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physics gcse > Magnetism and Electromagnetism > Flashcards

Magnetism and Electromagnetism Flashcards

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1
Q

what metals can be magnetised?

A

iron, nickel, cobalt, steel

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2
Q

what is an induced magnet?

A

a magnet produced when a magnetic material is placed in a magnetic field

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3
Q

what is a magnetic field?

A

a region around a magnet where a magnetic material will experience a force

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4
Q

why are field lines forced closer together at the poles of a bar magnet?

A

the magnetic field is strongest here - highest flux density

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5
Q

what do the lines on magnetic field lines show?

A

direction of magnetic field - lines of magnetic flux

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6
Q

what is the unit of magnetic flux density?

A

tesla, T

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7
Q

what is magnetic flux density?

A

Strength of a magnet

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8
Q

what does the strength of a magnetic field depend on?

A

the distance from the magnet

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9
Q

what is the direction of a magnetic field at any point given by?

A

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

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10
Q

what is the direction of the magnetic field line from?

A

the north (seeking) pole to the south (seeking) pole

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11
Q

why does the earth have a magnetic field?

A

it has an iron core

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12
Q

where is the magnetic north pole on the earth?

A

at the geographical south pole

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13
Q

what happens when a current flows through a conductor?

A

a magnetic field is generated around it

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14
Q

what is a solenoid?

A

a coil of wire

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15
Q

what is the direction of field around a wire, with current flowing through it, shown by?

A

right hand grip rule

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16
Q

what is the symbol for current into page?

A

cross in a circle

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17
Q

what is the symbol for current out of page?

A

dot in a circle

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18
Q

what can the strength of an electromagnetic field be increased by?

A
  • adding more turns on the coil
  • increasing the current passing through it
  • using an iron core
19
Q

what are the advantages of electromagnets over normal magnets?

A
  • variable strength
  • can be turned on and off
  • can reverse direction of magnetic field
20
Q

how does an electric bell work using electromagnets?

A
  • switch is closed to complete the circuit
  • current flows through the solenoid
  • solenoid becomes magnetic
  • iron striker is attracted to the electromagnet and strikes the bell
  • as striker moves towards the bell, the contact is broken
  • current stops flowing through the coil, which loses its magnetism
  • the spring returns the striker to its original position
  • circuit complete so current flows again
  • the bell will continue to ring as long as the switch is held closed
21
Q

how does an relay switch work using electromagnets?

A
  • relay contains 2 separate circuits: a low voltage one containing an electromagnet which is safe to be switched on and off and a high voltage one where the switch has been replaced with 2 metal contacts, one of which is connected to a spring which keeps the contacts apart and an iron block next to the spring
  • low voltage circuit is switched on
  • a current now flows around the circuit
  • there is now a magnetic field around the electromagnet
  • the magnetic field around the electromagnet attracts the iron block in the high voltage circuit, causing the contacts to close, switching on the high voltage circuit
22
Q

what happens if a wire carries a current at right angles to a magnetic field?

A

it experiences a force

23
Q

what does the size of the force produced by a wire carrying a current at right angles to a magnetic field depend on?

A

how the field and wire are angled:
- the force is greatest when the field direction and wire are at right angles
- the force is 0 when the direction of the field and wire are parallel

24
Q

what are the parts of Fleming’s Left Hand Rule?

A
  • thumb = motion or direction of force
  • First Finger = magnetic field direction
  • seCond finger = direction of current
25
Q

how is a current formed by induction?

A

a p.d. is induced when a coil of wire cuts through the magnetic field of a magnet

26
Q

how can the size of the p.d. (and current), formed in electromagnetic induction, be increased?

A
  • use a stronger magnet
  • move the magnet faster
  • add more turns on the wire coil
  • increase the area of the coil
27
Q

how do DC motors work?

A
  • a coil of wire is placed inside the field of a permanent magnet
  • the coil will turn when a current flows
  • there are opposite forces on each side, causing the coil to spin by turning electrical energy into kinetic
28
Q

why is a split ring commutator used in DC motors?

A
  • because the coil is vertical in between the magnets, the forces on it no longer make it turn, but pull it up and down on each side of the coil
  • the split ring commutator breaks the flow of current when the coil is vertical and reverse the direction of current every half turn to ensure continuous rotation
29
Q

what is the motor effect?

A

a current carrying wire in a magnetic field will experience a force (must be ppd to experience full force)

30
Q

explain how an electric motor works

A
  • coil of wire carrying a current is placed inside the field of a permanent magnet
  • the force acting on one side of the coil will act upwards, and the other downwards causing the coil to spin (clockwise)
  • once it has spun 180 degrees, the forces acting on the coil have swapped sides so will spin in the opposite direction (anticlockwise)
  • left like this, the coil would flip back and forth, not spin 360 degrees
  • so the current needs to change direction every 180 degrees (half turn) so coil will rotate in the same direction (full spin) - use a split ring commutator which moves as the wire moves to keep terminals the same
31
Q

how do loudspeakers work?

A
  • there is a varying current in the circuit which passes through a coil of wire
  • the coil experiences a force (magnetic field) and reversing the current reverses the force (magnetic field)
  • the magnetic field attracts to and then repels from the permanent magnet at the same frequency as the ac, meaning the coil vibrates at a specific frequency
  • the vibrating coil causes the cone to vibrate
  • the vibrating cone causes the air molecules to move
  • the movement of air molecules produces pressure variations in the air needed for a soundwave
  • air molecules bunch together forming compressions and spread out forming rarefactions
32
Q

how does changing the frequency of the ac in a loudspeaker change the sound?

A

it changes the frequency (pitch) of the sound produced

33
Q

how does changing the voltage (and therefore current) of the ac in a loudspeaker change the sound?

A

it changes the volume of the sound produced by changing the size of the force acting on the coil of wire

34
Q

HOW DO MICROPHONES WORK?

A
  • pressure variations in sound waves cause the flexible diaphragm to vibrate
  • the vibrations of the diaphragm cause vibrations in the coil
  • the coil moves relative to a permanent magnet, so a p.d. is induced in the coil
  • the coil is part of a complete circuit so the induced p.d. causes a current to flow around the circuit
  • the changing size and direction of the induced current matches the vibrations of the coil
  • the electrical signals generated match the pressure variations in the sound waves
35
Q

what is a transformer?

A

a device used to increase or decrease the p.d. of electricity

36
Q

what is a transformer made of?

A

two coils of wire wrapped around an iron core

37
Q

how does a transformer work?

A
  • a primary voltage drives an alternating current through the primary coil
  • the primary coil current produces a magnetic field, which changes as the current changes
  • the iron core increases the strength of the magnetic field
  • the changing magnetic field induces a changing potential difference in the secondary coil
  • the induced potential difference produces an alternating current in the external circuit
38
Q

do step up transformers have more turns on the primary or secondary coil?

A

secondary

39
Q

do step down transformers have more turns on the primary or secondary coil?

A

primary

40
Q

what does Nₛ / Nₚ = Vₛ / Vₚ stand for?

A

number of turns in secondary coil / number of runs in primary coil = voltage in secondary coil / voltage in primary core

41
Q

why are transformers assumed to have 100% efficiency?

A

energy can’t be created or destroyed, only transferred so when electrical energy from primary coil is transferred to secondary, it can’t increase in energy

42
Q

what is the generator effect?

A

the creation of p.d. therefore an induced current (flowing electrons) when a wire experiences a changing magnetic field

43
Q

how does a faster relative motion between magnetic and wire coil increase p.d. in the generator effect?

A

it increases the magnetic field change, and p.d = change in magnetic field / change in time

physics gcse (13 decks)

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