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AQA GCSE Physics > Magnetism > Flashcards

Magnetism Flashcards

1
Q

What materials can be made into permanent magnets?

A

ferromagnetic

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2
Q

What elements are ferromagnetic?

A

Iron, nickel and cobalt

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3
Q

What causes ferromagnetism?

A

Domains (mini-magnets) that exist as part of the metal’s structure aligning

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4
Q

What causes the alignments of the domains?

A

heating, working or applying an existing magnetic field

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5
Q

What is a magentic field?

A

A region surrounding a magnet in which another magnet or ferromagnetic material will experience a magnetic force

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6
Q

Which way do arrows point in a field pattern?

A

North to south

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7
Q

How can field patterns be plotted?

A
  • with a plotting compass
  • place it in area around magnet and the needle will line up along the field lines
  • head is a north pole so points towards a south pole
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8
Q

How do we know that the earth has a magnetic field?

A

Magnets suspended above the earth experience a force

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9
Q

How does the north pole of a compass point to geographic north?

A
  • north pole is attracted to and thus points at magnetic south pole
  • Earth’s magnetic south pole is the geographic north pole
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10
Q

What causes earth’s magnetic field?

A

The liquid nickel-iron outer core of the earth

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11
Q

Why does the solid iron core of the earth not exhibit magnetic properties?

A

it is at too high a temperature

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12
Q

How does induced magnetism take place?

A
  • When a magnetic material is in close proximity to, or in contact with, a permanent magnet
  • permanent magnet’s pole causes the opposte pole of the domains in the ferromagnetic material to attract to it and align.
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13
Q

Why is induced magnetism only temporary?

A

When the permanent magnet is removed, the domains fall back into random arrangement

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14
Q

Why does induced magnetism always result in attraction?

A

Since the induced magnet always has the opposite pole closest to the permanent magnet

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15
Q

other than magnets, what else can produce magnetic fields?

A

An electric current

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16
Q

What does the magnetic field in a current carrying wire look like?

A

Increasingly sized circles flowing the direction as shown by the right hand rule

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17
Q

What is a solenoid?

A

A current carrying coil

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18
Q

What determines the polarity of the filed in a solenoid?

A

The direction of current round the circuit

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19
Q

How do solenoids become electromagnets?

A

With the inclusion of a soft iron core

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20
Q

What is a soft iron core?

A

A pure iron core which isn’t alloyed

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21
Q

How can electromagnets be switched on and off?

A
  • domains in the iron core align to form a temporary, induced magnet when current flows through the coil
  • when the current is switched off, alignement of domains is lost so no magnetic effect
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22
Q

Why cant steel cores be used in electromagnets?

A

Sicne steel cores would become permanently magnetised even when the current is turned off

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23
Q

What does the interaction of magnetic fields result in?

A

A force being exerted

24
Q

What is the relationship between F and I? What variables are kept constant?

A

Directly proportional - the length of the current carrying wire in the magnetic field and the magnetic field strength of the magnets

25
Q

Whaat equation gives the magnitude of the force on a current carrying wire in a magnetic field?

A

F = BIL

26
Q

What is 1 tesla exactly equal to?

A

1 N/Am

27
Q

What does each finger represent in fleming’s left hand motor role?

A

thumb - force
first finger - magnetic field (north to south)
second finger - current direction

28
Q

What symbols represent a force into the plance and out of the plane?

A

A circle with a cross shows a force into the plane whereas a circle with a dot shows a force out of the plane

29
Q

Why does current need to keep flowing the same way round a motor?

A

To keep the motor spinning rather than turning back and forth

30
Q

What enables current to flow the same way round the coil in a motor? What does this mean?

A

A split ring communtator. Means the force on each side of the coil is kept in the same direction to maintain the rotation of the coil

31
Q

How can the rate of rotation in a motor be increases?

A
  • by increasing the number of turns on the coil
  • increasing the current in the coil
  • increasing the magnetic flux density of the permanent magnets
32
Q

How can the current of a coil in a motor be most effectively changed?

