Capacitance

Question 1

How is capacitance calculated?

Answer 1

C = Q / V

Question 2

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

Answer 2

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

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

Answer 4

● The capacitance of the capacitor, C.
● The resistance of the circuit, R.

Question 5

What is the time constant?

Answer 5

The time it takes for the charge in a capacitor falls to 37% of the initial value given by RC

A capacitor is considered fully discharged after 5 time constants.

Question 6

How does a capacitor charge up?

Answer 6

• Electrons move from negative to positive around the circuit.
• The electrons are deposited on plate A, making it negatively charged.
• Electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge.
• Electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a potential difference across the plates.
• When the p.d across plates = source p.d., the capacitor is fully charged and current stops flowing.

Question 7

Describe discharging in terms of electrons

Answer 7

• Electrons move in opposite direction than when the capacitor was charging up.
• Charge on one plate A decreases as it loses electrons, and plate B gains electrons, neutralising them.
• P.d. decreases exponentially across the plates

Question 8

Definition of Capacitance

Answer 8

charge stored (Q) by a capacitor per unit potential difference (V).

Question 9

Parallel Plate Capacitor

Answer 9

• Has two conducting parallel plates
• Separated by a dielectric
• Forms a uniform electric field when charged

Question 10

Why Dielectric Increases Capacitance

Answer 10

• Dielectric contains polar molecules
• Without field → random orientation
• With field → molecules align
• Their internal fields oppose the capacitor field
• This reduces overall electric field
• So less p.d. needed for same charge
• Since C = Q/V → capacitance increases