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fields Flashcards

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1
Q

what is a force field

A

area in which an object experiences a non-contact force

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2
Q

what two ways can force fields be represented as?

A

vectors or field lines

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3
Q

what does a vector in force fields show

A

direction of the force that would be exerted on the object

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4
Q

what does the distance between field lines represent

A

strength of the force exerted by the field in that region

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5
Q

how are force fields formed

A

interaction of masses, static charge or moving charges

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6
Q

what type of field is formed during the interaction of masses

A

gravitational fields

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7
Q

what type of field is formed during the interaction of charges

A

electric fields

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8
Q

what are the differences between gravitational fields and electric fields

A
  • in gravitational fields, the force is always attractive whereas in electric fields the force can be repulsive or attractive
  • electric force act on charge whereas gravitational force acts on mass
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9
Q

what are the similarities between gravitational fields and electric fields

A
  • both follow inverse square law
  • use field lines to be represented
  • have equipotential surfaces
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10
Q

what does gravity act on?

A

any object with mass

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11
Q

what type of force is gravity

A

attractive

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12
Q

what does newtons law of gravitation show

A

the magnitude of the gravitational force between two objects is directly proportional to the product of the masses and is inversely proportional to the square of the distance between them

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13
Q

newtons law of gravitation equation

A

F=Gm₁m₂/r²

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14
Q

what are the two types of gravitational fields

A

uniform field or radial field

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15
Q

how does a radial field exert gravitational force?

A

the force exerted depends on the position of the object in the field

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15
Q

how does a uniform field exert gravitational force?

A

the same gravitational force is exerted on a mass everywhere in the field

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16
Q

what Is gravitational field strength

A

force per unit mass exerted by a gravitational field on an object

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17
Q

what is gravitational potential?

A

work done per unit mass when moving an object from infinity to that point

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17
Q

what is gravitational potential at infinity

A

0

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18
Q

is gravitational potential always negative or positive

A

negative

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19
Q

what is the gravitational potential difference

A

energy needed to move a unit mass between two points

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20
Q

what is potential on an equipotential?

A

it is constant on the equipotential surface

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21
Q

what are equipotential surfaces

A

surfaces that are created through joining points of equal potential together

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22
Q

how much work is done when moving along an equipotential surface

A

none

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23
Q

what is gravitational potential and distance between centres of objects relationships

A

inversely proportional

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24
Q

how to calculate gravitational field strength from he graph of gravitational potential and distance

A

the gradient multiplied by -1

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25
Q

how do you calculate gravitational potential difference from the graph of gravitational field strength against distance

A

area under graph

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25
Q

what is Keplers third law

A

the square of the orbital period is directly proportional to the cube of the radius

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26
Q

what is the total energy of an orbiting satellite

A

its kinetic and potential energy

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27
Q

what is the escape velocity

A

the minimum velocity an object must travel at to escape the gravitational field at the surface of a mass

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28
Q

at escape velocity what is kinetic energy equal to

A

gravitational potential energy

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29
Q

what is a synchronous orbit

A

orbital period of the satellite is equal to the rotational period of the object it is orbiting

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30
Q

what is a geostationary satellite

A

their orbital period is 24hrs and they always stay above the same point as they orbit directly above the equator

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31
Q

uses of geostationary satellites

A

tv and telephone signals

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31
Q

what is a low orbit satellite

A

they travel much faster and their orbital periods are much smaller

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32
Q

if charges have the same sign they will be….

A

repulsive

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32
Q

what is coulombs law

A

magnitude of forces between two point charges in a vacuum is directly proportional to the product of their charges and inversely proportional to the square of the distance between charges

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32
Q

if charges have different signs the force will be….

A

attractive

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32
Q

uses of low-orbit satellites

A

monitoring weather, making scientific observations, military

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33
Q

what is electric field strength in uniform field Vs radial

A

uniform field is constant and varies in radial field

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33
Q

why is the magnitude of electrostatic forces between subatomic particles greater than the magnitude of gravitational forces

A

the masses of subatomic particles are incredibly small whereas their charges are much larger

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33
Q

what is electric field strength

A

force per unit charge experienced by an object in an electric field

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34
Q

What is absolute electric potential

A

potential energy per unit charge of a positive point charge at that point

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34
Q

what do electric field lines show

A

direction of force acting on a positive charge

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34
Q

when is the magnitude of electric potential greatest

A

at the surface of a charge

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34
Q

for uniform electric fields, what shape does a particle fired at a right angle make?

