7.4. Capacitance: Paper 2: Physics A Level AQA
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8 Terms
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.