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Physics MA > Magnetism and electromagnetism > Flashcards

Magnetism and electromagnetism Flashcards

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1
Q

what is the unit/name for current?

A

ampere (A)

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2
Q

what is the unit/name for voltage?

A

volt (V)

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3
Q

what is the unit for power?

A

watt (W)

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4
Q

what do magnets do?

A

magnets repel and attract other magnets and attract magnetic substances

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5
Q

can magnetism. be induced? how?

A

magnetism is induced in some materials when they are placed in a magnetic field

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6
Q

what happens when there is a current in a conductor

A

an electric current in a conductor produces a magnetic field around it

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7
Q

when is a voltage induced in a conductor/coil?

A

a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it and describe the factors that affect the size of the induced voltage

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8
Q

what is the relationship for input and output power? (efficiency)

A

input power = output power Vp Ip =Vs Is
for 100% efficiency
p being primary
s being secondary wire

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9
Q

what is the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer

A

input (primary) voltage/output (secondary) voltage = primary turns/ secondary turns

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10
Q

what do permanent magnets do? what are they main features

A

A permanent magnet always causes a force on other magnets, or on magnetic materials. Key features of a permanent magnet:
it produces its own magnetic field
the magnetic field cannot be turned on and off - it is there all the time

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11
Q

for a single bar magnet, where is the magnetic field strongest

A

nearest to the poles

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12
Q

summarise the magnetic domain theory

A

when placing an external domain, the atoms line u in a certain dimension

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13
Q

what is the uniform field

A

the region between the poles where there are equally spaced, parallel lines. The field strength remains constant as you move around this area. the lines of flux get further apart when moved out from the space

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14
Q

what happends to an iron nail when they are removed form a permanent magnet? what is this an example of?

A

an iron nail is not usually magnetic, it is magnetically soft. when is it removed from the permanent magnet, the domains go back to having different orientations (demagnetised)
this is an example of induced magnetism

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15
Q

what would happen if steel was put next to a permanent magnet

A

it woudl stay permanently magnetic after it was removed

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16
Q

what are the uses of electromagnets

A

uses in scrap yards - carlifts (as they are easy to control and regulate strength)
circuit breakers
door bell
the relay

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17
Q

how can electromagnets be used in scrap yards?

A

iron is a soft magnetic material. when current flows the iron becomes strongly magnetised, picking up scarp iron, steel. When the current is turned off, the iron loses its magnetised (its soft) so it releases the scrap

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18
Q

how does an electric bell work? what is it an example of?

A

it is an example of a use of an electromagnet
1. when the switch is closed, current flows around the circuit turning on the electromagnet
2. the soft armour armature is pulled towards the electromagnet and the hammer hits the gong
3. the contact switch to open cutting off the electric current
4. spring now pulls the armature back again, closing the contact switch
5. current now flows again and the hammer hits the gong again

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19
Q

how does a relay switch work?

A
  1. a relay switch is a way of using a low voltage circuit to switch remotely a high voltage (an possibly dangerous circuit)
  2. when switch A is closed, the small current provided by the cell causes the electromagnet to become magnetised
  3. the iron armature is then attracted to the electromagnet causing springy contact B to close in the high voltage circuits
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20
Q

what is s circuit breaker? how does it function?

A
  1. current normally flows between terminals A and B through the contact and the electromagnet
  2. when the current in a circuit increase, the strength of the electromagnet will also increase
  3. this will pull the soft iron armature towards the electromagnet
  4. as a result, spring 1 pulls apart the contact, disconnecting the circuit immediately, and stopping current flow
  5. the reset button can be pushed to bring the contact back to its original position to reconnect the circuit
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21
Q

what are the rules for magnetic field lines around a bar magnet?

A
  • magnetic field strength is weaker, further away from the magnet
  • the stronger the magnetic field strength, the closer the field lines at that point
  • the field lines loop around exiting the north pole and entering the south pole
  • field lines never cross
  • magnetic field lines are sometimes clled ‘lines of magnetic flux’
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22
Q

which elements are ferromagnetic?

