A-Level · A-Level Physics
A-Level Physics circuits
What is circuits?
A-Level Physics circuits combines Ohm's Law (V = IR), Kirchhoff's laws (current in = current out at a junction; sum of EMFs = sum of pds around a loop) and the EMF equation V = ε − Ir for cells with internal resistance. Most exam questions require applying multiple of these together — circuits with two cells, parallel branches, and the terminal voltage drop under load. Read the circuit diagram carefully before writing equations.
Worked example
A cell of EMF 6.0 V and internal resistance 0.50 Ω is connected to two resistors in parallel: 3.0 Ω and 6.0 Ω. Find the current drawn from the cell.
- Find the parallel resistance: 1/R = 1/3 + 1/6 = 2/6 + 1/6 = 3/6, so R_parallel = 2.0 Ω.
- Find the total resistance: R_total = r + R_parallel = 0.5 + 2.0 = 2.5 Ω.
- Apply V = IR with V = ε: I = ε / R_total = 6.0 / 2.5 = 2.4 A.
- Sanity check: terminal V = ε − Ir = 6.0 − (2.4 × 0.5) = 4.8 V, which equals I × R_parallel = 2.4 × 2.0 = 4.8 V. ✓
Series vs parallel
Series components share the same current; voltages add (ΣV = ε). Parallel components share the same voltage; currents add at the junction (ΣI). Resistors in series: R_total = R₁ + R₂ + ... Resistors in parallel: 1/R_total = 1/R₁ + 1/R₂ + ...
Kirchhoff's laws in practice
Most circuits with two or more cells, or branches with different resistor combinations, are best solved by setting up Kirchhoff equations:
- Current Law (KCL): label every current with a direction. At each junction, sum-in = sum-out.
- Voltage Law (KVL): pick a loop. Walk it. EMFs add when you go from − to + through a cell; potential drops add when you cross a resistor in the direction of current. Set the total to zero.
- Solve the resulting system of simultaneous equations for the unknown currents.
EMF, terminal pd and internal resistance
A real cell has EMF (ε) and internal resistance (r). When current I flows, the terminal voltage V is less than ε by Ir: V = ε − Ir. Plotting V (y) against I (x) for a cell gives a straight line with y-intercept ε and gradient −r. Required-practical questions examine this routinely.
Where it appears in the exam
AQA 7408, Edexcel 9PH0, OCR A H556 and OCR B H557 examine circuits on the Electricity paper plus synoptic questions on the final paper. Common variations: potential divider problems, LDR/thermistor sensing circuits, and combined-component networks.
Common mistakes
- Forgetting the cell's internal resistance — the terminal voltage V is NOT the EMF when current flows.
- Setting up Kirchhoff's loop equation with the wrong sign convention.
- Treating parallel resistors as adding (they don't — use the reciprocal formula).
- Confusing current and voltage when applying Ohm's Law to a branched circuit.
- Missing units — current in A, voltage in V, resistance in Ω.
Frequently asked
- When can I use Ohm's Law and when can't I?
- Ohm's Law (V = IR with constant R) applies to ohmic conductors — most metals at constant temperature. It does NOT apply to filament bulbs (R increases with temperature), diodes (current only flows above threshold voltage) or thermistors. Always check the I-V characteristic of the component.
- How do I find EMF and internal resistance from a graph?
- Plot terminal voltage V (y-axis) against current I (x-axis). The y-intercept is the EMF (V when I = 0). The gradient is −r (the magnitude of the internal resistance, with a negative sign because V decreases as I increases).
- Are potential divider questions covered on every board?
- Yes — all UK A-Level Physics boards (AQA, Edexcel, OCR A, OCR B) examine potential dividers explicitly. Often combined with LDRs (light-dependent resistors) or thermistors as a sensing circuit.
A-Level Physics glossary terms
- Ohm's LawOhm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided temperature is constant: V = IR. Where V is potential difference in volts, I is current in amps, and R is resistance in ohms. The law holds for ohmic conductors (most metallic conductors at constant temperature); non-ohmic components like filament bulbs or diodes do not obey it. A-Level Physics extends to internal resistance: V = ε − Ir.
- Kirchhoff's lawsKirchhoff's laws govern electrical networks. The Current Law (KCL): the sum of currents entering a junction equals the sum of currents leaving it (conservation of charge). The Voltage Law (KVL): the sum of EMFs around a closed loop equals the sum of potential differences (conservation of energy). A-Level Physics applies them to circuits with multiple loops, branched paths and cells in parallel or series.
- Internal resistanceInternal resistance (r) is the resistance inside a cell or battery that opposes current flow. The terminal potential difference is lower than the cell's electromotive force (EMF, ε) when current flows: V = ε − Ir. As current draw increases, terminal V drops. A-Level Physics examines this via plotting terminal V against I — the y-intercept is the EMF, the negative gradient is the internal resistance. The cell is also a closed loop in Kirchhoff analysis.
- Electromagnetic inductionElectromagnetic induction is the generation of an electromotive force (EMF) in a conductor when the magnetic flux through it changes. Faraday's law: ε = −dΦ/dt, where Φ is magnetic flux linkage. Lenz's law (the minus sign) states that the induced EMF opposes the change causing it (conservation of energy). Applications: dynamos, transformers, induction cookers, eddy-current braking. AQA, Edexcel and OCR all examine flux-linkage graphs and the derivation of induced EMF.
Related on StudyVector
Last updated: . StudyVector is independent and is not affiliated with AQA, Edexcel, OCR or JCQ. For canonical specification wording, see the awarding body's published documents.