6.1 Capacitors

Question 1

What is a capacitor?

Answer 1

● A capacitor is an electrical component that
stores charge on 2 separate metallic plates.
● An insulator, called a dielectric, is placed
between the plates to prevent the charge
from travelling across the gap

Question 2

What is capacitance?

Answer 2

The capacitance, C, is the charge stored, Q,
per unit potential difference, V, across the two
plates. Therefore we have C = Q / V. It is
measured in Farads, F (1F = 1CV-1)

Question 3

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

Answer 3

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

Question 4

What is the equation for the total
capacitance in series?

Answer 4

1/C(total) = 1/C(1) + 1/C(2) + …

Question 5

What is the equation for the total
capacitance in parallel?

Answer 5

C(total) = C(1) + C2 + …

Question 6

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

Answer 6

The energy stored by the capacitor.

Question 7

What is the time constant?

Answer 7

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 8

How was 37% derived when using the
time constant?

Answer 8

• Start with the formula Q=Qe^-t/RC
• When t = RC (after 1 time constant), the
  formula becomes Q = Q0e^-1
• e-1 ≈ 0.37, which is where 37% came from.

Question 9

What equations do we require for
charging a capacitor?

Answer 9

Charging up a capacitor produces Q = Q(1 - e-t/RC) &
V = V(1 - e-t/RC) where V
is the battery PD and Q=CV
.

Question 10

How does a capacitor charge up?

Answer 10

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 11

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 11

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

Question 12

State some uses of capacitors.

Answer 12

● Flash photography
● Nuclear fusion
● Backup power supplies
Also:
● DC blocking
● Smoothing AC to DC
● Tuning (Resonating magnetic field)

Question 13

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

Answer 13

● 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.