capacitors

Question 1

what is capacitance

Answer 1

the capacitance of a capacitor is the amount of charge it can store per unit potential difference

Question 1

what is capacitance measured in

Answer 1

farads

Question 2

equation for capacitance

Answer 2

C= Q/V

Question 3

what are capacitors

Answer 3

they consist of two metal plates separates by a dielectric

Question 4

what does the capacitance of a capacitor depend on?

Answer 4

surface area of plates,
separation of plates
dielectric being used

Question 5

what does a polar dielectric consist of?

Answer 5

lots of polarised molecules each with a positive end and a negative end.

Question 6

what happens when a dialectic is placed in a capacitor

Answer 6

when places in capacitorf the molecules align and all the positive ends are attracted to the negative plate and all negative ends are attracted to positive plates - results in capacitance increasing

Question 8

Why do the capacitance results in the capacitor increase

Answer 8

Polarised molecules each have in an electric field around them
This electric field opposes the electric field of the capacitor plates
Therefore potential difference required to change the plates decreases

Question 9

What happens to capacitance as potential difference is decreased

Answer 9

Capacitance increases

Question 10

equations to calculate energy stored in capacitor?

Answer 10

E = 1/2 QV
E = 1/2 CV ^2
E = Q^2/C

Question 11

how do we calculate energy stored by a capacitor using a graph?

Answer 11

area under a charge voltage graph

Question 12

what does the gradient of a charge voltage give us?

Answer 12

capacitance

Question 13

what is the time constant equal to?

Answer 13

product of resistance in the circuit and the capacitance of the capacitor

Question 14

time taken to charge capacitor to (1 -1/e) of its _____ value

Answer 14

final

Question 15

time taken to discharge the capacitor to 1/e of its ____ value

Answer 15

initial

Question 16

what is relative permittivity (dielectric constant)

Answer 16

Ratio of the charge stored with the dielectric between the plates to the charge stored when the dielectric is not present

Question 17

what increases when the relative permittivity increases

Answer 17

capacitance of the capacitor

Question 18

describe graph of discharging capacitor v against t

Answer 18

negative exponential graph

Question 19

describe graph of discharging capacitor I against t

Answer 19

negative exponential graph

Question 20

describe Q against t graph for charging a capacitor

Answer 20

starts from orgin and curves

Question 21

describe v against t graph for charging a capacitor

Answer 21

starts from orgin and curves

Question 22

what is the time constant

Answer 22

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

RC = resistance x capacitance

Question 23

how many constant to fully discharge a capacitor

Answer 23

after 5 time constants

Question 24

what is the half time of a capacitor

Answer 24

T1/2 = 0.69RC

Question 25

how does a capacitor charge

Answer 25

• electrons move from negative to positive around the circuit
  -electrons are deposited on Plate A which makes it negatively charged
  -electrons travel from plate B to positive terminal of the battery which gives the plate a positive charge
  -electrons build up on plate A and an equal amount of electrons are removed from plate B creating a pd across the plates
  -when the pd across plates = source pd , the capacitor is fully charged and current stops flowing

Question 26

describe and explain in terms of the movement of electrons how the pd across the capacitor changes when it discharges across a resistor

Answer 26

• electrons move in opposite direction than when the capacitor was charging up
• charge on one plate A decreases as it loses electrons and plate B gains electrons (making them neutral)
• pd decreases exponentially across plates

Question 27

2 factors that affect time taken for capacitor to charge/discharge

Answer 27

1. capacitance of capacitor
2. resistance of the circuit as it affects the current