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Science - continued > Magnetism and the motor effect > Flashcards

Magnetism and the motor effect Flashcards

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1
Q

Give examples of magnetic metals

A

Iron, nickel, cobalt and steel

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2
Q

What are the rules for north and south poles in magnets?

A

Opposites attract

Same repel

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3
Q

What is an induced magnet?

A

A material that is placed in a magnetic field which becomes a magnet itself and stops being a magnet when it’s taken out of the field again

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4
Q

Give examples of where magnets are used

A
  • motors
  • generators
  • loudspeakers
  • door latches
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5
Q

What do close together lines mean in a magnetic field diagram?

A

The magnetic field is strongest there

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6
Q

How can a magnetic field be found?

A

Using a plotting compass

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7
Q

What do magnetic field lines show?

A

The direction in which the magnetic force is acting at a specific point

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8
Q

What magnetic field is the earth’s similar to?

A

A bar magnet (they have a similar magnetic field shape)

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9
Q

What is the Earth’s magnetic field thought to be caused by?

A

Electric currents in the molten outer core (made of iron and nickel)

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10
Q

What does a current flowing through a wire cause?

A

A magnetic field

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11
Q

What does the direction of a magnetic field depend on?

A

The direction of the current - if the current changes direction, so does the direction of the magnetic field

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12
Q

What does the strength of the magnetic field depend on?

A

The size of the current - higher the current, stronger the field

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13
Q

Where will a magnetic field be the strongest

A

Nearest the wire and gets weaker with increasing distance

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14
Q

How can you use a thumbs up to work out the direction of the magnetic field

A

If you point your thumb in the direction of the current, the direction of your fingers curling round will show the direction of the magnetic field

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15
Q

What is a solenoid?

A

A coil of wire with electricity flowing through it

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16
Q

What is the magnetic field inside a solenoid like?

A

The lines on a solenoid magnetic field are very close together in the middle of the coil, and so is very strong

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17
Q

What is the magnetic field outside a solenoid like?

A

One side of the coil tends to balance out the other side of the coil and gives a weaker field outside

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18
Q

What is an electromagnet?

A

A coil of wire with a current flowing through it

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19
Q

How can the magnetic field of an electromagnet be made stronger?

A

By putting a piece of iron (iron core) inside the coil and this iron becomes a temporary magnet - it is only a magnet when the electromagnet is affecting it

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20
Q

What is the motor effect?

A

When a current-carrying wire or coil can exert a force on a permanent magnet

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21
Q

What happens to a current-carrying wire when it’s placed between 2 magnets?

A

It experiences a force - the current creates a magnetic field around the wire and this interferes with the magnetic field between the magnets

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22
Q

When is the force strongest with a wire between two magnets?

A

When the wire is at right angles to the magnetic field produced from the magnets `

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23
Q

When is the force weakest with a wire between two magnets?

A

When the wire is in the same direction as the magnetic field

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24
Q

What does the direction of the force depend on?

A

Directions of the magnetic field and current

25
Q

What does each of the three fingers represent in Fleming’s left-hand law?

A

Thumb- direction of force
Index- direction of magnetic field
Middle- direction of current

26
Q

What does the size of the force depend on?

A

Magnetic flux density (magnetic field strength), the current and the length of the wire in the field

27
Q

What is magnetic flux density measured in?

A

N/Am (newtons per amp metre)

28
Q

What causes a rotation in electric motors?

A

The force on a conductor in a magnetic field

29
Q

What does using a coil with many turns of wire do?

A

Increases the total force turning the coil

30
Q

What does the split-ring commutator do?

A

Ensures that the force on the coil always turns it in the same direction and that the current is always flowing in the correct direction so that the coil can continue to spin

31
Q

What does a changing magnetic field do?

A

Induce a voltage or potential difference in a wire - causes a current to flow

32
Q

How can a potential difference be induced?

A

By entering it into a magnetic field - the wire is often made into a coil so that there is more wire in the magnetic field

33
Q

What does the size of the induced potential difference depend on?

A
  • number of turns of coil in the wire
  • magnetic field strength
  • how fast the magnetic field changes or moves past the coil
34
Q

What does reversing the direction of the magnetic field change also reverse?

A

Reverses the direction of the induced potential difference

35
Q

When the potential difference causes a current to flow, what does the magnetic field of the current do?

A

The magnetic field of the current opposes the original change

36
Q

What does a generator consist of?

A

A coil of wire that is rotated inside a magnetic field - as the coil turns, a voltage is induced in the wire

37
Q

What are the ends of the coil connected to?

A

Slip rings

38
Q

How is contact made to the slip rings and what does this kind of generator produce?

A

Through an external circuit with carbon brushes pressing on the slip rings and it produces an alternating current (called an alternator)

39
Q

What does a commutator do?

A

Switches over the connections every half-turn of the coil and so produces a form of direct current

40
Q

What is a dynamo?

A

A generator with a commutator

41
Q

What do microphones convert?

A

Pressure variations in sound waves into variations in current in electrical circuits

42
Q

How does a moving coil microphone work?

A
  1. sound waves cause variations in air pressure
  2. pressure variations make a diaphragm vibrate which makes coil move back and forth
  3. coil moves with a permanent magnet and potential difference is induced
  4. coil is now part of the circuit and a current flows around it
  5. the changing size and direction of the current matches the vibrations of the coil
  6. the electrical signals produced match the pressure variations in the sound waves
43
Q

What do loudspeakers convert?

A

Variations in an electrical current into sound waves

44
Q

How do loudspeakers work?

A
  1. The varying current flows through a coil in a magnetic field
    2 this causes a force on the coil which moves back and forward as the current varies
  2. The coil is connected to the diaphragm and the movements of the diaphragm produce sound waves
45
Q

What is the national grid?

A

A network of wires and cables that electricity is sent through for everyday usage

46
Q

How much energy is wasted due to heating in the national grid?

A

When electricity flows through a wire, it heats up. The amount of energy lost in a building is small but the amount of energy lost in transmission lines is significant

47
Q

What is the relationship between voltage, current and energy transferred in a wire

A

If the voltage of electricty in a wire, increases, the current decreases and less energy is transferred by heating (efficiency improves)

48
Q

What do step up and step down transformers do?

A

Step-up: increases the voltage and decreases the current

step down: decreases voltage and increases current

49
Q

What do power stations produce electricity at (and what is it changed to)?

A

25 kW - changed to 400 kW by transformers before sent around the country

50
Q

What is a transformer made of?

A

2 coils of insulated wire wound onto an iron core ( no electrical connection between the two coils of wire

51
Q

What current does a transformer work with?

A

An alternating current (when the direction of the potential difference (also current) changes many times each second)

52
Q

Are the voltages at the primary and secondary coils the same?

A

No, they are at different voltages

53
Q

What does the alternating current in the primary coil create?

A

A continuously changing magnetic field - the iron core carries this magnetic field to the secondary coil

54
Q

What does the changing magnetic field induce and where?

A

A voltage on the secondary coil (the potential difference is greater in the secondary coil if it has more turns than the primary coil)

55
Q

the potential difference in primary / potential difference in secondary EQUALS WHAT?

A

number of turns in the primary / number of turns in the secondary

56
Q

What is voltage a measure of?

A

A measure of energy transferred by each coloumb of charge that flows through a wire

57
Q

How does conservation of energy tie into transformers?

A

The power supplied to a primary coil must be equal to the energy transferred away from the transformer in the secondary coil

58
Q

At what point would the power in the secondary coil equal the power in the primary coil?

A

If the transformer is 100% efficient

Science - continued (9 decks)

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