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A LEVEL PHYSICS > save my exams electromagnetism > Flashcards

save my exams electromagnetism Flashcards

1
Q

wha is a magnetic field?

A
  • it is a region of space where a magnetic pole will experience a force
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2
Q

how are magnetic fields created?

A
  • permanent magnets
  • moving electric charge ( ie current )
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3
Q

what is magnetic flux density?

A
  • the number of magnetic flux lines passing through a region of space per unit area
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4
Q

what is 1 tesla?

A
  • the flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the flux
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5
Q

what can we deduce about flux density and field lines?

A
  • higher flux density, stronger magnetic field so lines are closer together

vice versa

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6
Q

explain why a current carrying wire experiences a force?

A
  • current- carrying conductor produces its own magnetic field ( movement of electrons )
  • when interacting with an external B field it experiences a force
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7
Q

what is the equation for the force on a current-carrying wire?

A

F = BILsin@

F = force on current carrying wire ( N )
B = magnetic flux density ( B )
I = current ( A )
L = length of conductor ( m )
@ = angle between conductor and applied B field

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8
Q

how can we use flemings left hand rule to deduce the direction of the force on a current carrying wire?

A

thumb - direction of force
first finger - direction of magnetic field
second finger - direction of conventional current

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9
Q

why is there a force on a moving charge?

A
  • a moving charge produces its own magnetic field
  • when interacting with an external magnetic field it will experience a force
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10
Q

what is the equation for a force on a charged particle in a magnetic field?

A

F = BQVsin@

F = magnetic force on particle ( N )
B = magnetic flux density ( T )
Q = charge of the particle ( C )
v = speed of the particle (ms^-1)

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11
Q

how can we use FLHR for direction of force on a moving charge?

A

thumb - direction of force
first finger - direction of magnetic field
second finger - flow of positive charge

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12
Q

why do charged particles undergo circular motion?

A
  • charged particles will experience a force perpendicular to is direction of motion.
  • ie a centripetal force
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13
Q

derive the equation for a charged particles radius within a magnetic field

A

F = mv^2 / r and F = BQv

mv^2/r = BQv

mv/r = BQ

r = mv / BQ

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14
Q

what does the r = mv/BQ equation tell us?

A
  • radius is proportional to the velocity
  • radius is proportional to the mass
  • radius is inversely proportional to the charge
  • radius is inversely proportional to the magnetic field strength
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15
Q

what are cyclotrons?

A
  • particle accelerators that make use of circular trajectory to create a spiral path
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16
Q

what are cyclotrons made up of?

A
  • two semi circular dees
  • uniform magnetic field
  • alternating electric field
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17
Q

what are uses of cyclotrons in medical industry?

A
  • produce tracers for imaging ( as low half lives )
  • high energy beams for radiotherapy
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18
Q

what happens when the particles enter the dees?

A
  • magnetic field applied perpendicular to the path of the particles
  • magnetic force acts in a direction perpendicular to the field and path of the particles
  • particles undergo circular motion
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19
Q

what happens when a particle crosses a gap between the dees?

A
  • electric field causes particle to accelerate to opposite dee
  • causing the particles to enter the dee with a greater v so greater r
20
Q

what happens when a particle leaves the dee?

A
  • constant speed in the dees
  • time spent inside the dees are constant
  • direction of e field alternates each time particles reach a gap
  • process will repeat until they leave the cyclotron
21
Q

what is electromagnetic induction?

A
  • the process of inducing an emf in a conductor
  • when there is movement between a charge and a magnetic field
22
Q

when is there electromagnetic induction?

A
  • when a conductor CUTS through magnetic field lines
23
Q

what is magnetic flux?

A
  • the product of the magnetic flux density and the cross sectional area perpendicular to the direction of the magnetic flux density
24
Q

what is the equation for magnetic flux?

A

$ = BA

$ = magnetic flux ( Wb )
B = magnetic flux density ( T )
A = cross sectional area ( m^2 )

25
Q

what is magnetic flux linkage?

A
  • the product of the magnetic flux and the number of turns on the coil
26
Q

recap, what is electromagnetic induction?

A
  • the process by which an emf is induced when a conductor moves through a magnetic field
27
Q

what is faradays law?

A
  • the magnitude of the emf induced is proportional to the rate of change of flux
28
Q

what is lenzs law?

A
  • the direction of the induced emf is such that to oppose the motion that caused it
29
Q

what does the gradient of a flux time graph give us?

A
  • the emf
30
Q

what is the equation for induced emf for a straight conducting rod moving through a magnetic field?

A

emf = Blv

B = magnetic field strength
L = length of conductor
v = velocity

31
Q

how is an alternating voltage induced with a rotating coil?

A
  • when a coil rotates in a magnetic field, flux through coil will vary
  • as emf is rate of change of flux linkage, emf will change as it rotates
  • maximum when coil cuts through most field lines
  • minimum when coil cuts through least or zero
32
Q

explain the phase relationship between flux linkage and emf for a rotating coil

A

90 degrees out of phase as:

  • flux linkage maximum when angle = 0 or 180
  • zero when angle = 90 or 270

emf is zero at max flux, as flux isnt changing

33
Q

what is meant by alternating current?

A
  • current which periodically varies between a positive to a negative value with time
34
Q

what is peak voltage? or peak current?

A
  • maximum value of the alternating current or voltage
35
Q

what is peak-to-peak voltage?

A
  • distance between a positive and consecutive negative peak

( i.e double the magnitude of the peak voltage )

36
Q

what is root mean square current?

A
  • square root of the mean of the squares of all the values of current in one cycle

Irms = Io / root 2

37
Q

what is root mean square voltage?

A
  • the square root of the mean of the squares of all the values of the current in one cycle
38
Q

what is a transformer?

A
  • a device which changes high alternating voltage at low current, to low alternating voltage at high current
39
Q

why is the iron core necessary?

A
  • directs the magnetic field passing from primary coil to the secondary coil
40
Q

why is an emf induced in the secondary coil?

A
  • in primary coil, alternating current produces alternating voltage, creating an alternating magnetic field inside the core, therefore a changing magnetic flux
  • a changing magnetic field passes to the secondary coil through the iron core
  • resulting in a change of magnetic flux linkage within the coil so emf induced
41
Q

give the transformer equation?

A

Ns / Np = Vs / Vp

42
Q

what is a step up transformer

A

Ns > Np, increases voltage of power source
( used between power stations and transmission cables )

43
Q

what is a step down transformer ?

A

Np > Ns, decreases voltage of power source
( used between transmission lines and buildings )

44
Q

in reality transformers are not 100% efficient, give the equation for an ideal transformer efficiency

A

( IsVs / IpVp )x 100

45
Q

how do eddy currents arise?

A
  • a changing magnetic field from alternating current
  • creating a changing magnetic field in the core that acts against the field that induced them
  • emf is therefore induced
  • a current flows, as core is made from conducting material
46
Q

eddy currents dissipate energy by generating heat in the wires, how can we reduce this

A
  • laminate the core
  • use high resistivity material

A LEVEL PHYSICS (14 decks)

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