A

parabola

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35
Q

when is the absolute magnitude of electric potential at its greatest

A

when its at the surface of the charge

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35
Q

why does a particle fired at a right angle follow a parabolic shape through a uniform electric fiend

A

it experiences a constant force which causes particle to accelerate

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35
Q

what is the absolute electric potential

A

potential energy per unit charge of a positive point charge at that point in the field

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36
Q

what is electric potential at infinity

A

zero

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37
Q

when electric potential is positive, what is the charge

A

positive and repulsive

37
Q

when electric potential is negative, what is the charge

A

negative and attractive

38
Q

what happens when a charge moves along an equipotential surface

A

no work is done

38
Q

what is electric potential difference

A

energy needed to move a unit charge between two points

39
Q

what is the gradient of a tangent on potential against distance graph

A

electric field strength

40
Q

what is work done in moving a charge across a potential difference equal to

A

product of potential difference and charge

41
Q

when a charge moves along an equipotential surface, what work is done

A

none

42
Q

why is no work done when a charge moves along an equipotential surface

A

potential on an equipotential surface is the same everywhere

43
Q

what is the area under the graph on the graph of electric field strength against distance

A

electric potential difference

44
Q

what is capacitance

A

charge stored by a capacitor per unit PD

45
Q

what is a capacitor

A

an electrical component that stores charge

46
Q

what is a capacitor made of

A

two conducting parallel plates with a gap between them which may be separated by an insulating material

47
Q

what is the insulating material between the two plates of a capacitor called

A

dielectric

48
Q

what happens when a capacitor Is connected to power

A

opposite charges build up on the plates causing a uniform electric field to be formed

49
Q

what is permittivity

A

a measure of the ability to store an electric field in a material

50
Q

what is a dielectric formed of

A

polar molecules that align themselves with the field when an electric field is present, each molecule has its own electric field which the strength of which depends on the dielectrics permittivity

51
Q

what does the field caused by the dielectric do

A

opposes field formed by the capacitor reducing the field so the pd required to charge the capacitor decreases causing capacitance to increase

52
Q

what is represented by the area under a graph of charge against pd

A

energy stored by a capacitor

53
Q

how do you charge a capacitor

A

by connecting it in a circuit with a power supply and a resistor

54
Q

how does a capacitor charge

A

negative charge builds up on the plate connected to the negative terminal

electrons on the opposite plate are repelled by the negative charge so they move to the positive terminal

an equal but opposite charge is formed on each plate causing PD

as charge across plates increase, pd increases

electron flow decreases due to the force of electrostatic repulsion also increasing so current eventually reaches zero

55
Q

how do you discharge a capacitor

A

connect it to a closed circuit with a resistor

56
Q

how can you investigate a capacitor charging, discharging

A

use a data logger to measure values of pd and current to plot graphs of voltage and current against time which you can then draw a graph of charge against time

57
Q

what is time constant

A

the product of resistance and capacitance that is the value of the time taken to discharge a capacitor to 0.37 of initial value or charge to 0.63 of initial value

58
Q

what happens in terms of fields when a current passes through a white

A

a magnetic field is induced

59
Q

what do the field lines of an induced magnetic field when a current passes through a wire form

A

concentric rings

60
Q

what is magnetic flux density (B)

A

measure of the strength of a magnetic field

61
Q

what is the unit for magnetic flux density

A

Tesla

62
Q

what is the definition for one Tesla

A

force of 1N on 1m of a white carrying 1A of current perp to magnetic field

63
Q

what happens when a current carrying wire is placed in a magnetic field

A

a force is exerted on the wire

64
Q

if a current carrying wire is placed in a magnetic field and the current is parallel to the magnetic field what is the force

A

0N

65
Q

what formula is used for magnetic field if the field is perpendicular to current

A

F=BIL

66
Q

what do you use to find the direction of force exerted on a wire when field is perpendicular to the current

A

Flemings left hand rule

67
Q

what fingers are what for lemmings left hand rule

A

thumb - direction of force

first finger - field

second finger - direction of conventional current

68
Q

why is a force exerted on a current carrying wire

A

cause a force acts on charged particles moving in a magnetic field and the wire contains negatively charged moving electrons

69
Q

is the force exerted on a current carrying wire parallel or perpendicular to motion of travel

A

perpendicular

70
Q

why do charged particles follow a circular path when a current carrying wire is in a magnetic field

A

the force induced by the magnetic field acts as a centripetal force

71
Q

what is an application of the circular deflection of charged particles in a magnetic field

A

cyclotron

72
Q

what is a cyclotron and its uses

A

a particle accelerator which can produce ion beams for radiotherapy and radioactive tracers

