Capacitance

Question 1

What is the definition of Capacitance?

Answer 1

Capacitance is the charge stored per unit P.D across it

Question 2

What is a capacitor?

Answer 2

An electrical component used for storing electrical charge and energy

Question 3

What does a capacitor consist of?

Answer 3

A capacitor consists of two metal plates separated by an insulator

Question 4

How does charging a capacitor work?

Answer 4

1) Electrons will flow from the negative terminal of the cell to the negative terminal of the capacitor.

2) The excessive electrons on the negative plate of the capacitor will begin to repel the electrons on the positive plate, which will move towards the positive terminal of the cell.

3) Overall the plates will acquire an equal and opposite charge

Question 5

How does discharging a capacitor work?

Answer 5

When a capacitor is connected to a circuit, the excess electrons are going to flow from the negative plate towards the positive plate, causing the charge on the capacitor to decrease.

Question 6

When a P.D is placed across the plates of a capacitor, what happens?

Answer 6

When a P.D is placed across the plates of a capacitor, equal amounts of charge gather on the plates. The charges are equal and opposite

Question 7

What is the equation of capacitance?

Answer 7

C = Q/V

Question 8

What are the units of capacitance?

Answer 8

Farads (F) or Coulombs Volt^-1 (CV^-1)

Question 9

What is the equation for the energy stored (U), in a capacitor?

Answer 9

U = QV/2

Question 10

What is the only important area between the two plates of a capacitor?

Answer 10

The only important area between the two plates of a capacitor is the area of overlap.

Charges will only be acquired on the area overlap

Question 11

Charges will only be acquired on the area overlap of capacitor plates, why?

Answer 11

Because without the area overlap, there is no (repulsion / attraction) effect of (like / different) charges

Question 12

What is the equation for capacitance involving the area between the plates?

Answer 12

C = Ɛ0A/d

Question 13

What is Ɛ0?

Answer 13

Ɛ0 is the permittivity of free space

Question 14

What is a dielectric?

Answer 14

A dielectric is an insulating material that is placed between the plates of a parallel plate capacitor

Question 15

What is the effect of a dielectric?

Answer 15

The effect of a dielectric is increasing the capacitance of a capacitor

Question 16

How does a dielectric increase the capacitance of a capacitor?

Answer 16

By effectively reducing the P.D between the plates

Question 17

What is the equation for capacitance when there is a dielectric?

Answer 17

C = ƐrƐ0A/d

Question 18

The electric field between two plates of a capacitor being uniform means what?

Answer 18

The electric field between two plates of a capacitor being uniform means:

The electric field strength, E, will have a constant value everywhere between the plates.

Question 19

What is the equation for the electric field between the plates?

Answer 19

E = v/d Units: Vm-1

Where:
d = the separation of the plates
V = p.d. across the plates.

Question 20

What is the equation for capacitors in series?

Answer 20

Each capacitor will experience the same current so it will acquire the same charge.
The P.D or voltage is shared by Kirchhoff’s 2nd law.

1/C (total) = 1/C(1) + 1/C(2)

Question 21

What is the equation for capacitors in parallel?

Answer 21

The voltage is the same across each parallel branch of the circuit.
The charge is shared because the current is shared.

C (total) = C(1) + C(2)

Question 22

What is the time constant of a capacitor?

Answer 22

T = RC

R = Resistance
C = Capacitance

Question 23

The graph for a charging capacitor can be described by what equation?

Answer 23

Q=Q0(1-e^-t/RC)

When t = RC
Q = 0.632Q0

Question 24

The graph for a capacitor discharging can be described by what equation?

Answer 24

V=V0e^-t/RC OR Q=Q0e^-t/RC

When t=RC then V = 0.368V0