Chapter 21 - Capacitance

Question 1

Capacitor

Answer 1

A component used to store charge

Question 2

How does a capacitor work

Answer 2

• Made up of 2 electrically conducting plates separated by an air gap or a dielectric material
• When connected to a DC power source, negative charge builds on one plate, repelling electrons from the other plate to build a negative charge on the other
• Therefore a potential difference builds up in the air gap

Question 3

Capacitance formula

Answer 3

C (F) = Q/V

Question 4

Capacitance definition

Answer 4

The amount of charge stored per unit potential difference

Question 5

Charging a capacitor

Answer 5

1. Initially there is a high current
2. As the charge builds up on the plate, electrostatic repulsion makes it harder to deposit more electrons
3. When the p.d. across the capacitor equals the p.d. of the supply, the current falls to 0

Question 6

Discharging a capacitor

Answer 6

1. If the source of emf is removed, the capacitor will start to discharge
2. Current will flow in the opposite direction as the repulsive forces are initially high
3. The current and potential difference will fall to 0 over time

Question 7

Work done by a capacitor

Answer 7

W = 1/2 QV

Question 8

Parallel circuits

Answer 8

The p.d. across each capacitor is the same

The total capacitance is the sum of each individual capacitance

Question 9

Series circuits

Answer 9

The p.d. is shared across the capacitors
The charge of each capacitor is the came
1/C(T) = 1/C1 + 1/C2 + …

Question 10

Capacitor uses

Answer 10

Flash photography, smoothing peaks and troughs in p.d

Question 11

Using electricity equations for discharge

Answer 11

I = V0/R

Q = V0C

Question 12

Discharging Exponential equations

Answer 12

x = x0 e^-t/CR

x can be I, V, Q

Question 13

τ

Answer 13

TIme constant (CR) - the time for it to be 37% from full value- discharge to 37%, charge to 63%

Question 14

Current equation for a capacitor

Answer 14

I = -ΔQ/Δt

Question 15

ΔQ/Δt

Answer 15

-Q/CR

Question 16

Charging V and Q

Answer 16

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

Question 17

Charging I

Answer 17

I = I0 x e^-t/CR