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Physics P1-P5 > P4.2 - Uses of Magnetism > Flashcards

P4.2 - Uses of Magnetism Flashcards

in line with spec

1
Q

what is the ‘motor effect’?

A

when two ‘magnets’ exert a force upon each other

  • but remember a wire can also be a magnet (ie. have a magnetic field produced around it)
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2
Q

what is produced when a current flows in a wire?

A

a magnetic field around it

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3
Q

what is the equation to calculate the size of the force exerted on a wire?

  • first answer which factors it depends on
A

strength of magnetic field, current in wire, length of wire

F = B x I x L

b = magnetic flux density
i = current 
l = length of conductor
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4
Q

what is the unit for magnetic flux density?

A

Tesla (T)

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5
Q

which rule is used to calculate the direction of the force on the wire?

A

fleming’s left hand rule

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6
Q

which finger is which on fleming’s left hand rule?

A 
first = field 
second = current 
thumb = force
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7
Q

how does a magnet and current-carrying wire exert a force on one another?

A
  • whenever a current flows in a wire a magnetic field is generated around it
  • therefore it has magnetic field lines
  • the field lines of the magnet and current-carrying wire interact and exert a force on one another
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8
Q

how do you determine whether forces applied between magnets (or a wire) is repulsive or attractive?

A
  • if currents in wire in same direction = attractive

- if currents in wire in opposite directions = repulsive

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9
Q

what does a split ring commutator do?

and why does it do that

A
  • reverses the direction of the current in the coil every half turn of the coil
    (within a motor)
  • but allows current to flow the same way to the battery
  • to ensure the coil rotates in the same direction (and not flip-flop)
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10
Q

how would you increase the speed of a motor?

A
  • increase the size of the force acting on the wire

SO

  • increase current in coil
  • increase strength of magnet/ B-field
  • have more loops
  • longer length of wire
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11
Q

describe how a motor rotates

A

1) the motor contains a wire with a current running through it within a magnetic field
2) therefore the wire experiences a force upon it
3) as current flows in different directions of the coil, opposite forces act upon it which causes it to rotate
4) it reaches the middle and is now upright
5) as the current is no longer running perpendicular to the magnetic field, no force is produced
6) therefore, its momentum turns it over to the other side
7) once rotated, both sides would have switched position and would rotate the opposite direction to before
8) to prevent the coil from flip-flopping, a split ring commutator reverses the current direction every half turn (so the coil can rotate in the same direction)

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12
Q

how would increasing the current in a wire increase the force on the coil?

A
  • larger current = greater magnetic field line density around wire (current in wire = magnetic field lines)
  • and so greater interaction between wire and permanent magnet’s field lines
  • interaction is exerting a force on one another*
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13
Q

what does an electrical motor use the motor effect to do?

A

transfer electrical energy to kinetic energy

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14
Q

what is electromagnetic induction?

A

when a conductor cuts the magnetic field lines of a magnet, and a potential difference is induced across the end of the conductor

OPPOSITE OF MOTOR EFFECT
kinetic energy -> electrical energy

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15
Q

describe how electromagnetic induction produces a current

A

1) when a wire cuts magnetic field lines perpendicularly
2) electrons are moved to one side of the conductor
3) which produces a potential difference
4) and so if the wire is part of a complete circuit, a current would flow

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16
Q

what process is used for generators to produce electricity?

A

electromagnetic induction

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17
Q

what is the main principle to increase the size of a potential difference produced?

A

increase the RATE of cutting magnetic field lines

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18
Q

what 3 factors increase the size of p.d generated within a wire?

A
  • increase number of coils in the wire (more magnetic field lines produced)
  • use a stronger magnet
    (denser field lines)
  • move the wire quicker through the magnetic field (more field lines cut)
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19
Q

how would reversing the direction of the movement of a wire affect the voltage?

A

reverse direction of induced voltage

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20
Q

would a potential difference be induced if a wire was moving parallel to the magnetic field lines?

A

no, they must be perpendicular (to cut field lines)

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21
Q

the direction of an induced potential difference will always ………. the change producing it

A

oppose

LENZ’S LAW - so law of conservation of energy is not violated

22
Q

give an example of how the direction of induced potential difference will oppose the change producing it

A
  • bring a magnet with its north pole to the p.d induced coil of wire
  • the coil of wire would give rise to a north pole
  • which causes them both to REPEL
  • OPPOSING THE MOTION (so energy is conserved)
23
Q

what is an alternator?

A

a coil of wire rotating in a magnetic field which generates an AC current

24
Q

difference between AC and DC current?

A

AC current’s p.d constantly changes direction (+ and - sides switch)

  • direct current’s direction of potential difference does not change
25
Q

what does a graph of an alternator look like?

