diagnostic Resources

The following worksheets may help to identify whether students hold this particular misconception.

• Voltage in parallel Circuits (11-14)

As the second bulb is added, there is a current in both loops. The power in both bulbs is equal and set by the current in and the voltage across each bulb. Both bulbs are as bright as they would be in a simple loop (one battery, one bulb).

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• Predicting and measuring voltages in a parallel circuit (11-14)

The approach taken is to encourage the pupils to make predictions of voltage values before they make each measurement.

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• Loops in parallel circuits (14-16)

Circuits with only one parallel connection are best analysed by thinking of the two separate loops that make up the circuit. The loops have nothing in common, other than the battery, so you can treat them as two separate loops.

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• Lamps in parallel (11-16)

The current flowing in a circuit increases as more lamps are added in parallel with each other - but the pd across each lamp remains the same.

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• Episode 114: Components in series and parallel (16-19)

Have students remember the resistance equation (R = V / I). Show them a series circuit with increasing lamps, explaining reduced current at constant voltage. Repeat with lamps in parallel, demonstrating increased current and decreased effective resistance, resulting in ratios of 1:1/2:1/3 as lamps are added.

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## References

• Millar, R. and Beh, K. L. () Students’ understanding of voltage in simple parallel electric circuits. International Journal of Science Education, 15 (4), 351-361.