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Physics > P15 - Electromagnetism > Flashcards

P15 - Electromagnetism Flashcards

1
Q

Examples of magnetic materials

A

Iron, steel, cobalt and nickel

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2
Q

What is a permanent magnet

A

A magnet that produces its own magnetic field. Is usually made of steel as it doesn’t lose its magnetism easily

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3
Q

How can magnetic field lines be found?

A
  • Iron filings
  • Plotting compass which always points towards the South Pole
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4
Q

What is an induced magnet

A

A material that becomes a magnet when in a magnetic field. It always produces a force of attraction

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5
Q

What is a magnetic field

A

The region around a magnet where another magnet or a magnetic material experiences a force

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6
Q

What are magnetic field lines

A
  • Show the direction of the force that would act on a north pole at that point
  • They always point from a magnet’s north pole to its south pole
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7
Q

What does the strength of a magnetic field caused by a current in a wire depend on?

A
  • The current through the wire
  • The distance from the wire
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8
Q

How to remember the corkscrew rule

A

Right hand thumb rule

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9
Q

What is a solenoid?

A

A coil of wire that has a similar magnetic field to a bar magnet

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10
Q

What is the field like inside the solenoid?

A

Strong and uniform

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11
Q

Methods that increase the strength of a solenoid (3)

A
  • Add an irone core (makes it an electromagnet)
  • Increase current
  • Tighter turns / more truns on solenoid
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12
Q

Devices where electromagnets are used

A
  • Relays
  • Electric bells
  • Circuit breakers
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13
Q

How does a relay make use of electromagnets

A
  • A relay contains 2 separate circuits
  • There is a low voltage circuit containing an electromagnet which is safe to be switched on or off
  • On the high voltage circuit there are 2 metal contacts, an iron block and a spring
  • When the low voltage circuit is switched on, a current flows making a magnetic field around the electromagnet
  • The electromagnet then attracts the iron block in the high voltage circuit
  • This makes the contacts touch and a currnet can flow
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14
Q

How does a doorbell make use of an electromagnet

A
  • The switch closes when the buzzer is pressed and a current can flow through the circuit
  • The electromagnet then produces a magnetic field and the iron armature is attracted to the electromagnet
  • When this happens, the hammer hits the bell and this breaks the circuit so no current flowing and no electromagnet
  • This means the iron armature moves back to its original position and the circuit is complete again so the process repeats
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15
Q

Defenition of the motor effect

A

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

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16
Q

What factors increase the size of the force in the motor effect (3)

A
  • Increasing the current
  • Using a stronger magnet
  • Length of the wire
17
Q

What does each finger in Fleming’s left hand rule show

A
  • Thumb - Force
  • First finger - magnetic field
  • Second finger - current
18
Q

Equation that applies to Flemin’g left hand rule

A

Force = magnetic flux denisty x current x length

F = Bil

19
Q

What is magnetic flux denisty and give its unit

A

The strength of the magnetic field. It is measured in tesla (T)

20
Q

How does an electric motor work

A
  • A loop of wire in a magnetic field rotates due to a force experienced on both sides of the wire - one side moves up and one side moves down creating a moment (turning force)
  • When the coil gets to a vertical position, momentum keeps the wire turning and the split ring commutators change the direction of the current
  • This keeps the motor rotating in the same direction
21
Q

How does a moving coil loudspeaker work (5 steps)

A
  • You have a cone with a coil or wire wrapped around one end and the coil is connected to an A.C. supply. A permanent magnet goes inside the coil of wire
  • A.C through coil produces a magnetic field and this magnetic field interacts with the magnetic field of the permanent magnet
  • Causes a resultant force due to motor effect and pushes cone out
  • Current reverses, so force reverses, so cone moves back in
  • This causes cone vibrations which are sound waves
22
Q

What can you change to change the sound produced by a moving coil loudspeaker

A
  • By changing the frequency of the A.C. supply, you can change the frequency of the cone vibrations and therefore the frequency of the sound waves
  • If the size of the A.C. current changes, the amplitude and therefore volume of the sound changes
23
Q

Defenition of electromagnetic induction

A
  • Where a potential difference is induced across the ends of a conductor due to it moving through a magnetic field or the magnetic field is varying.
  • When the condctor stops moving, the potential difference is lost
  • If the conductor moves back the other away, a potential difference is induced again but it is in the opposite direction
24
Q

