P4 - magnetism and magnetic fields Flashcards
1
Q
what is an alternator?
A
also called AC generator
2
Q
what is current?
A
rate of flow of charge
3
Q
what is a diaphragm?
A
the part of a loudspeaker that moves to produce a pressure wave in the air i.e. sound
4
Q
what is a dynamo?
A
also called DC generator
5
Q
what is electromagnetic induction?
A
the process of producing an induced potential difference in a conductor due to the conductor cutting magnetic field lines
6
Q
what is a generator?
A
a device that converts KE into electrical energy; consists of a rotating wire in a magnetic field
7
Q
what does induced mean?
A
to bring about; make happen
8
Q
what is magnetic flux density?
A
same as magnetic field strength
9
Q
what is a motor?
A
a component that produces rotation from the combination of the fields due to magnets and current carrying wires
10
Q
what is a solenoid?
A
a coil of wire used in an electromagnet
11
Q
what is a split ring commutator?
A
keeps the coil rotating in the same direction by reversing the direction of current every half turn
12
Q
what is a transformer?
A
a component that uses two coils and an iron core to change the potential difference in a circuit
13
Q
what is a Tesla?
A
the unit of magnetic flux density
14
Q
what is a magnetic field?
A
a region where a magnetic material will experience a force
15
Q
magnetic field lines always point …
A
from north to south
16
Q
the strength of the field is shown by …
A
how close the field lines are to eachother
17
Q
the magnetic field is always strongest …
A
at the poles
18
Q
what are the only 3 magnetic elements?
A
- iron
- nickel
- cobalt
19
Q
what are bar magnets?
A
- permanent magnets
- contain aligned magnetic domains
- retain their magnetism and so are called magnetically hard materials
20
Q
what does an iron nail have?
A
randomly aligned domains
21
Q
what will happen to an iron nail when placed in a magnetic field?
A
- the domains will line up, producing an induced magnet
- by banging the magnet the domains then randomly align again because the nail is made of a ‘soft’ magnetic material
22
Q
how do we know the Earth has a magnetic field?
where does it come from?
A
- the Earth’s magnetic field is caused by iron ions moving (convection currents) inside the Earth’s core
- we know the shape of the Earth’s magnetic field because scientists take compasses around the world and are able to map the magnetic filed lines since the compasses line up with them
- the North Pole of a magnet is attracted to the geographic North Pole of the Earth because there is a magnetic South Pole there
23
Q
when an electric current flows in a wire, what shape is the magnetic field it produces?
A
circular
24
Q
what does the direction of the field depend on?
A
the direction of the current
25
as the distance away from the wire increases, what happens to the field strength?
it decreases
26
how do we show the magnetic field strength decreasing as distance increases?
the field lines getting further apart
27
what does the magnetic field pattern around a solenoid take the same shape as?
as that around a bar magnet
28
what can the strength of the magnetic field be increased by?
1. more turns on the coil/loops
2. more current/increase the current
3. add an iron core
29
investigation:
how does increasing the current in a solenoid affect the strength of the electromagnet?
- what are the variables in this investigation?
dependent variable = number of paperclips picked up
independent variable = current
control variables = size of paperclips, nail, no. of turns in solenoid
30
method for investigation for how does increasing the current in a solenoid affect the strength of the electromagnet?
1. set up circuit
2. use variable resistor to adjust/vary he current to chosen values e.g. 0, 0.25, 0.5, 0.75 etc
3. at each current, place the nail into a pile of paperclips and record how many are picked up
4. repeat the experiment so you have three values of paperclips for each current value
5. calculate the mean average paperclips and plot data onto a graph
31
what is formed when a current flows through a solenoid?
a magnetic field
32
what is the device called that consists of a current flowing through a solenoid and forming a magnetic field?
an electromagnet
33
what will happen if an iron core is added into the centre of an electromagnet?
the iron will be magnetised by the electromagnet's field, creating another magnetic field, this time around the iron core; these two fields add together to create one stronger magnetic field
34
what are 2 advantages of using electromagnets instead of permanent magnets?
1. they can be made very strong
2. they can be turned on and off rapidly
35
what is the motor effect?
when a current-carrying conductor (i.e. wire) is placed in a magnetic field, it experiences a force, which causes it to move
36
why does the motor effect occur?
the effect occurs because we are combining two magnetic fields and so each object experiences a magnetic force (similar to when you bring two magnets together)
37
how do we predict the direction of the force?
Fleming's left hand rule
38
what is Fleming's left hand rule?
thumb = motion
first finger = magnetic field
second finger = current
39
how can we increase the size of the force on the wire?
1. more current
2. stronger magnet
3. increase the length of the conductor in the magnetic field
40
how do we change the direction of the force on the wire?
1. swap the position of the north and south pole
2. change the direction of current (turn the cells around)
41
what does a DC motor contain?
- split ring commutator
- magnets
- battery
- brushes
- axle
- coil
42
how does a DC motor work?
- a current flows through the coil which is in a permanent magnetic field; this current causes its own magnetic field to form
- the right side of the coil experiences a downwards force due to Fleming's left hand rule
- on the left side of the coil, the current is flowing in the opposite direction relative to the magnetic field and so it experiences an upwards force
- this makes the coil rotate
- the split ring commutator keeps the coil rotating in the same direction by reversing the direction of current every half turn
43
why do motors use magnets with curved poles?
motors have curved magnets so that the distance between the coil and the magnet is kept constant and as small as possible (remember magnetic field strength decreases with distance)
44
how can the DC motor be made to rotate faster?
