G494/5 Flashcards
What is the convention for magnetic field lines
They point from north poles to south poles
What is the convention for North poles
Current travels anticlockwise when you look at a north pole
What is the field like within a solenoid?
Uniform, but around the edges it is like a bar magnet
How can we tell from the current direction what direction the mag field will act in
Right hand grip rule, thumb = current, fingers = field
What is flux and what is its unit
=BA = Webber
What does lenses law say
The direction of induced emf will be such that it opposes the changes creating it. A conservation of energy argument
What is permeability
A material property, how good is the material at confining magnetic fields to it
What is permeance
An object property, how good is the object at keeping flux in it
How is permeance calculated (2 ways)
Permeance = flux/current turns Permeance = permeability x (A/L)
What shape of object has a high permeance
Short and fat
What must be conserved in a transformer
Power in = power out, so IV in = IV out
What are the two ways induced currents can cause a system to lose energy
Can do work, as forces between poles can act over a distance. Also heat energy =I2R is lost due to currents
What are the primary and secondary voltages like in transformers (phase difference)
180
What is the transformer equation
Tp/Vp = -Ts/Vs
How are eddy currents generated in magnetic fields
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
How do we reduce the effects of eddy currents
Laminate the core, as increases resistance, and decreases current, and extend of currents
How is I related to flux
Proportional. Up to a limit, if increase I too much, B doesn’t keep increasing, but saturates, as all magnetic domains line up
How do we derive the transformer equation
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
What is a generator
Uses mechanical work to produce electricity
What is a motor
Uses electricity to produce torque
How does a generator work
Spin a permanent magnet near a coil, coil experiences a change in flux, emf induced
Why are generators hard to spin
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.
How do we quantify the direction of a force produced by a current in a conductor
Using left hand rule. First finger = field, which follows field lines from N to S. second finger = conventional current, and thumb = direction of force
How does a simple DC motor work
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
What does back emf do in a simple DC motor
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
How do we know to use the left hand rule for motors
We motor on the left in Britain
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
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
What is flux density?
Flux/A = magnetic field strength
What is the unit of mag field strength
Tesler, or Wbm-2
What is flux linkage
Nthi
What is a three phase generator
A stator with 3 pairs of coils. ?
How do we find max emf from a graph of flux against time
The max gradient of the flux graph x number of turns
How does a three phase generator work
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
How does a rotating field motor work
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
How does a squirrel cage motor work
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
What do flux lines try to do
Get shorter and straighter
Where should our arrows be on flux lines
From north poles to south poles
When is the max emf induced in comparison to flux cut
Max emf is when no flux is cut, as if flux is a sin graph, gradient is steepest at 0
What happens to back emf when a motor is first switched on
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
How is power out and back emf linked
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
Electric field strength definition
Force per unit charge
Electric potential definition
Energy required per unit charge to bring a charge from infinity to that point in the field
How should electric field lines be drawn
Show where a positive test charge would go, radiate from a pos charge, or point from + voltage -0V
What should potential lines look like
Intersect field lines at 90 degrees
Is field strength around a point charge (+) positive or negative
Positive, as force acts in same direction as R
Is field strength around a point charge (-) positive or negative
Negative, as attractive force
How do we prove that the field strength between 2 parallel plates is V/d
Work done =FxD = QV so force per unit charge = V/d
What is the KE gained by a particle moving through a voltage
=QV
In milikans oil drop experiment, what is the downwards and upwards forces
Down = mg Up = QE
What is relativistic momentum and how does its magnitude vary compared with normal momentum
=gamma mv, always bigger than normal momentum
What is the total energy of a particle equal to (2 things)
ɣmc2 or rest energy + kinetic energy
What is an expression for gamma in terms of energies
ɣ= Etotal/Erest
How do we find relativistic KE
Either re-arrange ɣ=Etotal/Erest, or use GV, or change in potential for grav fields
For a charge travelling in a mag field how do we find the radius
Mv2/r=Bqv
What is the force acting on a charge moving through a field, and how do we know its direction
F=Bqv (v= velocity) use left hand rule, current = flow of + charge
How does a linear accelerator work?
Has alternating electrodes with alternating signs. Particles travel into next electrode, as enter middle voltage suddenly swaps sign, so accelerated to the next one
How do electrodes in linear accelerators have to change
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
How does a cyclotron work
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.
