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A Level Physics Paper 2 > Electric Fields > Flashcards

Electric Fields Flashcards

1
Q

What is an electeic field

A

A region in which a charged body will experience a force

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2
Q

Like charges

A

Repel

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3
Q

Charges … electric fields and charges … with electric fields

A

Create

Interact

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4
Q

Opposite charges

A

Attract

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5
Q

Electric field lines

A

Show the direction of force that would be experienced by a small positive test charge if it were placed at that point on the field
Graphical representation of direction and relative strength of a field with arrows indicating direction
Closer means stronger force

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6
Q

Equipotentials

A

Perpendicular to field lines
Join points with equal potential
No work done on a test charge if it moves along an equipotential

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7
Q

Coulomb’s law of attraction

A

The force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of their separation

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8
Q

Express Coulomb’s law of attraction mathematically

A

F=1/4piĘ0 x Q1Q2/r^2

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9
Q

What does the coulomb charge depend on

A

The signs of the two charges

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10
Q

What is the permittivity of free space

A

A measure of how easily an electric field passes through a vacuum
Farad per meter as the unit

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11
Q

Electric field strength units

A

V/m

N/C

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12
Q

Absolute potential energy

A

Work done in moving a charged object from infinity to that point in an electric field

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13
Q

If the force is attractive then what happens to the potential energy

A

Decreases
More negative
Get closer

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14
Q

If a force is repulsive what happens to the potential energy

A

Increases
Less negative
Move away

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15
Q

Electric potential energy for two objects at infinite separation

A

Zero

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16
Q

Equation for electric potential energy

A

Ep=1/4piĘ0 x Q1Q2/r

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17
Q

Equation for electric potential energy

A

Dd

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18
Q

How do you handle signs for potential energy

A

Keep in equation

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19
Q

What happens when a positive charge is brought towards another positive charge

A

Charges repel
Work has to be done to overcome repulsion
Potential energy increases
So two positive charges have a positive potential energy

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20
Q

What happens if two negative charges are brought towards eachother

A

Charges repel
Work must be done to overcome this repulsion
So potential energy increases
So two negative charges have positive potential energy

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21
Q

What happens a positive charge is brought towards a negative charge or vice verse

A

Charges attract
So no work has to be done to bring them together
Instead kinetic energy of charges will increase as they move closer
And potential energy decreases from zero to be more negative
So two unlike charges have negative potential energy

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22
Q

Absolute electric potential

A

The work done per unit charge on moving a small positive test charge from infinity to that point in the field

