Physics 4.1 - Capacitance

Capacitor

A pair of conducting plates separated by an insulator. If a p.d. is placed across the plates, they acquire equal and opposite charges

Capacitance

Charge on either plate / p.d. between plates

Symbol used to denote capacitance

C

Unit of capacitance

F or CV^-1

Dielectric

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

Effect of using a dielectric on a capacitor

Increase C without changing the dimensions

Explain how a capacitor stores energy

Transferring charge from one player to the other, so that the plates carry equal but opposite charges so net charge is zero

Equation for capacitance if the insulator separating the plates is a vacuum or air

C = ε_{0 A/d}

Electric field within a parallel plate capacitor

Uniform inbetween plates

Equation used to calculate electric field within a parallel plate capacitor

E=V/d

Unit for electric field

Vm-1

Equation used for energy stored in a capacitor

U=1/2QV

How can energy stored in a capacitor charged from 0V to V from a Q-V graph

Area under the graph

Capacitance in series

• p.d. is shared

• Q is equal

• 1/C =1/C1 + 1/C2 + 1/C3

Capacitance in parallel

• p.d. is equal

• Q is shared

• C = C1 + C2 + C3 + …

Time constant

Time for Q to decrease to 1/e=0.37=37% of its original value

Unit for time constant

s

Symbol used to denote the time constant

τ = RC

explain what happens when a capacitor is charged

Current flows so electrons flow from one plate to another

Circuit when charging a capacitor

Explain what happens when a capacitor is discharged

Current flows in opposite direction so electrons move back

Circuit when discharging a capacitor

Graph for charging a capacitor

Graph for discharging a capacitor

Equation used when charging a capacitor

Equation used when discharging a capacitor