Capacitance

Question 1

How is capacitance calculated?

Answer 1

C = Q / V (C = Capacitance in F, Q = Charge in C, V = potential difference in V)

Question 2

What is the relative permittivity (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; εr = Q / Q0

Question 3

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

Answer 3

The energy stored by the capacitor

Question 4

Describe the Q against t graph for discharging of a capacitor through a resistor

Answer 4

Negative exponential from C axis

Question 5

Describe the V against t graph for discharging of a capacitor through a resistor

Answer 5

Negative exponential from V axis

Question 6

Describe the I against t graph for discharging of a capacitor through a resistor

Answer 6

Negative exponential from I axis

Question 7

Describe the Q against t graph for charging of a capacitor through a resistor

Answer 7

Positive exponential from origin

Question 8

Describe the V against t graph for charging of a capacitor through a resistor

Answer 8

Positive exponential from origin

Question 9

What is the time constant?

Answer 9

The time it takes for the charge in a capacitor to fall to 37% of the initial value; τ = RC

Question 10

How was 37% derived when using the time constant?

Answer 10

Using Q = Q0 e^-t/RC, when t = RC → Q = Q0 e^-1 ≈ 0.37 Q0

Question 11

What is the half time of a capacitor?

Answer 11

T½ = 0.69 RC

Question 12

What equations are required for charging a capacitor?

Answer 12

Q = Q0 (1 - e^-t/RC) and V = V0 (1 - e^-t/RC)

Question 13

How does a capacitor charge up?

Answer 13

Electrons move from negative to positive, accumulate on plate A, removed from plate B, creating pd until source pd is reached and current stops

Question 14

How does a capacitor discharge across a resistor in terms of electron movement?

Answer 14

Electrons move opposite to charging; plate A loses electrons, plate B gains, pd decreases exponentially

Question 15

State the 3 expressions for energy stored by a capacitor

Answer 15

E = ½ Q²/C = ½ QV = ½ CV²

Question 16

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

Answer 16

Capacitance (C) and Resistance (R) of the circuit