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.
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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- recall and use the circuit symbols shown in section 6 of this syllabus
- draw and interpret circuit diagrams containing the circuit symbols shown in section 6 of this syllabus
- define and use the electromotive force (e.m.f.) of a source as energy transferred per unit charge in driving charge around a complete circuit
- distinguish between e.m.f. and potential difference (p.d.) in terms of energy considerations
- understand the effects of the internal resistance of a source of e.m.f. on the terminal potential difference
- recall Kirchhoff’s first law and understand that it is a consequence of conservation of charge
- recall Kirchhoff’s second law and understand that it is a consequence of conservation of energy
- derive, using Kirchhoff’s laws, a formula for the combined resistance of two or more resistors in series
- use the formula for the combined resistance of two or more resistors in series
- derive, using Kirchhoff’s laws, a formula for the combined resistance of two or more resistors in parallel
- use the formula for the combined resistance of two or more resistors in parallel
- use Kirchhoff’s laws to solve simple circuit problems
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 Practical circuits
- › Check the polarity of every cell; subtract the e.m.f. of cells connected with reversed polarity.
- › Calculate resistance strictly using R = V/I for non-ohmic components; do not use the gradient of the I-V graph unless the component is ohmic.
- › Use the volume conservation relationship (V = AL) to determine that if length triples, area must become one-third, resulting in a nine-fold resistance increase.
- › Remember that potential difference is identical across all components connected in parallel; use P = V²/R to compare power dissipated.
- › Remember that at the balance point, no current flows through the cell being measured, so there are no 'lost volts' across its internal resistance.
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.
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.
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.
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.
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.
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.
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.
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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