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A-Level Physics > Fields > Flashcards

Fields Flashcards

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1
Q

What is a force field?

A

A region where an object will experience a non-contact force

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2
Q

What do force fields cause?

A

Interactions between particles

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3
Q

What force acts between masses?

A

Gravity

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4
Q

Is force a vector or a scalar?

A

Vector

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5
Q

What do field lines show?

A

Direction of the force

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6
Q

What is a radial field?

A

Where the force decreases the further away you are

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7
Q

What happens to field close to the Earth’s surface?

A

They become (almost) uniform

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8
Q

What is G?

A

The gravitational constant

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9
Q

What mathematical property does Newton’s Law of Gravitation have?

A

It is an Inverse Square Law

Force ∝ 1/(r^2)

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10
Q

What has a bigger impact in Newton’s Law of Gravitation; distance or mass?

A

Distance

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11
Q

What is gravitational field strength a measure of?

A

Force per unit mass

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12
Q

In a radial field, what is the relationship between g and r?

A

g is inversely proportional to r^2

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13
Q

What is gravitational potential a measure of?

A

Potential energy per unit mass

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14
Q

Why is gravitational potential negative?

A

You have to do work against the field to move the mass away

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15
Q

What is the gravitational potential energy an infinite distance from the mass?

A

Zero

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16
Q

Is gravitational potential positive or negative?

A

Negative

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17
Q

What is gravitational potential measured in?

A

J/kg

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18
Q

Why do you sometimes see positive gravitational potential energy?

A

It is the gain in potential energy

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19
Q

What does the gradient of a Gravitational Potential-Distance graph represent?

A

g

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20
Q

What is the energy required to move a mass through a gravitational potential difference (V)?

A

mV

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21
Q

What is an Equipotential?

A

A line/surface that joins all the points with the same potential (V)

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22
Q

What is the gravitational potential difference (V) when you travel along an Equipotential?

A

0

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23
Q

What is the work done when you travel along an Equipotential?

A

0

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24
Q

How are Equipotentials oriented in relation to field lines?

A

Perpindicular

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25
Q

What keeps an object moving in circular motion?

A

Centripetal force

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26
Q

What is the centripetal force for satellites?

A

Gravity

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27
Q

What is the force acting on an object in circular motion equal to?

A

F = (mv^2)/r

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28
Q

What is the speed of a satellite in a gravitational field?

A

v = sqrt((GM)/r)

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29
Q

What is the orbital period (T) of a satellite?

A

T = (2πr)/v

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30
Q

How do you find the relationship between T and r?

A

Substitute the satellite speed formula into the orbital period formula

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31
Q

In elliptical orbit, how does a satellite’s kinetic energy change with potential energy?

A

Kinetic energy (speed) increases as potential energy (height) decreases

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32
Q

What is the orbital period of a geostationary satellite?

A

24 hours

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33
Q

What kind of orbit does a geostationary satellite have?

A

Synchronous orbit

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34
Q

What is a synchronous orbit?

A

Orbital period of satellite = rotational period of primary

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35
Q

Where must a geostationary satellite be positioned above the primary?

A

Directly above the equator

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36
Q

Why are geostationary satellites useful for TV signals?

A

The satellite is stationary relatively, so the angle of the transmitter/receiver doesn’t need to change

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37
Q

How low does a Low Orbit Satellite orbit?

A

Below 2000 km

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38
Q

What are the benefits of Low Orbit Satellites?

A

Useful for communications

Cheaper to launch

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39
Q

What are the drawbacks of Low Orbit Satellites?

A

Multiple satellites working together needed

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40
Q

Are Imaging Satellites geostationary or Low Orbit?

A

Low Orbit

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41
Q

If the charged object is a sphere, where can you assume all of the charge acts?

A

At the centre

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42
Q

If force F acts on Q1, what is the force acting on Q2?

A

-F

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43
Q

What mathematical property does Coulomb’s Law of Gravitation have for point charges?

A

It is an Inverse Square Law

Force ∝ 1/(r^2)

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44
Q

What is electric field strength (E) a measure of?