A

iron, cobalt, nickel

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23
Q

what is ferromagnetic, and paramagnetic what can make a metal ferromagnetic

A

‘magnetic’ is short for ferromagnetic
‘paramagnetic’ means to become magnetic
a metal can become ferromagnetic if it is an alloy of one of the three ferromagnetic elements (nickel, cobalt, iron)

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24
Q

how do you make something magnetic?

A

rub a magnetic material on a non magnetic material to magnetise it

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25
Q

what are permanent magnets?

A
  • made from magnetically hard materials (such as steel) which are resistant to demagnetising
  • every magnet has a magnetic field we can detect
  • iron is a soft magnetic material and not a suibtable permanent magnet
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26
Q

what is an electromagnet?

A
  • a coil wrapped by a material with power pumping through it
  • iron would be more prefferable to use over steel because iron becomes demagnetised meaning it can be turned on and off and the strength can be controlled
  • this is cheaper and easeir (to make a strong magnet) rather that a hard magnet
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27
Q

explain magnetic domains

A

the ferromagnetic materials have regions called domains where the magnetisation of all the atoms line up in a certain direction
- different domains usually have different orientations but the line up placed in an external magnetic field
- hard magnetic materials stay magnetised evern after magnetic field removed
- soft magnetic materials demagnetise once magnetic field removed

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28
Q

how to you change the strength of an electro magnet?

A
  • increase current
  • no coils/solenoid
  • increase the mass of the soft iron core
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29
Q

what happens if a magnet is split?

A

2 more magnets are made

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30
Q

what is the right hand rule?

A

(thumbs up position)
the thumb points the direction of the conventional current. the way your fingers wrap around is the way the magnetic field it

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31
Q

what is the circuit breaker designed to do?

A

to protect an electrical curcuit from damage caused by a short circuit

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32
Q

explain how a circuit breaker works?

A

too much current passes through coil
the iron core of the electromagnet is magnetised
the switch is attracted to the core of the electromagnet
the circuit breaker switch is opened
the current is cut off

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33
Q

why does a buzzer, like an electric bell, work and why does it vibrate at a higher frequency?

A

the hitting of the core causes a vibration which is the source of the buzzing. it vibrates at a higher frequency becuase it has a smalle mass than a bell and responds quicker to the on and off of the electromagnet

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34
Q

what is a resultant field?

A

any moving charge in an external magnetic field feels a force

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35
Q

what is the formal name for the left hand rule? what is it?

A
  • fleming’s left hand rule
  • thumb shows motion - gives motion force
  • first finger (index) gives direction of field (N-S)
    -second finger gives the current - gives direction to current in wire
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36
Q

what factors affect force on current carrying conductor (wire)

A
  • current
  • strength of magnetic field
  • length of wire
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37
Q

what is the expression to define magnetic field strength

A

F=BIl
B=F/Il (capital ‘i’, lowercase l)
b is the symbol for magnetic field strength (in Tesla)
‘i’ is current, l is length, f is force

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38
Q

what is 1 tesla?

A

magnetic field strength in which current carrying wire of 1A, produces of force 1N for each 1 metre or length of wire in the field, at right angles to both wire and field

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39
Q

how does a loud speaker work?

A
  1. current in the electrical current is varying and passes through the coil causing the coil to experience a force (inwards or outwards)
  2. The current is reversed repeatedly to reverse the force
  3. The greater the current the greater the force
  4. As the current and force vary, the coil the vibrates
  5. The coil is attached to a cone which also vibrates and causes the air molecules to vibrate
  6. This movement of the air molecules produces the pressure variations in the air needed for a sound wave producing compressions and rarefactions
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40
Q

from what directions do magnetic field lines point?

A

north to south

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41
Q

what is the purpose of a split ring commutator?

A

the reverse the current every half turn (if you don’t it will just wobble)

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42
Q

how do you reverse the current of a DC motor?

A

reverse the current
reverse magnetic field

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43
Q

how do you increase the speed of a DC motor

A

increase the current, more coils, increase mass of soft iron core

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44
Q

how does fleming’s left hand rule apply to the motor?

