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Physics 2 - Mechancis, materials and waves > Capacitance > Flashcards

Capacitance Flashcards

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1
Q

What is a capacitor’s use

A

to store charge/cause a current for a specific time

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2
Q

What is the circuit symbol for a capacitor

A

–| |–

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3
Q

Structure of capacitor

A

two parallel metal plates with a small gap between

Most capacitors consist of two strips of aluminium foil separated by a layer of dielectric, all rolled up and put into a casing making the capacitance as large as possible because the area A is as large as possible and the spacing d is as small as possible.

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4
Q

What happens when a capacitor is connected to a battery in terms of electric flow

A

When a battery is connected to the plates, electrons from the negative terminal of the battery move onto the plate connected to that terminal. At the same time, electrons move from the other plate to the positive terminal of the battery leaving this plate with a positive charge. The two plates store equal and opposite amounts of charge

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5
Q

For a capacitor, storing a charge of Q means

A

+Q on one plate and –Q on the other

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6
Q

Uses of capacitors:

A

back up power supplies (which take over if the mains supply is interrupted), tuning circuits, touchscreen technology, camera flash

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7
Q

Work done in a capacitor =

A

W=QV

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8
Q

Why is the work done different to to the energy stored

A

Only half of energy transferred from battery (W = QV) is stored in capacitor.
The rest is wasted due to the resisitance in the circuit and is dissipated in the surroundings.

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9
Q

Doubling the charge does what to the energy stored

A

doubling the charge will double the voltage and as energy = 0.5CV^2 the stored energy will increase by a factor of 4

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10
Q

Describe the energy stored in a thundercloud

A

ground and cloud act like parallel plates d apart, ground positive, cloud negative, strong electric field of strength, E, between the two. Pd between cloud and ground V=Ed. Energy stored = 0.5QV = 0.5QED, higher clouds have more energy since work is done by the force of the wind overcoming the attraction between cloud and ground

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11
Q

How can you make the rate of charge/discharge slower

A

use a resistor

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12
Q

RC is the time constant (units = s) and occurs when

A

any of the quantities Q, V or I falls to 0.37 = 1/e of its initial value

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13
Q

Take taken for the initial value of V, Q or I to halve =

A

0.69RC

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14
Q

If the gap between the two parallel plates of a capacitor is filled with a dielectric, then

A

the capacitor can store more charge and has a higher capacitance

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15
Q

A dielectric is

A

Dielectrics are electrically insulating materials that increase the ability of a parallel-plate capacitor to store charge when it is placed between the plates of the capacitor e.g. Polythene

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16
Q

Give examples of dielectrics

A

polythene and waxed paper

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17
Q

What happens to each molecule in a dielectric

A

they become polarised

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18
Q

What happens to the electrons of the molecule in a capacitor

A

pulled towards positive plate

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19
Q

Negative charge is attracted to

A

the surface of dielectric facing the positive plate

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20
Q

Why is the surface of the dielectric left positively charged

A

The surface of the dielectric facing the negative plate loses negative charge and some positive charge is left on its surface

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21
Q

The relative permittivity, or dielectric constant of the dielectric, epsilon r =

A 
The charge stored with dielectric, Q
/
The charge stored without dielectric, Q0
= Q/Q0
=C/C0

Relative permitivity = charge stored by a parallel-plate capacitor when the space between the plates of the capacitor is completely filled with the dielectric substance, and Q0 = charge stored at the same pd when the space is completely empty. C, C0 also works.

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22
Q

For a parallel plate capacitor with dielectric filling the space between the plates, its capacitance C

A

e0 * er * A / d

d=plate spacing
A=surface area of plate/common area of the plates
er = dielectric constant
e0 = 8.85x10^-12 (F/m)

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23
Q

For large capacitance, what is needed:

A

Large A
Small d
Dielectric with large εᵣ

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24
Q

What are polar molecules

A

Molecules in a dielectric substance that are already polarised, but lie in random directions

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25
Q

When polar molecules are placed between the two charged plates, what happens and what does this cause

A

These molecules turn when the dielectric is placed between the charged plates because their electrons are attracted slightly to the positive plate. The surface of the dielectric near the positive plate gains negative charge, and the other surface gains positive charge

As a result, more charge is stored on the plates because the positive side of the dielectric attracts more electrons from the battery onto the negative plate and the negative side of the dielectric pushes electrons back to the battery from the postitive plate

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26
Q

In an alternating electric field, at low frequencies, the polarisation is…
bur as the frequency increases, polarisation… because of

A

In an alternating electric field, at low frequencies, the polarisation is in phase with the field, but as the frequency increases, polarisation ceases to work due to the inertia of the particles involved and the resistive forces that oppose the motion of the dipoles.