A

Force per unit charge

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45
Q

In what direction does the vector electric field strength (E) point?

A

The direction that a positive charge would move

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46
Q

What kind of field does a point charge have?

A

A radial field

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47
Q

What is the relationship between E and r in a Radial Field?

A

E ∝ 1/(r^2)

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48
Q

What is the relationship between E and d in a Uniform Field?

A

E ∝ 1/d

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49
Q

What is the formula for E in a uniform field?

A

E = V/d

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50
Q

What is the path of a charged particle moving through and electric field?

A

Curved

51
Q

In what direction does a positive charge curve through an electric field?

A

Direction of the field lines

52
Q

In what direction does a negative charge curve through an electric field?

A

Opposite to the direction of the field lines

53
Q

If a particle enters an electric field at right angles, what direction does it accelerate?

A

Perpendicular to initial motion

54
Q

When is electric potential (V) negative?

A

When Q is negative and the force is attractive

55
Q

What is the relationship between g and V?

A

g = -V/r

56
Q

In what direction do magnetic field lines go?

A

North to South

57
Q

What is induced around any wire carrying an electric current?

A

Magnetic Field

58
Q

How do you determine the direction of a magnetic field induced by an electric current?

A

Right-Hand Rule

59
Q

What shape does the magnetic field around a coiled wire take?

A

Doughnut-shaped

60
Q

What is a solenoid?

A

A cylindrical coil of wire

61
Q

What word describes a “coil with length”?

A

Solenoid

62
Q

Describe the magnetic field around a solenoid

A

Like a bar magnet, with a north and south pole

63
Q

What happens to a wire carrying a current in an external magnetic field?

A

It will experience a force

64
Q

Why does a current-carrying wire in an external magnetic field experience a force?

A

The field around the wire and the external magnetic field “add together”. Because the resultant field is not uniform, a force is produced.

65
Q

How do you determine the direction of the force on a current-carrying wire in an external magnetic field?

A

Fleming’s Left-Hand Rule

66
Q

What is the force on current-carrying wire parallel to an external magnetic field?

A

0N

67
Q

What is the relationship between the force on a current-carrying wire in an external magnetic field and the magnetic flux density?

A

F ∝ B

68
Q

Define magnetic flux density

A

The force on one metre of wire carrying a current of one amp at right angles to the magnetic field.

69
Q

What is the equation linking force, current, length of wire and flux density of a current-carrying wire in a perpendicular magnetic field

A

F = BIL

70
Q

Is flux density a scalar or a vector?

A

Vector

71
Q

What is magnetic flux density measured in?

A

Tesla

72
Q

Define the Tesla

A

Wb/(m^2)

73
Q

Why does a current-carrying wire experience a force in a magnetic field?

A

Force acts on all charged particles moving in a magnetic field

74
Q

What is the mathematical condition for circular motion?

A

Force is perpendicular to direction of travel

75
Q

For a charged particle moving through a magnetic field, how do you adapt Fleming’s left-hand rule?

A

Second finger is direction of motion of a POSITIVE charge (as opposed to current)

76
Q

For a negatively-charged particle moving through a magnetic field, in which direction does your second finger point with Fleming’s left-hand rule?

A

Opposite to direction of travel

77
Q

How do you find the radius of the circular path of a charged particle in a magnetic field?

A

Combine the equations for the force on a charged particle in a magnetic field and the force on a particle in circular motion

78
Q

What is magnetic flux density a measure of?

A

Strength of a magnetic field

like a “density” of field lines per area

79
Q

When does magnetic flux (Ф) = BA?

A

B is perpendicular to A

80
Q

What is N in magnetic flux linkage?

A

Number of coils

81
Q

What is induced by a flat coil/solenoid moving closer to/further away from a magnet?

A

e.m.f.

82
Q

How do you induce an e.m.f. with a flat coil or solenoid?

A

Move it closer or further away from a magnet

83
Q

What is the magnitude of induced e.m.f. equal to?

