capacitors

Question 1

capacitance

Answer 1

the amount of charge its able to store per unit potential difference
measured in farads

Question 2

capacitance equation

Answer 2

C = Q / V

Question 3

capacitor set up

Answer 3

an electrical components that can store electrical charge
made up of two electrical conducting plates and a dielectric between them

Question 4

when a capacitor is connected to a D.C.

Answer 4

Charge builds up on each plate, one plate becomes negatively charged, the other positive
The dielectric makes sure no charge travels across the plates so a potential difference builds up between the plates

Question 5

voltage rating

Answer 5

the voltage rating of a capacitor is the maximum potential difference that can safely be put across it

Question 6

relationship between potential difference and charge

Answer 6

CV = Q
capacitance is constant so charge is directly proportional to potential difference

Question 7

capacitors set up

Answer 7

can onlt store relivitely small amounts of charge and provide power for short amounts of time
useful as it can store charge until its needed and discharge it all in a fraction of a second
it can be very dangerous though

Question 8

uses of capacitors

Answer 8

camera flash
ultracapacitors - back up power supplies

Question 9

energy stored

Answer 9

when a capacitor is charged, one becomes negative, the other positive, this means the plates are being forced together which requires energy which is supplied by power source and and stored as electric potenctial as long as the charge is held
when the charge is released, the eletric potential energy is released

Question 10

p.d. - Q graph

Answer 10

proportional to eachother
area under equal to energy

Question 11

3 enegy equations

Answer 11

E = 0.5QV

E = 0.5CV^2

E = 0.5 Q^2 / C

Question 12

Where does current flow

Answer 12

positive to negative

Question 13

where does electrons in current flow

Answer 13

negative to positive

Question 14

permittivity

Answer 14

a measure of how difficult it is to generate an electric field in a medium
higher the permittivity of a material. the more charge is needed to generated an electric field of a given size

Question 15

relative permittivity

Answer 15

AKA dieletric constant
ratio of permittivity of a material to the permittivity of free space

Question 16

relative permittivity equation

Answer 16

εr = ε1 / ε0

Question 17

properties of capacitors that can change how much charge it can store at a given voltage

Answer 17

• the dielectric material separating the two conducting plates
  this changes the capacitance as different materials have different relative permittivity

Question 18

polar molecules

Answer 18

permittivity can be experienced by motion of molecules inside the dielectric
imagine a dielectric made up of polar molecules ( positive end / negative end )
when no charge is being stored, no electric field is being generated and molecules are aligned randomly
when a charge is applied, and electric filed is generated, negative end attracted to positive plate and vice versa
molecules rotate and align antiparalell to field

Question 19

polar molecules relationship with larger permittivities

Answer 19

each molecule has their own field opposing the applied field of the capacitor
the lareger the permittivity, the larger the opposing field
This reduces the overall eletric filed between the parallel plates reducing the p.d. needed to transfer a given charge to the capacitor so the capacitance increases

Question 20

capacitance equation with area

Answer 20

C = Aε0εr / d

Question 21

capacitance depends on

Answer 21

dimensions of the capacotor
dieletric

Question 22

charging a capacitor

Answer 22

when a capacitor is connected to a d.c. power supply, a current flows until the capacitor is fully charged
eletrons flow from - terminal of supply onto the plate connected to the capacitor so a negative charge builds up on the plate
eletrons then flow from other plate to positive terminal of supply making that plate positive
These eletrons are repelled by the negative charge on the negative plate and attracted to positive terminal of supply

Question 23

current change when capacitor is charging

Answer 23

initially the current is high
as charge builds up on plate, electrostatic repulsion makes it harder for more electrons to be deposited
when the p.d. across the supply equals the p.d. across the capacitor the current falls to 0 and the capacitor is fully charged

Question 24

charhging through a fixed resistor

Answer 24

the resistance affects the time taken to charge the capacitor
as current starts to flow, p.d across the capacitor is zero so there is no p.d opposing the current
the p.d of a battery causes an initially high current to flow (V/R)
as p.d across capacitor increases and p.d across resistor decrease and current drops

Question 25

I-T graph
capacitor through a fixed resistor

Answer 25

1 / x^2
starts at I0

Question 26

V-T and Q-t graph
capacitor through a fixed resistor

Answer 26

starts at Q0 and V0
x^2 graph, cut in half

Question 27

charging equation

Answer 27

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

V = V0 (1- e^-t/RC)

I = Io e^-t/RC

Question 28

What does Q0 / V0 / I0 mean

Answer 28

value when fully charged
initial current

Question 29

how to discharge through a fixed resistor

Answer 29

to discharge through a capacitor, take out the battery and reconnect the circuit

Question 30

discharging through a fixed resitor

Answer 30

the p.d drives a current through a circuit
this current flows in the oppsote direction from the charging current
current is initially high too
capacitor is fullt discharged when the p.d across the plates and the current falls to zero

Question 31

I-t / Q-t / V-t graph

Answer 31

all 1 / x^2

Question 32

discharging equations

Answer 32

Q = Q0 e^-t/RC

I = I0 e^-t/RC

V = V0 e^-t/RC

Question 33

time taken for capacitor to charge/ discharge depends on

Answer 33

• capacitance affect the amount of charge that can be transferred at a given voltage
  -resistance of a circuit ( larger resistance the longer)

Question 34

time constant

Answer 34

RC
time taken for charge on a discharging capacitor to fall to 1/e (roughly 37%) of initial value or charge to rise to about (63%) of full charge

Question 35

when t = RC

Answer 35

Q / Qo = 1/e = 0.37

Question 36

time taken to fully charge/discharge

Answer 36

in practice 5RC

Question 37

Time to half equation

Answer 37

0.5 = e^-t/RC

T0.5 = 0.69RC