Magnetism and Electromagnetism Flashcards
1
Q
features of magnets
A
north and south poles
same poles repel
opposite poles attract
2
Q
what are the poles of a magnet
A
where the magnetic forces are strongest
3
Q
what happens when two magnets are brought close together
A
they exert a force on each other
4
Q
features of a permanent magnet
A
always magnetic
always have poles
5
Q
features of induced magnets
A
materials that are magnetic but don’t have fixed poles
made into temporary magnets by stroking them with a permanent magnet
6
Q
how does stroking an induced magnet with a permanent magnet work
A
aligns the domains in the material in all the same direction creating a temporary magnet
7
Q
examples of induced magnets
A
iron
nickel
cobalt
8
Q
which direction do field lines go
A
north to south
9
Q
what happens to strength of the magnet with distance
A
strength decreases with distance from the magnet
10
Q
what happens when induced magnets are removed from the magnetic field
A
it loses all or most of its magnetism quickly
11
Q
what is a magnetic field
A
the region around a magnet where a force acts on another magnet or on a magnetic field
12
Q
what is the direction f the magnetic field at any point given by
A
the direction of the force that would act on another north pole placed at that point
13
Q
what does a magnetic compass contain
A
a small bar magnet
14
Q
how to map out magnetic fields using a plotting compass
A
- place the plotting compass near the magnet on a piece of paper
- mark the direction the compass needle points
- move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
- join the points to show the field lines
15
Q
where does the needle of a plotting compass point
A
to the south pole of the magnet
16
Q
how does the behaviour of a compass show the earth has a magnetic field
A
the earths core is made of iron and nickel and produces a magnetic field
17
Q
what happens when a current flows through a conducting wire
A
a magnetic field is produced around the wire
18
Q
what does the strength of the magnetic field produced by a current depend on
A
the current through the wire and the distance from the wire
19
Q
what does a solenoid do
A
increases the strength of the magnetic field created by a current through the wire
20
Q
how is the direction of a current shown
A
the right hand grip rule
21
Q
how does the right hand grip rule work
A
by gripping the object the fingers give the direction of the field and the thumb points in the direction of the current
22
Q
features of a magnetic field around bar magnet drawing
A
the magnetic field lines never cross eachother
the closer the lines the stronger the magnetic field
the lines have arrow heads to show the direction of the force exerted by the magnetic north pole
the arrowheads point from the north pole of the magnet to its south pole
23
Q
how to check if a magnet is a permanent magnet
A
it repels another magnet
24
Q
how can you show the direction of a current
A
right hand grip rule
plotting compasses on a piece of paper when a wire is pierced
25
what is a solenoid
wire coiled into a spiral
26
why do solenoids enhance the magnetic effect
makes magnetic field similar to field of a bar magnet
strong and uniform field
small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field
27
what does the iron core in an electromagnet do
increases its magnetic field strength
28
what is an electromagnet
a solenoid with an iron core
29
which factors of a solenoid affect strength
size of current
length
cross sectional area
number of turns (coils)
using a soft iron core
30
what is the motor effect
a wire carrying a current creates a magnetic field
which can interact with another magnetic field
causing a force that pushes the wire at right angles
31
when does the force on a given length of wire in a magnetic field increase
the current in the wire increases
the strength of the magnetic field increases
32
when is the force greatest on a given length of wire
when the direction of the current is 90 degrees to the direction of the magnetic field
33
when is there no motor effect force
if the current and magnetic field are parallel to eachother
34
how can the direction of a motor effect force be found
flemings left hand rule
35
how to use flemings left hand rule
hold your thumb, forefinger and second finger at right angles to eachother
the forefinger is lined up with the magnetic field lines pointing north to south
the second finger is lined up with the current pointing from positive to negative
the thumb shows the direction of the motor effect force on the conductor carrying the current
36
which effect makes an electric motor
a coil of wire carrying a current in a magnetic field experiencing a force that makes it rotate
37
what causes the coil in a dc motor to rotate anticlockwise
current in the left hand part of the coil causes a downward force and current in the right hand part of the coil causes an upward force
38
what happens when the coil in a dc motor is vertical
it moves parallel to the magnetic field producing no force
39
why does the motor not stop when the coil is vertical
the momentum of the motor carries it on round a little
a split ring commutator changes the current direction every half turn
40
what happens once the conducting brushes reconnect with the commutator after a half turn
current flows in the opposite direction through the wire in the coil
each side of the coil is now near the magnetic pole
41
equation for a conductor at right angles to a magnetic field and carrying a current
force = magnetic flux density x current x length
42
what is magnetic flux density measured in
tesla (T)
43
how do loudspeakers and headphones use the motor effect
to convert variations in current in electrical circuits to the pressure variations in sound waves
44
how do loudspeakers create sound waves
- a current in the coil creates an electromagnetic field
-the electromagnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
-the current is made to flow in the opposite direction
- the direction of the electromagnetic field reverses
-the force on the cone now pulls it back in
-repeatedly alternating the current direction makes the cone vibrate in and out
-the cone vibrations cause pressure variations in the air which are sound waves
45
what is needed for a loudspeaker cone to vibrate correctly
the electric current must vary in the same way as the desired sound
46
what do headphones contain
small loudspeakers
47
how does electromagnetic induction happen
