capacitance

Question 1

what is a capacitor

Answer 1

A capacitor is a passive electronic device which stores charge.
They consist of two or more conducting plates separated by an insulating material.

Question 2

what happens when a capacitor is connected to a battery

Answer 2

When connected to a battery, the plate connected to the positive side of the battery loses electrons to the battery and the other plate gains electrons from the battery.

Question 3

what is capacitance

Answer 3

The capacitance of a capacitor is the charge stored per unit potential difference

Question 4

what do capacitors store

Answer 4

There is no net charge stored. However, the charge difference has created an electric field that is capable of moving (doing work on) any other charged particle that is placed between the capacitor plates. The capacitor stores electrical potential energy.

Question 5

what are the applications of a capacitor

Answer 5

Store power; for back up purposes

Smoothing circuits: Noise on power supply rails can be removed by conducting away to ground

Tuned Circuits: In traditional radios and TVs the thing you twiddle to change station is a variable capacitor. This makes the tuned circuit sensitive to only the station you want

Filter Circuits: Remove unwanted frequencies

Question 6

What happens to potential difference (voltage) across the two plates of a capacitor when a capacitor is charging, in respect to the charge?

Answer 6

the charging process a capacitor from q to q + q.
Work must be done to force the extra charge onto the plates & this can be given by
E = vq
Where v is the average voltage during this step

Question 7

what is the full energy stored on a capacitor when the pd has gone from 0 to the full pd given by on the graph and in equation from

Answer 7

under the graph

E = ½QV

Question 8

if we were to double the charge on a capacitor what happens to the potential difference

Answer 8

it also doubles

Question 9

what happens if the charge or the pd on a capacitor is doubled

Answer 9

the energy stored quadruples

Question 10

why is the energy required to charge a capacitor not equal to the energy stored within it

Answer 10

half of the energy is lost through the resistor in the circuit and dissipated to the surroundings as heat

Question 11

how does a capacitor discharge through a fixed resistor

Answer 11

rapidly at first and then more and more gradually until the discharge current reaches 0

Question 12

what is the time constant for a capacitor

Answer 12

the time it takes for the charge on the capacitor to fall to 37% of the initial value

Question 13

how many time constants until the capacitor is considered fully discharged

Answer 13

5

Question 14

how can the charge on a capacitor be increased

Answer 14

by inserting a dielectric between the plates

Question 15

what happens to the molecules in the dielectric when a capacitor is charged

Answer 15

each molecule in the dielectric becomes polarised

Question 16

what happens when the molecules in a dielectric become polarised

Answer 16

they turn and line up with the field when the plates are charged because their electrons are attracted slightly to the positive plate

Question 17

why is more charge stored in the plate with a dielectric

Answer 17

the positive side of the dielectric attracts more electrons from the battery onto the negative plate

the negative side of the dielectric pushed electrons back to the battery from the positive plate

Question 18

what is the relative permittivity

Answer 18

the ratio of charge stored with the dielectric to charge stored without the dielectric is defined as relative permittivity

Question 19

how can the capacitance of a capacitor be increased

Answer 19

making the are of the plates as large as possible

making the distance between the plates as small as possible

Filling the gap between the plates with a dielectric with relative permittivity as large as possible