1. stronger magnets/magnetic field
2. increase the current
3. add more turns on the coil
45
demo 1: stationary magnet, moving conductor
- what happened when the wire was moved down between the poles of the magnet?
current was generated
46
demo 1: stationary magnet, moving conductor
- what happened when the wire was moved up between the poles of the magnet?
current flowed the other way
47
demo 1: stationary magnet, moving conductor
- what happened when the wire was moved more slowly?
less current
48
demo 1: stationary magnet, moving conductor
- what happened when the wire was held still between the poles of the magnet?
no current
49
demo 1: stationary magnet, moving conductor
- what happened when the wire was moved horizontally between the poles?
no current
50
demo 1: stationary magnet, moving conductor
- what happened when the magnet was turned around?
current flows the other way
51
demo 2: stationary conductor, moving magnet
- what happened when the magnet was pushed into the coil?
current was generated
52
demo 2: stationary conductor, moving magnet
- what happened when the magnet was at rest inside the coil?
no current
53
demo 2: stationary conductor, moving magnet
- what happened when the magnet was pulled out of the coil?
current flows the other way
54
demo 2: stationary conductor, moving magnet
- what happened when the magnet was moved faster?
more current
55
demo 2: stationary conductor, moving magnet
- what happened when the magnet was turned around?
current flows the other way
56
what is Faraday's Law?
the faster the magnetic field lines are cut the greater the induced potential difference (and therefore current)
57
what does a current in a magnetic field experience?
a force
58
what happens when a current-carrying conductor (like a wire) is put between magnetic poles?
the two magnetic fields affect one another and this results in a force on the wire
59
to experience the full force, what angle does a wire have to be at to the magnetic field?
a right angle
60
if a wire runs along a magnetic field what force will it experience?
none
61
what does a simple electric motor use?
magnets and a current-carrying coil
62
what do loudspeakers use?
magnets and a coil of wire
63
how does a loudspeaker work?
1. a loudspeaker contains a coil of wire which is surrounded by one magnet, another magnet is inside the coil
2. a.c. electrical signals are fed to the coil of wire, which is wrapped around the base of a cone
3. the interaction between the magnetic field and the current in the coil forces the coil to move in one direction; as its a.c., the current changes direction and forces the coil back in the other direction
4. as the current continues to alternate, the coil moves back and forth
5. these movements make the cone vibrate, which creates pressure variations in the air (sound)
64
what does a changing magnetic field do in a conductor?
induces a potential difference
65
what do alternators generate?
alternating current
66
what do dynamos generate?
direct current
67
how does an alternator work?
1. some alternators rotate a magnet in a coil of wire
2. as the magnet spins, an alternating p.d. is induced across the ends of the coil
3. the p.d. changes direction every half turn because the direction of the field changes as the magnet rotates
4. this produces an a.c. if coil is part of complete circuit
5. can also generate a.c. by rotating coil in magnetic field
6. slip rings at end of coil remain in contact with brushes that are connected to rest of circuit
7. this means the contacts don't swap every half turn so produce a.c.
68
how does a dynamo work?
1. dynamos rotate a coil in a magnetic field
2. the output p.d. and current change direction with every half rotation of the coil, producing a.c.
3. the coil is part of a complete circuit
4. a split-ring commutator swaps the connection every half turn to keep the current flowing in the same direction so it changes a.c. to d.c.
69
what do microphones work due to?
electromagnetic induction
70
how does a microphone work?
1. a dynamic microphone's structure is like a loudspeaker but the cone is replaced by a diaphragm
2. sound waves cause the diaphragm to move back and forth when hit by them
3. as the diaphragm moves, the coil of wire moves, inducing a p.d. across the ends of the coil of wire
4. the coil of wire is part of a circuit so induced p.d. means variations in current in electrical circuit
71
what do transformers do?
they change the potential difference
72
how do transformers work?
1. transformers change the size of the potential difference of an alternating current
2. they have two coils (primary and secondary) joined with an iron core
3. when an alternating p.d. is applied across the primary coil, it produces an alternating magnetic field
4. as iron is a magnetic material, the core also becomes magnetised; because the coil is producing an alternating magnetic field, the magnetisation in the core also alternates
5. a changing magnetic field induces a p.d. in the secondary coil
6. the power of a primary coil is given by power = p.d. x current
73
how efficient are transformers?
nearly 100%
74
what is the power in the primary coil of a transformer equal to?
power in primary coil = power in secondary coil
75
what are the two types of transformer?
- step-up transformer
- step-down transformer
76
what does a step-up transformer do and how?
- step the voltage up
- they have more turns on the secondary coil than the primary coil
77
what does a step-down transformer do and how?
- step the voltage down
- they have more turns on the primary coil than the secondary coil
78
what does the number of turns in a coil affect in a transformer?
the size of the induces potential difference across it
79
what is the ratio between the potential difference across the primary and secondary coils in a transformer the same as?
the ratio between the number of turns in the primary and secondary coils
