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Physics Paper 2 > P15 Electromagnetism > Flashcards

P15 Electromagnetism Flashcards

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1
Q

What is the force rule for two magnetic poles near each other?

A

Like poles repel. Unlike poles attract.

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2
Q

What is induced magnetism?

A

Magnetism created in an unmagnetised magnetic material when the material is placed in a magnetic field

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3
Q

At which part of a magnet are the magnetic forces strongest?

A

The poles of the magnet

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4
Q

What happens when two magnets are brought close to each other?

A

They exert a force on each other

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5
Q

What is the difference between a permanent magnet and an induced magnet?

A
  • A permanent magnet produces its own magnetic field
  • An induced magnet becomes magnetic when placed in a magnetic field
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6
Q

What type of force does induced magnetism always cause?

A

A force of attraction

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7
Q

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

A

The induced magnet loses most or all of its magnetism

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8
Q

What is a magnetic field?

A

The region surrounding a magnet where another magnet or magnetic material experiences a non-contact force

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9
Q

Four examples of magnetic materials

A

Iron, steel, cobalt, nickel

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10
Q

What can always be said about the force between a magnet and a magnetic field?

A

It is always attractive

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11
Q

How does the strength of a field vary with position?

A

The strength of the magnetic field depends on the distance from the magnet

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12
Q

How does the strength of a magnetic field alter as you move further away from the magnet producing it?

A

The magnetic field strength decreases the further you move away

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13
Q

In what direction does a magnetic field point?

A
  • In the direction that a North Pole would experience a force if placed in the field
  • From the north seeking pole to the south seeking pole of the magnet
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14
Q

What does a magnetic compass contain?

A

A small bar magnet that points in the direction of the Earth’s magnetic field

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15
Q

How can you determine the magnetic field of a bar magnet?

A
  1. Place the magnet in the middle of a piece of paper and trace around it
  2. Place the plotting compass near the magnet
  3. Mark the direction the compass needle points
  4. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
  5. Join the points to show the field lines
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16
Q

What are the properties of magnetic lines?

A
  • They never cross each other/overlap
  • Always point from the North Pole to the South Pole
  • They are continuous
  • The closer the lines, the stronger the force
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17
Q

Why is the Earth’s magnetic field important?

A
  • It play a big part in maintaining the Earth’s atmosphere
  • If there was no magnetic field the solar wind would strip away the Earth’s atmosphere leaving the Earth’s surface vulnerable to UV rays
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18
Q

How to show the magnetic effect of a current in a wire

A

Move a compass around the wire and the changing direction of the compass needle shows a magnetic field has been produced

OR

Sprinkle iron filings onto the card tapping the card will move the filings to show the magnetic field lines

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19
Q

The pattern of magnetic field around a straight wire carrying a current

A
  • The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire
  • The direction of the magnetic field is perpendicular to the wire and given by the right hand corkscrew rule
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20
Q

How can we increase the size of the force

A
  • Increase the strength of the magnetic field or
  • Increase the current
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21
Q

How can we alter the direction of the force

A
  • Alter the direction of the force or
  • Alter the direction of the current
22
Q

Equation for force

A

F = BIL

F = force in newtons
B = magnetic flux density measure in Tesla
I = current in amps
L = length in metres

23
Q

What is a commutator?

A
  • A copper ring split in two halves
  • In a simple electric motor each half is connected to the ends of the wire loop
  • In practice they are connected to the axle
24
Q

What is the axle?

A

In electric motors the commutator is attacher to the axle. The axle transfers the rotational motion

25
Q

What are the brushes in an electric motor?

A

The brushes connect the wire lop or armature to the power supply

26
Q

Explain how a coil rotates in a motor

A
  • The coil inside the magnetic field means that at any one time the current will flow in different directions inside the field
  • 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
  • So the coil rotates anticlockwise
27
Q

What are the 3 factors affecting the size of the force on a conductor?

A
  • Length of the conductor
  • Size of the current passing though the conductor
28
Q

Explain what is the role of the split-ring commutator in a motor

A
  • When the coil is vertical, it moves parallel to the magnetic field, producing no force
  • This would tend to make the motor stop
  • the split-ring commutator reverses the current every half turn so the coil keeps rotating
29
Q

What is the generator effect?

A
  • When there is relative motion between conductor and a magnetic field, a potential difference is induced
  • If a complete circuit is made then a current will flow
  • This is how an electrical current can be generated
30
Q

When does the size of the induced potential difference increase?

