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Physics > 3.7 Fields and their consequences > Flashcards

3.7 Fields and their consequences Flashcards

1
Q

What is the definition of a force field?

A

A region in which a body experiences a non-contact force.

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2
Q

In which situations to force fields arise?

A

Between objects of mass
Between static charges
Between moving charges

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3
Q

What are the similarities between gravitational and electrostatic forces?

A

Both involve inverse square law
Use of field lines
Use of potential
Equipotential surfaces

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4
Q

What are the differences between gravitational and electrostatic forces?

A

Masses always attract but charges can also repel

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5
Q

What is the definition of gravitational force?

A

The force of attraction between 2 masses is proportional to the product of the 2 masses and inversely proportional to the distance between their centres squared.

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6
Q

What is gravity?

A

A universal attractive force acting between all matter.

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7
Q

What is Kepler’s 1st law?

A

The path of any object in an orbit follows the shape of an ellipse, with the orbited body at one of the foci.

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8
Q

What is Keplers 2nd Law?

A

An imaginary line from the sun to the planet sweeps out equal areas in equal times.

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9
Q

What is Kepler’s 3rd law?

A

The square of a planet’s orbital period is directly proportional to the cube of the average radius from the sun.

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10
Q

What is gravitational potential?

A

Work done per unit mass when a mass is moved from infinity to a point which is a distance, r, from the centre of a body. It is scalar.

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11
Q

What is an equipotential surface?

A

A surface of constant potential. (No work needs to be done to move across it.)

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12
Q

What are the properties of equipotential lines?

A

The gravitational potential is the same anywhere on the line.
They are perpendicular to field lines.
Concentric circles
Distance between them increases

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13
Q

What is potential gradient?

A

At a point in a gravitational field, it is the change of potential per metre at that point. (It is equivalent to g, it decreases as you get further form the earths surface.)

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14
Q

Why is gravitational potential negative?

A

At infinity, potential is 0. So, energy must be put in to move from infinity.

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15
Q

What is the definition of a gravitational field?

A

A region of space where an object of mass experiences a force.

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16
Q

What is the definition of gravitational energy?

A

The energy of an object due to its position in a gravitational field.

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17
Q

What is the work done on a body in a gravitational field?

A

It is equal to the gravitational potential energy gained.

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18
Q

What is escape velocity?

A

The minimum velocity an object must be given to escape from the planet when projected vertically from the surface.

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19
Q

What is the total energy for an object in orbit?

A

Total energy is half the gravitational potential energy. (You add kinetic energy and gravitational potential.)

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20
Q

What are the 3 different types of orbits?

A

Geostationary
Low/ polar
Geosynchronous

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21
Q

What are the properties of a geostationary orbit?

A

The orbital period of the satellite is the same as the rotational period of the object.
The axis of rotation of the satellite is the same as the axis of the planet.
The satellite must orbit in the plane of the equator.

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22
Q

What are the uses of geostationary orbits?

A

Communication
TV
Weather

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23
Q

What are the features of a geosynchronous orbit?

A

The period of the orbit is the same as the time period of one rotation of the planet on its axis.
The satellite return to the same point in the sky at the same time each day.

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24
Q

What are the features of a low orbit?

A

Travel relatively close to the earth and take around 90min to complete one orbit.
An example of a low orbit is a polar orbit which orbits the poles of the earth.

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25
Q

What are the uses of low orbits?

A

Spying
Imaging
Weather
Geological prospecting
GPS

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26
Q

What factors affect orbital period?

A

Speed
Circumference

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27
Q

What is the effect of increasing distance on orbital speed and period?

A

↑ distance → ↓ orbital speed

↑ distance → ↑ orbital period

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28
Q

What is the definition of a coulomb? Is charge scalar or vector?

A

The charge transported by 1 amp of current in 1 second. Charge is scalar.

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29
Q

What is Coulomb’s law?

A

The magnitude of force, F, between charges Q1 and Q2 is proportional to the product of Q1 and Q2 and inversely proportional to the square of the distance, r, between their centres.

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30
Q

What is an electric field?

A

Where a charged particle feels a force.

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31
Q

What is the definition of electric field strength?

