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A Level Physics > Capacitance > Flashcards

Capacitance Flashcards

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1
Q

What is Capacitance?

A

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

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2
Q

What equation is the definition of Capacitance?

A 
Capacitance = Charge / Potential Difference
C = Q/V
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3
Q

What are the Units and Symbol of Capacitance?

A

Symbol: C
Units: Farad (F)

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4
Q

Why are Capacitors generally measured in μF ?

A

Because one Farad is a very large amount

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5
Q

What is the total charge of a Capacitor?

A

Zero

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

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6
Q

What is Relative Permittivity?

A

The ratio of a materials permittivity to that of free space

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7
Q

What is Permittivity?

A

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

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8
Q

What can Relative Permitivitty also be known as?

A

Dielectric Constant

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9
Q

What is a Dielectric?

A

insulator

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10
Q

How do Dielectric relate to Capacitors?

A

Dielectrics are placed between the two plates of a capacitor

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11
Q

What is the equation of Relative Permittivity?

A

ε(r) = ε(of material) / ε(0)

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12
Q

How do Polar Molecules reduce an E-field?

A

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

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13
Q

What is the equation for Capacitance with a Dielectric?

A 
Capacitance = Area x ε(0) x ε(of material) / Distance of seperation
C= Aε(of material)ε(0)/d
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14
Q

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

A

CV= It

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15
Q

What are uses of Capacitors?

A

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

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16
Q

What are the equations for Energy in a Capacitor?

A 
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
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17
Q

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

A

Capacitance

18
Q

What is the Energy on a Q-V graph

A

The area between the line and Charge axis

19
Q

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

A

1/Capacitance

20
Q

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

A

The other half is lost thermally due to resistance

21
Q

What is present in a Chargeing Circuit of a Capacitor?

A 
Power Supply (battery, mains, cell)
Resistor
Capacitor
Ammeter
Voltmeter
22
Q

What is present in a Discharging Circuit of a Capacitor?

A

Resistor (to create load)
Capacitor
Ammeter
Voltmeter

23
Q

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

A

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

24
Q

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

A

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

25
Q

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

A

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

26
Q

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

A

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

27
Q

What is the gradient of a Charge-Time graph?

A

Current

28
Q

What is the area under a Current-Time graph?

A

Charge

29
Q

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

A

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

30
Q

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

A

Discharge time is unaffected

31
Q

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

A

Discharge time is doubled

twice as long to remove twice the amount of charge

32
Q

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

A

Discharge time is doubled

as half the current, half the rate of charge

33
Q

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

A

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

Q(0) = maximum charge

34
Q

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

A

V=V(0) x e^(-t/RC)

V(0) = maximum p.d

35
Q

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

A

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

I(0) = maximum I

36
Q

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

A 
Q = Q(0) x (1-e^(-t/RC))
Q(0) = Maximum Charge
37
Q

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

A 
V = V(0) x (1-e^(-t/RC))
V(0) = Maximum p.d
38
Q

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

A 
I = I(0) x (1-e^(-t/RC))
I(0) = Maximum Current
39
Q

What is the Time Constant?

A

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

40
Q

What is 1/e roughly equal to?

A

0.37

41
Q

What is the relationship between Half Life and Time Constant?

A

T(0.5) ≈ 0.69τ

42
Q

What is the Units and Symbol for Time Constant?

A

Symbol: τ
Units: ΩF or s

A Level Physics (16 decks)

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