A

shows the direction of force, current, N+S magnetic field

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45
Q

name a pice of technology utilising the motor effect

A

loudspeakers

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46
Q

which way does a compass point when there is no current

A

north

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47
Q

which way does a compass point if the current is very large (e.g., 30 A)

A

they go the same way as the current. if there are two compass’, one above and one below the wire, and the magnetic field goes in a circular motion around the wire.
the north points to the way the current goes

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48
Q

which way does the current go when there is a very small current (1A)

A

the compasses point half where earth north is and the owhere the magnetic field it

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49
Q

draw the magnetic field near a long straight wire, solenoid, a circular flat coil (whatever that is)

A

check pg. 193, 194 for correct diagrams (in textbook)

50
Q

how can voltage be induced with coils

A

voltage is induced across the ends of the coil when it crosses through magnetic field lines. If the conductor is part of a complete circuit, the induced potential difference makes an electric current to flow ina. circuit

51
Q

how can induced voltage be increased?

A

increasing the number of turns on the coil of wire, increasing the strength of the magnet, making the magnet coil pass on another more quickly (increase rate at which magnetic field lines are cut)

52
Q

what is the Lorenz force?

A

when there is a magnetic field, a moving charge is produced and vice versa

53
Q

what is the main use of electromagnetic induction?

A

AC genetrator

54
Q

what happens in electromagnetic induction when the coil rotates faster?

A

larger frequency of alternating current, the bigger the peak value of induced current (more flux lines are being cute per second)

55
Q

what is the generator effect?

A

when a conductor cuts through magnetic field lines or vice versa and a current is induced

56
Q

what is ‘cutting’?

A

moving through each other at right angles

57
Q

what is an alternating current?

A
  • not a constant output the entire time
  • varies in magnitude, but can generate electrical power regardless of direction
  • direction of current is switched periodically so volatage is also switched
58
Q

what is a split ring?

A

used in DC commutators in motors and generators to reverse direction of current, they are not needed for AC generatators

59
Q

what is a slip ring commutator?

A

used in AC generators, they don’t reverse any directions, but keeps rotating components in contact

60
Q

name some examples of DC motor

A

fans, toys , toothbrushes, hairdryers

61
Q

what is the name of the electricity flowing through thte wire in a DC motor?

A

conventional flow

62
Q

what is an electromagnet?

A

e.g., bolt wrapped in wire (current in wire causes domains to line up and become magnetic)

63
Q

describe a dc motor

A
  1. two curved magnets
  2. a metal (loop) armature with a commutator attatched to reverse current
  3. brushes with spring attatched to make sure they are always connected
  4. the brushes and armature are connected to wires
    whent he commutator goes round, the brushes go to the other side and polarity is switched
64
Q

how can you make the speed oft he dc motor regulated?

A

you can add another metal loop and commutator

65
Q

what is torque?

A

Torque is a measure of the force that can cause an object to rotate about an axis

66
Q

how would you increase torque?

A

increase the number of metal wires in the loop (more electromagnets = stronger
more electricity also increases torque

67
Q

how can voltage (and current be increased) in a wire (with induced current)?

A

moving the wire more quickly , using a magnet sot here ar emore field lines cute, wrapping wire into a coil so more pieces of wire move through mangetic field

68
Q

how cna you induce voltage?

A

pushing a magnet through a coil of wireor pushing a wire through a magnet (U shaped)

69
Q

what is the purpose of an alternator?

A

to conduct electricity regardless of direction

70
Q

in an ac generator, when is the most cutting

A

when the block with wire is flat. plane of coil is parallel to direction of magnetic field so at this position the sides of the coil cross through field lines cutting more than is the plane of coil is perpendicular to magnetic field lines so the sides of the coild move parallel to field and magnetic field lines don’t cross through them

71
Q

what is induction

A

basically making electricity

72
Q

what is a transformer? describe how it works.

A

a transformer transfers electrical energy from one circuit to another and can also step up (increase) or step down (decrease) voltage
1. an. alternating current passing through the primary coil generates an alternating magnetic field in the iron core
2. the field lines fo the alternating magnetic field in iron core pass (cut) through the secondary coil
3. the mgnetic field it the secondary coil induces an alternating potential different between the terminals of the secondary coil

73
Q

why do we use the national grid? how does it work?

