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Physics P2 > P15 - Electromagnetism > Flashcards

P15 - Electromagnetism Flashcards

(69 cards)

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1
Q

What are the poles of a magnet?

A

The places where the magnetic forces are strongest

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2
Q

What happens when two magnets are brought close together?

A

They exert a force on each other

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3
Q

What happens when two like poles are brought together?

A

They repel each other

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4
Q

What type of force is attraction and repulsion between magnetic poles?

A

Non-contact

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5
Q

What is a permanent magnet?

A

A magnet that produces its own magnetic field

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6
Q

What is an induced magnet?

A

A material that becomes magnetic when placed in a magnetic field

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7
Q

What happens when an induced magnet is removed from a magnetic field?

A

The magnet loses most/ all of its magnetism quickly

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8
Q

What is the difference between permanent magnets and induced magnets?

A

The permanent magnets maintain magnetic properties while induced magnets become magnetic when placed in a magnetic field

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9
Q

What is a magnetic field?

A

The region around a magnet where a force acts on another magnet or on a magnetic material

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10
Q

What is the force between a magnet and magnetic material?

A

Attraction

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11
Q

What does the strength of a magnetic field depend on?

A

The distance from the magnet

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12
Q

What is the direction of the magnetic field given by?

A

The direction of the force that would act on another north pole placed at that point

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13
Q

Where is the direction of a magnetic field line from?

A

From the north pole of a magnet to the south pole

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14
Q

Where is the strongest magnetic field?

A

At the poles of the magnets

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15
Q

What happens when a current flows through a wire?

A

A magnetic field is produced

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16
Q

How can the strength of a magnetic field created by a current through a wire be increased?

A

Shaping the wire to make a solenoid

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17
Q

How can we increase the strength of the magnetic field of a solenoid?

A

Add an iron core

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18
Q

How does a magnetic compass work?

A
  • Contains a small bar magnet on a pivot so it can rotate
  • the compass needle points in the direction of the Earth’s magnetic field or the magnetic field of a magnet
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19
Q

Explain how magnetic fields can be mapped out(4)

A
  1. Place the plotting compass near the magnet on a piece of paper
  2. Mark the direction the compass needle points
  3. Move plotting compass to many different positions in the magnetic field, marking the needle direction
  4. Join points to show field lines
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20
Q

What does a magnetic field diagram show?(4)

A
  • Magnetic field lines never cross each other
  • the closer the lines the stronger the magnetic field
  • arrows to show direction of force exerted by a magnetic north pole
  • arrows point from north pole to its south pole
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21
Q

What happens when a current flows in a wire?

A

It creates a circular magnetic field around the wire, magnetic field can deflect the needle of a magnetic compass

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22
Q

When is the strength of magnetic field greater?

A
  • When it is closer to the wire
  • if current is increased
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23
Q

What is a solenoid?

A

A wire coiled up into a spiral shape

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24
Q

What is the magnetic field like in a solenoid?

A

The smaller ones caused by the current in each coil add together to make a stronger overall magnetic field

