Magnetism + Electromagnetism Flashcards
1
Q
where are the magnetic forces the strongest in a magnet?
A
the poles
2
Q
what happens when two magnets are brought together?
A
- force is exerted (non-contact)
- opposite poles attract
- like poles repel
3
Q
what is a permanent magnet?
A
a magnet produces its own magnetic field
4
Q
what is an induced magnet?
A
a material that becomes a magnet when it is placed in a magnetic field
- causes a force of attraction
- quickly loses most/all of its magnetism when it isn’t in the field
5
Q
what is a magnetic field?
A
the region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel)
6
Q
what is the force between a magnet and a magnetic field?
A
attraction
7
Q
what affects the strength of a magnetic field?
A
distance from magnet
further away=weaker
strongest at poles
8
Q
what way does a magnetic filed go?
A
north to south
9
Q
how do we find the direction of a magnetic field using a compass?
A
- magnetic compass has a small bar magnet
- place compass near the north pole of a magnet
- draw a cross at the north pole of the compass
- move compass so the south pole is at the cross
- draw a cross on the north pole of the compass
- continue until you have a complete magnetic field line
- join the points with a line
- repeat at different points of the north pole
10
Q
how do we know the earth’s core is magnetic?
A
- hold a compass away from any magnets, the needle always points north to south= earth has its own magnetic field due to its core
11
Q
how is a magnetic field created around a wire?
A
when current flows in a circuit
- large current=stronger field
- close to wire=stronger field
12
Q
right-hand thumb rule
A
- thumb=direction of current
- direction fingers curl=direction of magnetic field
13
Q
what is a solenoid?
A
a coil of wire which increases the strength of a magnetic field
14
Q
what happens to solenoid when current is turned on?
A
- when current is turned on, a strong and uniform magnetic field is formed inside the solenoid which is similar to one of a bar magnet
15
Q
how to increase the strength of a solenoid’s magnetic field
A
- increase size of current
- increase no. of turns in a coil
- iron core
16
Q
what is a solenoid with an iron core called?
A
electromagnet
17
Q
why are electromagnets useful?
A
- change strength of magnetic field by changing current
- can be turned on and off
18
Q
uses of electromagnets
A
- relay
- electric doorbell
19
Q
how do relays work?
A
- 2 separate circuits (one low voltage, one high voltage)
- low voltage circuit has an electromagnet=safe to turn on and off
- switch of high voltage circuit is replaced by 2 metal contacts
- one of the contacts is connected to a spring=keeps contacts apart
- iron block next to spring
- low voltage circuit turned on=current flowing=magnetic field around electromagnet=field attracts iron block=contacts close=high voltage circuit is on
20
Q
how do electric doorbells work?
A
- buzzer pressed=switch closes=current flows=electromagnet produces a magnetic field=iron contact attracted to field=clapper hits bell+breaks circuit
- iron contact springs back to original position=circuit complete again=repeats
21
Q
what is the motor effect?
A
a conductor carrying a current is placed in a magnetic field at right angles, the magnet producing the field and the conductor exert a force on each other
- if the conductor is parallel to the magnetic field= x experience a force
22
Q
motor effect equation
A
F = B I l (capital i, lowercase l)
- only works for wire at right angles to magnetic field
23
Q
what is magnetic flux density?
A
a measure of the strength of a magnetic field
24
Q
fleming’s left hand rule
A
thumb - motion (direction of force)
first finger - field (N to S)
second finger - current (+ to -)
25
how do electric motors work?
- current runs in opposite directions on either side of the loop
- loop placed in magnetic field=wire experiences a force on both sides
- force on left=up, force on right=down
- moment on left + moment on right=loop wire rotate clockwise
- when loop is at 90 = x rotating
- loop rotates beyond 90 = force on left is down, force on right is up=forces push loop back to 90 position
- split-ring commutator=switches direction of current when loop reaches 90
- split-ring is connected to conducting brushes=current pass onto ring
- current produces a turning force on the motor=motor rotates on clockwise direction
- current is broken for a tiny fraction of a sec when it's at 90, but wire keeps turning due to momentum
- current switches direction=motor rotates in the same direction
26
how is the motor effect used in loudspeakers and headphones?
