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Physics GCSE > Section 22: Magnetic Fields > Flashcards

Section 22: Magnetic Fields Flashcards

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1
Q

what are the poles of the magnet

A
  • north
  • south
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2
Q

what do all magnets produce

A

magnetic field

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3
Q

what is a magnetic field

A

a region where all magnets or other magnetic materials experience force

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4
Q

what direction do the magnetic fields go

A

north to south

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5
Q

what happens if the lines are close or further away from eachother

A

the closer the lines are together, the stronger the magnetic field
the further away from eachother, the weaker the magnetic field

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6
Q

where is the magnetic field the strongest

A

the magnetic field is the strongest at the poles of the magnetic meaning the magnetic forces are the strongest at the pole

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7
Q

how do you get the shape of a magnetic field

A

place a card over a magnet and sprinkle iron fillings onto it. the fillings line up with the field lines
HOWEVER it wont show the DIRECTION of the field

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8
Q

what do magnetic fields cause

A

it causes forces between magnets

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9
Q

what happens between the 2 magnets

A

between the 2 magnets the magnetic force can be REPULSIVE or ATTRACTIVE.

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10
Q

what happens if you place the north and south poles of two bar magnets near eachother

A

it creates a UNIFORM FIELD between the 2 poles

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11
Q

do the same poles repel or attract

A

repel

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12
Q

do the opposite poles attract or repel

A

attract

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13
Q

how do you find the direction of magnetic fields

A

inside a compass is a tiny bar magnet called a needle. a compass needle ALWAYS lines up with the magnetic field that it is in.

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14
Q

how do you use a compass to find what the field around a magnet looks like

A

you can use a compass to build up a picture of what the field around a magnet looks like:
1. put a magnet on a piece of paper and draw around it.
2. place the compass on the paper near the magnet, the needle will point in the direction of the field line at this position.
3. mark the direction of the compass needle by drawing 2 dots at each end of the needle.
4. then move the compass so that the tail end of the needle is where the tip of the needle was in the previous position and put a dot by the tip of the needle. repeat this and then join up the marks youve made, you will end up with a drawing of one field line around the magnet.
5. repeat this method at different points around the magnet to get several field lines.
- make sure to draw the arrows from north to south.

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15
Q

what happens when youre not near a magnet

A

the compasses always points towards the earths north pole.

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16
Q

why does the compass always point towards the earths north pole

A

this is because earth generates its own magnetic field. this shows that the core of the earth must be magnetic

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17
Q

what are the 3 magnetic elements

A
  • iron
  • nickel
  • cobalt
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18
Q

what happens if you put a magnetic material near a magnet

A

the magnetic material is attracted to the magnet. some alloys and compounds of the magnetic elements are magnetic as it contains the element. the magnetic force between a magnet and a magnetic material is always attractive.

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19
Q

what are the 2 types of magnets can a metal be

A

permanent of induced

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20
Q

what is a permanent magnet

A
  • magnets that produce their own magnetic field ALL THE TIME
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21
Q

what is a induced magnet

A

also known as temporary.
- magnets that only produce a magnetic field while theyre in another magnetic field. if you put any magnetic materials into a magnetic field- it will become an INDUCED magnet

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22
Q

why is the force between a magnet and a magnetic material always attractive

A

the south pole of the magnet INDUCES a north pole material

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23
Q

what happens when you take away the magnetic field from the induced magnets

A

the induced magnet will return back to normal and stop producing a magnetic field
- how quickly they lose their magnetism depends on the materials theyre made from

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24
Q

how long do magnetically SOFT materials lose their magnetism

A

very quickly
- pure iron
- nickel-iron alloys

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25
Q

how long do magnetically HARD materials lose their magnetism

A

very slowly
- permanent magnets are made from magnetically hard materials

26
Q

some uses of magnetic materials

A
  • fridge doors
  • doorbells
  • MRI machines
27
Q

how are magnetic materials used in fridge doors

A

a permanent magnetic strip in your fridge door to keep it closed

28
Q

how are magnetic materials used in doorbells

A

they use electromagnets which turn on and off rapidly, to repeatedly attract and release an arm which strikes the metal bell to produce a ringing noise.

29
Q

how are magnetic materials used in MRI machines

A

they use magnetic fields to create images of the inside of your body without having to use ionising radiation

30
Q

what does a moving charge create

A
  • a magnetic field
    when a current flows through a long, straight conductor, a magnetic field is created around it.
31
Q

what is the magnetic field made up of

A

concentric circles perpendicular to the wire, with the wire in the centre.