a coil of wire is moved in a magnetic field
a magnet is moved in a coil of wire
as a potential difference is needed to make electric current flow in a circuit
48
what is electromagnetic induction also referred to as
the generator effect
49
what are the results of the induced voltage
an induced current is produced if the conductor is connected in a complete circuit
the induced current creates a magnetic field around itself that opposes the original charge
50
example of a magnet moving into a coil of wire
induced magnetic field repels the magnet back out of the coil
51
what does the direction of the induced current depend on
the direction of movement of the magnet relative to the coil
52
when is the current reversed in the generator effect
the magnet is moved out of the coil
the other pole of the magnet is moved into the coil
53
ammeter reading of a magnet moving into a coil of wire
positive
54
ammeter reading if a magnet is stationary in the coil of wire
no current flow
55
ammeter reading when a magnet moves out of the coil of wire
negative current flow
56
what factors increase the induced potential difference or current
speed of movement increased
magnetic field strength increased
the number of turns on the coil is increased
57
what is the generator effect used in
an alternator to generate ac
a dynamo to generate dc
58
what is an ac generator
a device that produces a potential difference
59
why do cars use an alternator
to keep the battery charged
to run the electrical system while the engine is working
60
features of a simple alternator
the coil is rotated in the magnetic field
current is induced in the rotating coil
slip rings connected to the coil
brushes make continuous contact between the external circuit and the slip rings
current flows in external circuit
61
what do slip rings do in an alternator
maintain constant contact with the same sides of the coil
62
what happens as one side of the coil moves up through the magnetic field in an alternator
a potential difference is induced in one direction
63
how does the alternator cause an ac
as one side of the coil moves up through the magnetic field a potential difference is induced in one direction
as the rotation continues and that side of the coil moves down the induced potential difference reverses direction
this means the current is constantly changing
64
how can the output of an alternator be represented
potential difference time graph
voltage on y axis
time on x axis
65
what does the graph of an alternator look like
alternating sine curve
66
how can the maximum potential difference or current be increase in an alternator
increasing the rate of rotation
increasing the strength of the magnetic field
increasing the number of turns on the coil
67
how is a dc generator different to an ac generator
it uses a split ring commutator rather than two slip rings
68
how do bike lights use dc generators
use a dynamo
to run the lamps while the wheels are turning
69
features of a dynamo in a bike
wheel of dynamo rubs against tyre to make magnet turn
magnet is surrounded by coils of wire and wire is connected to lamp
70
how does the current flow in only one direction in a dynamo
a split ring commutator changes the coil connections every half turn
as the induced potential difference is about to change direction the connections are reversed
71
how can the output of a rotating dynamo be shown
potential difference time graph
72
what does the graph of a dynamo look like
a sine curve that stays in the same direction all the time
73
how can the maximum potential difference or current in a dynamo be increased
increasing the rate of rotation
increasing the strength of the magnetic field
increasing the number of turns on the coil
74
what is a microphone
a device that converts sound waves into electrical signals
75
how does a microphone use the generator effect
to induce a changing current from the pressure variations of sound waves
76
moving coil microphone process
-pressure variations in sound waves cause the flexible diaphragm to vibrate
-the vibrations of the diaphragm cause vibrations in the coil
-the coil moves relative to a permanent magnet so a potential difference is induced in the coil
-the coil is part of a complete circuit so the induced potential difference causes a current to flow around the circuit
-the changing size and direction of the induced current matches the vibrations of the coil
-the electrical signals generated match the pressure variations in the sound waves
77
what is a transformer
a device that can change the potential difference or voltage of an alternating current
78
what does a step up transformer do
increases the voltage
79
what does a step down transformer do
reduces the voltage
80
what is a basic transformer made of
primary coil from the ac input
secondary coil leading to ac output
wound around an iron core
81
why are the coils wrapped around an iron core
it is easily magnetised and an carry magnetic fields from the primary coil to the secondary coil
82
how does a transformer work
a primary voltage drives an ac through the primary coil
the primary coil current produces a magnetic field which changes as the current changes
the iron core increases the strength of the magnetic field
the changing magnetic field induces a changing pd in the secondary coil
the induced potential difference produces an alternating current in the external circuit
83
what do transformers need to work
an alternating current
84
ratio of voltage to no of turns on a transformer
primary voltage / secondary voltage
=
number of turns on p coil/number of turns on s coil
85
electrical power equation
power = potential difference x current
86
power output from transformer equation
potential difference across primary coil x current in primary coil
=
potential difference across secondary coil x current in secondary coil
87
what does the national grid do
carries electricity around britain
88
what does a higher current in a cable mean
the greater the energy transferred to the surroundings by heating
so high currents waste more energy than low currents
89
how does the national grid reduce energy transfers to the environment
step up transformers are used to increase voltage to power stations to thousands of volts
lowering the current in the transmission cables
90
how are step down transformers used in the national grid
to decrease the voltage from the transmission cables so it is safer to distribute to homes and factories
