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Physics Paper 2 > Capacitors > Flashcards

Capacitors Flashcards

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1
Q

What is capacitance

A

Charge stored by a capacitor per unit potential difference

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2
Q

Capacitance formula

A

C = Q / V

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3
Q

What is a capacitor

A

Electrical component that stores charge

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4
Q

What are capacitors made up of

A

2 conducting parallel plates with a gap between them, may be seperated by inulsating material called a dielectric

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5
Q

How do capacitors cause uniform electric fields

A

When capacitor is connected to a source of power, opposite charge builds up on the two parallel plates, causing an electric field to be formed

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6
Q

What is the permittivity of a dielectric

A

Measure of a materials ability to store an electric field in the material

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7
Q

What is the relative permittivity of a dielectric

A

Also known as dielectric constant of the material, used to calculate capacitance of capacitor, ration of permittivity of dielectric to permittivity of free space

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8
Q

Dielectric constant formula (ratio)

A

Er = E / E0

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9
Q

Er = E / E0 - what is Er

A

Relative permittivity

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10
Q

Er = E / E0 - what is E

A

Permittivity of dielectric

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11
Q

Er = E / E0 - what is E0

A

Permittivity of free space

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12
Q

Capacitance formula using area of plates

A

C = AE0Er / d

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13
Q

C = AE0Er / d what is A

A

Area of plates

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14
Q

C = AE0Er / d what is d

A

Distance between plates

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15
Q

C = AE0Er / d what is E0

A

Permittivity of free space

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16
Q

C = AE0Er / d what is Er

A

Relative permittivity

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17
Q

How to find electrical energy stored by capacitor on a Q-V graph

A

Area under graph

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18
Q

Link between charge and potential difference on capacitors

A

Potential difference is directly proportional to charge

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19
Q

Q-V graph line

A

Straight line, through origin

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20
Q

Electrical energy stored, charge and potential difference formula

A

E = (1/2)QV

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21
Q

Electrical energy stored, charge and capacitance formula

A

E = Q^2 / 2C

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22
Q

What does a capacitor need to be connected to in order to charge

A

Power supply and resistor

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23
Q

What is the gradient on a Q-t graph

A

Current

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24
Q

Shape of I-t graphs for a charging capacitor

A

Decreasing exponentially, l shape - area underneath is the charge

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25
Shape of V-t graphs for a charging capacitor
Increasing exponentially, r shape, asymptote above x-axis
26
Shape of Q-t graphs for a charging capacitor
Increasing exponentially, r shape, asymptote above x-axis, gradient = current
27
How to find charge on an I-t graph
Area under graph
28
How to capacitors charge
When connected to power supply, current flows, negative charge builds up on plate connected to negative terminal, on opposite plate, electrons are repelled by negative charge built up so electrons flow to positive terminal and equal but opposite charge formed on both plates creates a pd, as charge across plates increases the pd increases but electron flow decreases due to force of electrostatic repulsion also increasing, so current decreases and eventually reaches 0
29
What is used to measure the capacitance
Charge stored, pd between plates
30
What is the capacitance
Charge stored per volt
31
What are the units for capacitance
Farads - usually measured in microfarads
32
What are dieletrics formed of
Polar molecules
33
What are polar molecules
Molecules that are slightly positively charged on one side and slightly negatively on the other
34
How doe polar molecules act when there is no electric field
Arranged in random directions
35
Where are polar molecules found in capacitors
The insulator
36
How do polar molecules move in electric fields
Rotate until they rest in line with the field - negative ends go towards positive plate and vice versa
37
What is it called when all polar molecules rest symmetrically
Polarised
38
Relationship between permittivity and capacitance
Directly proportional
39
Relationship and explanation between area and capacitance
Directly proportional - electrons spread out, less repulsion, so more electrons fit on
40
What is the dielectric
Insulating material that polarises in the presense of an electric field
41
Charge breakdown of polarised dielectric - what is it equivalent to
One side positive, one side negative, middle is neutral - equal to 2 plates (1 -ve and 1 +ve)
42
What happens after the dielectrics are polarised
A secondary electric field is created around each molecule in the opposite direction to the first - this weakens the first field, so less potential difference is required to charge the capacitor, causing capacitance to increase
43
What is permittivity
How easy it is for a dielectric to polarise
44
How does permittivity influence the primary electric field
Higher permittivity weakens the electric field (polarised = second field)
45
How to calculate the relative permittivity
Permittivity of material / permittivity of free space
46
How is a potential difference created across a capacitor when connected to a power supply
Current starts to flow, negative charge builds up on plate connected to negative terminal, due to this electrons are repelled from other plate, so electrons move to positive terminal and an equal and opposite charge is formed on each plate, hence creating a potenital difference
47
How the charge of a capacitor increasing effects the current
Charge increases so pd increases but electron flow decreases due to electrostatic force increasing, so current decreases and eventually reaches 0
48
How to discharge a capacitor through a resistor
Must connect it to a closed circuit with just a resistor
49
What is the shape if an I-t graph for a discharging capacitor
Decreasing exponentially
50
What is the shape if an V-t graph for a discharging capacitor
Decreasing exponentially
51
What is the shape if an Q-t graph for a discharging capacitor
Decreasing exponentially
52
Why do the current, charge and pd all fall exponentially when a capacitor is discharging
Current flows in opposite direction, will take the same amount of time for all the values to half
53
Capacitor charging formula for current
I = I_0 x e^(-t / RC)
54
I = I_0 x e^(-t / RC) what is I
Current at a given time
55
I = I_0 x e^(-t / RC) what is I_0
Intial current
56
I = I_0 x e^(-t / RC) what is t
Time
57
I = I_0 x e^(-t / RC) what is R
Reseistance of the circuit
58
I = I_0 x e^(-t / RC) what is C
Capacitance
59
Capacitor charging formula for potential difference
V = V_0 (1 - e^(-t / RC))
60
Capacitor charging formula for charge
Q = Q_0 (1 - e^(-t / RC))
61
Capacitor discharging formula for current
I = I_0 x e^(-t / RC)
62
Capacitor discharging formula for potential difference
V = V_0 x e^(-t / RC)
63
Capacitor discharging formula for charge
Q = Q_0 x e^(-t / RC)
64
What is RC
Time constant
65
What is the time constant equal to
Time to discharge a capacitor to 37% of its initial value (charge, current or voltage) or to charge to 63% of its initial value (charge or voltage)
66
How to find time constant using lnQ
Gradient of lnQ-t graph is -1 / RC so RC is -1 / gradient
67
Time to half when discharging formula
T_1/2 = 0.69RC
68
When does a capacitor stop charging
When the pd across the plates is equal to the pd of the battery
69
What are the 3 factors affecting capacitance
Permittivity, area, distance
70
Potential difference, electric field and distance between plates formula
Electic field = voltage / distance
71
Explanation for distance and electric field strength relationship
E = V/D - E stays the same, D increases so V increases to, C = Q/V Q stays the same, V increases, so C increases to
72
Relationship between distance and capacitance
Indirectly proportional
73
Relative permittivity symbol formula
ɛ_r = ɛ/ɛ_0
74
Charge / time =
Current
75
Formula for charge stored against time when charging a capacitor
Q = Q_0 (1-e^-t/RC)
76
How much charge is stored in a capacitor at the time constant whilst charging
0.63
77
Relationship between voltage across capacitor and charge stored
Proportional

Physics Paper 2 (7 decks)

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