4. Capacitors

Question 1

Define a capacitor.

Answer 1

A capacitor is an energy storage device, using electric fields.

Usually made up of two parrallel plates with a gap or material in between

Question 2

How does Charging of a capacitor work?

Answer 2

• Electrons move round the circuit, creating a positive and negative end on the two parallel plates with a gap. This generates an electric field. Once ‘steady state’ is reached (where Vs = Vc) the capacitor is fully charged and the current can’t flow through.
• We can find the amount of charged moved round for a given voltage by the equation Q = CV

Question 3

What is capacitance, C?

Answer 3

• It is the measure of the capacitors ability to store energy. Measured in Farads (F)
• C = Q/V

Question 4

How can we find the capacitance of a parallel plate capacitor?

Answer 4

• Either C = Q/V
• Or: C = εA/d

Where Q = charge, V = voltage, ε = permitivitiy, A = area of plates, d = distance between plates.

Question 5

How can we relate capacitors to V and I?

Answer 5

• If we differnetiate Q = CV, we get I = dQ/dt => I = C dv/dt.
• If we integrate Q = CV , we get Q = ∫Idt, thus V = 1/C ∫Idt

diff and integrate with respects to t, time.

Question 6

What is the equation for finding energy stored in a capacitor?

Answer 6

E = 1/2 CV^2