Capacitance

Question 1

What is a capacitor?
How does a charge build up on a plate

Answer 1

2 conducting plates separated by a gap or a dielectric.

Positive and negative charge builds up on opposite plates , creating an electric field.

Question 2

What is capacitance

Answer 2

CHarge per unit potential difference.

C = Q /V

Farads

Question 3

How can you increase capacitance/
What is permitivity

Answer 3

Use a dielectric

Measure of how difficult it is to generate an electric field

Question 4

WHat happens in a dielectric when no charge is applied

Answer 4

Made up of polar molecules
( positive and negative end )

There is no electric field , molecules point in random directions

Question 5

What happens in a dielectric when a charge is applied

Answer 5

An electric field is generated
Negative ends of the molecule are attracted to the +ve charged plate

Cause molecules to rotate and align themselves

Each molecule creates their own electric field , which opposes the applied electric field.

Reduces the overall electric field , reducing potential difference. so capacitance increases.

Question 6

What are the 3 expressions for energy stored by a capacitor

Answer 6

E = 1/2 QV
E = 1/2 CV^2
E = 1/2 Q^2/C

Question 7

WHat happens to the capacitor when it begins charging

Answer 7

Electrons flow onto negative terminal , negative charge builds up

Negative charge build up repels electrons off positive plate , making it positive.

Equal and opposite charge builds up on each plate , causing a potential difference.

Charge builds up on plates , electrostatic repulsion makes it harder more electrons to be deposited.
Pd across capacitor = Pd across power supply
so current falls to 0

Question 8

Describe the graphs of I , V , Q against t

Answer 8

I / t = Expo decay

V / t = Expo growth

Q / t = Expo growth

Question 9

WHat happens when a capacitor discharges

Answer 9

When current and potential difference across plate are zero.

Current flows in the opposite direction from charging current .

Potential difference dec , current dec

CHarge and pd falls exponentially with time

Question 10

2 Factors that affects the time to discharge and charge

Answer 10

• Capacitance of the capacitor - affect amount of charge transfered

-Resistance of circuit - affects the current

Question 11

What is the equation for time constant

Answer 11

T = RC