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P4.2 Flashcards

1
Q

What happens when a current carrying conductor (wire) is out between magnetic poles

A

The result is a force on the wire

The wire exerts and equal and opposite force on the magnet

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2
Q

How can the wire experience full force

A

The wire has to be at 90 degrees to the magnetic field as of it runs along it, it will experience no force at all but at angles in between it will feel some force
- magnetic field around wire and poles affect each other, causing a resultant force

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3
Q

How can we make the force stronger

A

We can do so by making the current or the magnetic field stronger

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4
Q

Which way does the force act

A

The force acts in the same direction relative to the magnetic field of the magnets and the direction of current in the wire. Changing the direction of either the magnetic field or the current will change the direction of the force

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5
Q

What is a good experiment to show the direction of the force

A

Apply a set of rails inside a horseshoe magnet. A bar is placed onto the rails, which completes the circuit. This generates a force that rolls along the bar.

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6
Q

What is a catapult field

A

A current passed through a wire creates a circular magnetic field which creates a third catapult field also called the motor effect. It creates a stretched field in which the lines are either above the wire and only a few below it. As the wire moves down, all the field lines straighten as the movement of the wire acts to shorten the field lines. It can also have fewer wires on top which will make it go upwards. To change the direction of force the current can be reversed

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7
Q

What is Flemings left hand rule

A

ThuMb is Movement F = force
First finger is Magnetic field B = field
SeCond finger is the Current I = current

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8
Q

How do you calculate the force on the wire and when is it used

A

Force on a conductor carrying a current (N) = magnetic flux density (T) x length (m) x current (A) or F = BxIxL

Used only when the wire is at right angles to a magnetic field

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9
Q

What type of currents are split ring commutators used for

A

Direct current

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10
Q

What is a motor

A

Two wire carrying current in the opposite direction

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11
Q

How do you change the direction of motor rotation

A

Reverse the direct current’s polarity/ reversing the current or swap the magnetic poles

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12
Q

How can you make a motor faster

A

Increase the magnitude of the current flowing in the wire
Increase the strength of the magnetic field
Increase the number of coils of wire
Increase the length of coil

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13
Q

What is the job of a split ring commutator in a d.c. motor

A

It’s reverses the direction of the current flowing through the coil every half turn to keep the motor rotating in the same direction.

It enables the current to flow the same way from the battery but change to different halves of the coil as it spins.

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14
Q

How does a simple d.c motor/ electrical motor work?

A

The current through the wire flows in different directions on each side of the coil. The wires are attached to an axle (spindle) and sit between two opposite poles of a magnet. Each side of the coil is perpendicular to the magnetic field. Due to this they experience force in opposite directions making it spin. To make it keep spinning a split ring commutator is used.

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15
Q

Why would the d.c motor/ electrical motor stop spinning at 90 degrees

A

Normally when the coil is vertical, it is parallel to the magnetic field. When this happens it produces no force, which would make it stop.

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16
Q

Why does the d.c motor/ electrical motor carry on spinning after 90 degrees

A

After 90 degrees the current stops flowing and the momentum of the motor causes it to carry on spinning
The split ring on the right loses contact and makes contact with the left hand brush and the split ring on the left left makes contact with the right hand and it makes sure that the current direction will stay the same

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17
Q

What happens when wires are in a changing magnetic field

A

An induced potential difference is produced which cuts the field lines or flux lines

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18
Q

Why is the p.d induced when a conductor cuts the flux lines?

A

Electrons in the wire get pushed to one end so one is positive and one is negative. This is as charged particles like electrons experience a force when they move through a magnetic field.

  • in the motor effect the e- are the current flowing through the wire so there’s a force in the wire
  • in the generator effect the wire is moving through the field and the e- in the wire experience the same magnetic forces and get pushed one way
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19
Q

How can you make an induced p.d

A

When you move a conductor in a magnetic field or make a magnetic field around a conductor, it causes an induced potential difference across its ends if there is relative motion between the wire, or coil and the magnets.

It cuts the field lines as you move it. The p.d induced depends on the length of wire in the field and the rate at which to cut the field lines.

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20
Q

What happens when no field lines are cut

A

There is no induced p.d

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21
Q

How can you increase the induced p.d

A
  • move the wire faster
  • using a stronger magnetic field
    This means more field lines are cut per second
  • using more wires (more loops/coils)
    This means that you induce a p.d in each loop so the total p.d increases
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22
Q

How can you reverse the induced p.d and make the opposite end of the conductor positive

A
  • reverse the magnetic field
  • move the conductor the opposite way through the fields
    —> doing both at the same time means they cancel each other out and the p.d stays the same way so only do one at a time
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23
Q

What happens when you connect a wire cutting the flux lines in a circuit

A

A current flows which then produces a magnetic field and this causes a force to act on it.

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24
Q

What would happen if the force acting on the wire is in the same direction as the force applied

A

The wire would fly off and you would get energy from nowhere. This can’t happen because energy is conserved. This causes the magnetic field to be produced in the opposite direction that produces the potential difference.

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25
Q

Why does a magnet fall slowly through a copper pipe?

