IGCSE Physics: Electric Circuits Practice Questions
In a series circuit the current is the same everywhere and the potential differences add up to the supply voltage. In a parallel circuit the potential difference across each branch is the same and the branch currents add up to the total current.
Sub-topic 4.3 of Cambridge IGCSE Physics 0625 is where the electrical equations get used. Almost every mistake comes from applying a series rule to a parallel circuit or the reverse, so the questions below force you to identify the arrangement before calculating.
What you need to know for Electric Circuits
- Series rulesThe current is the same at every point. The potential differences across the components add up to the supply potential difference. Total resistance is the sum of the individual resistances.
- Parallel rulesThe potential difference across each branch equals the supply potential difference. The currents in the branches add up to the current from the supply. The total resistance is less than the smallest individual resistance.
- Combining resistors in parallel1 divided by R total equals 1 divided by R1 plus 1 divided by R2. For two equal resistors in parallel the total is half of one of them.
- MetersAn ammeter is connected in series and should have very low resistance. A voltmeter is connected in parallel across a component and should have very high resistance.
- Potential dividerTwo resistors in series across a supply. The potential difference is shared in the same ratio as the resistances, so the larger resistor takes the larger share.
- Input componentsA thermistor's resistance falls as temperature rises. A light dependent resistor's resistance falls as light intensity rises. Both are used in potential dividers to make sensing circuits.
IGCSE Physics Electric Circuits questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Two resistors of 6.0 ohms and 3.0 ohms are connected in parallel across a 12 V supply. Calculate the total resistance and the current drawn from the supply.
Show the worked answer
- Use 1 divided by R total = 1 divided by R1 plus 1 divided by R2.
- 1 divided by R total = 1 divided by 6.0 plus 1 divided by 3.0 = 0.1667 + 0.3333 = 0.5.
- R total = 1 divided by 0.5 = 2.0 ohms. Note this is less than the smaller of the two resistances, as it must be.
- Current from the supply: I = V divided by R = 12 divided by 2.0 = 6.0 A.
Three identical lamps are connected in series to a 12 V battery. State the potential difference across each lamp and explain what happens to the others if one lamp breaks.
Show the worked answer
- In a series circuit the potential differences across the components add up to the supply potential difference.
- The lamps are identical, so the 12 V is shared equally: 12 divided by 3 = 4.0 V across each lamp.
- There is only one path for the current in a series circuit.
- If one lamp breaks the circuit is no longer complete, so no current flows anywhere and all three lamps go out.
A 4.0 ohm resistor and an 8.0 ohm resistor are connected in series across a 6.0 V supply. Calculate the current in the circuit and the potential difference across the 8.0 ohm resistor.
Show the worked answer
- In series, total resistance is the sum: R total = 4.0 + 8.0 = 12 ohms.
- Current: I = V divided by R = 6.0 divided by 12 = 0.50 A.
- The current is the same through both resistors in series.
- Potential difference across the 8.0 ohm resistor: V = IR = 0.50 x 8.0 = 4.0 V. As a check, the 4.0 ohm resistor takes 2.0 V, and 4.0 + 2.0 = 6.0 V.
A thermistor is connected in series with a fixed resistor across a supply, with the output voltage taken across the fixed resistor. Explain what happens to the output voltage as the temperature rises.
Show the worked answer
- As temperature rises, the resistance of the thermistor decreases.
- The two components form a potential divider, so the supply voltage is shared between them in the same ratio as their resistances.
- With the thermistor's resistance now smaller, it takes a smaller share of the supply voltage.
- The fixed resistor therefore takes a larger share, so the output voltage across it increases. This can be used to switch on a cooling fan when the temperature is high enough.
Common mistakes in this topic
- Applying series rules to a parallel circuit or the reverse.
- Forgetting the final reciprocal when combining parallel resistors.
- Using the supply voltage instead of the component's own potential difference in V = IR.
- Connecting an ammeter in parallel or a voltmeter in series.
- Assuming a total parallel resistance can be greater than one of the branches. It is always less than the smallest.
Exam tips
- Label the circuit as series or parallel before writing any equation. That one step prevents most errors.
- In series the current is the constant. In parallel the potential difference is the constant. Fix those two facts and everything else follows.
- Check parallel answers for plausibility: the total resistance must be smaller than the smallest branch.
- Use the potential divider ratio to check answers quickly. The larger resistance always takes the larger share.
- Remember that both thermistors and light dependent resistors decrease in resistance as their input increases.
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Electric Circuits FAQs
What are the rules for series and parallel circuits?
In series the current is the same everywhere, the potential differences add up to the supply voltage, and the resistances add. In parallel the potential difference across each branch equals the supply, the branch currents add to give the total, and the combined resistance is less than the smallest branch resistance.
How do I calculate total resistance in parallel?
Add the reciprocals of the individual resistances, then take the reciprocal of the result. For a 6 ohm and a 3 ohm resistor, one sixth plus one third is a half, so the total is 2 ohms. Always check that the answer is smaller than the smallest individual resistance.
Why do all lamps go out when one fails in a series circuit?
A series circuit has only one path for the current. A broken lamp creates a gap in that single path, so the circuit is no longer complete and no current flows anywhere. In a parallel circuit each lamp has its own path, so the others continue to work.
How does a potential divider with a thermistor work?
The thermistor and a fixed resistor share the supply voltage in the same ratio as their resistances. As temperature rises the thermistor's resistance falls, so it takes a smaller share and the voltage across the fixed resistor rises. That rising output voltage can be used to trigger a switching circuit.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.