3.7.4 Capacitance

Question 1

What is capacitance?

Answer 1

The charge stored per unit potential difference (between plates)

Question 2

What is a capacitor?

Answer 2

An electrical device used to store energy in electric circuits, they are made up of parallel plates

Question 3

What is the formula for capacitance?

Answer 3

C=Q/V

Question 4

What is a dielectric?

Answer 4

A material made up of many polar molecules, when a charge is applied, one of the parallel plates becomes positively charged, the other negatively charged, hence, an electric field is generated between the plates.

Question 5

What is relative permitivity?

Answer 5

Also known as the dielectric constant:
The ratio of the permittivity of a material to the permittivity of free space.
(permittivity is how well the polar molecules in a dielectric align with an applied electric field)

Question 6

What is the formula for capacitance in terms of permittivity?

Answer 6

C=(A ε_0 ε_r)/d

Question 7

How does the potential difference across the plates vary with charge?

Answer 7

The potential difference across the capacitor increases as the amount of charge increases. (more work must be done to get electrons on the other plate when there is a current due to electron repulsion)

Question 8

What is the formula for energy stored in a capacitor?

Answer 8

From graph;
E=1/2QV =
E=1/2CV^2 =
E=Q^2/2C

Question 9

What does each graph look like for a charging capacitor?

Answer 9

I-t : exponential decay
V-t : increases to a maximum value
Q-t : Increases to a maximum value

Question 10

What does each graph look like for a discharging capacitor?

Answer 10

All: exponential decay curve

Question 11

What is the time constant for a discharging capacitor?

Answer 11

The time taken for the charge, current or voltage of a discharging capacitor to decrease to 37% of its original value.

Question 12

What is the time constant for a charging capacitor?

Answer 12

The time taken for charge or voltage of a charging capacitor to rise to 63% of its maximum value

Question 13

What is the equation for the time constant?

Answer 13

τ=RC

Question 14

What is the time to half for a discharging capacitor?

Answer 14

Time taken for the charge, current or voltage of a discharging capacitor to reach half of its initial value.

Question 15

What is the equation for time to half?

Answer 15

τ =0.69RC

Question 16

What are the capacitor discharge equations?

Answer 16

I=I_0e^(-t/RC)
Q=Q_0e^(-t/RC)
V=V_0*e^(-t/RC)

Question 17

What are the capacitor charge equations?

Answer 17

Q=Q_0(1-e^(-t/RC))
V=V_0(1-e^(-t/RC))