A 
goes from positive to negative 
|\            / 
|  \         /
|---\-----/----------------------
|     \   /
|      U

(imagine those lines were curved)

26
Q

what is the role of the slip ring commutators on an alternator?

A

allow current to pass out of the coil

27
Q

what is the role of brushes in an alternator?

A

brush against commutators so that voltmeter is connected to end of coil, but coil does not become tangled

28
Q

how can you increase the size of the potential difference in an alternator?

A
  • rotate the coil faster
  • use a stronger magnet
  • use a coil with a larger surface area/ more coils
29
Q

what happens if you rotate a coil faster?

A
  • greater rate of cutting field lines
  • so a greater pd induced (at peaks)
  • and a greater frequencey of pd

REMEMBER ON GRAPH

30
Q

why is the potential difference zero when the coil is vertical in an alternator?

A
  • coil is parallel to poles of magnet
  • so no field lines are cut when it moves
  • no pd induced
31
Q

what is a dynamo?

A

a direct current generator

32
Q

what is the role of split ring commutators in a dynamo?

and how does it affect it

A

reconnects the coil the other way round every half turn of the coil

  • so p.d induced does not reverse direction

SO CURRENT DOES NOT CHANGE DIRECTION (only the magnitude) - (ignore this line)

33
Q

what do alternators and dynamo have in common?

A

both use electromagnetic induction to produce an electric current from kinetic movement

34
Q

define a transformer

A

a device that uses an alternating pd (current) in a primary circuit to induce an alternating pd (current) in a secondary circuit

35
Q

do transformers use the motor effect or electromagnetic induction?

A

electromagnetic induction

36
Q

how does a transformer work?

A

1) primary coil has an AC current running through it
2) and so p.d is continually reversed
3) the magnetic field lines produced around the wire continually expand and collapse

4) and so a p.d (and therefore current) is continually induced in the secondary coil
* AN AC CURRENT*

37
Q

why is there an iron core present in a transformer?

A

to ‘trap’ the magnetic field so that it can guide it to the secondary circuit

(iron has a greater ability to carry magnetic flux than air)

38
Q

can transformers work with a DC supply?

A
  • no, as the direction remains constant.
  • and so the magnetic field lines would not expand and collapse continuously
  • so a p.d would be generated in the secondary coil only once (and current would not be able to flow)
39
Q

what does a step-up transformer do?

and how

A

increase the potential difference
- as it as a greater number of turns in the secondary circuit compared to the primary circuit

  • and so more magnetic field lines are cut per second
40
Q

what is the relationship between the potential difference of primary and secondary coils, and the number of coils?

A

pd (primary coil) turns (primary coil)
——————— = ————————-
pd (secondary coil turns(secondary coil)

OVERALL
- ratio of p : s (pd) = ratio of p : s (coils)

41
Q

what does it mean if a transformer is 100% efficient?

A

the power remains constant

power on primary coil = power on secondary coil

42
Q

what does a microphone do?

A

converts pressure variations in sound waves to variations in current in electrical circuits

43
Q

how does a microphone work?

A

1) sound wave hits a diaphragm and causes it to move in and out
2) this causes the wire attached to the diagram to also move in and out
3) the coil moves within a permanent magnet’s field
4) and so cuts its field lines
5) and a potential difference is produced across it
6) and therefore an AC current flows

44
Q

how would the current produced by the microphone have the same frequency as the sound wave?

A
  • sound waves have rarefactions and compression
  • so at an area of high pressure (compression), the diaphragm would be pushed inwards
  • and the frequency would alternate accordingly
45
Q

how do loudspeakers work?

A

1) changing p.d in coil produces a changing current within the wire
2) as wire has a current running through it, a CHANGING magnetic field is generated around it
3) the permanent magnet’s magnetic field and wire’s magnetic fields interact
4) and they either attract/repel, causing the diaphragm to move in and out
5) a sound wave is generated of the same frequency at which the diagram vibrates

46
Q

what happens when AC current changes direction in a loudspeaker)?

A

the cone is pulled inwards (which creates a pressure difference in the air)

47
Q

how do you increase the volume in a loudspeaker?

A

increase the voltage/p.d (as it increases the motor effect produced, and therefore the amplitude)

48
Q

what effect do microphones use?

A

electromagnetic induction

kinetic -> electric
movement of diaphragm to pd, current

49
Q

what effect do loudspeakers use?

A

the motor effect

electric -> kinetic
alternating p.d causes diaphragm to move and produce sound waves

50
Q

In what way is a carbon microphone better than a conventional one (do we need to know? no)

A

Carbon microphones are lighter

51
Q

Tell me about a carbon microphone

A
  • In a carbon microphone there are carbon granules behind the diaphragm.
  • As the sound wave hits the diaphragm it changes the resistance of the carbon.
  • A current passing through the carbon granules increases or decreases as the resistance changes.

Physics P1-P5 (14 decks)

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