About D.C. circuits in A Level Physics
D.C. circuits is one of the 13 main topics in our A Level Physics (Cambridge (CAIE) 9702) question bank, and about 8% of its real questions come from it. Tayyari Karo has 56 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2, Paper 4; 45 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 26 hard, 22 medium, 8 easy.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
Try 6 real questions
These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2025 · Paper 1 · D.C. circuits
Two resistors of resistances R₁ and R₂ are connected in parallel between X and Y.
What is the combined resistance between X and Y?
- R₁ + R₂
- R₁R₂/(R₁ + R₂)
- (R₁ + R₂)/R₁R₂
- R₁/R₂
Show answer and explanation
Answer: B. R₁R₂/(R₁ + R₂)
For resistors in parallel, 1/R = 1/R₁ + 1/R₂, so R = R₁R₂/(R₁+R₂).
October/November 2024 · Paper 1 · D.C. circuits
Kirchhoff's second law is a consequence of a basic principle.
What is this principle?
- The charge flowing in an electric circuit is conserved.
- The energy in an electric circuit is conserved.
- The sum of the electric currents entering a point in an electric circuit is equal to the sum of the electric currents leaving that point.
- The sum of the potential differences in an electric circuit is equal to the sum of the products of the current and resistance.
Show answer and explanation
Answer: B. The energy in an electric circuit is conserved.
Kirchhoff's second law (sum of e.m.f.s = sum of p.d.s around a loop) follows from conservation of energy.
October/November 2024 · Paper 1 · D.C. circuits
A cell with internal resistance is connected to a light-dependent resistor (LDR), a fixed resistor and a voltmeter (measuring the p.d. across the fixed resistor). The voltmeter reading increases.
Which quantity decreases as the voltmeter reading increases?
- the charge moving through the cell per unit time
- the energy transferred to the fixed resistor per unit charge
- the intensity of the light incident on the LDR
- the terminal potential difference across the cell
Show answer and explanation
Answer: D. the terminal potential difference across the cell
If the p.d. across the fixed resistor increases, the current has increased (LDR resistance decreased, e.g. more light), so more p.d. is dropped internally, meaning the terminal p.d. across the cell decreases.
October/November 2023 · Paper 1 · D.C. circuits
Kirchhoff's first and second laws are a consequence of the conservation of which quantities?
- charge and energy
- charge and resistance
- mass and energy
- mass and resistance
Show answer and explanation
Answer: A. charge and energy
Kirchhoff's first law follows from conservation of charge (current in = current out at a junction) and the second law from conservation of energy (sum of e.m.f.s = sum of p.d.s around a loop).
May/June 2022 · Paper 1 · D.C. circuits
A cell of internal resistance r and e.m.f. E is connected in series with an ammeter and a variable resistor R. When R is 10 Ω the ammeter reads 0.3 A. When R is 5 Ω the ammeter reads 0.4 A. What is the value of E?
- 0.5 V
- 2 V
- 3 V
- 6 V
Show answer and explanation
Answer: D. 6 V
E = I(R+r). From the two readings: E = 0.3(10+r) = 0.4(5+r). Solving: 3+0.3r = 2+0.4r → 1 = 0.1r → r = 10 Ω. Then E = 0.3(10+10) = 6 V.
May/June 2021 · Paper 1 · D.C. circuits
Three resistors of 120 Ω, 180 Ω and 150 Ω are connected in series with a 6.0 V battery of negligible internal resistance. What is the potential difference across the 180 Ω resistor?
- 1.6 V
- 2.4 V
- 3.6 V
- 4.0 V
Show answer and explanation
Answer: B. 2.4 V
Total resistance = 120+180+150 = 450 Ω. I = 6.0/450 = 0.0133 A. V₁₈₀ = 0.0133 × 180 ≈ 2.4 V.