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G494/5 Flashcards

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1
Q

What is the convention for magnetic field lines

A

They point from north poles to south poles

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2
Q

What is the convention for North poles

A

Current travels anticlockwise when you look at a north pole

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3
Q

What is the field like within a solenoid?

A

Uniform, but around the edges it is like a bar magnet

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4
Q

How can we tell from the current direction what direction the mag field will act in

A

Right hand grip rule, thumb = current, fingers = field

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5
Q

What is flux and what is its unit

A

=BA = Webber

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6
Q

What does lenses law say

A

The direction of induced emf will be such that it opposes the changes creating it. A conservation of energy argument

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7
Q

What is permeability

A

A material property, how good is the material at confining magnetic fields to it

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8
Q

What is permeance

A

An object property, how good is the object at keeping flux in it

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9
Q

How is permeance calculated (2 ways)

A 
Permeance = flux/current turns
Permeance = permeability x (A/L)
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10
Q

What shape of object has a high permeance

A

Short and fat

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11
Q

What must be conserved in a transformer

A

Power in = power out, so IV in = IV out

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12
Q

What are the two ways induced currents can cause a system to lose energy

A

Can do work, as forces between poles can act over a distance. Also heat energy =I2R is lost due to currents

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13
Q

What are the primary and secondary voltages like in transformers (phase difference)

A

180

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14
Q

What is the transformer equation

A

Tp/Vp = -Ts/Vs

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15
Q

How are eddy currents generated in magnetic fields

A

In the iron, get a changing flux that induces an emf, as iron is a conductor this causes currents to flow. These currents act as to oppose the changes causing them, ie the reduce the voltage in the primary coil, which reduces I which reduces flux.
Also cause heating effects = I^2R

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16
Q

How do we reduce the effects of eddy currents

A

Laminate the core, as increases resistance, and decreases current, and extend of currents

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17
Q

How is I related to flux

A

Proportional. Up to a limit, if increase I too much, B doesn’t keep increasing, but saturates, as all magnetic domains line up

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18
Q

How do we derive the transformer equation

A

Say that in the primary coil, the back emf = the forward emf. Say in the secondary coil that the emf induced= Ndthi/dt. Same flux goes through both coils, so make dthi/dt the subject for both, eliminate this

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19
Q

What is a generator

A

Uses mechanical work to produce electricity

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20
Q

What is a motor

A

Uses electricity to produce torque

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21
Q

How does a generator work

A

Spin a permanent magnet near a coil, coil experiences a change in flux, emf induced

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22
Q

Why are generators hard to spin

A

The current induced follows the right hand rule, the current wants to reduce the change causing it, ie wants to make the generator spin slower. Current in a conductor has a force =bil, force acts in opposite direction as you are spinning it.

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23
Q

How do we quantify the direction of a force produced by a current in a conductor

A

Using left hand rule. First finger = field, which follows field lines from N to S. second finger = conventional current, and thumb = direction of force

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24
Q

How does a simple DC motor work

A

Have a current going around in a square shaped piece. Induces a force, force takes it to top, has momentum, would flip, but then flip back again
So have a split ring commutator, when the wire reaches the top, it becomes in contact with the other bit of the circuit, so the current changes direction, so it keeps spinning in one direction . split ring brushes wear out

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25
Q

What does back emf do in a simple DC motor

A

As the wire spins in the mag field, emf induced, works to counteract the emf/current going in, so reduces force. So spins but draws no power as long as no friction. Once loaded, the back emf decreases a lot so power starts to be drawn, and the force is larger

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26
Q

How do we know to use the left hand rule for motors

A

We motor on the left in Britain

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27
Q

What is an equation for flux cut by a spinning ring when theta is the angle between the normal to the ring and the mag field. And how can we get an expression for max emf from this

A

Flux cut = BACosθ as when theta = 0 no flux cut
Θ = ωt, ie angle covered per second x time
Emf = -dthi/dt = + ωtBasin ωt
Emax = NBA ω

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28
Q

What is flux density?