A

By increasing or decreasing the applied p.d. (proportionate)

33
Q

How do AC motors keep current and force in the same direction?

A

By using slip rings and brushes

34
Q

Describe the set up of a loudspeaker:

A

Permanent magnet within a cylinder, wrapped in coils which are connected to the source signal and it all leads to a (paper) cone

35
Q

Describe how a loudspeaker works:

A
  • singles from the amplifier drives a varying current in the coil
  • this current produces a varied magnetic field
  • this magnetic field interacts with the field of the permanent magnet
  • this generates attractive and repulsive forces that act on the coil and the magnet
  • these forces cause the cylinder and the coil to vibrate
  • Vibration of the cone causes the vibration of air particles at the same frequency as the varying current driven by the signal from the source
36
Q

What is the generator effect?

A

Conductor + magnetic field + movement = current

37
Q

What does magnetic field and movement manifest in in electromagnetic induction?

A

A changing magnetic field

38
Q

What is the first law of electromagnetic induction?

A

A potential difference is induced only when there is a change in the magnetic field around the conductor

39
Q

What is the second law of electromagnetic induction?

A

The magnitude of the induced p.d. Is proportional to the rate of change of the magnetic field around the conductor

40
Q

What is Lenz’s law?

A

the direction of an induced emf is such that it induces a magnetic field that opposes the change in magnetic flux which is causing it

41
Q

At what angle to magents have to be to induce an emf?

A

90 degrees

42
Q

Explain why the coil in a motor has a maximum rate of rotation:

A
  • as the coil rotates in the magnetic field, an emf is induced in the coil that drives a current round the coil
  • however, the direction of this induced emf and hence current (according to Lenz’s law), will oppose the motion of the coil
  • the current driving the motor and the induced current are oppostie in direction meaning the overall current in the coil is reduced
  • the induced emf also results in forces on the coil in the opposite direcction to the forces driving the coil
  • so the coils rate of roatation will be decreased by both these factors
43
Q

Describe the layout of a transformer:

A
  • contains a soft iron core with a primary coil at one end and a secondary coil at the other
  • these have different numbers of turns
44
Q

Describe the magnetic field lines in a transformer: Why are they like so?

A
  • magnetic field lines which are in the same shape as the core, in layers
  • this is as they are focussed and confined by the soft iron core
  • the magnetic field contains no arrows as it is constantly changing due to the suppied AC
45
Q

How do transformers induce a p.d.?

A
  • an alternating p.d. is applied to the primary coil
  • this induces an alternating magnetic field in the priary coil
  • the soft iron core focusses this alternating magnetic field through the secondary coil
  • the alternating magnetic field in the secondary coil induces an alternating p.d. in the secondary coil
46
Q

What does a step up transformer do? How?

A

Increases the p.d. by having more turns of coil in the secondary coil than the primary coil

47
Q

What does a step down transformer do? How?

A

Decreases the p.d. by having fewer turns of coil in the secondary coil than the primary coil

48
Q

What is the transformer equation for an ideal transformer?

A

(Ns/Np) = (Vs/Vp) = (Ip/Is)

49
Q

What is an ideal transformer?

A

A transformer which increases or decreases p.d. without energy losses between the primary and secondary coil

50
Q

What cause the energy losses in a transformer?

A
  • current loops induced in the iron core

- back e.m.f in the primary coil by the magnetic field associated with the induced p.d. in the secondary

51
Q

Why are eddy currents formed?

A

Around magnetic lines in a transformer

52
Q

How does a current loop cause energy losses?

A
  • energy is lost by driving the currents in the core

- current loops induce a magnetic field in the opposite direction to the field inducing the current

53
Q

How does the back e.m.f in the primary coil incurr an energy loss?

A
  • reduces the input p.d. as it appears to increase the resistance in the primary coil
54
Q

Why is the soft iron core in a transformer laminated?

A

to reduce energy losses due to current loops

55
Q

What happens to current in a step up transformer?

A

It is decreased

56
Q

What happens to current in a step down transformer?

A

It is increased

AQA GCSE Physics (9 decks)

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