A
  • the changing magnetic field produces an induced p.d. across the pipe
  • this causes a current to flow in the pipe as it’s a complete circuit
  • current produces a magnetic field
  • these fields are in opposite directions so the magnet is repelled and it falls slower
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26
Q

What are some examples of things that use electromagnetic induction

A

Metal detectors - detect the field around a magnet
Remote charger for phone
Induction hobs in kitchens

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27
Q

What are the two different situations that you get electromagnetic induction?

A
  • electrical conductor (like a coil of wire) and a magnetic field move relative to each other
  • magnetic field around electrical conductor changes (gets bigger /smaller/reverses)
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28
Q

How are generators the opposite of motors?

A
  • they use the relative motion of a conductor and magnetic field induce a potential difference and a current
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29
Q

What happens when the direction of rotation is reversed in a generator?

A
  • If rotation reversed then direction of induced potential difference/current is also reversed
30
Q

How does the current inducing dynamo or alternator increase?

A
  • more turns on the coil
  • magnetic flux density is increased
  • speed of rotation is increased
31
Q

What type of current do alternators generate and what are the two ways of doing so?

A
  • alternating current as the p.d changes direction
  • some alternators rotate a magnet in a coil of wire
  • some alternators rotate a coil in a magnetic field
32
Q

How do you alternators generate an a.c. by rotating a magnet in a coil of wire?

A
  • as magnet spins an alternating p.d is induced across the ends of the coil
  • p.d. changes direction very half turn as the direction of the field changes as the magnet rotates
  • only produce a.c if coil is part of a circuit
33
Q

How can you generate an a.c. by rotating a coil in a magnetic field?

A
  • slip rings at end of coil remain in contact with brushes connected to rest of circuit
  • this means contacts don’t swap every half turn so they produce and a.c
  • circuit is always connected but doesn’t tangle
34
Q

What is the graph like for a alternator?

A
  • connected to wire = positive p.d
  • cuts through m.f as spins = current (slip ring)
  • coil is horizontal = cuts most field lines = max positive p.d (high induced current and emf)
  • coil is vertical = cuts no lines = ammeter is 0 (no emf and current )
  • coil rotates past vertical = sides of coil cut magnetic fields in opposite direction = current direction changes = max negative current
35
Q

What type of current do dynamos generate?

A
  • direct current
  • p.d that drives current doesn’t change direction
  • only magnitude of p.d. can change
36
Q

How do dynamos create a d.c.?

A
  • dynamos rotate coil in magnetic field
  • output p.d and current change direction with every half rotation of the coil producing a.c.
  • coil is part of complete circuit
  • split ring commutator swaps the connection every half turn to keep current flowing in same direction (so it changes a.c. to d.c.)
37
Q

How can you increase the output of generators?

A
  • use stronger magnetic field
  • use more turns on the coil
  • spin the coil faster
38
Q

What is the graph like for a dynamo?

A
  • always positive current due to split ring so positive pulse of current
39
Q

What are the differences between a generator and motor?

A
  • motor = electrical energy to mechanical energy, flemings left hand rule, less maintenance, used in bikes, cars and hairdryers
  • generator = mechanical to electrical energy, Flemings right hand rule, more maintenance, used in power stations and industrial work (alternators = less weight for machine for 1kW output than generators)
40
Q

Why do transformers only work with an alternating current?

A
  • A changing magnetic field is needed for a potential difference (and so current) and electromagnetic induction in the secondary coil
  • A direct current won’t produce an alternating magnetic field
  • an a.c in the primary coil will produce an alternating magnetic field
  • a.c allows it to be stepped up and down
41
Q

What do transformers do and what is their structure like?

A
  • change size of p.d. of an a.c.

- have two coils, primary and secondary, joined with an iron core

42
Q 
How does a transformer work? - detailed der
Alt = alternating
P = primary 
S= secondary 
Mag = magnetic
A
  • alternating p.d is applied across the P coil = creates an alt current ( alternating direction) and mag field around coil
  • alternating mag field in P coil = induces alt mag field in iron core as iron becomes magnetised = alternating magnetisation in core
  • changing mag field of iron core = induces a p.d in S coil
  • part of circuit = current flows around S coil
43
Q

How do transformers work? - simpler definition

A
  • primary coil = alternating p.d = alternating mag and current around coil
  • iron core produces alternating mag field as iron = magnetic
  • iron mag field = induces p.d on secondary coil
  • part of circuit = current flows through secondary coil
44
Q

How can risk of injury be reduced in transformers?

A
  • insulating coils as they have very high voltages/p.d
45
Q

Why can’t we just conduct electricity from primary to secondary coil?

A
  • wires are insulated in plastic

- electricity can’t pass through the wire to core

46
Q

What is the power of a primary coil and how efficient is a transformer?

A

Power = p.d x current
Nearly 100% efficient but some energy is lost due to heating as mag, field in soft iron core produces its own small current which just results in heat being lost

47
Q

What does it mean if the secondary coil has more turns?

A
  • power in primary coil = power in secondary coil

- more turns in secondary coil = p.d increases and current decreases as more turns means more electromagnetic induction

48
Q

Why is an iron core used?

