# magnetic fields

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 Author: ghoran ID: 293378 Filename: magnetic fields Updated: 2015-01-20 05:54:22 Tags: physics unit Folders: Description: revision Show Answers:

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1. permanent magnets have been widely used in
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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