magnetic fields

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Author:
ghoran
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293378
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magnetic fields
Updated:
2015-01-20 05:54:22
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physics unit
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  1. permanent magnets have been widely used in
    navigational compasses
  2. the compass needle is a permanent magnet supported so it
    can rotate freely in a plane
  3. when the compass is placed on a horizontal surface
    the needle rotates until one end points approx to the north
  4. magnets exert
    forces on each other
  5. like poles
    repel each other
  6. unlike poles
    attract each other
  7. an electric field exists in the space around
    electric charges
  8. magnetic fields exist in the space around
    a magnet
  9. the magnetic field is a
    vector
  10. the direction of a magnetic field at any point in space is the
    • direction indicated by the pole of a small compass needle placed at that point 
    • so from north to south
  11. a ... has a magnetic field around it
    current
  12. the greater the current the ... the field
    stronger
  13. right hand grip rule
  14. a charge placed in a magnetic field experiences a magnetic force if
    • the charge is moving 
    • the velocity of the moving charge has a component perpendicular to the direction of the magnetic field
  15. the magnetic force is proportional to
    • the field 
    • the charge 
    • the velocity of the particle 
    • the sine of the angle between the field and the direction of motion of the particle 
    • F ∞ qvsin∅
    • F = Bqvsin∅
    • where B is a constant
  16. if the field is perpendicular to the velocity then
    ∅ = 90 so F= Bqv
  17. magnetic filed strength =
    magnetic flux density B
  18. B depends on
    the strength of the magnet
  19. the stronger the magnet the ... the value of B
    greater
  20. B =
    a measure of the strength of a magnetic field
  21. the flux density is equal to
    the magnitude of the force on a unit length of current carrying wire per unit current , unit Tesla T
  22. B is a
    vector and its direction can be determined by using Flemings left hand rule
  23. an alternative unit for the Tesla is
    Wbm-2
  24. one tesla is
    the strength of the magnetic field in which a unit test charge , travelling perpendicular to the magnetic field with a speed of 1ms-1 experiences a force of 1N
  25. Fleming's left hand rule
  26. if B is directed into the page we use a
    X
  27. if B is directed out of the field we use a
    .
  28. current flows in the opposite direction to
    electrons
  29. see diagram from notes but A = cross sectional area , n = number of particles , L = length of wire  
    for each charged particle ..
    • F = Bqvsin∅
    • field is perpendicular to velocity so F=Bqv 
    • total charge Q = nALq 
    • F = BnALqv 
    • from AS nAqv = I 
    • F = BIL when the field is perpendicular to velocity 
    • F = BILsin∅ when magnetic field is at an angle ∅ relative to the velocity of the particle (current of the wire)
  30. charged particles in circular orbits
    • TV tubes
    • accelerators 
    • mass spec
  31. model ans why we get charged particles in circular orbits
    • magnetic field applied is perpendicular to velocity of charged particles
    • magnetic force acts perpendicular to velocity of charged particles 
    • force provides centripetal acceleration towards center of circle
    • r = mv/BQ
    • r is constant when B and v are constant
  32. other points about charged particles in circular orbits
    • direction of motion is impacted by force 
    • speed not impacted by force 
    • kinetic energy not changed by force 
    • force is perpendicular to magnetic field 
    • no work is done by the field on the particle because the force is perpendicular to velocity and W = F X distance moved in direction of force
  33. the cyclotron is a
    particle accelerator and has many uses in medicine , for example they can be used to produce radioactive tracers or high energy beams of radiation for use in radiotherapy
  34. a cyclotron is made up of
    two hollow semicircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes , and an alternating pd applied between the electrodes
  35. charged particles are produced and fired into
    one of the electrodes where the magnetic field makes them follow a semicircular path and then leave the electrode
  36. an applied pd between the electrodes then
    accelerates the particles across the gap until they enter the next electrode
  37. because the particle's speed is slightly higher
    it will follow a circular path with a radius larger before leaving the electrode again
  38. at this point the direction of the pd will have
    been reversed and so the particle is accelerated again before entering the next electrode . this process repeats as the particle spirals outwards
  39. picture on the screen of a color TV is formed as the result of
    electron beams in the tube scanning the screen inside the tube
  40. beams are controlled by
    magnetic fields produced coils outside the tube
  41. changing current through a coil changes the
    strength of the magnetic field
  42. hence the
    direction of the electron beams is changed
  43. the path of the electron beams can be seen where it
    passes over the fluorescent screen in the tube
  44. the beam is deflected downwards when
    a magnetic field is directed into the plane of the plane of the screen
  45. hall probes measure
    B
  46. a piece of semiconducting material is placed in a a magnetic field with field lines perpendicular to the flat side a constant current passes through the slice and electrons are deflected downwards as a result
    • a pd is created between top and bottom edges of the slice 
    • F = BQv 
    • F = VQ/d because F = EQ and E = V/d 
    • VQ/d = BQv
    • V/d = Bv 
    • B and d are constant so V is proportional to B
  47. beam of electrons produced by an
    electron gun
  48. electron gun consists of
    an electrically heated filament wire near positively charged metal anode which attracts electrons emitted by the hot filament wire
  49. emission is called
    thermionic emission
  50. electrons pass through a small hole in the anode to form beam . the greater the pd between the anode and and filament wire
    the higher the speed of electrons when they reach the anode
  51. what rely on thermionic emission
    oscilloscope , cathode ray TV tube , magnetron valve used in microwave cookers and radar systems
  52. mass spec used to analyse the
    type of atom present in a sample
  53. how mass spec works
    • atoms ionized
    • directed at same velocity into uniform magnetic field 
    • deflected in semicircle by magnetic field onto a detector 
    • path of ion depends on Q/m in accordance with equation r = mv/BQ 
    • detector linked to pc which shows relative abundance of each ion
  54. velocity sector consist of a magnet and pair of parallel plates at a spacing d and voltage V
    F =
    • BQv
    • F also = QV/D
  55. ions moving a such a velocity that
    • BQv = QV/d experience equal and opposite forces so pass through undeflected 
    • all other ions are deflected and pass through collimeter slit 
    • beam emerging from collimeter consists of different ions but they all have same speed v = V/dB
  56. electric motor
    simple electric motor consists of a coil of insulated wire which spins between the the poles of a U shaped wire
  57. when a dc current passes round the coil :
    • the wires at opposite edges of the coil are acted on by forces in opposite directions
    • the force on each edge makes the coil spin about its axis
    • current is supplied to coil via a split ring commutator 
    • the direction of the current round the coil is reversed by the split ring commutator each time the coil rotates through half a turn 
    • this ensures the current along the edge changes direction when it moves from one pole to another
    • the result is the force on each edge continues to turn the coil in the same direction
  58. magnetic field lines are called
    • flux (∅) in Webber's (Wb)
    • magnetic flux density = magnetic field strength
    • B =  ∅/A = Wbm-2
  59. in circular orbit time for one full cycle =
    • r = mv/BQ 
    • v = rBQ/m 
    • v = s/t 
    • T = s/v 
    • T = 2pir x m/BQr 
    • T = 2pim/BQ
  60. this tells us that
    the period needed by the charged particles to complete an orbit is constant regardless of the values of r and v

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