Capacitance — A-Level Physics Revision

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What is Capacitance?

Capacitance is the ability of a component, called a capacitor, to store electrical energy in an electric field. It is defined as the charge stored per unit potential difference. This topic covers the function of capacitors, the factors affecting their capacitance, and how to calculate the total capacitance for capacitors connected in series and parallel. You will also study the exponential nature of capacitor charging and discharging through a resistor, introducing the concept of the time constant (RC).

Board notes: Capacitance is a major A-Level topic for all exam boards (AQA, Edexcel, OCR). All specifications cover the definition of capacitance, energy storage, and the analysis of charging and discharging curves using the time constant. The rules for combining capacitors in series and parallel are also fundamental across all boards.

Worked example

Common mistakes

• 1Applying the resistor rules for series and parallel to capacitors. The rules are reversed: for capacitors in series, you add the reciprocals (1/Ct = 1/C1 + 1/C2), and for capacitors in parallel, you add them directly (Ct = C1 + C2).
• 2Confusing the equations for energy stored in a capacitor. There are three equivalent forms: E = ½QV, E = ½CV², and E = ½Q²/C. Students must choose the most appropriate one based on the information given in the problem.
• 3Misunderstanding the time constant (τ = RC). The time constant is the time it takes for the charge, current, or voltage to fall to approximately 37% of its initial value during discharge, or to rise to 63% of its final value during charging. It is not the time to fully charge or discharge.

Capacitance exam questions

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

Frequently asked questions

• What does a capacitor do in a circuit?

  A capacitor stores electrical charge and energy. It can be used in timing circuits, for smoothing out variations in direct current, in filter circuits, and to store energy for a rapid release, such as in a camera flash.

• Why can't you add the capacitances directly for capacitors in series?

  In a series circuit, each capacitor stores the same amount of charge, but they share the total potential difference. This sharing of voltage leads to a total capacitance that is less than any of the individual capacitances.