# AQA A-Level Physics/Capacitance

Capacitance- The capacitance of a capacitor is measured in farads (F). One farad is one coulomb of charge per volt.

C = capacitance measured in farads

Q = charge measured in coulombs

V = potential difference measured in volts

A capacitor is a charge storing device. Charges (Q) are measured in coulombs. A single electron carries a charge of 1.6x10^-19 C. A capacitor is two metal plates separated by an insulator (or dielectric).

1 farad is a very large capacitance. Typically capacitors have a capacitance around the micro-farad, nano-farad and pico-farad sizes.

Energy Stored by a Capacitor: The charge stored by a capacitor is directly proportional to the potential difference across the plates of the capacitor.

The area underneath the line on the graph is equal to the energy stored by the capacitor measured in Joules.

E=energy stored by the capacitor measured in Joules

Q=charge within the capacitor measured in coulombs

V=potential difference measured in volts

C=the capacitance of the capacitor in farads

Capacitor discharge: As the capacitor discharges the electrons from the negative plate flow to the positive plate of the capacitor and a current flows in the circuit. The graph below shows the exponential decay curve when the charge is plotted against time for a discharging capacitor.

Q=the charge of the capacitor at time t measured in coulombs

Q0=the charge of the capacitor when t=0 seconds measured in coulombs

t=the time the capacitor has been discharging measured in seconds

C= the capacitance of the capacitor measured in coulombs

R= the resistance in the discharging circuit measured in ohms

RC time constant: The RC time constant gives us an idea of how quickly the capacitor will discharge. If the RC time constant is large, the capacitor will take a greater time to discharge than if the RC time constant is small. RC time constant is worked out by multiplying the resistance of the circuit and the capacitance of the capacitor.

The time constant is equal to the time it takes for the charge on a capacitor to reach 1/e (37%) of its initial value. We can find the RC time constant from the graph.

Because RC =37% of the charge, to find an estimate for the time taken for the capacitor to be empty, we multiply the RC time constant by 5.