A
  • iron is magnetic so it guides the magnetic field

- no current flowing in the iron due to the coils

49
Q

What is the structure of a step up transformer?

A
  • more turns on secondary coil than primary coil
  • step voltage up (p.d = less)
  • reduce current needed
50
Q

Why are step up transformers useful?

A
  • reduce the current needed as p = V x I
  • reduce the heating effect in the wires and reduce the power lost due to heating in the wires
  • minimise energy loss when power stations send electricity to wire and pylons
51
Q

What is the structure of a step down transformer?

A
  • more turns on primary coil then secondary coil
  • step voltage down
  • increase the current needed
52
Q

How can you increase the efficiency of a transformer?

A
  • use low resistance windings (little power= i^2r happening)
  • soft iron core = more easily magnetised
  • laminating the core = chop transformers into lots of squares and layer them together = reduces eddy current (current in the core due to it magnetising = energy lost)so the amount of current that can flow in the soft iron core itself so less heat is lost
53
Q

How does the number of turns in the coils effect the size of induced p.d?

A
  • ratio between p.d. across primary and secondary coils is same as ratio between number of turns in primary and secondary coils
54
Q

What is the equation linking potential difference and the number of turns in the two coils of a transformer?

A
  • p.d across primary coil (V) / p.d. across secondary coil (V) = number of turns in primary coil / number of turns in secondary coil
  • Vp/Vs = Np/Ns
55
Q

What is the input voltage and output voltage?

A
  • input voltage - p.d. across the primary coil

- output voltage - p.d. across the secondary coil

56
Q

What is the equation linking potential difference and the current the two coils of a transformer?

A
  • p.d in primary coil x current in primary coil = p.d in secondary coil x current in secondary coil
  • Vp x Ip = Vs x Vp (
57
Q

Why are step down transformers useful?

A
  • increase the current needed as p = V x I

- decreases voltage to around 230 V when it goes to a town so it’s safe to use in houses

58
Q

Compare generators and motors:

A
  • both use magnets that produce a magnetic field
  • motor = current in wire (that flows due to p.d across ends of wire) produces movement
  • generator = movement produces p.d in a wire (and current flows if there is a circuit)
59
Q

What type of variations are sound waves?

A

They are pressure variations in the air

60
Q

What is the structure of dynamic microphones?

A
  • like a generator
  • coil of wire surrounded by a magnet
  • another magnet is inside the coil
  • coil of wire is attached to a diaphragm (thin sheet of plastic)
61
Q

How does a microphone work?

A
  • work due to electromagnetic induction
  • sound waves = diaphragm to move back and forth when hit by it
  • diaphragm moves = coil of wire moves = moves over magnet so cuts mag field lines = induces a p.d across ends of wire (p.d switches direction as coil of wire moves back and forth)
  • if coil = part of circuit then the induced p.d = variations in current electrical circuit = electrical signal produced by microphones
62
Q

How does high and low pressure effect the diaphragm?

A
  • low pressure = sound waves hit diaphragm and push it out = rarefactions
  • high pressure = sound waves hit diaphragm and pull it out = compressions
63
Q

What is the frequency of the p.d equal to in a microphone and how is volume increased from a microphone?

A
  • Hz of changing p.d = Hz of sound waves

- changing pattern of p.d passes through amplifier and moving coil loudspeaker = increases volume of sound

64
Q

How does a carbon microphone work?

A
  • carbon granules behind diaphragm
  • sound waves hit diaphragm = changes resistance (R) of carbon
  • current passing through carbon granules = increases/decreases as R changes
65
Q

What is the structure of a loudspeaker?

A
  • like a motor
  • coil of wire surrounded by a magnet
  • another magnet is inside the coil
  • coil of wire is wrapped around the base of a cone
66
Q

When can you hear sound in a loudspeaker?

A
  • if the ends of the wire are connected to a changing p.d of a suitable frequency = sound
67
Q

Why does the cone of a loudspeaker move?

A
  • force between current carrying coil of wire and magnetic field can be used to make things move back and forth
68
Q

How does a loud speaker work?

A
  • alternating p.d. = a.c. electrical signal = fed into coil of wire (wrapped around cone)
  • interaction between the magnetic field and current in the coil forces the coil to move in one direction (Fleming’s left hand rule)
  • a.c. = the current changes direction = coil moves back in other direction
  • current alternates = coil moves back and forth = cone vibrates = creates pressure variations in the air = sound
69
Q

How do loud speakers work - simpler version

A
  • alternating p.d. = a.c. electrical signal = into coil of wire
  • mag field of coil and magnet interact = repel or attract = resultant force = cone moves in and out as the current is a.c.
  • creates pressure variations in the air = sound waves
70
Q

What does changing the frequency of a.c of a loudspeaker do?

A
  • change Hz of a.c = change Hz that cone vibrates
  • high Hz = higher pitch sound
  • low Hz = lower pitch sound
  • change p.d = increase size of current = increase amplitude of vibration = increases volume of sound as larger force on coil and cone

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