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25
What is an electromagnet?
A solenoid with an iron core
26
How is an electromagnet made?
Simple electromagnetic made by coiling wire around an iron nail
27
Where are electromagnets used?
Devices such as electric bells and door locks that can be controlled remotely
28
Magnetic field pattern for a straight wire carrying a current and for a solenoid?
29
What is the motor effect?
When a conductor is carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other
30
When does the force on a wire in a magnetic field increase?
The current in wire increases, strength of magnetic field increases
31
How can the direction of a motor effect force be found?
Using Fleming’s left hand rule
32
What does the thuMb show?
- Movement(direction of the motor effect force on the conductor carrying the current)
33
What does the Forefinger show?
Field(N to S)
34
What does the seCond finger show?
Current(+ to -)
35
What is the equation for a conductor at right angles to a magnetic field and carrying a current?
Force = magnetic flux density * current * length
36
What is the unit for magnetic flux density?
In tesla, T
37
What is the basis of an electric motor?
A coil of wire carrying a current in a magnetic field tends to rotate
38
What happens in this diagram of the dc motor?
Current in the left part of the coil causes a downward force, current in right part of the coil causes an upward force and the coil rotates anticlockwise because of these forces
39
What happens when the coil is vertical?
Moves parallel to the magnetic field, producing no force
40
Why does it keep rotating even with no force?
- The momentum of the motor carries it on round a little - split ring commutator changes the current direction every half turn
41
What do loudspeakers and headphones use the motor effect to do?
- Convert variations in electrical to the pressure variations in sound waves
42
How do moving-coil loudspeakers and headphones work?
Convert electrical signals into sound waves using the motor effect 1. Current in the coil creates an electromagnetic field 2. Interacts with a permanent magnet generating a force, pushing the cone outwards 3. Current is made to flow in the opposite direction 4. Direction of the electromagnetic field reverses 5. Force on the cone now pulls it back in 6. Repeatedly alternating current direction makes the cone vibrate in and out 7. Cone vibrations cause pressure vibrations in the air - sound waves
43
What is the generator effect?
Process of generating a potential difference and a current in a conductor when it moves through a magnetic field or when a magnetic field changes around a conductor
44
What does the induced current do?
Creates a magnetic field which opposes the original change, either the movement of the conductor or the change in magnetic field
45
What factors affect the size of induced potential difference or current?
Current is reversed when - magnet is moved out of the coil and the other pole of the magnet is moved into the coil
46
What does the direction of the induced current depend on?
The direction of movement of the magnet relative to the coil
47
When will an induced current or potential difference increase?
Speed of movement, magnetic field strength or number of turns on the coil is increased
48
In what two situations is the generator effect used?
- In an alternator to generate ac - in dynamo to generate dc
49
What is an alternating current generator?
A device that produces a potential difference - simple one consists of a coil of wire rotating in a magnetic field
50
What do alternators consist of?
- Coil rotating in magnetic field - current induced in rotating coil - slip rings connected to coil - brushes make continuous contact between the external circuit and slip rings - current flows in external circuit
51
How is the generator effect used in an alternator to generate ac?
- As one side of the coil moves up through the magnetic field, a potential difference is induced in one direction. - As rotation continues that side of the coil moves down, the induced potential difference reverses direction - Produces a current that is constantly changing - alternating current
52
What does alternator output on a graph look like?
Sine curve
53
How can the maximum potential difference or current be increased?
Increasing the rate of rotation, increasing strength of the magnetic field, increasing number of turns on the coil
54
What is the difference between ac generator and dc generators?
Dc use a split ring commutator rather than the two slip rings in ac generator
55
How is the generator effect used in a dynamo to generate dc?
- A split ring commutator changes the coil connections every half turn - as induced potential difference is about to change direction the connections are reversed - current to the external circuit always flows in the same direction
56
What does the dynamo output on a graph look like?
Sine curve that stays in the same direction
57
Dynamo output on a graph points
A - Coil moves parallel to the direction of magnetic field - no potential difference induced B - coil moves at 90 degrees to the direction of magnetic field so induced potential difference is at its maximum
58
What is a microphone?
A device that converts sound waves into electrical signals
59
How do microphones use the generator effect?
Convert the pressure variations in sound waves into variations in current in electrical circuits
60
How does a moving-coil microphone work?
1. Pressure variations in sound waves cause the flexible diaphragm to vibrate 2. Vibrations of the diaphragm cause vibrations in the coil 3. Coil moves relative to a permanent magnet, so a potential difference is induced in the coil 4. Coil is part of a complete circuit so induced potential difference causes a current to flow 5. Changing size and direction of induced current matches vibrations of the coil 6. Electrical signals generated match the pressure variations in the sound waves
61
What is a transformer?
A device that can change the potential difference or voltage of an alternating current
62
What does a basic transformer consist of?
A primary coil from the ac and a secondary coil leading to the ac output wound on an iron core
63
Why is iron used?
Easily magnetise, can carry magnetic fields from each coil
64
What happens within a transformer?
1. A primary voltage drives an ac through the primary coil 2. primary coil current produces a magnetic field, which changes as the current changes 3. Iron core increases the strength of the magnetic field 4. Changing magnetic field induces a changing potential difference in the secondary coil 5. Induced potential difference produces an alternating current in the external circuit
65
What does the ratio of the potential difference across primary and secondary coils of a transformer depend on?
The ratio of number of turns on each coil
66
What equation can be used for the ratio of potential difference and number of turns?
Vp/Vs=Np/Ns
67
What is the potential difference in a step up transformer?
Vs>Vp
68
What is the potential difference in a step down transformer?
Vs
69
What is the equation for power output(secondary coil) and input(primary coil)?
Vs * Is=Vp * Ip

Physics P2 (10 decks)

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