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Physics > Topic 7 - Magnetism and Electromagnetism > Flashcards

Topic 7 - Magnetism and Electromagnetism Flashcards

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1
Q

how many poles does a magnet have?

A
  1. north and south
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2
Q

what is a magnetic field?

A

a region where other magnets or magnetic materials experience a force. all magnets produce one

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3
Q

how can a magnetic field be drawn?

A

by drawing magnet field lines. they always go from north to south. the closer the lines are the stronger the magnetic field

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4
Q

where is a magnetic field the strongest?

A

at the poles of a magnet

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5
Q

what kind of force is there between a magnet and a magnetic material?

A

it is always attractive, no matter the pole

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6
Q

what happens when 2 poles are near each other?

A

they will exert a force on each other this can be attractive or repulsive. 2 poles that are the same repel and opposite poles attract

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7
Q

what is inside a compass?

A

a tiny bar magnet. the north pole is always attracted to the south pole of any other magnet it is near. so it always points the direction that the magnetic field is in

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8
Q

how to draw magnetic field lines?

A

move a compass around the magnet and trace its position

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9
Q

what direction do compasses point?

A

they point north when they are not near a magnet because earth generates its own magnetic field, which shows the core of the earth is magnetic

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10
Q

what are the 2 types of magnet?

A

permanent magnets and induced magnets

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11
Q

what are permanent magnets?

A

they produce their own magnetic field

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12
Q

what is an induced magnet?

A

magnetic material that turns into a magnet when it is put next to a magnetic field. when you take away the magnetic field, they lose their magnetism and stop producing a magnetic field

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13
Q

what force is there between a permanent magnet and an induced magnet?

A

it is always attractive

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14
Q

what happens when a current flows through a wire?

A

a magnetic field is created around the wire. the field is made up of concentric circles perpendicular to the wire

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15
Q

how can the direction of the magnetic field be worked out?

A

using the right hand rule. your thumb should point the direction of the current in the wire is going, the direction your fingers are pointing are the direction the magnetic field is going. e.g. up is anti-clockwise

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16
Q

how can you increase the strength of a magnetic field around a wire?

A

by wrapping the wire into a coil called a solenoid. this causes the field lines around each loop of wire to line up so there are lots of field lines pointing in the same direction that are very close together

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17
Q

what is the magnetic field like inside a solenoid?

A

its strong and uniform, it has the same strength and direction at every point in that region

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18
Q

what is the magnetic field like outside a solenoid?

A

the magnetic field is just like the one round a bar magnet

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19
Q

what increases the strength of a solenoid?

A

increasing the number of coils
putting a block of iron in the centre, which becomes an induced magnet
increase the current of the wire

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20
Q

what is an electromagnet?

A

a solenoid with an iron core, or a magnet whose magnetic field can be turned on and off

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21
Q

why are electromagnets useful?

A

magnets you can switch on and off are really useful. the’re quick to turn on and off and they can create a varying force

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22
Q

what can electromagnets be used for?

A

it can be used in a crane to carry materials like iron and steel e.g. in scrap yards
they can be used as switches, where the magnet can be used to pull a part of a circuit into place and complete it

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23
Q

what happens when a current-carrying wire is put between magnetic poles?

A

the magnetic field around the wire interacts with the magnetic field it has been placed in. this means the magnet and the conductor exerts a force on each other. this is the motor effect and can cause the wire to move

24
Q

how must a wire feel the full force in the motor effect?

A

the wire has to be at 90* to the magnetic field. if it is parallel to the magnetic field it won’t experience any force. the force always works at right angles to the magnetic field of the magnets and the direction of the current of the wire

25
Q

what is a good way of showing how force works with magnetic field lines and the current?

A

apply a current to a set of rails inside a horseshoe magnet. a bar is placed on the rails and completes the circuit. this generates a force and the bar rolls across the rails

26
Q

what increases the strength of a force in the motor effect?

A

increasing the strength of the magnetic field
increase the amount of current passing through the conductor
increasing length of conductor thats in the magnetic field

27
Q

what is the equation to find the force acting on a wire when it is at 90* to the magnetic field?

A

F=BIl

force (N) = magnetic flux density (T, tesla) X current (A) X length (m)

28
Q

how can the direction of a force in the motor effect be worked out?

A

using the Fleming’s left hand rule

29
Q

what is the Fleming left-hand rule?

A

first finger = direction of field lines (N to S)
second finger = direction of the current
thumb = direction of the force
if current or field lines are reversed force is reversed

30
Q

how does a basic dc motor work?

