AQA GCSE Physics > Magnetism > Flashcards

Magnetism Flashcards

(56 cards)

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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?

Study These Flashcards
A

A force being exerted

24
Q

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

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A

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

25
Whaat equation gives the magnitude of the force on a current carrying wire in a magnetic field?
F = BIL
26
What is 1 tesla exactly equal to?
1 N/Am
27
What does each finger represent in fleming's left hand motor role?
thumb - force first finger - magnetic field (north to south) second finger - current direction
28
What symbols represent a force into the plance and out of the plane?
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
Why does current need to keep flowing the same way round a motor?
To keep the motor spinning rather than turning back and forth
30
What enables current to flow the same way round the coil in a motor? What does this mean?
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
How can the rate of rotation in a motor be increases?
- 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
How can the current of a coil in a motor be most effectively changed?
By increasing or decreasing the applied p.d. (proportionate)
33
How do AC motors keep current and force in the same direction?
By using slip rings and brushes
34
Describe the set up of a loudspeaker:
Permanent magnet within a cylinder, wrapped in coils which are connected to the source signal and it all leads to a (paper) cone
35
Describe how a loudspeaker works:
- 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
What is the generator effect?
Conductor + magnetic field + movement = current
37
What does magnetic field and movement manifest in in electromagnetic induction?
A changing magnetic field
38
What is the first law of electromagnetic induction?
A potential difference is induced only when there is a change in the magnetic field around the conductor
39
What is the second law of electromagnetic induction?
The magnitude of the induced p.d. Is proportional to the rate of change of the magnetic field around the conductor
40
What is Lenz's law?
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
At what angle to magents have to be to induce an emf?
90 degrees
42
Explain why the coil in a motor has a maximum rate of rotation:
- 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
Describe the layout of a transformer:
- 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
Describe the magnetic field lines in a transformer: Why are they like so?
- 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
How do transformers induce a p.d.?
- 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
What does a step up transformer do? How?
Increases the p.d. by having more turns of coil in the secondary coil than the primary coil
47
What does a step down transformer do? How?
Decreases the p.d. by having fewer turns of coil in the secondary coil than the primary coil
48
What is the transformer equation for an ideal transformer?
(Ns/Np) = (Vs/Vp) = (Ip/Is)
49
What is an ideal transformer?
A transformer which increases or decreases p.d. without energy losses between the primary and secondary coil
50
What cause the energy losses in a transformer?
- 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
Why are eddy currents formed?
Around magnetic lines in a transformer
52
How does a current loop cause energy losses?
- 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
How does the back e.m.f in the primary coil incurr an energy loss?
- reduces the input p.d. as it appears to increase the resistance in the primary coil
54
Why is the soft iron core in a transformer laminated?
to reduce energy losses due to current loops
55
What happens to current in a step up transformer?
It is decreased
56
What happens to current in a step down transformer?
It is increased
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