Capacitor

What is a capacitor?

• Two metal plates separated by an insulator/air gap

• Charge does not flow across the gap between plates

What is capacitance? (C)

• The charge stored per unit potential difference across it

• C = Q/V

Farad (F)

• Unit of capacitance, 1 coulomb by volt

What circuit is used to demonstrate the discharge of capacitors?

RC circuit

What effects the time taken of discharging a capacitor?

• It’s capacitance

• The resistance of the resistor

Time constant (τ)

• τ = CR

• time constant = capacitance x resistance

• The time taken for the p.d to fall to 37% (e^-1) of it’s initial value

Exponential decay

• y = e^-x

• X = X₀e^-t/CR, where X is a quantity

Uses for capacitors:

• Supply a small amount of energy for a long time and quickly discharge it

• Circuits (converting alternating current to direct current by smoothing it’s path)

• Used to create a camera flash

• Used to release energy during fusion

• Used as a power supply

P.d, charge and total capacitance of capacitors in series

• V = V1 + V2 + …

• Q is constant

• 1/C = 1/C1 + 1/C2 + …

P.d, charge and total capacitance of capacitors in parallel

• V is constant

• Q = Q1 + Q2 + …

• C = C1 + C2 + …

Derivation of capacitance in a series circuit

Derivation of capacitance in a parallel circuit