Capacitors

Question 1

What is a capacitor?

Answer 1

A device designed to store charge.

Question 2

How does a capacitor charge?

Answer 2

Two parallel metal plates placed near each other. When connected to a battery, electrons move through the battery and are forced onto one of the plates. this makes the plate become negative. On the other plate, an equal number of electrons are repelled making that plate positive.

Question 3

What is the symbol for a capacitor in a circuit?

Answer 3

Two equal length parallel lines.

Question 4

What would a charge against pd graph for a capacitor look like? what can be found from this graph?

Answer 4

A straight proportional line passing through the origin.

Area = work done = energy stored

Question 5

Define capacitance

Answer 5

The charge stored per unit pd.

Question 6

How is the energy stored in a capacitor obtained from a graph?

Answer 6

Charge against pd graph

Area under is a triangle of QV/2

Question 7

In what proportionality does current, charge and pd of a capacitor decrease when discharging?

Answer 7

Exponentially

Question 8

What is the time constant of a circuit?

Answer 8

the rate of discharge of a capacitor. the time take for the voltage/charge/current to fall to 1/e (37%) of its original value
time constant = RC

Question 9

When t=RC in a capacitor’s discharge, what percentage charge remains?

Answer 9

37%

Question 10

When t=2RC in a capacitor’s discharge, what percentage charge remains?

Answer 10

14%

Question 11

At what point, is a capacitor considered discharged?

Answer 11

When t=5RC

Question 12

Define a farad

Answer 12

One coulomb of charge stored per volt

Question 13

What factors affect the capacitance of a capacitor?

Answer 13

surface area of plates
Separation of plates
Dielectric

Question 14

Define permittivity

Answer 14

Permittivity is a measure of how difficult it is to generate an electric field in that medium. The higher the permittivity of the material the more charge needed to generate an electric field.

Question 15

Define relative permittivity

Answer 15

Relative permittivity is the ratio of the permittivity of a material to the permittivity of free space, it is sometimes known as the dielectric constant.

Question 16

Explain permittivity

Answer 16

When no charge is stored by a capacitor, no electric field is generated, the polar molecules are randomly aligned.
When a charge is applied to the plates of a capacitor an electric field is generated between them. The negative ends of the molecule are attracted to the positive plate and vice versa. This causes the molecules to rotate and align them selves anti-parallel to the field.
The molecules each have their own electric field, which now opposes the applied electric field of the capacitor. The larger the permittivity, the larger the opposing field. This reduces the overall electric field between the plates and therefore reduced the p.d. needed to transfer a given charge to the capacitor. This increases the capacitance (Q = CV).

Question 17

If you measure voltage and time for a capacitor discharging, what graph would you plot to obtain a straight line? What could you find from this graph?

Answer 17

ln V against t
Gradient = - 1/RC
Intercept = ln Vo

Question 18

What are the units of time constant

Answer 18

seconds

Question 19

How can you prove a graph is exponential?

Answer 19

An exponential graph always shows a constant ratio property. This means in equal amount of time the Y axis will always decrease by the same ratio.