A

Rate of change of flux linkage

84
Q

What is the rate of change of flux linkage equal to?

A

Magnitude of induced e.m.f.

85
Q

What is Faraday’s Law?

A

The induced e.m.f. is directly proportional to the rate of change of flux linkage

86
Q

Why does the electrons in a conducting rod accumulate at one end of the rod when there is relative motion with a magnetic field?

A

Force acts on all charged particles moving in a magnetic field

87
Q

What is electromagnetic induction?

A

Electrons accumulate at one end of a conducting rod moving in a magnetic field, inducing an e.m.f. across the ends of the rod

88
Q

What is magnetic flux linkage?

A

The product of the number of turns in a coil and the magnetic flux

89
Q

What is the unit of both flux linkage and magnetic flux?

A

Weber (Wb)

90
Q

What is Lenz’s Law?

A

The induced e.m.f. is always in such a direction as to oppose the movement that caused it

91
Q

How do you use Fleming’s left-hand rule to find the direction of induced e.m.f. in a conductor?

A

Thumb = resistive force (Lenz’s Law)

92
Q

What is the equation for the force on a charged particle in a magnetic field?

A

F = BQv

93
Q

How does a generator convert kinetic energy into electric energy?

A

Rotating a coil in a magnetic field

94
Q

What flows through a coil that is part of a complete circuit moving in a magnetic field?

A

An induced current

95
Q

What transfer of energy occurs in a generator?

A

Kinetic to electrical

96
Q

What is an Alternator?

A

A generator of alternating current

97
Q

How does an alternator connect the coil to an external circuit?

A

Slip rings and brushes

98
Q

What is the phase difference between flux linkage and induced voltage in an Alternator?

A

90 degrees

99
Q

Where do you measure θ between for BANcosθ?

A

Normal to the coil and B

Coil is perpendicular when θ = 0

100
Q

How do you know that the field lines are going into the paper?

A

Circle with a cross in it

101
Q

What is the waveform for voltage with an alternating current?

A

Sinusoidal

102
Q

What is the waveform for e.m.f. with an alternating current?

A

Sinusoidal

103
Q

What is the waveform for flux linkage with an alternating current?

A

Sinusoidal

104
Q

What is the time period of an alternating current?

A

The time between peak voltages

105
Q

What kind of current is Mains Electricity?

A

Alternating current

106
Q

What kind of current does a battery supply?

A

Direct current

107
Q

What does a transformer do?

A

Changes the size of the voltage for an alternating current

108
Q

What happens in a transformer’s primary coil?

A

An alternating current produces magnetic flux

109
Q

What is another name for the primary coil?

A

Input coil

110
Q

Describe a transformer

A

A primary coil (connected to an a.c. current) around one side of an iron core, with a secondary coil on the other side

111
Q

How is an alternating voltage induced in a transformer’s secondary coil?

A

Changing magnetic field is passed through the iron core, inducing an alternating voltage in the secondary coil

112
Q

How do transformers change the voltage?

A

Having a different number of turns on each coil

113
Q

What causes small losses of power from the transformer?

A

Eddy currents in the iron core

114
Q

How do eddy currents dissipate energy?

A

By generating heat

115
Q

How can you reduce the effect of eddy currents?

A

Laminate the core with layers of insulation

116
Q

How is heat generated in a transformer?

A

Resistance in coils, eddy currents

117
Q

How do you reduce the resistance in coils in a transformer?

A

Use a thick copper wire, which has low resistance

118
Q

What is the efficiency of a transformer?

A

Ratio of power out to power in

119
Q

How must you express transformer efficiency?

A

As a percentage

120
Q

Are transformers 100% efficient?

A

No

121
Q

What current is electricity from power stations sent round the country in the national grid?

A

Lowest possible current

122
Q

Why is electricity sent in the national grid at the lowest possible current?

A

Reduced the power lost due to resistance

123
Q

How are transformers used in the national grid?

A

Step-up voltage to the nation grid

Step-down voltage for domestic use

A-Level Physics (13 decks)

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