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Physics A - Level > Fields (3): Capacitors > Flashcards

Fields (3): Capacitors Flashcards

1
Q

What is the definition of a capacitor?

A

A device designed for storing charge (controls the variable flow of charge)

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2
Q

Describe the set up of a capacitor

A
  • Two conducting parallel plates connected to a battery
  • Insulating material between them (sometimes a dielectric)
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3
Q

The charges stored on the plates of the capacitor are…?

A

Equal but opposite (+Q and -Q)

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4
Q

What is the circuit symbol of a capacitor?

A

Two separated parallel lines of the same length

  • Like a battery but lines are the same length
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5
Q

Which side is the positive side of a cell when drawn as its circuit symbol?

A

Longer side - +ve -> electrons flow into this side
Shorter side - -ve -> electrons flow out of this side

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6
Q

What is the direction of conventional current?

A

+ve -> -ve (the opposite way of the flow of electrons)

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7
Q

How can you measure the potential difference across a capacitor in a circuit?

A

Attach a high resistance voltmeter in parallel

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8
Q

How can you investigate the change in capacitor pd with respect to time for a constant current?

A

In series place a:
- capacitor
- cell/battery/powerpack
- switch
- Ammeter
- Variable resistor

In parallel to the capacitor place a HIGH RESISTANCE VOLTMETER or a DATALOGGER (for more precise readings) in order to record voltage readings at measured times

NB: The variable resistor is used in order to keep the current constant

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9
Q

How do you calculate the charge stored by a capacitor in a measured time when current is constant?

A

Q = It

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10
Q

If you plotted a graph of charge against voltage for a capacitor of constant current, what relationship would you observe?

A

Directly proportional

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11
Q

What is the definition of Capacitance, C?

A

The charge stored per unit of potential difference

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12
Q

What is the general equation for capacitance?

A

C = Q/V

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13
Q

What are the units for capacitance?

A

Farads (normally micro x10^-6, nano x10^-9 or pico x10^-12)

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14
Q

What are 3 uses of capacitors?

A

Back-up power supplies - When the mains supply is interrupted

Smoothing circuits - Circuits that smooth out unwan ted variations in voltage

Timing circuits - Circuits that switch on or off automatically after a preset delay

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15
Q

Describe how a capacitor is charged

A

When connected to a DC power source there is a current as the supply draws electrons from one plate to the other. This leaves one plate with a +Q charge and the other with a -Q charge so that charge stored by the capacitor is Q. Current flows until the potential difference across the capacitor is the same as the emf across the power source.

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16
Q

What is formed between the two plates of a capacitor when it is charged?

A

A uniform electric field

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17
Q

How is energy stored in a capacitor?

A

As electric potential energy

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18
Q

Describe a general set up for charging and then discharging a capacitor?

A

In parallel place a:
- Voltage supply
- Capacitor
- torch bulb/joulemeter/high resistance resistor (whatever you are measuring or showing)

Place a switch in the lop of the capacitor that can switch between the voltage supply loop to charge and the other loop to discharge

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19
Q

Is work done when electrons are forced around a circuit and onto the capacitor plates?

A

YES

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20
Q

What does the graph of potential difference against charge look like for a capacitor? Why?

A

Directly proportional relationship

work done/energy transferred = VQ
- Q and V are proportional

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21
Q

What is the area under a Q against V graph of a capacitor representative of?

A

Total energy stored by the capacitor

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22
Q

What are the other equations for the energy stored by a capacitor, derived from E = 1/2QV?

A

C = Q/V
Q = VC
therefore, E = 1/2V^2C

V = Q/C
therefore, E = 1/2Q^2/C

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22
Q

What is the equation for the energy stored by a capacitor used the graph of Q against V?

A

E = 1/2QV

23
Q

How would you determine the energy stored in a capacitor?

A

In parallel place a:
- Voltage supply
- Capacitor
- torch bulb/joulemeter

Place a switch in the loop of the capacitor that can switch between the voltage supply loop to charge and the other loop to discharge

Record the initial pd and joulemeter reading before the discharge starts -> once discharged record the joulemeter reading again -> The difference between the joulemeter readings is the total energy stored by the capacitor before it was discharged

24
Q

What situation acts like a capacitor in day to day life?

A

A thundercloud and the earth

25
Q

What type of graph does the decay/discharging of a capacitor follow through a fixed resistor?