- cone with a coil of wire wrapped around one end
- coil of wire=connected to an a.c supply
- permanent magnet goes inside the coil of wire
- current passes through coil=generates a magnetic field
- magnetic field from the coil now interacts with the magnetic field from permanent magnet=fields either attract or repel each other=produces a resultant force=coil vibrates
- current switches direction=direction of force on coil reverses
- cone vibrates=generating sound waves
- changing frequency of AC supply=change frequency of cone vibrations
- higher frequency=higher pitch sound
- increase size of the current=increase amplitude of the vibration=increases volume of the sound
27
what is induced potential?
- wire moves up through a magnetic field=p.d induced across ends of the wire
- wire stops moving=p.d is lost
- wire moves down through a magnetic field=p.d induced again but in a reverse direction
28
what is the generator effect?
The induction of pd or current in a wire moving relative to a magnetic field
29
what factors affect size of induced potential?
- stronger magnetic field=larger induced p.d/current
- move wire quicker=larger induced p.d/current
- wire is shaped into a coil=more turns in coil=larger induced p.d/current
30
what affects the direction of induced current/p.d?
- direction of movement
- switching the poles of a magnet
31
what does induced current do?
- induced current creates its own magnetic field which opposes the movement of the magnet
eg. north pole of magnet inserted=that end of the coil becomes the north pole=magnet repelled=hard to push magnet in
north pole pulled out=that end of coil becomes south pole=attracts magnet=hard to pull out
- induced current=hard to move magnet=work is done
32
uses of generator effect
- alternator (a.c)
- dynamo (d.c)
33
how does an alternator work?
-an alternator is a coil of wire rotating in a magnetic field
- coil is connected to two metal rings called commutator=allow the current to pass out of the coil a potential difference is induced when the wire passes through the magnetic field
- red side of the wire always connects onto ring A and the orange side of the wire always connects onto ring B
- maximum potential difference is when the coil is horizontal=wire is sweeping directly through the magnetic field lines are the fastest possible rate
the red side is moving downwards and the orange side is moving upwards when the coil is vertical, the potential difference fall to zero because the coil is now moving parallel to the field
- as a coil continues moving round we get a potential difference again however in this case the potential difference has reversed direction that's because the two sides of the coil are now moving in a different direction to before now the red side is moving up and the orange side is moving down and once again when the coil is vertical it's moving parallel to the magnetic field so the potential difference is zero
34
how does a dynamo work?
- a dynamo produces a direct current
- split ring commutator=two sides separated by a gap A and B
- the side of the coil that's moving down is connected to Part A of the split ring commutator and the side of the coil thats moving up is connected to side B because the coil is cutting
through the magnetic field lines a
potential difference and current are
induced
- when the coil is vertical it's moving parallel to the magnetic field so the potential difference is zero
- coil has moved around at this
point the red side is moving up and the orange side is moving down but side of the car that's moving down is still connected to Part A
of the split ring commutator and the side that's moving up is still connected to Part B so because of this the direction of the potential difference in current do not reverse when the coil rotates=dc
- 2 peaks for each full rotation of the coil= each side of the coil passes through the magnetic field twice during each cycle of rotation once passing down through the field and once passing up
35
how does a microphone work?
- coil of wire attached
to diaphragm
- end of the coil of wire sits over a permanent magnet
- sound waves hit the diaphragm=vibrate=coil of wire moves in and out to the magnetic field=induces a potential difference across the end of the wire
- the frequency of the changing potential difference is the same as the frequency of the sound waves
- the changing pattern of potential
difference is now passed to an amplifier and then into a moving coil loudspeaker=increases the volume of
the sound
35
how do transformers work?
- 2 separate coils of wire wrapped around an iron core
- primary coil is connected to a.c
- current flows through primary coil, it generates a changing magnetic field
- magnetic field is transmitted through iron core and passes through secondary coil
- changing magnetic field passes through secondary coil=p.d induced
- iron core increases strength of magnetic field
- only work with a.c=changing magnetic field needed to induce p.d
36
transformer equation (power)
VsIs=VpIp
36
transformer equation (turns in coil)
Vp/Vs=np/ns
