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GCSE Physics > Electromagnetism > Flashcards

Electromagnetism Flashcards

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1
Q

what is a magnetic field

A

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

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2
Q

where is the field strongest around a bar magnet

A

The poles - the field lines are the closest together. As you get further away, the fields get weaker.

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3
Q

what do magnetic field lines show

A

show the direction of the force that would be exerted on another north pole placed at that point - north to south on a field diagram

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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
  • two unlike poles attract
  • similar poles repel
  • examples of non-contact force
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5
Q

difference between permanent and induced magnet

A

A permanent magnet produces its own magnetic field whereas an induced magnet becomes a magnet when it is placed in a magnetic field. Induced magnets can only repel and quickly lose magnetism after being removed from magnetic field.

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6
Q

how does a compass show Earth has a magnetic field

A
  • compasses contain a small bar magnet
  • Earth has a magnetic field so compass points in direction of Earth’s magnetic field
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7
Q

how could you SHOW magnetic fields around bar magnet

A
  • place magnet on sheet of paper
  • sprinkle iron filings onto sheet around magnet
  • filings will line up with the field
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8
Q

how could you PLOT magnet field around bar magnet

A
  • place bar magnet on sheet of paper
  • place plotting compass on paper and mark points and end of needle
  • move compass to one of the points and repeat until you have a loop connecting the poles
  • join lines with ruler
  • repeat a different distances from the bar magnet until you have a field diagram
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9
Q

how is a magnetic field produced around a wire

A

when current flows through a conducting wire, a circular magnetic field is produced around the wire

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10
Q

how do you remember direction of field lines around a current carrying wire

A

RIGHT HAND corkscrew rule

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11
Q

what does strength of magnetic field around a wire depend on

A

the distance from the wire - the field gets weaker as you increase distance

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12
Q

how to make field around a wire stronger

A

shaping wire into a solenoid (coil) and increasing current

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13
Q

describe magnetic field of a solenoid

A

inside a solenoid it is strong and uniform with a similar shape to that of a bar magnet

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14
Q

how can magnetic field of solenoid be strengthened

A

increasing current and adding an iron core, making it an electromagnet

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15
Q

describe the motor effect

A

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

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16
Q

describe Fleming’s left hand rule

A

thumb - force
index - direct of magnetic field lines
middle finger - direction of current

17
Q

how can you increases size of force on a motor

A
  • increase the current in a wire
  • increase magnetic field strength
  • increase number of turns on the coil to increase the length of wire within the field
18
Q

how does a motor rotate

A
  • a coil of wire carries a current in two opposite directions
  • one side of a coil experiences a force in one direction, the other in the opposite direction
  • with an axle through the middle, the coil will rotate
19
Q

how are electromagnets used in scrapyard cranes

A

Steel frames of cars are attracted to the electromagnet when a current passes through the coil as it generates a magnetic field. When the current is switched off, the vehicle falls off.

20
Q

how are electromagnets used in circuit breakers

A
  • switch is held closed by a spring
  • if current gets too great, magnetic field strength will increase enough to pull the switch open, and the flow of current stops
21
Q

how are electromagnets used in electric bells

A
  • current flows and armature is attracted to the electromagnet, and so the bell rings as the hammer hits the bell
  • because the armature is attracted, it also opens the switch so current stops flowing in the circuit
  • the armature then springs back to its original position, closing the circuit so the cycle can repeat
22
Q

how are electromagnets used in relays

A
  • relay is an electric switch
  • small current magnetises the coil which attracts the armature
  • this then closes the switch gap, and connects the circuit
23
Q

how do loudspeakers and headphones work

A

Loudspeakers and headphones use the motor effect to convert variations in current from electrical circuits to pressure variations in sound waves. The varying current provides different forces to make a diaphragm oscillate, vibrating the air which produces sound.

24
Q

what is the generator effect

A
  • an electrical conductor moves relative to magnetic field and cuts through the field lines
  • a potential difference is induced across ends of conductor
  • if the conductor is part of a complete circuit, a current is induced

This can also be done if there is a change in magnetic field around the conductor.

25
Q

what happens as a result of the induced current

A

it generates a magnetic field that opposes the original change, either the movement of conductor or change in magnetic field

26
Q

how could current induced by a generator be increased

A
  • increase speed of conductor
  • increase magnetic field strength
  • increase number of turns on the coil
27
Q

how could current induced by a generator be reversed

A
  • reverse motion of conductor
  • reverse polarity of magnetic field
28
Q

difference between AC alternator and DC dynamo

A
  • dynamo uses split-ring commutator to reset current each half turn of the coil so induced current remains in one direction
  • alternator uses complete ring commutator so the induced current changes direction every half turn
29
Q

how does moving coil microphone work

A
  • microphones use generator effect to convert pressure variations into current
  • vibrating air makes diaphragm vibrate which is attached to a coil which moves through a fixed magnetic field, inducing a current
30
Q

what is the structure of a transformer

A

consists of a primary and secondary coil wound on an iron core - iron is used as it is a soft magnetic material so easily ionised

31
Q

what is a step up transformer

A

more turns on secondary coil so output potential difference across secondary coil will be greater than the input potential difference across the primary coil

32
Q

what is a step down transformer

A

more less on secondary coil so output potential difference across secondary coil will be lower than the input potential difference across the primary coil

33
Q

how does power output of a transformer relate to its power input

A

if transformers were 100% efficient, power output would equal power input

34
Q

why are transformers necessary

A
  • step up transformers increase potential difference and lower current whilst keeping the same power
  • transmission is more efficient as less energy is wasted through resistive heating

GCSE Physics (10 decks)

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