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27
Q

Give an example of a use of capacitor discharge through a fixed resistor

A

any electronic timing circuit or time-delay circuit

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28
Q

What do dielectrics do

A

increase the charge stored in a capacitor for any given pd across the capacitor terminals i.e. its effect is to increase the capacitance of the capacitor

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29
Q

For a parallel plate capacitor with dielectric filling the space between the plates how is a large capacitance achieved

A

making the area A as large as possible, making the plate spacing d as small as possible, filling the space between the plates with a dielectric which has a relative permittivity as large as possible.

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30
Q

Charge discharge circuit

A

Capacitor centre with two way switch to the left

Battery above with voltmeter

Bulb with joulemeter below

Switch towards battery for charge, switch towards bulb for discharge through resistor?

Resistor is used to slow charge/discharge down but must be put in right place for each different type

Energy is released from the capacitor as shown in the above diagram.
The capacitor is charged by the battery.
It is then discharged through the bulb.
This energy can be measured using a joulemeter across the bulb

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31
Q

A couple consists of

A

two conductors insulated from each other

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32
Q

Graph of charge against voltage is

A

Q=CV
y=mx
straight line

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33
Q

When a capacitor is charged, how is energy stored

A

energy is stored in it because electrons are forced onto of its plates and taken for the other plate. The energy is stored in the capacitor as electrical potential energy

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34
Q

Energy stored in a capacitor =

A

0.5QV = 0.5CV^2 = 0.5Q^2/C = area under charge against pd graph

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35
Q

A joulemeter is used to

A

measure the energy transfer from a charged capacitor when the capacitor discharged. Measured before and after and the difference is the energy transferred from the capacitor during the discharge process.

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36
Q

Milli m
Micro mu
Nano n
Pico p

A

Milli 10^-3
Micro 10^-6
Nano 10^-9
Pico 10^-12

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37
Q

Why cannot a steady dc current pass through a capacitor

A

Because the dielectric is an insulator

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38
Q

Capacitance unit =

A

Farad (F)

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39
Q

Relative permittivity, er is usually between

A

2 and 10 but is 1 for air/vacuum

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40
Q

Capacitors in parallel and series

A

Parallel: Ct = C1 + C2
Series: 1/Ct = 1/C1 + 1/C2

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41
Q

The time for Q, V and I to fall to half their original values is given by

A

RC*ln2

42
Q

Between the plates of the capacitor…

A

no charge flows

43
Q

Charge –Q flows onto the plate connected to the negative terminal of the supply and

A

flows off the place connected to the positive terminal of the supply, leaving it with charge +Q

44
Q

Why is the total charge of a capacitor actually zero, and why is there energy

A

As the capacitor plates have equal amounts of charge of the opposite sign, the total charge is actually zero. However because the charges are separated they have energy and can do work when they are brought together

45
Q

The amount of energy stored in a capacitor depends on its

A

the amount of charge on the capacitor plates. The voltage required to place this charge on the capacitor plates, i.e. the capacitance of the capacitor.

46
Q

The time constant represents

A

time it takes for the charge on a capacitor to fall to 1/e of its initial value when a capacitor is discharging. The time it takes for the charge on a capacitor to rise to 1-1/e of its final value when the capacitor is charging

47
Q

Give two reasons why a capacitor is not a suitable source for powering a cordless telephone

A

capacitor would be impossibly large (to fit in phone), capacitor would need recharging very frequently, capacitor voltage would fall continuously while in use

48
Q

Charging capacitor, what happens from t=0 until fully charged in terms of electron flow

A

net electron flow is round the circuit from right hand side of capacitor clockwise to left hand side. Rate of electron flow or current decreases as time increases (repulsion from electrons already on left hand side makes flow decrease), current ceases when pd across the capacitor = emf, current decreases exponentially

49
Q

Charging capacitor, what happens from t=0 until fully charged in terms of potential differences across the capacitor and resistor

A

pd across capacitor increase, pd across resistor decreases, sum of pd of resistor and capacitance = emf, capacitance increases exponentially with time to the emf, resistor voltage decreases exponentially with time to 0V

50
Q

Conducting sphere in a capacitor with P1 the positive plate, p2 the negative plate why does the sphere shuttle between the two plates

A

sphere gains electrons (or is charged –ve) at p2 and is repelled by p2 (or attracted by p1 or experiences correct force in field), sphere loses electrons at (or negative charge) P1, explanation of return of sphere from p1 to p2, sphere reaches same potential as plate on contact

