Capacitance definitions

Capacitor

A pair of conducting plates separated by an insulator. When a PD is placed across the plates, the acquire equal and opposite charges.

Capacitance

Charge on either plate/pd between plates

Unit: F

Dielectric

Insulator between the plates of a capacitor, also serving to make the capacitance larger than if there were just empty space.

Equation for Charge

Q = CV

Capacitance of parallel plate

C = eºA/d where e = 8.85×10^-12 Fm ^-1

Capacitors in parrallel

Add - C= C1+C2+C3…

Capacitance in series

1/CT = 1/C1 + 1/C2 + 1/C3 ….

(remember to fine the reciprocal for C).

To calculate the charge and voltage across a variety of parrallel/ series combinations.

1. Find total capacitance.

2. Find charge.

3. Find pd for one capacitor. (voltage across parallel is the same).

4. Find charge for individual capacitors.

Energy stored in a capacitor

∆W= ∆QV

Energy Equations

• U =1/2 QV

• U = ½ CV²

• U = ½ Q²/C

time constant

RC time taken for quantities to fall to 37%