CIE A Level Physics
CIE A Level Physics D.C. Circuits Exam Questions
Free CIE A Level Physics exam questions for D.C. Circuits, with a mark scheme for every pack. Covers circuit symbols, e.m.f. and internal resistance, Kirchhoff's laws, series and parallel resistors and potential divider circuits.
D.C. Circuits exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Physics D.C. Circuits
D.C. circuits is Topic 10 of the Cambridge International AS & A Level Physics (9702) syllabus. It builds on Electricity by analysing whole circuits. You apply Kirchhoff's first and second laws, combine resistors in series and parallel, include the internal resistance of a source, and design potential dividers with thermistors and light-dependent resistors as sensors. These ideas return in Capacitance at A Level. Examiners like questions on terminal potential difference and lost volts, multi-loop Kirchhoff problems, and explaining how the output of a potential divider changes with temperature or light level.
What you need to know
- Practical circuits Know the standard circuit symbols. Terminal p.d. V = E - Ir, where E is e.m.f. and r is internal resistance; the 'lost volts' is Ir.
- Kirchhoff's first law The sum of currents into a junction equals the sum of currents out. It is a consequence of conservation of charge.
- Kirchhoff's second law Around any closed loop, the sum of e.m.f.s equals the sum of p.d.s. It is a consequence of conservation of energy.
- Combining resistors Series: R = R1 + R2 + ...; parallel: 1/R = 1/R1 + 1/R2 + ...
- Potential dividers Vout = Vin × R2/(R1 + R2). Replacing a resistor with a thermistor or LDR makes the output depend on temperature or light intensity.
Common mistakes in D.C. Circuits
- Ignoring internal resistance when the question gives it. Include r in the total resistance: I = E/(R + r).
- Linking Kirchhoff's first law to conservation of energy. The first law is conservation of charge; the second law is conservation of energy.
- Adding parallel resistances directly. Use the reciprocal formula, and remember the total is less than the smallest resistor.
- Getting the direction of change wrong in sensor circuits. As a thermistor warms, its resistance falls, so the p.d. across it falls.
- Assuming a voltmeter has a low resistance. An ideal voltmeter has infinite resistance and an ideal ammeter has zero resistance.
Try three D.C. Circuits exam-style questions
1. A cell of e.m.f. 12 V and internal resistance 0.50 Ω is connected to a 5.5 Ω resistor. Calculate the current and the terminal potential difference. (3 marks)
Show the mark scheme answer
I = E/(R + r) = 12/6.0 = 2.0 A; V = IR = 2.0 × 5.5; V = 11 V.
2. A 6.0 Ω resistor and a 3.0 Ω resistor are connected in parallel, and this combination is in series with a 4.0 Ω resistor. Calculate the total resistance. (2 marks)
Show the mark scheme answer
Parallel: 1/R = 1/6.0 + 1/3.0 so R = 2.0 Ω; total = 2.0 + 4.0 = 6.0 Ω.
3. A potential divider consists of a fixed resistor and an NTC thermistor, with the output taken across the fixed resistor. Explain what happens to the output p.d. when the temperature rises. (3 marks)
Show the mark scheme answer
Thermistor resistance decreases as temperature rises; total resistance decreases so the current increases (or thermistor takes a smaller share of the supply p.d.); p.d. across the fixed resistor increases.
D.C. Circuits FAQs
What does Cambridge AS & A Level Physics D.C. circuits cover?
It covers circuit symbols, e.m.f. and internal resistance, Kirchhoff's two laws, series and parallel combinations of resistors, and potential divider circuits including sensors.
Which papers is D.C. circuits examined in?
It is an AS Level topic assessed in Paper 1 and Paper 2, and finding e.m.f. and internal resistance is a common practical task.
How do I find the internal resistance of a cell from a graph?
Plot terminal p.d. V against current I; the gradient is -r and the y-intercept is the e.m.f. E, since V = E - Ir.