51
Q

Gradient of Q against t curve =

A

Current as Q/t = I

52
Q

Discharging capacitor graphs against time for

Current
Voltage
Charge

A

All exponential with decreasing gradients from y axis tending towards zero cutting the y axis at maximum value

53
Q

Charging capacitor graphs against time for

Current
Voltage
Charge

A

Current is same as discharging, decreasing gradient from y axis tending to zero

Voltage and charge is inverted, starts from zero and asymptope at supply charge/voltage

time constant occurs when V rises to 0.63 of its initial value

54
Q

Explain the shape of the current time graph

A

As capacitor is charged there is more charge so Q increases. It is then harder to find places to store charge so electrons do not flow as fast. So current equals charge flowing per second decreases and the amount of rate that charge is added to the capacitor decreases

55
Q

Charging equations

A 
V = Vₒ(1-e^-t/RC)
Q = Qₒ(1-e^-t/RC) 
I = Iₒ(e^-t/RC)
56
Q

Discharging equations

A 
V = Vₒ(e^-t/RC)
Q = Qₒ(e^-t/RC) 
I = Iₒ(e^-t/RC)
57
Q

Charging capacitor energy/time exponential

A

-1/x

58
Q

When t=RC the charge falls to

A

37% of its initial value or 63% of its final value

59
Q

The time taken to discharge by over 99% is equal to

A

t=5RC

60
Q

Equipment for charging a capacitor at constant current

A

Ammeter, switch, high resistance voltmeter, variable resistor (to keep current constant)

61
Q

Capacitor discharge equipment

A

voltmeter, resistor, switch, capacitor

62
Q

Define capacitance

A

Capacitance = charge stored per unit potential difference = ratio of the magnitude of the charge on one plate to the potential difference across the capacitor = Q/V

63
Q

What is the role of a capacitor

A

to store energy/charge and release it/cause a current when needed

64
Q

What is an advantage to the capacitor over batteries

A

they charge much better

65
Q

What is the composition of capacitors

A

two metal plates separated from each other by an insulating, dielectric material. A wire from each plate connects the capacitor to the circuit

66
Q

Why are the two metal plates in a capacitor separated by a dielectric material

A

so electrons do not flow between them

67
Q

What is meant by permittivity

A

How hard it is for an electron to pass through it

68
Q

What reduces the strength of an electric field and what does this mean

A

Materials reduce the strength of the electric field meaning more charge can be stored

69
Q

What happens when another material is put in between the plates of a capacitor

A

the capacitance charges. Since the dipole field in the dielectric tends to partially cancel the capacitors field, the net field is reduced.

70
Q

When a charge of Q coulombs is transferred to the capacitor, it means that

A

one capacitor plate will have a charge of +Q coulombs while the other plate has charge –Q coulombs. The charge stored by the capacitor is Q coulombs

71
Q

Dielectric action in the capacitor, or capacitance, is equal to Ae0er/d where d is the perpendicular diameter, er is the dielectric constant =

A

permittivity of the dielectric material in between the plates, e0 is permittivity of free space and A is the area of the smallest capacitor

72
Q

Why does the voltage across a capacitor decrease with time

A

Electrons slow down due to repulsion so the rate of flow decreases therefore the voltage across the capacitor decreases with time

73
Q

What way do electrons flow around a capacitor

A

positive terminal to negative terminal

74
Q

Explain the Voltage time graph for a charging capacitor

A

asymptope at batteries voltage

75
Q

Define the volt

A

energy per unit charge so 1V = 1J/C

76
Q

When a capacitor is fully charged the pd =

A

Vs

77
Q

When a capacitor begins to charge the pd across the plates is

A

zero

78
Q

Derivation of 0.5QV = E

A

Q on x axis, E=QV, area under graph = total energy = 0.5Qv

79
Q

The rate at which electrons leave the capacitor is dependent on

A

how many electrons on the capacitor

80
Q

At which point during the discharging process is the capacitor losing charge

A

at them smallest rate: when the pd across R is least

81
Q

Explain why there is an increase in the energy stored by the capacitor when er is reduced

A

Energy increases because in the polar dielectric molecules align in the field with positive charged end toward the negative plate and so work is done on the capacitor separating the positively charged surface of the dielectric from the negatively charged plate (or vice versa)

82
Q

Charging circuit equipment

A

battery, switch, capacitor, voltmeter, resistor, rectangle with battery on the left, switch at top, capacitor on right, resistor on bottom and voltmeter in parallel to capacitor.

83
Q

Discharging circuit

A

resistor moved in between the capacitor and voltmeter

84
Q

Explain the charging capacitor circuit

A

As soon as the switch is closed towards the battery, the battery is connected across the capacitor, current flows, and the potential difference across the capacitor begins to rise but, as more and more charge builds up on the capacitor plates, the p.d. across the capacitor equals that of the supply voltage. The capacitor is then fully charged.