A

exponential

26
Q

Describe the graph of current against time as a capacitor charges

A

Initially current flowing around the capacitor circuit will decrease rapidly as the flow of electrons from one plate to another is easy and uncontested. However, as time progresses the current will tend towards 0 as the electrostatic repulsion increases between electrons and more work is required to push them onto the negatively charged plate

27
Q

Describe the graph of voltage/charge against time as a capacitor charges

A

Initally, there is a rapid increases in the charge and voltage across the capacitor. However, as time progresses the change in voltage/charge per unit of time will decrease as it tends towards the emf across the power supply/ no more electrons can flow

28
Q

What does the gradient of a Q against time curve of a capacitor tell you?

A

The current at that point

29
Q

What are the equations for Q and V for a capacitor charging?

A

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

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

30
Q

What is the equation for I for a capacitor charging?

A

I = I0 x e^(-t/RC)

31
Q

What is the equation that proves that a small fractional drop of (delta)Q/Q when discharging is the same for any short time period?

A

(delta)Q/Q = -(delta)t/RC

32
Q

How do you prove (delta)Q/Q = -(delta)t/RC?

A

C = Q/V
V = Q/C = IR
I = -Q/RC = (delta)Q/(delta)t

rearrange
(delta)Q/Q = -(delta)t/RC

33
Q

When discharging what direction is the current?

A

Negative/ opposite to charging

34
Q

What do the graphs of Q, I and V look like when discharging?

A

They are all exactly the same and they exponentially decay to 0

35
Q

What is the definition of the time constant, RC?

A

The time taken for the discharge charge/voltage/current to fall to 0.37 of its initial value

36
Q

What is 0.37 equal to?

A

e^-1

37
Q

What do R and C stand for in time constant, t = RC?

A

R = resistance of the circuit
C = capacitance of the capacitor

38
Q

How can you calculate the time constant from a discharging graph?

A
  • Draw across from 0.37Q/I/V
  • Draw down to the time = time constant for that specific set up
39
Q

If you plot a graph of ln(Q/I/V) against time what is the gradient of the graph?

A

-1/RC

40
Q

What does Q0/V0 stand for in the capacitor charging equation?

A

The maximum charge or voltage a capacitor can have when fully charged

41
Q

What is the definition of RC when charging a capacitor?

A

The time taken for the Q/V to be 0.63 of its initial value

42
Q

How would you determine a capacitor discharge graph?

A

In parallel place a:
- Voltage supply
- Capacitor
- High resistance fixed resistor

Place a switch in the loop of the capacitor that can switch between the voltage supply loop to charge and the other loop to discharge

Place an oscilloscope/data logger/digital voltmeter across the fixed resistor and record values of Q/I/V for measured times

You can then compare the graph value or RC against the calculated value of RC

43
Q

What can be placed between the plates of a capacitor to increase the charge stored/capacitance?

A

A dielectric

44
Q

What materials are dielectric typically?

A

Insulating materials e.g polythene and waxed paper

45
Q

Describe how dielectrics work to increase the capacitance of a capacitor

A
  • When a dielectric is placed between the plates of a capacitor and it is charged the molecules in the dielectric become polarised due to the uniform electric field
  • This causes the the molecules to rotate and align with the electric field as the negative ends of the molecules are attracted to the positive plate and vice versa

2 WAYS OF THINKING:
1: As a result more charge is stored as the positive side of the dielectric attracts more electrons onto the negative capacitor plate and the negative side of the dielectric pushes electrons back to the battery from the positive plate. Therefore there is an increase in the charge stored by the capacitor THEREFORE CAPACITANCE INCREASES.

2: The molecules each have their own electric fields which, in this alignment, now opposes the applied electric field of the capacitor. This reduces the overall electric field which reduces the potential difference required to charge the capacitor. THEREFORE CAPACITANCE INCREASES.

46
Q

Why are some capacitors bad?

A

As electrons are able to move form one plate to the other - the gap in between the plates conducts electricity so current can flow and charge is not sotred on the plates

47
Q

What is the definition of permittivity of a dielectric?

A

The ability of a dielectric to store an electric field

48
Q

What is the definition of relative permittivity?

A

The ratio of the charge stored with the dielectric to the charge stored without the dielectric

49
Q

What is the equation for relative permittivity?

A

epsilon(r) = Q/Q0

50
Q

What units does relative permittivity have?

A

No units as it is a ratio

51
Q

What is another equation for relative permittivity?

A

epsilon(r) = C/C0

52
Q

What is the relative permittivity of a substance also known as?

A

Its dielectric constant

53
Q

What is the equation for capacitance when a dielectric is used?

A

C = Aepsilon(0)epslion(r)/d

epsilon 0 - absolute permittivity of free space
A - surface area of each plate
d - the distance between the plates

54
Q

What are the ways that you can maximise the capacitance of a capacitor?

A
  • Surface area as large as possible
  • Distance between the plates as small as possible (without conduction)
  • Dielectric permittivity as large as possible

Physics A - Level (22 decks)

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