A

The electric force per unit charge on a positive charge. It is a vector quantity.

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32
Q

What is the null point?

A

In the case of two like charges, there’s a point between the 2 charges where the electric field strength is 0. It is farther from the greater charge.

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33
Q

What is a uniform electric field?

A

A charge will feel the same force wherever it’s placed in the field and the force acts parallel to the field lines. The electric field between 2 parallel charged plated is uniform.

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34
Q

Which factors affect uniform electric field strength?

A

Potential difference between the plates
The distance between the plates

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35
Q

What happens when a charged particle enters a uniform field?

A

The horizontal component of its velocity remains unchanged. It accelerates in the vertical direction. Gravity is negligible.

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36
Q

Describe Millikan’s oil drop experiment.

A

The oil drops were placed in a uniform electric field.
The drops were given a negative charge.
This produces an upwards force in the field, which balances the weight.
This enables the charge on an electron to be calculated.
All charges were found to be a multiple of e.

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37
Q

What happens inside an electron gun?

A

The positive anode attracts the e-. It is circular with a hole in the center.
The attractive force on the e- is horizontal.
The hot negative cathode releases e- by thermionic emission.

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38
Q

What is the definition of electric potential energy?

A

The work done, on a positive test charge, when it is moved, from infinity, to that point in the field.
It is a scalar quantity.

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39
Q

What is the definition of electric potential?

A

At a point in an electric field, it is the work done per unit positive charge on a positive test charge when it’s moved from infinity to that point in the field.
It is scalar.

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40
Q

In which situations will potential difference be negative or positive?

A

V is positive when a force is required to move a positively charged object to a point.
V is negative when the charge will move anyway due to the electrostatic attraction of the other charge.

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41
Q

What is potential gradient?

A

The gradient of a V-r graph which is also equal to electric field strength. As you go up a potential gradient, you travel in the opposite direction to the field.

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42
Q

What does the area under an E-r curve represent?

A

The electric potential at that point.

43
Q

What is the definition of capacitance?

A

The ability of a body to hold an electric charge.

44
Q

What is a farad?

A

The capacitance when a p.d of 1V appears across a capacitors plates when a charge of 1C is stored.

45
Q

What is the basic structure of a capacitor?

A

It contains 2 parallel plates separated by an insulating material, a dielectric.

46
Q

What are the uses of capacitors?

A

The flash in a camera
Smoothing surges in electric currents
Allows data to be stored in a computer if the power is cut
Used to start nuclear reactions, by powering intense laser beams
(Some hybrid cars)

47
Q

How is a capacitor charged?

A

Electrons gather on the plate attached to the negative terminal of the battery.
So, electrons are drawn from the positive plate of the capacitor.
So, a net negative charge forms on one plate and a positive on the other.
A current flows until the p.d across the capacitor is the same as the EMF.

48
Q

What does the gradient and area under the curve of a a charge-voltage graph represent?

A

Gradient= capacitance
Area under the curve= energy stored

49
Q

What happens when the voltage across the capacitor doubles?

A

The charge also doubles causing energy stored to quadruple.

50
Q

What factors affect capacitance?

A

Area
Distance
Permittivity

51
Q

What effect does increasing the area of the plates have on capacitance?

A

By increasing the area of the plates, the charge that can be stored increases for a particular voltage. So, capacitance increases.

52
Q

What effect does increasing the distance between the plates have on capacitance?

A

The effect that the plates have on each other decreases. So, capacitance decreases.

53
Q

What effect does increasing the permittivity of the dielectric have on capacitance?

A

This increases the strength of the electric field between the plates and increases the capacitance.

54
Q

What are the common dielectrics?

A

Mica
Wax paper

55
Q

What is the purpose of the dielectric?

A

To insulate the 2 plates when storing a charge.
To allow an electric field to form.

56
Q

What is the permittivity of air?

A

It is equivalent to the permittivity of free space.

57
Q

How do dielectrics work?

A

The material is made up of polar molecules.
When charge is applied, an electric field forms between the plates, so, molecules rotate and align with the electric field.
The molecules have their own electric field, opposing that of the capacitor.
The larger the permittivity, the larger the opposing field as electric field decreases which reduces the p.d needed to charge the capacitor, increasing the capacitance.