A

forces work on the loop of wire that cause one side to be pushed up and the other down. a split ring commutator is used to swap the current every half turn so it keeps spinning

31
Q

how does a split-ring commutator work?

A

half of the ring is attached to the positive side of the battery and the other is connected to the negative side. whenever the wire is switching to the other side of the motor it will have a moment with no current then it will touch the other side and the current will be switched

32
Q

how do loudspeakers and headphones work?

A

an ac current is sent through a coil of wire attached to the base of a paper cone.
the wire should be wrapped around 1 pole and surrounded by the other.
a current causes a force to act of the wire which moves the cone, the alternating current means the forces keep changing so the cone vibrates

33
Q

how is a sound created from headphones or speakers?

A

vibrations make the air around the cone vibrate and creates the variations in pressure that cause a sound wave. the frequency of the sound is the same frequency as the current

34
Q

what is the generator effect?

A

the induction of a potential difference (and current if its a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in a magnetic field

35
Q

how does the generator effect create a potential difference in a wire and a current if its part of a circuit?

A

the magnet in a coil of wire can be moved or the wire can be moved in a magnetic field. moving the magnet from side to side creates a small amount of current if the wire is part a complete circuit

36
Q

how can the potential difference of a wire be reversed in the generator effect?

A

if the magnet or wire is moved in the opposite direction
or the polarity of the magnet is reversed.
an alternating current is produced if the magnet or coil keeps moving back and forth

37
Q

how can an ac generator be created?

A

as the magnet turns, the magnetic field in a coil changes, this change induces a potential difference which can make a current flow in the wire.
when the magnet has turned through half a turn, the direction of the magnetic field through the coil reverses so the pd reverses and the current will change direction
if the magnet keeps spinning an alternating current will be formed

38
Q

what happens when a change in magnetic field induces a current in the wire?

A

a magnetic field is created around the wire that acts against the change that created it e.g. movement of a wire or change in the field its in

39
Q

how can an induced potential difference be increased?

A

increasing speed of movement
increasing strength of the magnetic field
increasing number of coils

40
Q

how does an alternator work?

A

they rotate a coil in a magnetic field
as the coil or magnet spins a current is induced that changes direction every half term
they produce an alternating pd

41
Q

what do alternators have instead of a split-ring commutator?

A

they have slip rings and brushes so the contacts don’t swap every half turn.

42
Q

how do dynamos work?

A

they work in a similar way to alternators except they have a split-ring commutator which swaps the connection every half term to keep the current flowing in the same direction

43
Q

what does an oscilloscope do?

A

it shows how potential difference changes over time

44
Q

what does an oscilloscope show for an alternator?

A

the line goes up and down, crossing the horizontal axis

45
Q

what does an oscilloscope show for a dynamo?

A

the current still goes up and down but it does not cross the axis, it is always positive

46
Q

what does the height of the line in an oscilloscope mean?

A

the height of the line at any given point is the generated potential difference at the time. increasing the frequency of revolutions increases the overall pd and creates more peaks

47
Q

what are microphones?

A

they are basically loudspeakers in reverse

48
Q

how do microphones work?

A

as sound waves hit a flexible diaphragm thats attached to a coil of wire around a magnet the coil moves in the magnetic field which generates a current
the louder the sound, the more the diaphragm moves

49
Q

what do transformers do?

A

they change the size of the potential difference of an alternating current

50
Q

what are transformers?

A

they all have 2 coils of wire. the primary and the secondary, joined with an iron core

51
Q

how do transformer work?

A

when an alternating pd is applied to the primary coil the iron core magnetises and demagnetises quickly. the changing magnetic field induces an alternating pd in the secondary coil if it is part of a complete circuit a current is induced

52
Q

what are the 2 types of transformer?

A

step-up transformers

step-down transformers

53
Q

what are step-up transformers?

A

they step up the potential difference because they have more turns on the secondary coil than the primary

54
Q

what are step-down transformers?

A

they step the potential difference down because they have more turns on the primary coil than the secondary

55
Q

how can the pd or number of turns on a coil be worked out on a transformer?

A

Vp/Vs=np/ns

input pd/output pd=turns on 1st coil/ turns on 2nd coil

56
Q

how can you work out the amount of current produced or inputted into a transformer?

A

assuming that the transformer is 100% efficient the power going in= power coming out so:
Vs Is = Vp Ip
current in 1st coil X pd in 1st coil = current in 2nd coil X pd in 2nd coil

57
Q

why does the national grid transmit power at high pd’s?

A

low current means less energy is wasted by heating the wires and surroundings. a high pd is needed for a low current to produce the same amount of power

Physics (11 decks)

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