What is the generator effect

A
  • Same as defenition of induced electromagnetism
  • If the conductor is connected to a circuit, an induced current is produced which creates its own magnetic field
  • The induced current’s magentic field opposes themovement of the magnet (the original change)
  • We only get the generator effect when the wire passes through the magentic field and not along it

These together make up the generator effect

25
Q

What factors affect the direction of the induced current/ p.d. (2)

Generator Effect

A
  • If the magnet is moved out of the coil
  • If the other pole of the magnet enters the coil
26
Q

What factors affect the size of the induced current/ p.d. (3)

Generator Effect

A
  • Strength of magnetic field
  • Number of turns on the coil
  • Speed of magnet through coil
27
Q

How does an alternator produce an alternating current

A
  • An alternator is a coil or wire rotating in a magnetic field which is connected to 2 commutator rings
  • One side of th wire always attaches to one commutator ring and the other side attaches to the other ring
  • The commutators allow the current to pass out of the coil
  • A potential difference is induced when the wire passes through the magnetic field
  • This is alternating and as it is connected to a circuit, an A.C. is produced
28
Q

Describe the potential difference graph produced by an alternator

A
  • The maximum potential difference is when the coil is horizontal as the wire is moving through the magentic field at the fastest rate
  • When the coil is vertical, the potential difference is 0 and the coil is moving parallel to the field
  • As the coil moves round, we get a potential difference again but it has reversed direction because the two sides of the coil are moving in different directions to before
29
Q

How does a dynamo produce a direct current

A
  • It has a split-ring commutator so one side of the coil is connected to one side of the ring and vice versa
  • A potential difference and current are induced
  • When the coil is vertical, there is no potential difference as the coil is moving parallel to the magentic field
  • When the coil moves round, the side of the coil moving down is connected to the same side of the split ring commutator and vice versa
  • Therefore, the direction of the p.d. doesn’t change so the direction of the current doesn’t either
  • 2 peaks equate to one rotation of the coil as each side of the coil passes through the field twice
30
Q

How does a moving coil microphone work?

A
  • The coil is attached to a diaphragm which is placed inside a permanent magnet
  • Sound waves cause the diamphragm to vibrate
  • This causes the coil to vibrate
  • The coil moves in the magnetic field so a potential difference is induced across the ends of the wire
  • The potential difference switches direction as the coil moves backwards and forwards
  • The frequency of the changing potential difference is the same as the frequency of the sound waves
  • The coil is part of a circuit so an induced current is produced
31
Q

What is a transformer and how do they work generally

A
  • 2 coils of wire - a primary coil and a secondary coil that are wrapped around an iron core (easily magnetised) which increases the strength of the magnetic field
  • The primary coil is connected to an alternating current and as this flows through the coil, it generates an changing magnetic field
  • This A.C. passes through the iron core and into the secondary coil and induces a potential difference in it
  • Transformers can only work with A.C. as we need a changing magnetic field to induce a potential difference (generator effect)
32
Q

Structure of a step up transformer

A
  • The secondary coil has more turns that the primary coil
  • This means that the induced p.d. in the secondary coil will be greater than the p.d. in the primary coil
33
Q

Structure of a step down transformer

A
  • The primary coil has more turns than the secondary coil
  • Therefore, the induced p.d. in the secondary coil is less than the p.d. in the pirmary coil
34
Q

Equation linking potential difference and turns in the coils of transformers

A

Vp/Vs = Np/Ns

35
Q

Equation for the power in the coils in a transformer

A
  • As P = VxI:
  • Vp x Ip = Vs x Is
  • This is bceause power is conserved between transformers (asssuming it is 100% efficient)
36
Q

How are transformers used in the national grid

A
  • First, a step up transformer increases the p.d. to 400000V
  • This is because P = VI so to transfer the same amount of electrical power, a lower current is needed
  • This makes the Natioanl grid more efficient because less current measn the cables are heated less so less energy is dissipated to the surroundings
  • Then, a step down transformer decreases the voltage to 230V for UK homes
  • This si to make it safer for everyday use

Physics (15 decks)

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