Capacitance

Capacitance is a fundamental concept in A-Level Physics that describes the ability of a system to store electric charge. A capacitor consists of two conducting plates separated by an insulator (dielectric). When a voltage is applied, charge accumulates on the plates, creating an electric field and storing energy. The capacitance (C) is defined as the ratio of charge stored (Q) to the potential difference (V) across the plates: C = Q/V, measured in farads (F). This topic is crucial for understanding how electronic circuits filter signals, smooth power supplies, and store energy temporarily.

In the WJEC A-Level specification, capacitance is studied in the context of DC circuits and exponential decay. You will learn how capacitors charge and discharge through resistors, deriving and using the time constant τ = RC. The equations Q = Q₀e^(-t/RC) and V = V₀e^(-t/RC) describe the exponential decay of charge and voltage. Practical skills include plotting graphs of ln(V) against time to determine the time constant. Capacitors also introduce the concept of energy storage: E = ½CV², which links to electric fields and work done.

Mastering capacitance builds a foundation for more advanced topics like alternating current (AC) circuits, where capacitors exhibit reactance, and for understanding how electronic devices such as timers, filters, and flash cameras operate. In the exam, you will be expected to recall definitions, derive equations, interpret graphs, and solve problems involving charge, voltage, and energy. A strong grasp of exponential functions and logarithms is essential.