32
Q

how do you change the direction of the magnetic field

A

changing the direction of the current

33
Q

how do you work out which way the magnetic field goes

A

using the right hand thumb rule

34
Q

how do you know if the field is strong or weak

A

the larger the current through the wire, or the closer to the wire you are the stronger the field is

35
Q

what is the motor effect

A

when a current carrying wire in the presence of a magnetic field experiences a force

36
Q

what happens if you put a wire between magnetic poles

A

when a current carrying conductor is put between magnetic poles, the two magnetic fields interact. the result is a force on the wire

37
Q

how to experience the full force

A
  1. the wire has to be 90° to the magnetic field. if the wire runs along the magnetic, it wont experience any force at all.
  2. the force will always act in the same direction relative to the magnetic field, and the direction in the wire.
    - changing the direction of either the current or magnetic field will change the direction of the force
38
Q

how do you change the direction of the force

A

changing the direction of either the magnetic field or the current

39
Q

what does the force acting on the conductor in a magnetic field depend on

A
  1. the magnetic flux density (field) - how many field lines there are in a region
    - this shows the strength of the magnetic field
  2. the size of the current through the conductor
  3. the length of the conductor thats in the magnetic field
40
Q

how do you find the force acting on the current when it is at 90° to the magnetic field

A

F=BIL
F= force (N)
B=magnetic flux (T-tesla OR N/Am)
I=current (A)
L=length (m)

41
Q

what is a solenoid

A

a long coil of wire.
you can increase the strength of the magnetic field produced by a length of wire- by wrapping it onto a long coil with lots of loops called a solenoid
it is an example of an electromagnet

42
Q

what happens inside the solenoid

A
  • inside the solenoid, you get lots of field lines pointing in the same direction. the magnetic field is strong and almost uniform. the magnetic field is the strongest inside
43
Q

what happens outside the solenoid

A

outside the solenoid, the overlapping field lines cancel eachother out, so the field is weak apart from the ends of the solenoid

44
Q

what is the direction of the field in a solenoid

A

the direction of the field depends on the direction of the current.

45
Q

what is an electromagnet

A

a magnet with a magnetic field that can be turned on and off using an electric current

46
Q

how do you increase the field strength of a solenoid

A

you can increase the field strength of the solenoid even more by putting a block iron in the centre of the coil. this iron core becomes an induced magnet whenever the current is flowing

47
Q

what happens if you change the magnetic field

A

it induces a potential difference in a conductor

48
Q

electromagnetic induction

A

the induction of a potential difference in a wire which is experiencing a change in magnetic field

49
Q

what are the situations where you get electromagnetic induction

A

there are 2 situations where you get electromagnetic induction:

  1. if an electrical conductor and a magnetic field moves relative to eachother.
    - you can do this by moving/rotating either a magnet in a coil of wire or a conductor in a magnetic field.
    - if you move or rotate the magnet in the opposite direction, then the potential difference/current will be reverse
    - if you keep the magnet moving backwards and forwards, or keep it rotating in the same direction you produce an alternating current
50
Q

what are the situations where you get electromagnetic induction

A

you can also get induced potential difference when the magnetic field through an electrical conductor changes (gets bigger or smaller)
this is what happens in a transformer
1. you can increase the size of the induced potential difference by increasing the strength of the magnetic field, increasing the speed of the movement or change of the field or having more turns per unit length on the coil of wire.
2. the induced potential difference or current always opposes the change that made it:
- when a current is induced in a wire, that current produces its own magnetic field
- the magnetic field created by an induced current always acts against the change that made it.
so it is basically trying to return things to the way they were

51
Q

what do transformers do

A

they change the potential difference but only for alternating currents.
- they use induction to change the size of the potential difference of an alternating current

52
Q

features of a transformer

A
  1. primary coil
  2. secondary coil
  3. an iron coil in the middle of the coils
53
Q

what happens when an alternating p.d. is applied across the primary coil

A

it produces an alternating magnetic field

54
Q

what is the iron core

A

it is a magnetic material that is easily magnetised and demagnetised as the coil is producing an alternating magnetic field, the magnetesiation in the core also alternates.

54
Q

what happens if you change the magnetic field

A

it induces a potential difference in the secondary coil

55
Q

what is a step up transformer

A

increases the potential difference- steps the transformer up
- they have more turns on the secondary coil

56
Q

what is a step down transformer

A

decreases the potential difference- steps the transformer down
- they have more turns on the primary coil

57
Q

how do you assume the input power is equal to the output power

A

Vp x Ip = Vs x Is
Vp= p.d. in primary
Ip = current in primary

58
Q

what do transformers in the national grid do

A

produces a high p.d. difference and a low current
1. once the electricity has been generated, it goes into the national grid
2. the national grid has to transfer loads on energy each second which means it transmits electricity at a high power
3. electrical power=current x p.d. so to transmit the huge amounts of power needed, you need a high potential difference or a high current
4. but a high current makes the wire heat up, so loads of energy is wasted to thermal stores
5. to reduce these losses and make the national grid more efficient, high voltage, low resistance cables and transformers are used.
6. STEP UP transformers at POWER STATIONS boost the p.d. up really high and keep the current really low
7. STEP DOWN transformers decreases it and brings it back down to safe, usable levels at the consumers end

59
Q

what is the national grid

A

a network of wires and transformers that connects uk power stations to consumers

Physics GCSE (7 decks)

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