85
Q

Charge discharge circuit how they are changed to each other

A

battery on the left, capacitor in the middle, resistor on the right. For charging: switch above capacitor towards the battery to the left while for discharging the switch is towards the resistor to the right

86
Q

Explain the discharging capacitor circuit

A

As soon as the switch is put to the resistors side, a large current starts to flow and the potential difference across the capacitor drops. As charge flows from one plate to the other through the resistor the charge is neutralized and so the current falls and the rate of decrease of potential difference also falls. Eventually the charge on the plates is zero and the current and potential difference are also zero – the capacitor is fully discharged. Note that the value of the resistor does not affect the final potential difference across the capacitor – only the time it takes to reach this value. The bigger the resistor, the longer the time taken

87
Q

VIQ /t graphs asymptopes and axes

A

Current - zero
Charge - discharging = 0, charging = Qs
Voltage - discharging = 0, charging = Vs

VQ charging cuts at zero

IVQ charging cut at IVQmax

88
Q

Explain the shape of the current time graph for a capacitor

A

When a capacitor discharges through a fixed resistor, the discharge current decreases gradually to zero. The pd across the capacitor decreases as it loses charge so the current decreases gradually. Because the resistors connected directly to the capacitor, the resistor current decreases as the pd decreases

89
Q

Quantity decreases by the same factor in equal intervals of time e.g. if the charge is Q0 after t seconds then it is … after nt seconds

A

x^nQ0

90
Q

In an alternating electric field what happens to dipoles

A

the polar dipoles rotate and the non-polar dipoles oscillate one way then the opposite way as the field strength increases and decreases

91
Q

At low frequencies what happens to dipoles in an alternating electric field

A

At low frequencies, the three polarisation mechanics alternate in phase with the field at low frequency. However, as the frequency increases, each mechanism ceases to work due to the inertia of the particles involved and the resistive forces that oppose the motion of the dipoles.

92
Q

The mass of the particles being moved by the field determines

A

which mechanism ceases first as the frequency increases.

93
Q

The greater the mass of each particle

A

the greater its inertia is, and so the frequency at which ceases first as the frequency increases, then ionic polarisation, then electronic polarisation

94
Q

When a dielectric is placed between two oppositively charged parallel plates connected to a battery what happens and what is its implication

A

each molecule of the dielectric becomes polarised. This means that its electrons are pulled slightly towards the positive plate so the surface of the dielectric facing the positive plate gains negative charge at the expense of the other side of the dielectric that faces the negative plate. The other surface of the dielectric loses negative charge so some positive charge is left on the surface

95
Q

What is the voltage in the circuit at any instant

What is the initial current

A

instant during the charging process, the source pd V0 = the resistor pd + the capacitor

Hence V0 = IR + Q/C at any instant.

The initial current I0 =V0/R assuming the capacitor is initially uncharged

96
Q

Explain why the vt curve for a charging capacitor is shaped like that

A
  • When a capacitor is charged by connecting it to a source of constant pd, the charging current decreases as the capacitor charge and pd increases.
  • When the capacitor is fully charged, its pd is equal to the source pd and the current is zero as no more charge flows in the circuit.
  • The capacitor charge builds up until the capacitor pd V is equal to the source pd V0.
97
Q

The current at any stage is equal to

A

the rate of change of charge

Therefore the current is given by the gradient of the charge-time graph and so it decreases exponentially.

98
Q

Explain the shape of a charge time graph for a charging capacitor

A

In a DC circuit a capacitor functions by the removal of some electrons on one plate and the addition of electrons on the other plate. The electrons are moved through the wire attached to the plates, and this is driven by a battery or a DC power supply. As more and more electrons are moved, it becomes harder and harder to move more, and the maximum charge that can be placed on each plate is approached over time. Once “charged” in this way, a capacitor stores energy in the electric field between the plates, and this energy can later be used in many different ways as the capacitor is discharged.

99
Q

Why does a capacitor allow AC current but not DC current

A

Direct current can’t jump the gap between plates, because it would take a massive amount of voltage to force the electron to jump the gap between plates. The electrons hit the plate and stop.

Alternating current, on the other hand, is moving the electrons back and forth in place – so the plate on one side of the capacitor is constantly having electrons pushed in and then pulled back out. This motion creates a small electric field which induces the same alternating current in the other plate, because electric fields can jump the gap between plates.

100
Q

Permitivity of free space

A

8.85*10^-12 F/m

Physics 2 - Mechancis, materials and waves (10 decks)

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