A

Flux/A = magnetic field strength

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29
Q

What is the unit of mag field strength

A

Tesler, or Wbm-2

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30
Q

What is flux linkage

A

Nthi

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31
Q

What is a three phase generator

A

A stator with 3 pairs of coils. ?

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32
Q

How do we find max emf from a graph of flux against time

A

The max gradient of the flux graph x number of turns

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33
Q

How does a three phase generator work

A

A stator with 3 pairs of coils. A permanent magnetic spins and agitates each coil in turn, produced 3 emf outputs 120 degrees out of phase from each other

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34
Q

How does a rotating field motor work

A

Have a stator with 3 pairs of coils, and a permanent magnet in the middle. Permanent magnetic is pulled around by the poles created in the coil

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35
Q

How does a squirrel cage motor work

A

Have 3 pairs of coils, and an iron rotor covered in conducting plates and bars. The rotor experiences a change in flux, which creates an emf in the conducting end plates, that creates a current that pulls the rotor round, trying to minimise the change in flux. Tries to keep up with the rotating flux arrow

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36
Q

What do flux lines try to do

A

Get shorter and straighter

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37
Q

Where should our arrows be on flux lines

A

From north poles to south poles

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38
Q

When is the max emf induced in comparison to flux cut

A

Max emf is when no flux is cut, as if flux is a sin graph, gradient is steepest at 0

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39
Q

What happens to back emf when a motor is first switched on

A

To start with, draws a lot of current but quickly drops because as the speed increases, so does the back emf. Gets to the point where draws no current. Then as loaded, starts drawing current. Effective voltage = voltage in – back emf

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40
Q

How is power out and back emf linked

A

Voltage in –back emf) =IR
Voltage in x I –backemf x I = power lost through heating
So power in –power lost = back emf x I
So useful power = back emf x current

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41
Q

Electric field strength definition

A

Force per unit charge

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42
Q

Electric potential definition

A

Energy required per unit charge to bring a charge from infinity to that point in the field

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43
Q

How should electric field lines be drawn

A

Show where a positive test charge would go, radiate from a pos charge, or point from + voltage -0V

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44
Q

What should potential lines look like

A

Intersect field lines at 90 degrees

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45
Q

Is field strength around a point charge (+) positive or negative

A

Positive, as force acts in same direction as R

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46
Q

Is field strength around a point charge (-) positive or negative

A

Negative, as attractive force

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47
Q

How do we prove that the field strength between 2 parallel plates is V/d

A

Work done =FxD = QV so force per unit charge = V/d

48
Q

What is the KE gained by a particle moving through a voltage

A

=QV

49
Q

In milikans oil drop experiment, what is the downwards and upwards forces

A 
Down = mg
Up = QE
50
Q

What is relativistic momentum and how does its magnitude vary compared with normal momentum

A

=gamma mv, always bigger than normal momentum

51
Q

What is the total energy of a particle equal to (2 things)

A

ɣmc2 or rest energy + kinetic energy

52
Q

What is an expression for gamma in terms of energies

A

ɣ= Etotal/Erest

53
Q

How do we find relativistic KE

A

Either re-arrange ɣ=Etotal/Erest, or use GV, or change in potential for grav fields

54
Q

For a charge travelling in a mag field how do we find the radius

A

Mv2/r=Bqv

55
Q

What is the force acting on a charge moving through a field, and how do we know its direction

A

F=Bqv (v= velocity) use left hand rule, current = flow of + charge

56
Q

How does a linear accelerator work?