A

When:
- The speed of the movement increases
- The strength of the magnetic fields increases/ use a stronger magnet
- The number of turns on the coil increases
- The area of the coil is greater
- The rate at which the magnetic flux is cut increases

31
Q

Explain the similarities and differences between a motor and a generator

A
  • Generators use movement to create current but motors use current to generate movement
  • Both use a stationary magnetic field
  • Both have a rotating coil
  • A motor uses direct current, a generator produces alternating current
  • A motor makes use of a split ring commutator, in a generator there is no need for a commutator
  • A generator uses split rings, but in a motor there is no split rings
32
Q

Explain how a microphone works

A
  • an incoming longitudinal sound wave causes the membrane to move forwards and backwards
  • uses the generator effect
  • when a magnet moves into a coil of wire, a potential difference is induced and a current can flow
  • this happens only when the magnet or coil is moving
  • moving coil microphones convert sound waves into an electrical signal
  • a coil moves past a magnet in proportion to the pressure vibrations from the sound and an alternating potential difference is induced
  • the frequency of the sound waves which hit the diaphragm will produce an alternating current with the same frequency as the incoming sound waves
33
Q

Explain how a loudspeaker works

A
  • A current flows through the coil
  • Because the coil is in a magnetic field, there is a force on the coil
  • The current is alternating, and so the force on the coil constantly changes direction
  • This causes the coil to vibrate
  • The coil is attached to the cone
  • The cone vibrates producing sound waves
34
Q

Primary coil

A
  • Will have a number of turns on it and a certain potential difference applied
  • It is connected to the power source
35
Q

Secondary coil

A
  • Will then give a certain potential difference based on the number of turns on it
  • Will provide alternating current for the circuit it is connected to
  • Energy is transferred to the secondary coil even though they aren’t electrically connected
  • The potential difference produced is proportional to the number of turns on the coil
36
Q

What is a transformer?

A

A soft iron laminated core that is wrapped in insulated wire on each side

37
Q

Why is iron used for the core of a transformer?

A

It is easily magnetised and demagnetised and so provides a path for the induced changing magnetic field to pass to the other coil

38
Q

What are transformers used for and what are they made of?

A
  • A basic transformer consists of a primary coil and a secondary coil wound on an iron core
  • Transformers are used to increase or decrease the size of an alternating potential difference
39
Q

What is a step-up transformer used for?

A

Increase the size of an alternating potential difference

40
Q

What is a step-down transformer used for?

A

Decrease the size of an alternating potential difference

41
Q

What type of a current does a transformer work for and why?

A
  • Only works for an alternating current
  • Because a changing magnetic field is needed to induce alternating current in the secondary coil
42
Q

What is the motor effect?

A

When a conductor carrying current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other

43
Q

Describe the differences between a step-up transformer and a step-down transformer

A
  • Step-up transformers have more turns on the secondary coil, but step-down transformers have more turns on the primary coil
  • Step-up transformers are used to increase the size of an alternating potential difference, but a step-down transformers are used to decrease the size of an alternating potential difference
  • In a step-up transformer the output potential difference is greater than the input potential difference, but in a step-down transformer the output potential difference is fewer than the input potential difference
44
Q

Describe the similarities between a step-up transformer and a step-down transformer

A
  • Both are transformers and consist of a primary coil and a secondary coil
  • Both contain a laminated core
  • Both uses iron to make the core
  • The coils are covered in plastic
  • The coils are made from an electrical conductor, usually copper
45
Q

How can you calculate either the number of turns needed or the potential difference in coils

A

Vp / Vs = Np / Ns

46
Q

Why is power constant in a transformer?

A

Transformers cannot create or destroy energy therefore they must have the same power on each side (law of conservation of energy)

47
Q

How to calculate power of a transformer?

A

Ip x Vp = Is x Vs assuming the transformer is 100% energy efficient

48
Q

What is the reason for the step up transformer?

A
  • To reduce the current and transmit the same power
  • Using a lower current but higher voltage means that less energy is wasted due to the heating effect of the current and the resistance in the cables
  • So there’s less power loss and the efficiency is increased
49
Q

what is a permanent magnet?

A

a magnet that produces its own magnetic field

50
Q

what is an induced magnet?

A
  • ​a material that becomes a magnet when it is placed in an existing magnetic field
  • but loses its magnetism quickly once it is removed
  • induced magnetism always produces attractive forces.
51
Q

explain how a circuit breaker works

A
  • too much current passes through the coil
  • the iron core of the electromagnet is magnetised
  • the switch is attracted to the core of the electromagnet
  • the circuit breaker switch is opened
  • the current is cut off

Physics Paper 2 (11 decks)

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