A2 Unit 19 Capacitance

9702

Define capacitor

Device designed to store charge

Define capacitance

Charge stored per unit potential difference

Give the equation for the capacitance of a capacitor

C = Q/V

What does a simple capacitor consist of?

Two parallel metal plates opposite each other with a gap between them

Give the equations used to calculate the energy stored on a capacitor

E = 1/2QV

E = 1/2CV²

How can the energy stored on a capacitor be found from a charge potential difference graph?

Area under the graph

Give the equations for how current, potential difference and charge vary with time for a discharging capacitor?

I = I₀e^-t/τ

V = V₀e^-t/τ

Q = Q₀e^-t/τ

What is the area under a current-time graph equal to?

Charge that has flowed off the capacitor

What is the gradient of the tangent of a charge-time graph at any point equal to?

Current at that time

Define the time constant of a capacitor discharge circuit

The time taken for the charge, current and potential difference to fall to approximately 1/e (~37%) of its original value

Give the equation to calculate the time constant

τ = RC

How can the time constant be found from graphs of lnI, lnV and lnQ against time?

τ = -1/gradient