73
Q

what is a cyclotron formed off

A

two semi circular electrodes called dees with a uniform magnetic field acting perpendicular to the electrodes
high frequency alternating voltage applied between the electrodes

74
Q

what happens to the charged particles in the cyclotron

A

they move from the centre of one of the electrodes and are deflected in a circular path, the speed will not increase due to the magnetic field

75
Q

what happens when particles reach the edge of the electrode in a cyclotron

A

they move across to the other electrode and are accelerated by the electric field so the radius of their circular path will increase. when they move across again the alternating electric field changes direction allowing particles to be accelerated again. repeats until required speed is reached and they exit cyclotron

76
Q

How do you work out the escape velocity

A

Ek = Ep therefore

1/2(m)(v^2)=(GMm)/r

77
Q

What orbit do geostationary satellites follow

A

Geosynchronous

78
Q

What is magnetic flux

A

A value which describes the magnetic field passing through a given area when the field is perpendicular to the area

79
Q

What is magnetic flux linkage

A

Magnetic flux multiplied by the number of turns of a coil

80
Q

What happens with calculating magnetic flux if the field is not perpendicular to a coil of wire

A

Use trigonometry to resolve the components

81
Q

What is the value of magnetic flux when the field is parallel to the coil

A

0

82
Q

What is electromagnetic induction

A

A conducting rod moves relative to a magnetic field, electrons in the rod experience a force and build up on one side of the force causing emf to be induced

83
Q

What are the two laws for electromagnetic induction

A

Faraday and lenz’s law

84
Q

What is faradays law

A

The magnitude of induced emf is equal to the rate of change of flux linkage

85
Q

What is lenz’s law

A

Direction of induced current is such as to oppose the motion causing it

86
Q

How do you demonstrate lenz’s law

A

By measuring the speed of a magnet falling through a coil of wire and its speed when falling from the same height without falling through the coil

87
Q

How does lenz’s law work

A

As magnet approaches coil, there is a change of flux through the coil
so an emf and a current is induced.

The direction of induced current is such as to oppose the motion of the magnet which causes magnet to slow down

As magnet passes through the centre of the coil there is no change in flux so no emf is induced.

As magnet leaves coil there is a change in flux so a current is induced that opposes the motion of magnet

88
Q

What happens when a coil rotates at a constant frequency

A

The emf induced can be calculated using a formula derived from the formula for magnetic flux linkage with respect to time

89
Q

How does flux linkage and angular speed of a coil relate

A

Flux linkage varies depending on the angular speed of the coil

90
Q

What type of current is the emf induced by a coil rotating in a magnetic field

A

Alternating

91
Q

What does an oscilloscope show for a direct current

A

A straight line parallel to the axis

92
Q

For an alternating current what does an oscilloscope shoe

A

Sinusoidal waveform or when time base is turned off it’ll be a vertical line

93
Q

How to measure the peak voltage from an oscilloscope

A

Distance from the equilibrium to the highest/lowest point

94
Q

How to measure peak-to-peak voltage from an oscilloscope

A

Distance from minimum point to maximum point

95
Q

How to measure the root mean square voltage from an oscilloscope

A

Average of al the squares of the possible voltages

96
Q

How to measure the time period from an oscilloscope

A

Distance from one point on a curve to the point where the curve repeated

97
Q

What is electricity supplied to homes in the uk voltage

A

230v

98
Q

What are transformers used for

A

To change the voltage of alternating currents

99
Q

What are transformers made up of

A

A primary coil attached to input voltage and a secondary coil connected to the output voltage and an iron core

100
Q

What is the relation between the voltage in the primary coil and secondary coil to the number of turns on primary coil to the secondary coil

A

They are the same

101
Q

What is a step-up transformer

A

Increases the input by having more turns on the secondary coil than primary

102
Q

What is a step-down transformer

A

Decreases input by having less turns on the secondary coil

103
Q

What are the causes of energy loss in a transformer

A

Eddy currents, resistance, if the core is not easily magnetised

104
Q

What are eddy currents

A

Currents induced by alternating magnetic fields in the primary coil

due to lenz’s laws they oppose the field that produced them

reducing flux density and generating heat causing energy to be lost

105
Q

How can eddy currents effects be reduced

A

By using a laminated iron core because eddy currents cannot pass through an insulator so their amplitude is reduced.

High resistivity material also works

106
Q

How can energy lost in transformers from resistance reduced

A

By using a thick wire with low resistance

107
Q

When transferring electrical power, what is the power lost due to resistance equal to

A

I^2R

108
Q

As voltage is stepped up what happens to current

A

It decreases

109
Q
A

physics (4 decks)

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