7.4. Capacitance: Paper 2: Physics A Level AQA

How is capacitance calculated?

C = Q \ V

where:

C is Capacitance (F)

Q is Charge in the plates (C )

V is potential difference across the plates (V)

What is the relative permittivity (a.k.a. dielectric constant)?

● The ratio of the charge stored with the dielectric between the plates to the charge stored when the dielectric is not present.

● ε_r = Q / Q_0

● The greater the relative permittivity, the greater the capacitance of the capacitor.

What does the area under the graph of charge against pd represent ?

The energy stored by the capacitor.

What is the half time of a capacitor?

T½ = 0.69RC

How does a capacitor charge up?

1. Electrons move from negative to positive around the circuit.

2. The electrons are deposited on plate A, making it negatively charged.

3. Electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge.

4. Electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a potential difference across the plates.

5. When the p.d across plates = source p.d., the capacitor is fully charged and current stops flowing.

Describe and explain in terms of the movement of electrons how the p.d across a capacitor changes, when it discharges across a resistor.

1. Electrons move in opposite direction than when the capacitor was charging up.

2. Charge on one plate A decreases as it loses electrons, and plate B gains electrons, neutralising them.

3. P.d. decreases exponentially across the plates.

State the 3 expressions for the energy stored by a capacitor.

E = ½ (Q^2 /C) = ½ (QV) = ½ (CV^2 )

What 2 factors affect the time taken for a capacitor to charge or discharge?

● The capacitance of the capacitor, C. This affects the amount of charge that can be stored by the capacitors at any given potential difference across it.

● The resistance of the circuit, R. This affects the current in the circuit and how quickly it flows, hence how quickly the capacitor charges/discharges.