How do we find the time period in a cyclotron and what does this tell us about the supply that needs to drive it
R = mv/Bq t=2pimv/bq/v = 2pim/bq
Same regardless of speed, so a constant freq ac needed
What is a synchrotron
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
What are edge effects
Field strength line bow out edges, as weaker field, so spacing between lines gets smaller
What is the field strength around a point charge
E=kQ/r2
What is the potential around a point charge
V = kQ/r
What is the field strength inside a hollow sphere of charge
Constant, as no energy required to move between points in the sphere
How do you add field strengths
As vectors
How do you add potential energies
As scalars
How does a velocity selector work
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
What is antimatter
Something that combines with matter leaving only energy. Every property opposite to matter except mass
What rules does annihilation follow
Conservation of energy, and momentum, conservation of lepton, baryon and charge
How does annihilation work
A particle and its antiparticle collide and produce 2 gamma particles. 2 must be produced to conserve momentum
What is the momentum of a photon
P=E/c actually true for anything going near speed of light so rest mass is negligible
What is the energy transfer in annihilation
2mc2= 2hf
What is creation
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
what is the energy transfer in creation
Hf>2mc2
How do PET scans work
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
What changes the length of a quantum arrow
Quantum arrow length is proportional to charge
What gives the probability of an event happen
Probability = length of quantum squared ^2
How do electromagnetic forces get carried
Boson is the virtual photon, which has only small amounts of mass, and no charge
What does a feynmann diagram represent
2 particles trying everything allowed, the arrows for all the paths combined give the quantum amplitude for an event
On feynman diagrams how do we represent bosons
With curly arrows
On feynman diagrams how do we represent fermions
With a straight arrow
What is a fermion
½ integer spin, ie made of three quarks. Basically all normal matter, protons, electrons, neutrinos, and all antimatter
What is a boson
A force carrier particle, with integer spin (hence mesons)
What are the main types of bosons
Photon = carrier of EM force
Gluon = carrier of strong force
Z W+ and W- = weak force
Graviton = gravitational force
What is the pauli exclusion principle
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
What does the pauli principle have to do with electrons
2 electrons can only be in same orbital if of opposite spin
Do bosons obey the pauli principle?
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
What was the problem with energy conservation in beta decay
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
What is the weak interaction due to (simply)
A quark changing flavour
What quarks are protons made of
uud
What quarks are neutrons made of
udd
What is the w+ boson, and when is it formed
Formed as an up quark goes to a down quark, Ie in B+ decay, and is very massive. Later decays to a positron + neutrino
What is the w- boson, and when is it formed
Formed as a down quark goes to a up quark, Ie in B- decay, and is very massive. Later decays to a electron + antineutrino
What is a Z0 boson
A large photon, a neutrino interacts with an electron in an atom by exchanging a Z boson
What are the subcategories of fermions
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
Charge on an up quark
+2/3
Charge on a down quark
-1/3
6 types of quark
Up, down, strange, charm, top, bottom
Charge on strange
-1/3
Charge on charm
+2/3
Charge on top
+2/3
Charge on bottom
-1/3
what quarks are -1/3
Bottom down strange
What quarks are +2/3
Charm up top
How many generations of particles are there
3
What quarks fit each generation
Up/down = 1st generation Strange/charm = 2nd generation Top/bottom = 3rd generation
What leptons fit each generation
Electron/electron neutrino = 1st generation
Muon/muon neutrino= 2nd generation
Tau/tauneutrino= 3rd generation
What do higher generations have
Higher energies, and more unstable
What is colour charge
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
What are gluons and how are they different from virtual photons
They are bosons, but as well as exchanging colour charge they carry colour charge as well whereas photons don’t carry charge.
Why is it impossible to separate quarks
Quark field gets stronger as you pull apart, eventually put enough energy in, new quark/antiquark pairs get created, and field rebounds
What was surprising about the results of Rutherford scattering
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.
What further tests was run with Rutherford scattering
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
What has happened at the point of closest approach
All KE has been converted to potential energy.
How much KE is needed to get to a distance r?
KE >PE=kZe2e/r
How do you find out size of nucleus in Rutherford scatt
Know KE of alpha particles, so know Z, so can work out r
What is the force between an alpha particle and a nucleus, what is its direction and how strong is it
Acts on a line between centres, F=kqQ/r2