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23
Q

Units fir electric potential

A

J/C

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24
Q

Electric potential equation

A

V=1/4piĘ0 x Q/r

Where Q is the charge of the thing creating the field

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25
Positive value for change in potential
Work has been done on the charge to move it from its initial to its final position
26
What does a negative value of potential mean
The field does work on the charge and it moves on its own
27
Force and separation ratio
F1r1^2=F2R2^2
28
Field strength and separation ratio
E1r1^2=E2r2^2
29
Potential and separation ratio
V1r1=V2r2
30
Potential energy and separation
Ep1r1=Ep2r2
31
Area under a force separation graph
Work done moving between two points W=Fs
32
Area under a field strength separation graph
Change in potential between two points E=-V/r
33
Gradient of a potential separation graph
Magnitude of field strength at that point E=-V/r
34
Gradient of a electric potential energy separation graph
Force acting at that point F=W/s
35
Explain the force and field strength inside a hollow conducting sphere and the potential and potential energy
Force is zero Field strength is zero Potential is constant Potential energy is constant 9Since no further work is done in bringing the charge from infinity once you get past the surface of the sphere)
36
Similarities between electric and gravitational fields
Force and field strength are inversely proportional to the separation squared Potential and potential energy are inversely proportional to separation
37
Differences between electric and gravitational fields
Gravitational fields are always attractive but electric can be attractive or repulsive Gravitational potential/potential energy is always negative but electric potential/potential energy can be positive or negative G is the constant for gravitational fields whereas e0 is the constant for electric fields Electric fields usually more important on a small scale in atoms and nuclei but gravitational is usually more important on a larger scale with respect to planets and galaxies
38
How did Millikan conduct an experiment to measure the charge of an electron
Adjusted the potential difference across two parallel plates | To make a charged oil drop hover in the uniform electric field between the plates
39
How does field strength vary between two parallel plates
Doesn't Same everywhere Uniform
40
Equation to calculate the force on an electron passing through two parallel plates
F=Eq E=V/d
41
How do you calculate the speed of an electron as it enters two parallel plates
E=1/2mv^2 | E=QVolts
42
Why are capacitors good
Don't have resistance associated with them | So good for storing and quickly releasing energy
43
What is a capacitor
A device that stores and releases charge Pair of metal plates separated by a thin layer of an insulator (dielectric) So charge can be stored on the plates but can't flow across the gap
44
Capacitance
The charge stored per unit potential difference across the plates of the capacitor Must include the numbers if given
45
Equation for capacitance
C=Q/V Capacitance is measured in Farads, C/V
46
Gradient of a charge potential difference graph
Capacitance
47
Area of a charge potential difference graph
Energy stored in capacitor
48
When will a capacitator stop charging
When its potential difference is the same as the power supply
49
Equations for energy stored in a capacitor
E=1/2QV E=1/2CV^2 E=Q^2/2C
50
Capacitance given the area and separation
C=Ae0er/d
51
What is a dielectric
Material placed between two parallel plates in a capacitor | Which increases the capacitance
52
What makes a good capacitor
Large surface area Small separation Large relative permeability of dielectric
53
Does the capacitance change in Q=CV
No
54
Structure of a dielectric
Consists of polar molecules | That line up in the electric field between the plates
55
Explain the effect of a dielectric on a fixed charge capacitor in isolation
The presence of molecules negative charges near to the positive plates makes it positive potential less The presence of molecules positive charges near the negative plates makes its negative potential less negative So the potential difference across the plates has now decreased Since Q is fixed because it still stores the same charge and C=Q/V, decreased potential difference means an increased capacitance Energy stored, given by 1/2Q^2/C is now less Some energy has been used to align the polar molecules
56
Explain the effect of a dielectric on a fixed potential difference cell
The presence of molecules negative charges near to the positive plates makes it positive potential less The presence of molecules positive charges near the negative plates makes its negative potential less negative The potential difference between the cell and the capacitor plates now drives a current Current increases the charge on the capacitor plates which in turn increases the potential difference The potential difference across the fully charged capacitor is the same as before but the charge stored has increased The insertion of a dielectric between the capacitor plates increases the energy stored Better to insert the dielectric before charging
57
Equations for discharging a capacitor
Q=Q0e^-t/RC I=I0e^-t/RC V=V0e^-t/RC
58
Explain RC in charging and discharging
The time constant | The time in seconds it takes for the charge to fall or rise to 1/e of its original value (37%)
59
Higher the RC...
Longer capacitor takes to discharge or charge
60
What is the theoretical discharge time
5RC The theoretical time to discharge a capacitor After 5RC only 0.69% of original remains
61
5RC
Time to fully discharge or charge a capacitor
62
Gradient of a discharging charge time graph
Discharge current
63
Area of a discharging current time graph
The charge lost
64
How much charge of original remains after 3RC
5.07%
65
Explain the exponential discharge curve
The potential difference across the capacitor drives a current through the resistor Current is the rate of flow of charge, so the charge on the capacitor plates decreases as energy is transferred As the capacitors charge decreases so does the potential difference So the current driven by the potential difference decreases So the rate of change of charge on the plates decreases Rate of change of potential difference also decreases as the capacitor discharges
66
How does charging a capacitor work
When a capacitor is in a circuit with a cell, resistor and closed switch, current is driven by the potential difference The current transports electrons from one capacitor plate to the other This increases the potential difference across the capacitor until it is equal to the p.d Which is when the current has dropped to zero and the p.d across the resistor is zero
67
Charging capacitor equations
Q=Q0(1-e^-t/RC) V=V0(1-e^-t/RC) I=I0e^-t/RC
68
Gradient for a charging charge time graph
Current
69
Area for a charging current time
Charge gained
70
Theoretical charge time
5RC The theoretical time to charge a capacitor After 5RC 99.31% of maximum charge
71
Explain the exponential charging curve
The potential difference across the resistor drives a current through the resistor The current is the rate of flow of charge so the charge on the plates increases as electrons are transferred from the positive plate to the negative plate As the charge on the capacitor plates increases so does the potential difference As the potential difference across the capacitor plates increases, Kirchhoff's Second Law requires the potential difference across the resistor to decreases So current through the resistor decreases which means the rate of increase in the charge on the capacitor plates decreases So the rate if change of potential difference, charge and current all decrease as the capacitor charges
72
Give a method to explain how you can demonstrate constant rate discharge
Close switch to charge capacitor Until voltmeter reading = supply p.d Open switch and close other switch to discharge capacitor Decrease resistance of variable resistor to maintain a constant current Record p.d every 20 seconds until ~0V and capacitor is discharged Plot a graph
73
How do you show a graph is exponential
``` Read initial value on y axis Half Find time taken for this decrease by 50% Repeat for 50% to 25% etc Use mean time as best estimate If the graph is exponential then the time to reduce by half should be constant ```
74
How do you graphically show that a curve of discharging is exponential
Plot a graph of Ln Q V or I against time | If the graph is exponential then it should be linear
75
How can you chose a suitable resistor to perform the capacitance experiment
Effect of resistance influences time constant Increasing resistance increases time constant so increases discharging time Too small and you won't have enough time to get enough readings since decrease too fast Too big and the readings will hardly change and produce a very large graph

A Level Physics Paper 2 (7 decks)

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