Capacitors

Question 1

What happens when a capacitor is charging from a d.c supply?

Answer 1

There is a net flow of electrons from one plate to the other, which decrease exponentially over time and ceases when the p.d across the capacitor equal that if the supply. Also the current drops to zero when the capacitor is fully charged, as does the p.d in the resistor.

Question 2

What is capacitance?

Answer 2

The charge stored by a capacitor per volt of potential difference applied across it.

Question 3

What is directly proportional when charging a capacitor?

Answer 3

The charge stored is directly proportional to the potential difference across it.

Question 4

What affect capacitance and is it constant?

Answer 4

It is constant for a given capacitor and depends on the area and separation of the plates as well as the material between them

Question 5

What is capacitance measured in?

Answer 5

Farads

Question 6

What is dielectric?

Answer 6

The insulation between the two plates

Question 7

What happens when a capacitor is connected in circuit

Answer 7

A charge flows onto the capacitor, the battery has to transfer energy to work against the repulsive forces from the charge already on the capacitor

Question 8

Energy and capacitance question

Answer 8

The work done is equal to the energy supplied by the power source and the work done in moving a charge through a potential difference. This is double the energy stored by the capacitor as the energy is not constant when charging

Question 9

What is the time constant?

Answer 9

It is equal to the resistance times by the capacitance or is the time taken for the current to drop to 1/e of its current value

Question 10

What happens if you have a large resistance in the circuit?

Answer 10

There is a small current flowing through the circuit, so it takes longer to charge

Question 11

What happens when discharging?

Answer 11

It begins with a maximum current value which decreases exponentially over time. Voltage and charge also decrease exponentially over time when the capacitor is discharging

Question 12

How do you find out the number of photons released in a camera flash?

Answer 12

Divide the energy change taking place in the camera by the energy of one photon

Question 13

What are the uses of a capacitor?

Answer 13

Camera flashes
Ultracapacitors - to back up power supplies for short periods of time.
To smooth out variations in d.c voltage supplies.

Question 14

What is a capacitor?

Answer 14

A component that can store electrical charge in electrical fields, which simply consists of two metal plates separated by an air gap.