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Physics: Uplearn > Fields and Their Consequences P2 > Flashcards

Fields and Their Consequences P2 Flashcards

1
Q

What does the electrostatic force tell us if the sign is negative?

A

The force is attractive

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2
Q

What is the magintude of the electric field strength at a given point?

A

The force per unit charge that a positive test charge would experience without effecting the field strength.

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3
Q

What occurs when a positive test charge is moved closer to a negative point charge?

A

Work is done by the electrostatic force therefore the elelctric potential of the particle decreases

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4
Q

What is the electric potential energy of a postive test charge in an electric field defined as?

A

The work done required to move the test charge from infinity to the position it is in

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5
Q

What happens to the electric potential energy of a positive test charge if you move it closer to a positive source charge?

A

It becomes more positive (Increases)

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6
Q

What has done the work if the electric potential energy of a small positively charge particle at a given position is -3J?

A

3J of work is done by the electrostatic force to move the particle from infinity to that point

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7
Q

What is the potential gradient at a point on a potential graph?

A

The change in potential per unit distance at that point

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8
Q

What is the magnitude of the electric potential gradient at a point equal to?

A

The magnitude of the electric field strength at that point

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9
Q

What graph can we find the potential difference from by calculating the area underneath it?

A

Electric field strength against distance graph

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10
Q

What is the difference between a good conductor and a good insulator?

A

A good conductor has lots of free electrons whereas a good insulator doesn’t have many (e.g. Silver and Diamond)

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11
Q

What is a capacitor made of?

A

Two conducting components, seperated by an insulated material

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12
Q

What is the potential difference across a battery?

A

The difference in potential energy per coulomb between one terminal and the other

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13
Q

What is electric current?

A

The coulombs of charge moving past a given point per second

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14
Q

When does a capacitor stop charging?

A

When the potential difference across the capacitor is the same as the potential difference across the battery

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15
Q

What is the capacitance of a capacitor?

A

The charge per unit volt between the capacitor plates

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16
Q

How can we find the capacitance from a graph?

A

Plot a graph of charge against potential difference and find the gradient

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17
Q

What are dielectrics?

A

Insulating materials that polarise in the presence of an electric field

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18
Q

What happens as we increase the permittivity of a dielectric?

A

It becomes easier to polarise therefore the weaker the electric field is that passes through it

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19
Q

What is the relative permittivity of a material? (Dielectric Constant)

A

The ratio of the permittivity of the material to the permittivity of free space

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20
Q

What relationship does capacitance have with permittivity and the area of each plate?

A

They are directly proportional

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21
Q

Why does increasing the area of each plate increase the capacitance?

A

More charge can be stored on each plate

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22
Q

What are the three factors which affect capacitance?

A

The permittivity of the insulator, the area of the plates and the distance between each plate?

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23
Q

Why is the distance between the plates inversely proportional to the capacitance?

A

Since the electric field strength remains constant, the potential difference increase which means the capacitance decreases.

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24
Q

How can a constant current be achieved when discharging a capacitor?

A

Use a variable resistor and decrease the resistance as the capacitor discharges

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25
Q

What is the relationship between charge and time when a capacitor discharges through a fixed resistor?

A

Exponential

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26
Q

What is the time constant?

A

The time it takes the charge stored in the capacitor to fall by 37% of the charge stored in the fully-charged capacitor.

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27
Q

How do we find the time taken for the charge on a capacitor to halve?

A

Multiply the time constant (RC) by 0.69

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28
Q

What does the area represent underneath a current/time graph?

A

The initial charge of a capacitor

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29
Q

When we start charging an uncharged capacitor through a fixed resistor, what happens after a time of one time constant?

A

The charge stored increases by 63% and the current decreases by 37%

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30
Q

What does the magnitude of the Force of a current carrying wire depend on?

A

The length of wire in the field, the current, the magnetic field strength and the angle between the current and the magnetic field lines

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31
Q

What is the magnetic flux density defined as?

A

The force per unit current per length of wire when the wire is perpendicular to the magnetic field.

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32
Q

How do you find the current required to levitate a wire in a magnetic field?

A

Set F as the weight of the wire in F = BIL

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33
Q

What conditions must be met for a particle to experience a force in a magentic field?

A

It has a charge and is not moving parallel with the field

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34
Q

What must be changed if we use Fleming’s left hand rule for negatively charged particles?

A

The velocity must be in the opposite direction

35
Q

What is the formula for the radius of a charged particle’s circular path in a uniform magnetic field?

A

r = mv/BQ

36
Q

What is within and between the D’s of a cylcotron?

A

There is a uniform magnetic field within the D’s and an alternating electric field between them.

37
Q

What is the purpose of the alternating electric field and uniform magnetic field in the cyclotron?

A

The electric field increases the proton’s speed and the magnetic field forces the protons round a circular path.

38
Q

What must the time period of the alternating electric field equal to in a cyclotron?

A

The time taken for a proton to complete one full cycle around the cyclotron

39
Q

What is the formula for the time period for a proton to complete a full cycle around the cyclotron?

A

T = 2πm/BQ

40
Q

What happens when a wire cuts through magnetic field lines?

A

An EMF is induced in the wire (Electromagnetic Induction)

41
Q

What is Lenz’s law?