A

to improve reliability of demand - if one power station fails the other ones connected tot he grid can still deliver electricity. it also reduces cost and if all share the cost it works out cheaper.
1. energy produced at power station -> step up transformer (increases voltage, decreases current and improves efficiency of supply+carrying electricity at low currents increases efficiency) -> transmission cable -> step down transformer (safety) -> houses

74
Q

which equation tells up that we lose more power as heat with high current

A

P = I^2 R

75
Q

what is the equation for efficiency in an 100% efficient transformer?

A

power primary=power secondary
Vp x Ip = Vs x Is

76
Q

what is the equation linking number on coils and voltage?

A

Np/Ns=Vp/Vs

77
Q

what is the equation linking voltage, current, resistance?

A

V=IR

78
Q

name 2 uses for an induced magnet

A

relay switch, circuit breaker, bell

79
Q

2 uses of the motor effect

A

loudspeaker, DC motor

80
Q

what are properties of magnetically hard and soft materials?

A

hard - resitant to demagnetising
soft- can be induced (temporary magnet) - onlystays magnetic when near a magnetic field

81
Q

what does a magnetic fiel line represent?

A

magnetic field (always start at north pole and finish at south, when lines are closer field is stronger)

82
Q

what is a magnetic field?

A

area around a magnet (where there is a force on a magnetic object and if a field is strong, force is big and vise versa)

83
Q

do field lines of magnets ever cross?

A

no

84
Q

how to construct an electormagnet?

A

wrap coil (solenoid) around a soft magnetic material e.g., iron and swithc on current so the iron becomes magnetised

85
Q

draw the magnetic field line around current carrying wire

A
86
Q

when can magnetism be induced?

A

when some materials are placed in a magnetic field

87
Q

what happens when there is electric current in a conductor?

A

it produces a magnetic field around it

88
Q

describe the left hand rule.

A

thumb pointing upwrds indicates motion
indicating finger (1st finger) shows polarity N-S
2nd finger (middle finger) shows current

89
Q

why is step up and step down useful in large scale generation and transmition of energy?

A

by stepping up volage when tranfereing energy form power station to transmssion cable you save power as high voltage=low current and low current means less power is lost via heat and it is then stepped down for household use

90
Q

describe a transformer

A

primary coil (with an AC supply)
Secondary coil
soft iron core connecting them (rectangle/circle with wire wraping round it from both coils seperately)

91
Q

why is power stepped up/down in a generator?

A

different number of turns on the wire (coils) affect sht eamount of cutting and therefore different amount of voltage ( electricity)

92
Q

what colour is the north/south pole

A

north-red
blue- south

93
Q

where is magnetic field strongest?

A

poles of the magnet

94
Q

what happens with two like poles? two unlike poles?

A

2 like poles - attract
unlike- repel

95
Q

how can you show the direction (+shape) of magnetic field?

A

put a bar magnet on a piece of paper, near the magnet, and mark down where the compass (north points) goes. Do one 2 dots, one on each end of the compass. repeat this so you get multiple field lines (seperate ones). Connedt the points and you will get the shape of magnetic field. make sure to draw N-S arrows.

96
Q

what is the tiny bar magnet in a compass called?

A

needle

97
Q

how can you see the shape of a magnetic field?

A

get a current carrying wire, card, iron filings. place the card around the wire (as if you were sticking the pole into the card -> if it was siedways: —l—) . sprinkle iron filings around current carrying wire and tap the card to evenly distribute it. this will show shape

98
Q

what does the wire need to be if the current is higher?

A

thicker because a thinner one would melt (not be able to carry charge ((also potentially bend)))

99
Q

what happens if there are 2 magnetic fields

A

they interact with each other

100
Q

what is deflection?

A

when an arrow moves less (e.g., on a voltmeter)

101
Q

according to what do the current carrying wire and bar magnets interact?

A

fleming’s left hand rule

102
Q

in and induced magnet, why does dropping a magnet, instead of pushing it increase volage?

A

there are MORE MAGNETIC FIELD LINES CUT PER SECOND

103
Q

what are the 3 magnetic materials?

A

cobalt, nickel, iron

104
Q

what is an alloy of iron that is magnetic?

A

steel

105
Q

when do incuced magnets produce a magnetic field?