A

Has alternating electrodes with alternating signs. Particles travel into next electrode, as enter middle voltage suddenly swaps sign, so accelerated to the next one

57
Q

How do electrodes in linear accelerators have to change

A

Have to get longer, and further apart, as particles travel more distance in same time as they get faster. But as approach speed of light, can stay the same distance apart, as speed constant

58
Q

How does a cyclotron work

A

Has two half circles with opposite charges with a mag field at 90 degrees to the circle. Charge accelerated by opposite sign plate, and mag field works to accelerate in a circle. As arrives at other side, suddenly changes sign of plate.

59
Q

How do we find the time period in a cyclotron and what does this tell us about the supply that needs to drive it

A

R = mv/Bq t=2pimv/bq/v = 2pim/bq

Same regardless of speed, so a constant freq ac needed

60
Q

What is a synchrotron

A

Like CERN, particles accelerate, but keep same radius, so freq of AC constantly changes until particles up to speed of light. Large radius needed to reduce energy losses due to synchrotron radiation

61
Q

What are edge effects

A

Field strength line bow out edges, as weaker field, so spacing between lines gets smaller

62
Q

What is the field strength around a point charge

A

E=kQ/r2

63
Q

What is the potential around a point charge

A

V = kQ/r

64
Q

What is the field strength inside a hollow sphere of charge

A

Constant, as no energy required to move between points in the sphere

65
Q

How do you add field strengths

A

As vectors

66
Q

How do you add potential energies

A

As scalars

67
Q

How does a velocity selector work

A

Have a mag field and a electric field at 90 degrees to each other. Up force = QE, down force = BqV, only for particles E=BV are particles not deflected, so only ones with these velocities pass through the small slit

68
Q

What is antimatter

A

Something that combines with matter leaving only energy. Every property opposite to matter except mass

69
Q

What rules does annihilation follow

A

Conservation of energy, and momentum, conservation of lepton, baryon and charge

70
Q

How does annihilation work

A

A particle and its antiparticle collide and produce 2 gamma particles. 2 must be produced to conserve momentum

71
Q

What is the momentum of a photon

A

P=E/c actually true for anything going near speed of light so rest mass is negligible

72
Q

What is the energy transfer in annihilation

A

2mc2= 2hf

73
Q

What is creation

A

When a particle-antiparticle pair are created from just gamma photons. Can technically collide 2 gamma photons, but highly unlikely to be in same tiny region of space. Happens when a gamma photon travels near a nucleus, nucleus interacts, and this allows momentum to be conserved

74
Q

what is the energy transfer in creation

A

Hf>2mc2

75
Q

How do PET scans work

A

Label an organic molecule with O-15. Decays by B+ decay. Positron quickly meets an electon in the brain, emmiting 2 gamma photons, which are detected by scintillators and then photomultipliers. Shows brain activity

76
Q

What changes the length of a quantum arrow

A

Quantum arrow length is proportional to charge

77
Q

What gives the probability of an event happen

A

Probability = length of quantum squared ^2

78
Q

How do electromagnetic forces get carried

A

Boson is the virtual photon, which has only small amounts of mass, and no charge

79
Q

What does a feynmann diagram represent

A

2 particles trying everything allowed, the arrows for all the paths combined give the quantum amplitude for an event

80
Q

On feynman diagrams how do we represent bosons

A

With curly arrows

81
Q

On feynman diagrams how do we represent fermions

A

With a straight arrow

82
Q

What is a fermion

A

½ integer spin, ie made of three quarks. Basically all normal matter, protons, electrons, neutrinos, and all antimatter

83
Q

What is a boson

A

A force carrier particle, with integer spin (hence mesons)

84
Q

What are the main types of bosons

A

Photon = carrier of EM force
Gluon = carrier of strong force
Z W+ and W- = weak force
Graviton = gravitational force

85
Q

What is the pauli exclusion principle

A

2 fermions cannot exist at exactly the same point in space time and in the same quantum state, as their phases are opposite, so the quantum arrow length is 0, so 0 probability

86
Q

What does the pauli principle have to do with electrons

A

2 electrons can only be in same orbital if of opposite spin

87
Q

Do bosons obey the pauli principle?

