Capacitors

Question 1

Describe the function and structure of a capacitor

Answer 1

An electrical reservoir that stores charge and energy.

It is made of two parallel metal plates separated by an insulating material called a dielectric

Question 2

Explain why the overall charge on a capacitor is always zero

Answer 2

Electrons travel to the negative plate
Electrons cannot traverse dielectric because it is an insulator
Charge does not flow. Plate gains negative charge
As negative charge increases, negative charge is repelled from the other plate so it is left with a positive charge

Question 3

What happens if a charged capacitor is placed in another circuit?

Answer 3

It discharges and negative charge will flow back to the opposite plate

Question 4

Define farad

Answer 4

1 coulomb of charge is stored for every 1 volt of potential difference across it

Question 5

Define capacitance

Answer 5

How much charge a capacitor can store for every 1 volt of potential difference across it

Question 6

Define permittivity

Answer 6

The measure of how difficult it is to generate an electric field in a medium

Question 7

Define relative permittivity

What is the equation for relative permittivity?

Answer 7

The ratio of the permittivity of a material to the permittivity of free space

Er = E1
—
E2

Question 8

What happens to the polar molecules in a dielectric when a charge is applied

What is the effect of a higher permittivity

Answer 8

The molecules that originally had a random orientation align antiparallel to the electric field between the plates

Higher permittivity = larger opposing field of molecules = smaller overall electric field of plates = less potential difference needed to transfer a given charge and Q=CV So C increases

Question 9

Explain the shape of a current time graph of a charging capacitor

Answer 9

Current is the rate of flow of charge

As charge on the plate increases, there is a stronger NEGATIVE repulsion so it becomes more difficult for charge to flow

Question 10

Explain the shape of a Q against time graph of a charging capacitor

Answer 10

Q=CV so same shape as V against t graph where Q increase at the same rate V increases
Q=It where Q is proportional to current
As time increases, current decreases so the gradient of this curve flattens

Question 11

Define time constant

What is the unit?

Answer 11

Time taken for I, V or Q to reach
1/e of its original value

Seconds

Question 12

What happens if R increases (time constant)

What happens if C increases?

What happens to the discharge time if time constant increases?

Answer 12

R increases, Tc increases and Initial current decreases

C increases, Tc increases and initial current remains the same

It increases