A

when in another magnetic field

106
Q

why sithe force between an amgnet and magnetic material always attractive?

A

magnetic induction shows that because south pole induces north pole and vice versa

107
Q

descirbe the magnetic field lines around a wire

A

consentric circles perpendicular to the wire

108
Q

how can you see how current in a wire affects compass?

A

get a wire, 2 compasses, power source
put one compass above and one below wire. First with no voltage, compasses will point north. when there is a high voltage (20A) then the compass north will point int magnetic field direction. when there is a small voltage (e.g., 3A) the voltage will be inbetween north and magnetic field lines of wire.

109
Q

wha is the motor effect?

A

a current carrying wire being put between two magnetic poles which interact which recults in a force. try and draw this

110
Q

how does a current carrying feel the full effect of force (moto effect) when between 2 magnetic poles?

A

it had to be 90 degrees/right angle to magnetic field. if the wire runs along manetic field there wont be any ofrce at all.

111
Q

draw the magnetic field of a solenoid (long coil of wire)

A

impagin an upside down ‘U’. this is the solenoid. around the 2 bases touching the surface will be rings. inbetween you have feiel lins going through , pointing in the same direction. It is strong and almost uniform. OUtside the coil there are overlapping fields which cancel each other out (they are wear apart form the nds of solenoids). if there are multiple solenoids together, instead of there being multiple rings around the bases, they cancel eachother out and become one (inside the ‘arch’ they add to become strong).

112
Q

how do you increase the strength of a solenoid?

A

it is an electromagnet. You can increase it more by putting a block of iron at the centre of the coil which becomes an induced manget whenever current it flowing

113
Q

does induced current oppose the change that made it?

A

yes (trying to return back to normal)

114
Q

draw an ac generator

A

pg 166 of CGP book

115
Q

when is there a force on charged particles?

A

when it moves in a magnetic field as long as its motion is not parallel to the field

116
Q

how does a loudspeaker work?

A
  1. In a loudspeaker, the diaphragm is replaced with a paper cone.
  2. The coil is wrapped around one pole of a permanent magnet, so the a.c. signal causes a force on the coil, which moves the cone. (This is the motor effect — see page 160.)
  3. When the current is reversed, the force acts in the opposite direction.
  4. These movements make the cone vibrate, which makes the air around the cone vibrate and creates the variations in pressure that cause a sound wave (p.66).
117
Q

why is a force exerted on a current carrying wire in a magnetic field?

A

Current flows in the wire/coil. This creates a magnetic field around the wire/coil. This magnetic field interacts with the field from the permanent magnet. This produces a force on the wire/coil which moves the wire/coil
this is appliclable to dc motors and loudspeakers.

118
Q

decribe the generation of electricity by rotation of a magnet within a coil of wire (generator example).

A

Model answer for a generator (Rotating magnet)
· Magnet is rotated within a coil
· As it turns the coil cuts the constantly changing magnetic field lines from the magnet.
· This induces a voltage (or current) in the coil.
· This can then be connected to an existing circuit.
· In a generator, energy is being converted from kinetic (mechanical) energy into electrical energy.
· The size of the induced voltage (or current) can be increased by:
· Using a stronger magnet
· Having more turns in the coil
· Spinning/moving the magnet faster

119
Q

decribe the generation of electricity by rotation of a coil of wire within a magnetic field (generator example).

A

Model answer for a generator (Rotating coil):
· Coil is rotated within a magnetic field
· As it turns the coil cuts the magnetic field lines.
· This induces a voltage (or current) in the coil.
· This can then be connected to an existing circuit.
· In a generator, energy is being converted from kinetic (mechanical) energy into electrical energy.
· The size of the induced voltage (or current) can be increased by:
· Using a stronger magnet
· Having more turns in the coil
· Spinning/moving the coil faster.

120
Q

how to find magnetic field between 2 bar magnets

A

to find the magnetic fiel line of 2 bar magnets use same method as single bar magnet (compass, paper, marking points)

121
Q

describe how the force on a current-carrying conductor in a magnetic field changes with the magnitude and direction of the field and current.

A

If you increase the magnitude of the current through a wire or the size of the magnet being used, you increase the force on the wire.

122
Q
A

Physics MA (3 decks)

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