58
Q

What happens when the dielectric is removed from a charged capacitor?

A

εr falls to 1
The capacitance falls by the same factor
The p.d increases by the same factor
Q remains constant
E increases by the same factor
Energy is conserved as work is done to separate the negatively charged surface of the dielectric from the positively charged plate.

59
Q

How is a capacitor discharged?

A

When it is connected to a complete circuit, it discharges the stored charge, creating a current.

60
Q

Which factors affect discharge time?

A

Capacitance
Resistance

61
Q

What effect does capacitance have on discharge time?

A

↑Capacitance → ↑ Discharge time

62
Q

What effect does resistance have on discharge time?

A

↑Resistance → ↑ Discharge time

63
Q

What is the time constant?

A

The time taken for the charge, current or voltage to decrease by a factor of e^-1

64
Q

How do you find the time constant from a graph?

A

Find V_0
Find e^-1V_0
The value for time at e^-1V_0 is the time constant

65
Q

What is the time taken for Q,I and V to halve when a capacitor is discharged?

A

0.69RC

66
Q

What is the definition and unit of magnetic field strength/magnetic flux density, B?

A

The force per unit current per unit length on a current carrying wire in a magnetic field.
Unit is Tesla

67
Q

What is Tesla in base units?

A

N A^-1 m^-1

68
Q

What is the equation for the force on a charged particle moving in a magnetic field when field is perpendicular to velocity?

A

F = BQv

69
Q

In what direction does magnetic force act on positively and negatively charged particles?

A

They both travel in a circular path but the force acts in opposite directions so, their paths are opposite.

70
Q

Describe how a cyclotron works.

A
  • A particle accelerates fur to the uniform alectric field between the D’s.
    • The force acts in the direction of motion so work is done on the charged particle by the electric field .
    • So, the particle gains Ek
    • The electric field reverses everytime the particle enters the field.
    • Inside the D’s, the magnetic field creates centripetal acceleration.
    • But, the magnitude of the velocity doesn’t change and the force acts perpendicularly to motion and no work is done on the particle.
    • Everytime the particle enters the D, the velocity is larger, so, radius of the path is larger.
71
Q

What is the equation for magnetic flux?

A

Magnetic flux = BA

72
Q

What are the properties of magnetic field lines?

A

Go from north to south
The closer they are, the stronger the field
They’re vectors

73
Q

What factors can increase the strength of an electromagnetic?

A

Increase the number of turns
Increase the current
Insert a soft iron core
Decrease the diameter of the coils

74
Q

What is the definition of permeability?

A

The measure of the ability of a material to support the formation of a magnetic field within it.

75
Q

How can you measure the magnetic force on a wire?

A

Use a rigid rod instead of a wire
Place the yoke on the balance
Turn on the current
Record the reading on the balance
Convert it to newtons

76
Q

What is the definition of magnetic flux linkage?

A

A measure of the magnetic flux through a coil with N turns.

77
Q

What is the equation for magnetic flux linkage?

A

Magnetic flux linkage = BAN
Where N is the number of turns cutting the flux

78
Q

What is the equation for flux linkage at an angle to the field?

A

Flux linkage = BAN cosθ

79
Q

What is Faraday’s law?

A

The magnitude of the induced EMF in a circuit is equal to the rate of change of magnetic flux linkage through the circuit.

80
Q

What is the generator effect?

A

When the flux through the coil changes, an EMF is induced.
The greater the speed of movement, the greater the rest of change of flux and the greater the EMF.

81
Q

What is Lenz’s law?

A

The direction of the induced EMF is always such as to oppose the change that caused the EMF.

82
Q

What happens when a magnet moves through a conducting rod?

A

The moving magnet induces an EMF in the coil, which creates a current, producing magnetic flux.
The flux opposes the flux of the magnet due to Lenz’s law.
So, magnetic repulsion occurs.
So, work is done to move the magnet against the force.
Work done is converted to electrical energy.

83
Q

What happens when a magnet falls through a conducting rod?

A

An EMF is induced in the rod.
So, a circular current is induced in the tube.
Current produces magnetic flux, which opposes the flux of the magnet, creating magnetic repulsion.
Work is done by gravity to move the magnet against this force, this is converted to electrical energy so, energy is conserved.