A

No, their quantum arrows add with the same phase, they are inclusive, like to be found in the same point in space/time. Ie in a laser lots of photons exist in same place and polarisation

88
Q

What was the problem with energy conservation in beta decay

A

Neutron -> proton + electron releases energy, as neutron is heavier than proton, so all electrons should come off with the energy difference, all at the same speed, but they didn’t. as antineutrino takes some energy

89
Q

What is the weak interaction due to (simply)

A

A quark changing flavour

90
Q

What quarks are protons made of

A

uud

91
Q

What quarks are neutrons made of

A

udd

92
Q

What is the w+ boson, and when is it formed

A

Formed as an up quark goes to a down quark, Ie in B+ decay, and is very massive. Later decays to a positron + neutrino

93
Q

What is the w- boson, and when is it formed

A

Formed as a down quark goes to a up quark, Ie in B- decay, and is very massive. Later decays to a electron + antineutrino

94
Q

What is a Z0 boson

A

A large photon, a neutrino interacts with an electron in an atom by exchanging a Z boson

95
Q

What are the subcategories of fermions

A

Split into leptons (which have a lepton number and are not affected by the strong nucleur force) and hadrons which are affected by the strong nucleur force, and are made of quarks. These are then subdivided into baryons (3 quarks) and mesons (2 quarks) which aren’t actually fermions at all

96
Q

Charge on an up quark

A

+2/3

97
Q

Charge on a down quark

A

-1/3

98
Q

6 types of quark

A

Up, down, strange, charm, top, bottom

99
Q

Charge on strange

A

-1/3

100
Q

Charge on charm

A

+2/3

101
Q

Charge on top

A

+2/3

102
Q

Charge on bottom

A

-1/3

103
Q

what quarks are -1/3

A

Bottom down strange

104
Q

What quarks are +2/3

A

Charm up top

105
Q

How many generations of particles are there

A

3

106
Q

What quarks fit each generation

A 
Up/down = 1st generation
Strange/charm = 2nd generation
Top/bottom = 3rd generation
107
Q

What leptons fit each generation

A

Electron/electron neutrino = 1st generation
Muon/muon neutrino= 2nd generation
Tau/tauneutrino= 3rd generation

108
Q

What do higher generations have

A

Higher energies, and more unstable

109
Q

What is colour charge

A

A property quarks have. Colour is red, green or blue (RGB) or antired, antigreen, antiblue.
The exchange of colour charge uses gluons, which are responsibly for holding the nucleus together

110
Q

What are gluons and how are they different from virtual photons

A

They are bosons, but as well as exchanging colour charge they carry colour charge as well whereas photons don’t carry charge.

111
Q

Why is it impossible to separate quarks

A

Quark field gets stronger as you pull apart, eventually put enough energy in, new quark/antiquark pairs get created, and field rebounds

112
Q

What was surprising about the results of Rutherford scattering

A

Only 1 in 10,000 particles rebounded. They were extremely high energy, something very small and charged must be in the nucleus. Only head on collisions deflect, so showed most collisions weren’t head on, so not many nucleus’ per unit area, but ones that do exist are very very small.

113
Q

What further tests was run with Rutherford scattering

A

Slow particles down using a mica sheet, more should be deflected, as force acts for a longer time
Use a less + charged nucleus, less should be deflected
Number deflected at each angle should follow 1/r2 rule

114
Q

What has happened at the point of closest approach

A

All KE has been converted to potential energy.

115
Q

How much KE is needed to get to a distance r?

A

KE >PE=kZe2e/r

116
Q

How do you find out size of nucleus in Rutherford scatt

A

Know KE of alpha particles, so know Z, so can work out r

117
Q

What is the force between an alpha particle and a nucleus, what is its direction and how strong is it

A

Acts on a line between centres, F=kqQ/r2

physics a2 (10 decks)

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