Capacitance

Question 1

What is Capacitance?

Answer 1

The amount of charge that can be stored per unit Potenial difference

Question 2

What equation is the definition of Capacitance?

Answer 2

Capacitance = Charge / Potential Difference
C = Q/V

Question 3

What are the Units and Symbol of Capacitance?

Answer 3

Symbol: C
Units: Farad (F)

Question 4

Why are Capacitors generally measured in μF ?

Answer 4

Because one Farad is a very large amount

Question 5

What is the total charge of a Capacitor?

Answer 5

Zero

Conservation of charge means that the opposite chrges must be created in equal quanity

Question 6

What is Relative Permittivity?

Answer 6

The ratio of a materials permittivity to that of free space

Question 7

What is Permittivity?

Answer 7

How difficult it is to generate an electric field in a medium

Question 8

What can Relative Permitivitty also be known as?

Answer 8

Dielectric Constant

Question 9

What is a Dielectric?

Answer 9

insulator

Question 10

How do Dielectric relate to Capacitors?

Answer 10

Dielectrics are placed between the two plates of a capacitor

Question 11

What is the equation of Relative Permittivity?

Answer 11

ε(r) = ε(of material) / ε(0)

Question 12

How do Polar Molecules reduce an E-field?

Answer 12

Tge E-field of a polar molecules aligns against the larger E-field. Reducing the resultant E-field

Question 13

What is the equation for Capacitance with a Dielectric?

Answer 13

Capacitance = Area x ε(0) x ε(of material) / Distance of seperation
C= Aε(of material)ε(0)/d

Question 14

What equation should be used if a Capacitor is disharged at a constant current?

Answer 14

CV= It

Question 15

What are uses of Capacitors?

Answer 15

Measuring small time intervals
Quickly discharging charge in a defibrilator
Tuning circuits
Smoothing AC → DC power supplies

Question 16

What are the equations for Energy in a Capacitor?

Answer 16

Energy = 0.5 x Charge x p.d = 0.5 Capacitance p.d² = 0.5 x Charge²/Capacitance
E = 0.5QV = 0.5CV² = 0.5Q²/C

Question 17

What is the Gradient if pd is on th X axis and Q is on the Y axis?

Answer 17

Capacitance

Question 18

What is the Energy on a Q-V graph

Answer 18

The area between the line and Charge axis

Question 19

What is the Gradient if pd is on th Y axis and Q is on the X axis?

Answer 19

1/Capacitance

Question 20

Why is only half the energy of the circuit in the Capacitor?

Answer 20

The other half is lost thermally due to resistance

Question 21

What is present in a Chargeing Circuit of a Capacitor?

Answer 21

Power Supply (battery, mains, cell)
Resistor
Capacitor
Ammeter
Voltmeter

Question 22

What is present in a Discharging Circuit of a Capacitor?

Answer 22

Resistor (to create load)
Capacitor
Ammeter
Voltmeter

Question 23

What shape is Capacitor Current-time, Charge and Discharge graph?

Answer 23

Charge:
Exponetial decay from maximum current
Discharge:
Exponential decay towards zero from negative maximum current

Question 24

What shape is Capacitor V(r)-time, Charge and Discharge graph?

Answer 24

Charge:
Exponetial decay from EMF
Discharge:
Exponential decay towards zero from negative EMF

Question 25

What shape is Capacitor V(c)-time, Charge and Discharge graph?

Answer 25

Charge: 1-e curve toward maximum EMF Discharge: Exponential decay from maximum positive EMF

Question 26

What shape is Capacitor Q(c)-time, Charge and Discharge graph?

Answer 26

Charge: 1-e curve toward maximum Q Discharge: Exponential decay from maximum positive Q

Question 27

What is the gradient of a Charge-Time graph?

Answer 27

Current

Question 28

What is the area under a Current-Time graph?

Answer 28

Charge

Question 29

What is the Shape of a graph on the combined p.d of both the Resistor and Capacitor?

Answer 29

Charge: Constant at EMF Discharge: Constant at 0 (as no overall p.d )

Question 30

What happens to the Discharge time if supply p.d is doubled?

Answer 30

Discharge time is unaffected

Question 31

What happens to the Discharge time if the Capacitor was doubled?

Answer 31

Discharge time is doubled | twice as long to remove twice the amount of charge

Question 32

What happens to the Discharge time if the Resistance was doubled?

Answer 32

Discharge time is doubled | as half the current, half the rate of charge

Question 33

What is the equation for Charge on a Discharging Capacitor as a function of time?

Answer 33

Q=Q(0) x e^(-t/RC) | Q(0) = maximum charge

Question 34

What is the equation for p.d on a Discharging Capacitor as a function of time?

Answer 34

V=V(0) x e^(-t/RC) | V(0) = maximum p.d

Question 35

What is the equation for Current on a Discharging Capacitor as a function of time?

Answer 35

I=I(0) x e^(-t/RC) | I(0) = maximum I

Question 36

What is the equation for Charge on a Charging Capacitor as a function of time?

Answer 36

``` Q = Q(0) x (1-e^(-t/RC)) Q(0) = Maximum Charge ```

Question 37

What is the equation for p.d on a Charging Capacitor as a function of time?

Answer 37

``` V = V(0) x (1-e^(-t/RC)) V(0) = Maximum p.d ```

Question 38

What is the equation for Current on a Charging Capacitor as a function of time?

Answer 38

``` I = I(0) x (1-e^(-t/RC)) I(0) = Maximum Current ```

Question 39

What is the Time Constant?

Answer 39

The time taken for the Charge(or Current or p.d) to reduce to 1/e of its initial amount

Question 40

What is 1/e roughly equal to?

Answer 40

0.37

Question 41

What is the relationship between Half Life and Time Constant?

Answer 41

T(0.5) ≈ 0.69τ

Question 42

What is the Units and Symbol for Time Constant?

Answer 42

Symbol: τ Units: ΩF or s