Capacitance

Question 1

How is capacitance calculated?

Answer 1

C = Q \ V
C = Capacitance (F)
Q = Charge in the plates (C )
V = potential difference across the plates (V)

Question 2

What is the relative permittivity

Answer 2

● The ratio of the charge stored with the
dielectric between the plates to the charge
stored when the dielectric is not present.
● εr = Q / Q0
● The greater the relative permittivity, the greater
the capacitance of the capacitor.

Question 3

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

Answer 3

The energy stored by the capacitor.

Question 4

What is the time constant?

Answer 4

The time it takes for the charge in a capacitor falls to 37% of
the initial value (explained in the following slide) given by RC
(resistance x capacitance).
A capacitor is considered fully discharged
after 5 time constants.

Question 5

How was 37% derived when using the
time constant?

Answer 5

● Start with the formula Q = Q0^e-t/RC
● When t = RC (after 1 time constant), the
formula becomes Q = Q0^e-1
● e-1 ≈ 0.37, which is where 37% came from.

Question 6

What is the half time of a capacitor?

Answer 6

T½ = 0.69RC

Question 7

What equations do we require for
charging a capacitor?

Answer 7

Charging up a capacitor produces Q = Q0^(1 - e-t/RC) &

V = V0(1 - e-t/RC) where V0is the battery PD and Q0=CV0

Question 8

How does a capacitor charge up?

Answer 8

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.

Question 9

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

Answer 9

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.