Unit 4: Fields and options

Question 1

Define a capacitor.

Answer 1

A pair of conducting plates separated by an insulator. If a pd, is placed across the plates, they acquire equal and opposite charges.

Question 2

Define capacitance, C, of a capacitor.

Answer 2

Capacitance = charge on either plates/ pd between plates
Unit: F (farad) [=CV⁻¹]

Question 3

Define dielectric.

Answer 3

Insulator between the plates of a capacitor, also serving to make the capacitance larger than if there were just empty space.

Question 4

What happens when a pd is applied to a capacitor?

Answer 4

Charge is transferred by the power supply around the circuit. The plates then carry an equal and opposite charge.

Question 5

State the formula for charge.

Answer 5

Q = CV

Question 6

Give 2 factors for the quantity of charge on each plate.

Answer 6

Pd applied and the size of the capacitor.

Question 7

State the formula for capacitance of metal plate capacitors.

Answer 7

C = E₀ (8.85 x 10⁻¹²) A/ d

Question 8

Define E₀. in capacitance formula.

Answer 8

Permittivity of free space.

Question 9

Define A in capacitance formula.

Answer 9

Area of the plates.

Question 10

Define d in capacitance formula.

Answer 10

Distance between the plates.

Question 11

What is capacitance proportional to?

Answer 11

Plate area

Question 12

What is capacitance inversely proportional to?

Answer 12

Separation of the plates.

Question 13

State the formula for the energy of a capacitor (1).

Answer 13

U = ¹/²QV

Question 14

State the formula for the energy of a capacitor (2).

Answer 14

U = ¹/²CV²

Question 15

State the formula for the energy of a capacitor (3).

Answer 15

U = ¹/² Q²/C

Question 16

What happens when air is subjected to an electric field greater than 3 x 10⁶Vm⁻¹?

Answer 16

The air itself is said to ‘break down’ and a spark will jump across the gap of the field.

Question 17

State the formula for the E-field between capacitors.

Answer 17

E = V/d