Practical circuits
Cambridge A-Level Physics (9702) · Unit 10: D.C. circuits · 8 flashcards
Practical circuits is topic 10.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 10 — D.C. circuits , alongside Potential dividers. In one line: Electromotive force (e.m.f.) is the total energy transferred per unit charge in driving charge around a complete circuit. It is measured in volts (V) and represents the potential difference provided by a source.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 3 definitions, 2 key concepts and 3 calculations — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Some cards in this deck are on other syllabus topics: 10.2 Kirchhoff’s laws. Each card is labelled with its topic.
Electromotive force (e.m.f.)
Electromotive force (e.m.f.) is the total energy transferred per unit charge in driving charge around a complete circuit. It is measured in volts (V) and represents the potential difference provided by a source.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Recall the circuit symbols listed in section 6 of the syllabus, and use them correctly.
- Draw circuit diagrams, and interpret ones you are given, using the symbols listed in section 6 of the syllabus.
- Define the electromotive force (e.m.f.) of a source as the energy it transfers per unit charge in pushing charge all the way round a complete circuit, and use this definition.
- Use energy ideas to explain how e.m.f. differs from potential difference (p.d.).
- Understand how the internal resistance of an e.m.f. source affects the p.d. across its terminals.
Also in this deck: 10.2 Kirchhoff’s laws
- Recall Kirchhoff's first law; understand that conservation of charge is the reason it holds.
- Recall Kirchhoff's second law; understand that conservation of energy is the reason it holds.
- Derive, by applying Kirchhoff's laws, an expression for the total resistance when two or more resistors are connected in series.
- Use the formula giving the total resistance when two or more resistors are connected in series.
- Derive, by applying Kirchhoff's laws, an expression for the total resistance when two or more resistors are connected in parallel.
- Use the formula giving the total resistance when two or more resistors are connected in parallel.
- Solve simple circuit problems by applying Kirchhoff's laws.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in these topics
- › Explain that e.m.f. is the total energy transferred per unit charge, while terminal p.d. is the energy available to the external circuit.
- › Apply the equation V = E - Ir; as current increases, the 'lost volts' (Ir) increase, causing terminal potential difference (V) to decrease.
- › Check the polarity of every cell; subtract the e.m.f. of cells connected with reversed polarity.
Topic 10.1 · Practical circuits
Define electromotive force (e.m.f.).
Electromotive force (e.m.f.) is the total energy transferred per unit charge in driving charge around a complete circuit. It is measured in volts (V) and represents the potential difference provided by a source.
Topic 10.1 · Practical circuits
Explain the difference between e.m.f. and potential difference (p.d.) in terms of energy considerations.
E.m.f. is the energy supplied *by* a source per unit charge, whereas potential difference is the energy dissipated *by* a component per unit charge. E.m.f. is a source of energy, while p.d. represents energy used.
Topic 10.1 · Practical circuits
What effect does the internal resistance of a source have on the terminal potential difference?
Internal resistance causes the terminal potential difference to be less than the e.m.f. When current flows, some potential is 'lost' across the internal resistance (V = Ir), reducing the voltage available to the external circuit.
Topic 10.2 · Kirchhoff’s laws
State Kirchhoff’s first law and explain its significance.
Kirchhoff's first law (junction rule) states that the total current entering a junction equals the total current leaving the junction. This is a direct consequence of the conservation of charge.
Topic 10.2 · Kirchhoff’s laws
State Kirchhoff’s second law and explain its significance.
Kirchhoff's second law (loop rule) states that the sum of the e.m.f.s in a closed loop is equal to the sum of the potential drops. This law is a direct consequence of the conservation of energy.
Topic 10.2 · Kirchhoff’s laws
Derive the formula for the combined resistance of two resistors in series using Kirchhoff's laws.
In series, the current is the same through both resistors. Using Kirchhoff's second law: V = V₁ + V₂ = IR₁ + IR₂ = I(R₁ + R₂). Therefore, the equivalent resistance is R = R₁ + R₂.
Topic 10.2 · Kirchhoff’s laws
Derive the formula for the combined resistance of two resistors in parallel using Kirchhoff's laws.
In parallel, the voltage is the same across both resistors. Using Kirchhoff's first law: I = I₁ + I₂ = V/R₁ + V/R₂ = V(1/R₁ + 1/R₂). Therefore, 1/R = 1/R₁ + 1/R₂.
Topic 10.1 · Practical circuits
A battery with an e.m.f. of 6.0 V and an internal resistance of 0.5 Ω is connected to a 2.5 Ω resistor. Calculate the terminal potential difference.
First, find the current: I = e.m.f. / (R + r) = 6.0 / (2.5 + 0.5) = 2.0 A. Then, calculate the terminal potential difference: V = IR = 2.0 * 2.5 = 5.0 V.
Review the material
Read full revision notes on Practical circuits — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 10 — D.C. circuits
Practical circuits sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Practical circuits deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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