3.7.4.1 - Capacitance

Question 1

What is capacitance?

Answer 1

The charge required to cause unit potential difference in a conductor.

Question 2

What is capacitor made up of?

Answer 2

Two conducting plates separated by a gap or a dielectric.

Question 3

What is created in a capacitor and how?

Answer 3

A uniform electric field due to positive and negative charge building up on opposite plates.

Question 4

What is permittivity?

Answer 4

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

Question 5

What does a higher permittivity indicate?

Answer 5

The more charge is needed to generate an electric field of a given size.

Question 6

What is a dielectric?

Answer 6

Material made up of polar molecules, which have a positive and negative end.

Question 7

When no charge is applied what happens in the dielectric material?

Answer 7

No charge is being stored so no electric field is being generated. The molecules align randomly.

Question 8

What happens in a dielectric material when charge is applied?

Answer 8

An electric field is generated between the plates. The negative ends of the molecules are attracted to positively charged plates and vice versa. This causes the molecules to rotate and align.

Question 9

What happens to the electric field in a capacitor when charging with a dielectric material and why?

Answer 9

The overall electric field strength reduces as each of the polar molecules have their own electric field which opposes the applied electric field of the capacitor.

Question 10

What does a larger permittivity result in for the electric field strength?

Answer 10

The larger the permittivity, the larger the opposing field is. This reduces the overall electric field.

Question 11

What does larger permittivity overall result in?

Answer 11

Reduced electric field, therefore reduced potential difference but higher capacitance.

Question 12

From what can you calculate the energy stored by a capacitor?

Answer 12

Underneath a Q V graph.

Question 13

What happens during charging?

Answer 13

As soon as the switch is closed, current starts to flow. The electrons on the plate connected to the negative terminal causes negative charge to build up. The build up of negative cage repels electrons off the plate connected to the positive terminal, making that plate positive. An equal but opposite charge builds up on each plate, causing a potential difference between the plates.

Question 14

What is the IT graph for charging?

Answer 14

Exponentially decreasing.

Question 15

Graph for charging of QT

Answer 15

Exponentially increasing

Question 16

Graph of VT for charging?

Answer 16

Exponentially increasing

Question 17

How can you find the time constant for an exponentially increasing graph?

Answer 17

Read off at 63% of original y axis value.

Question 18

How can you find the time constant for exponentially decreasing graph?

Answer 18

Read off at 37% of original y axis value.

Question 19

What happens during discharging?

Answer 19

Current flows in the opposite direction from the charging current. As the potential difference decreases, the current decreases as well. When a capacitor is discharging the amount of charge on and potential difference between the plates fall exponentially with time. That means it always takes the same length of time for the charge or potential difference to halve, no matter what value it starts at.

Question 20

What are all the graphs for discharging?

Answer 20

Exponentially decreasing.

Question 21

What is the time to half?

Answer 21

The time taken for the charge, current or potential difference of a discharging capacitor to reach half the value it was when fully charged.