capacitors (everything)

Question 1

what are the propities of a dialectric

Answer 1

-insolator
-polerisable

Question 2

capacitence equation

thereofre
give unit of capacitence

Answer 2

C=Q/V

=CV^-1
1 F (farad) = 1 CV^-1

Question 3

what is the capacitence eqauation (not C=Q/V)

what does this equation assume

Answer 3

C=Aε0εr/d
A is area of parrelel plates
d is distence between parellel plates
ε0 is permetivity of free space
εr is reletive permativity

the plates are parallel

Question 4

what are 3 ways to increse C

Answer 4

incresase A
incresease εr
decrease d

Question 5

reletive permativity (or dialetic constent) equation

Answer 5

εr=ε1/ε0
where ε1 is the reletive permativity of meterial 1

Question 6

molecules in the dielectric of an uncharged capacitor point __ _______ ______.
this means…

Answer 6

in random directions

the molecules and therefore the larger dielectric are unpolerised

Question 7

Explain and decribe, with refrence to molecules, the effect of polarisation within a dialectric

Answer 7

• when charge is applied to a capacitor, an electric field is generated
• this means all the molecules in the dialectric will be polerised and orentate the negetive end of the molecules to the positeve charged plate and visa versa
• the polerised molecules now generat an elecric field of there own that opposes the capacitors elecrtic feild
• this reduces the overall electric feild, which reduces the pd across the capacitor, which reduces the p.d needed to charge the capacitor allowing more charge to be deposited at the plates, increaseing capacitence

↑C=Q/↓V

Question 8

Q ∝…

draw a Q over p.d graph

what is the area

therefore
give an energy (E) equation

Answer 8

Q∝p.d

see pg133

enegy stored

E=1/2 QV

Question 9

give all 3 E equations

Answer 9

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

Question 10

decribe the process of charging a capacitor in terms of the electrons

(Dubble check)

Answer 10

• when power supply is turned on charge starts to flow, negetive charge starts to build up on the capacitor plate conneted to the negetive terminal (e- flow opposite to flow of current)
• the build up of negetive charge repelse electrons off the opposing plate
• an equal but opposite charge builds up on each plate
• as there is more charge built up on each plate it becomes harder for charge to be deposited because of eectrostatic repulsion
• when p.d across the capacitor = p.d across the power supply
  current fals to 0

Question 11

for capacitors to build up charge the type of power supply must be…

Answer 11

dc

Question 12

what is the area beneth a I/t graph

what is the graidint of a Q/t graph

Answer 12

charge Q

current I

Question 13

when dischargeing current flows…

Answer 13

opposit direction to charging current

Question 14

time taken to charge or discharge depends on…

Answer 14

capacitence C
and resistnece R

Question 15

give all charging equations

sketch the graphs

Answer 15

Q = Q0(1-e^-t/RC)
V = V0(1-e^-t/RC)
I=Ioe^-t/RC

Question 16

give all discharging equations

sketch the graphs

Answer 16

Q=Qoe^-t/RC
V=Voe^-t/RC
I=Ioe^-t/RC

Question 17

time constent equation

time constent is…

derive it

Answer 17

τ=RC

time taken for Q, V or I to fall to 37% when dicharging
also
time taken for Q or V to reach 63% when charging

Question 18

in practice time taken for a capacitor to charge or discgarge fully is given by…

Answer 18

5RC

Question 19

what is the gradiant of a log(Q,V or I) against t graph

Answer 19

-1/RC

Question 20

time to half T(1/2) =

Answer 20

0.69RC
OR
ln(2)RC