Electrical Circuits — A-Level Physics Revision

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What is Electrical Circuits?

This topic builds on the fundamentals of current and resistance to analyse complete circuits. It introduces Kirchhoff's laws, which are essential for solving complex circuits: the first law relates to the conservation of charge at a junction, and the second law relates to the conservation of energy in a closed loop. You will apply these laws to series and parallel circuits, and also study potential dividers and the effects of internal resistance in power sources.

Board notes: Kirchhoff's laws and their application to series, parallel, and more complex circuits are fundamental to all A-Level Physics specifications (AQA, Edexcel, OCR). The concepts of potential dividers and internal resistance are also universally covered. Edexcel and AQA often feature more intricate circuit problems requiring careful application of these principles.

Worked example

Common mistakes

• 1Confusing the rules for resistors in series and parallel. For resistors in series, the total resistance is the sum of individual resistances (Rt = R1 + R2 + ...). For resistors in parallel, the reciprocal of the total resistance is the sum of the reciprocals of individual resistances (1/Rt = 1/R1 + 1/R2 + ...).
• 2Incorrectly applying Kirchhoff's second law. A common mistake is getting the signs wrong. When going around a loop, the sum of the EMFs must equal the sum of the potential drops (V=IR) across the components. A consistent direction must be chosen.
• 3Forgetting to account for internal resistance. The terminal potential difference of a power source is the EMF minus the 'lost volts' due to its own internal resistance (V = ε - Ir). This is often overlooked in calculations.

Electrical Circuits exam questions

Exam-style questions for Electrical Circuits with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is a potential divider circuit?

  A potential divider is a simple circuit that uses two resistors in series to produce an output voltage that is a fraction of the source voltage. The output voltage depends on the ratio of the resistances.

• What are 'lost volts'?

  Lost volts is the potential difference across the internal resistor of a power source when current is flowing. It represents the energy per unit charge that is converted into heat within the source itself.