84
Q

What happens when a cut is made in the rod that a bar magnet is falling in to?

A

An EMF is induced but current can’t flow, so a field isn’t formed so, there’s no magnetic force on the magnet.
So, the magnet will continue to fall with acceleration due to gravity.

85
Q

How do you calculate the magnitude of induced EMF?

A

EMF = (magnetic flux linkage) / (change in time)

86
Q

What is electromagnetic induction?

A

When an EMF is produced in a rod when it cuts through a magnetic field.

87
Q

How do you increase the EMF of a wire undergoing EM induction?

A

Move the wire faster
Use a stronger magnet
Use a coil of wire and increase the number of turns

88
Q

How can you obtain an accurate value for EMF from a CRO?

A

Adjust the V/div setting to give the tallest trace.
Adjust the y position so a peak of trough in on a division line
Turn off the time base
Read the peak to peak voltage value and half it.

89
Q

What happens when the time base is switched off on a CRO?

A

A vertical line is formed of length equal to peak to peak voltage as the trace no longer moves horizontally.

90
Q

What is the RMS voltage value?

A

The effective voltage delivered.

91
Q

How do you find the RMS value?

A

Square the values
Find the mean of the squares
Square root the mean

92
Q

What is a step-up transformer?

A

It contains 2 coils connected by an iron core, where the secondary coil has more turns.
It increases voltage

93
Q

What is a step-down transformer?

A

It contains 2 coils connected by an iron core, where the secondary coil has less turns.
It decreases voltage

94
Q

How do transformers work?

A

There is a current in the primary coil, producing magnetic flux.
The iron core transfers the flux as it provides good linkage.
The flux links to the secondary coil.
The ac current in the primary coil changes and the flux changes too.
The flux changes.
This induces an alternating EMF in the secondary coil.
So, an ac is then induced in the secondary coil.

95
Q

How do transformers work when using DC currents?

A

Switching the current on and off in the primary coil causes a rapid change in current.
The flux in the coil also changes rapidly.
So, the flux of the iron core also changes rapidly.
The flux in the iron core links to the secondary coil.
The changing flux linkage induces an EMF in the secondary coil.
The flux opposes the flux in the primary coil.

96
Q

What is an eddy current?

A

Unwanted circular current of electricity which are induced in the iron core of a transformer.
They act like electromagnets, generating flux.

97
Q

What are the causes of energy loss in a transformer?

A

Heating of the coils
Energy required to magnetise and demagnetise the core
Eddy currents
Magnetic losses

98
Q

Explain how heating of the coils in a transformer causes energy loss and how this can be avoided.

A

The wires of the coils have some resistance and the current causes them to heat. Leading to energy loss.
This effect can be reduced by using thicker wires with a lower resistivity.

99
Q
  • Explain how magnetising and demagnetising the core in a transformer causes energy loss and how this can be avoided.
A

The AC current in the primary coil magnetises and demagnetises the core continuously in opposite directions, which requires energy and the energy is wasted to the heating of the core.
This can be reduced by using a magnetically soft material for the core which can be easily magnetised and demagnetised.

100
Q

Explain how eddy currents in a transformer causes energy loss and how this can be avoided.

A

Magnetic flux passing though the core is changing continuously and the core is being cut by flux. The constant change in flux induces an EMF in the core, creating eddy currents. This heats the core and causes energy to be wasted.
The effect can be reduced by laminating the core, the laminations are separated by this layers of insulation. Current can’t flow in a discontinuous conductor.

101
Q

Explain how magnetic losses causes energy loss in a transformer and how this effect can be reduced.

A

For a transformer to be efficient, as much flux by the primary current must pass through the secondary coil.
So, magnetic losses can be reduced by having a design where the 2 coils are as close as possible.

102
Q

Why is electricity transferred at a high voltage across the national grid?

A

We want the current to be as low as possible as a high current leads to heat loss which decreases efficiency.

103
Q

Why can’t we transfer electricity across the grid at higher voltages?

A

More insulation will be required
And you increase the risk of sparking
So, more expensive equipment is required.

Physics (7 decks)

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