A-Level Physics AQA 7408

Capacitance

Definition of capacitance: $C = \dfrac{Q}{V}$

Parallel plate capacitor

Dielectric action in a capacitor $C = \dfrac{Aε_0ε_r}{d}$

Relative permittivity and dielectric constant.

Students should be able to describe the action of a simple polar molecule that rotates in the presence of an electric field.

Energy stored by a capacitor

Interpretation of the area under a graph of charge against pd.

$E = \dfrac{1}{2}QV = \dfrac{1}{2}CV^2 = \dfrac{1}{2}\dfrac{Q^2}{C}$

Capacitor charge and discharge

Graphical representation of charging and discharging of capacitors through resistors. Corresponding graphs for Q, V and I against time for charging and discharging.

Interpretation of gradients and areas under graphs where appropriate.

Time constant RC.

Calculation of time constants including their determination from graphical data.

Time to halve, $T_½ = 0.69RC$

Quantitative treatment of capacitor discharge, $Q = Q_0e^{-\dfrac{t}{RC}}$

Use of the corresponding equations for V and I.

Quantitative treatment of capacitor charge, $Q = Q_0\Big(1- e^{-\dfrac{t}{RC}}\Big)$

Required practical 9:

Investigation of the charge and discharge of capacitors. Analysis techniques should include log-linear plotting leading to a determination of the time constant, RC