A

The direction of the induced emf opposes the change that caused it

42
Q

What pole does the thumb point towards in the right hand grip rule?

A

The north pole

43
Q

What happens if we move a magnet towards a coil?

A

The coil develops a magnetic field which repels the magnet therefore, if the end of the magnet is the north pole, the part of the coil closest to the magnet will also become a north pole.

44
Q

If a coil is repelling a magnet, which direction is the initial velocity of the magnet?

A

Towards the coil

45
Q

What is Faraday’s law?

A

The magnitude of the emf induced in a coil is equal to the rate of change of flux through the coil

46
Q

What is the magnitude of the emf in a wire dependent on?

A

The rate at which it cuts through magnetic field lines

47
Q

What are the four ways to induce an EMF in a coil?

A

Change the flux density.
Change the area of the coil.
Rotate the coil.
Move a coil into or out a magnetic field.

48
Q

In what graph is the gradient the EMF?

A

Flux linkage against time

49
Q

In what graph is the area the change in flux linkage?

A

EMF against time

50
Q

At what point is the EMF of a coil at its maximum when moving a magnet through it?

A

Just as it enters and leaves

51
Q

What happens when a coil is rotated within a magnetic field?

A

An alternating current is created

52
Q

How do you find the mean induced EMF when generating AC?

A

Divide the flux linkage by the time taken for a 90 degrees turn (BAN/t1/4)

53
Q

How do you find the maximum EMF?

A

BANω

54
Q

How do you reduce the uncertainty when measuring voltage from an oscilloscope?

A

Increase the Y-Gain so it just fits on the screen

55
Q

How do you find the mean power?

A

Half the peak power

56
Q

How can a coil provide an AC to another coil without being connected to it?

A

An AC is passed through the the first coil which creates a magnetic field which is constantly changing, this then means the 2ns coils oppose to change is also constantly changing which means so does the current across it

57
Q

What is the purpose of the iron core in a transformer?

A

To maximise the flux through the secondary coil

58
Q

What happens as you increase the number of coils on a coil in a transformer?

A

The alternating voltage across the coil increases

59
Q

How can you reduce eddy currents in an iron core?

A

The core is made from alternating layers of iron and insulation (Lamination)

60
Q

What are the four main sources of inefficiency in a transformer?

A

Eddy currents in the core.
Heat loss through coils.
Not all flux from the primary coil going through the secondary coil.
Magnetising and demagnetising the core.

61
Q

What are the four solutions to stop the previous inefficiencies?

A

Laminate the core to stop eddy currents.
Use thick copper wires with high resistance to stop heat loss.
Wrap both coils around the same part of the core.
Use a soft, magnetic core like iron.

62
Q

How are transformers used in the national grid?

A

Step-up transformers are used between power stations and transmission lines which reduces current and power lost as heat.
Step-downs are then used to reduce the voltage to a safe level for domestic use.

63
Q

What is the magnitude of the gravitational force of two objects proportional to?

A

The mass of both objects

64
Q

What is the magnitude of the gravitational force of two objects inversely proportional to?

A

The distance from the centre of both objects squared (Inverse square law)

65
Q

What is a point mass?

A

Where the mass is concentrated at a single point

66
Q

What is gravitational field strength?

A

The gravitational force per unit mass of a test mass

67
Q

What is a test mass?

A

A particle with a very small mass used to test a gravitiational field without affecting it

68
Q

What is gravitational potential energy?

A

The amount of energy/work done required to move a mass from infinity to a given point in a gravitational field

69
Q

What is gravitational potential energy at infinity?

A

Zero

70
Q

What happens to the GPE as it gets closer to the source mass?

A

The more negative the GPE becomes

71
Q

What is escape velocity?

A

The minimum velocity required for an object to move from a point in a planet’s gravitational field to a point where the gravitational field is negligible.

72
Q

What is gravitational potential?

A

The work done to move a 1kg mass from infinity to a point in the field

73
Q

What are equipotentials?

A

A set of points which have the same gravitational potential

74
Q

What is potential gradient?

A

The change in potential per unit distance at that point.

75
Q

What does the area between two points below a gravitational field strength graph tell us?

A

The gravitational potential difference

76
Q

At what seperations are equipotentials when the gravitational field strength is high?

A

Large seperations

77
Q

What do satellites do in low Earth orbits?

A

They move over lots of different points on the surface and orbit relatively close to the surface

78
Q

What do satellites do in geostationary orbits?

A

They appear to stay still in the sky over a single point on the surface and orbit at distances relatively far from the surface

79
Q

How do you derive the escape velocity?

A

Set the equations of kinetic energy and gravitational force equal to each other

80
Q

What is the formula for the total energy of a satellite?

A

-GMm/2r

81
Q

Why is gravitational potential difference a negative value?

A

Because work needs to be done to move an object from the inside of the field to the outside. Since infinity is zero, below infinity must be negative

82
Q

How is the orbital period realted to the radius of a circular orbit?

A

T^2 = k x r^3

83
Q

How could you find the speed of an orbiting satellite?

A

Set the centripetal force to the gravitational force and sub in v=2πr/T

84
Q

What is an eddy current?

A

As the primary coil induces emf in the secondary coil, it induces mini currents within the core.

Physics: Uplearn (12 decks)

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