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Physics Revision > P4 > Flashcards

P4 Flashcards

1
Q

How do magnetic fields explain the behaviour of magnets?

A

-like poles repel, unlike poles attract just like electric charges
-you can explain the movement of magnets with magnetic field lines
-Field lines are a way of modelling a field
-Field lines represent magnetic flux and the number of field lines passing through a particular area is called a magnetic flux density
-we also call magnetic flux density, magnetic field strength
-the magnets move in the direction that makes the field lines shorten
-the density of the field lines show you the strength of the magnetic field

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2
Q

What is an induced magnet?

A

-A steel paper clip is attracted to a magnet
-even though a paper clip is not a permanent magnet, it is an induced magnet
-In the domain model of magnetism, a permanent magnet is made up of many small magnetic regions (domains) that all line up.
-steel or iron has regions that are not lined up, but when they are in a magnetic field they do line up
-in some (hard) magnetic materials, the domains continue to be lined up when you remove the magnetic field.
-in other (soft) magnetic materials the domains return to their original direction.

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3
Q

Why do compasses point “north”?

A

If you hang a magnet up it will line up so that one end of it points towards the magnetic North Pole.
-This point in the surface of the earth is not the same as the North Pole, which is the point about which the earth spins.
A compass points towards the magnetic North Pole
-it is a north seeking pole
-The earth behaves as if it has a large bar magnet at its centre
-this is a model of the earth and it’s magnetic field
-Scientists are not sure what causes the field
-It could be produced by convection currents in the molten iron core of the earth
-many compasses are “weighted” so that they lie horizontally. They need to be weighted because the earths magnetic field is like that of a bar magnet
- the angle between the field lines and a line horizontal to the surface of the Earth is the “dip”
-it is 90 degrees at the north and south magnetic poles, and zero at the magnetic equator. In the UK the field has a dip of about 70 degrees

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4
Q

What dimension is a magnetic field?

A

3 dimensional

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5
Q

What is the magnetic field around a wire?

A

An MRI scanner needs to produce a very strong magnetic field
-the field is stronger than any produced by a permanent magnet
-in 1820, Hans Christian oersted discovered that there is a magnetic field around a wire that is carrying a current

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6
Q

What does it mean if the current in a wire is coming towards you?

A

The field lines are anti-clockwise

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7
Q

What does the strength of the field around a wire depend on?

A

The strength of the field, which is called magnetic field strength, or magnetic flux depends on:

-the magnitude of the current: a bigger current = a stronger field

-the distance from the wire: nearer the wire = a stronger field

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8
Q

What is the strength of a magnetic field measured in and what are they like?

A

The strength of a magnetic field is measured in teslas (T).
-The strength of the Earths magnetic field is about 0.01 mT the strength of the field 1 cm from a wire carrying a current of 5A is about 0.1mT.
-A very strong permanent magnet has a magnetic field near its poles of about 1T

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9
Q

How does a solenoid enhance the magnetic effect?

A

-You can make a loop of wire that carries a current. The magnetic field in the centre is a straight line
-Many parallel loops make a coil. Or solenoid
-Adding together many fields produces a much stronger field than that of a single wire
-You can make the field even stronger by putting a magnetic material inside the core
-the field produces an induced magnet
-You can make an electromagnet that is much stronger than any permanent magnet.
-the electromagnets used in MRI scanners have a strength of 10T, and the strongest continuous magnetic field produced is over 40T
-You can model the direction of the current in wires using darts

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10
Q

What happens when you combine fields?

A

-You can combine the field due to a wire with the field due to a permanent magnet. This produces a force on the wire

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11
Q

Why is there a force on the wire?

A

-If you work out what is happening in the combined fields you can predict what will happen to the wire.
-When you put two fields together that are in the same direction they add up, but if they’d re in opposite directions they will cancel out

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12
Q

The force on a wire creates a catapult field:

A

-The current is into the paper and the uniform field is left to right.
-You get a catapult field with stretched field lines above the wire and fewer stretched lines below it
-the wire moves down, and all the field lines straighten
-the movement of the wire has acted to shorten the field lines
-the current, magnetic field, and force are all at right angles to eachother

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13
Q

How can you remember which way the wire moves?

A

-drawing the fields and combining them
Or
-use Flemings left hand rule

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14
Q

What is Flemings left hand rule?

A

-Thumb= force(movement)
-first finger = field (north to south)
-second finger = current (positive to negative)

-start by lining up your first finger with the field, then put your second finger in line with the current.
-the direction of your thumb tells you the direction of the force

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15
Q

How do you calculate the size of force?

A

-you know that the strength of the field around a wire depends on the current through it.
-the five in the wire depends on the current, the field that it is in, and the length of wire in the field (assuming the wire and the field are at 90 degrees)
-you need to be able to apply this equation:

Force on a conductor ( at right angles to s magnetic field) carrying a current (N) = magnetic flux density (T) x current (A) x length (m)
-this is how scientists define one ampere (amp).
-One amp is the size of current that produces a force of 0.2 micronewtons on two wires 1 metre apart
-you saw how parallel wires carrying current experience forces

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16
Q

What do you need to make a robot move?

A

An electric motor

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17
Q

How can you get a coil or wire to spin?

A

-A motor needs to spin
-You know that a combination of magnetic fields can cause a force on a wire that has current flowing in it.
-You can make a very simple motor by making a piece of wire into a loop and placing the loop in a magnetic field
-when you connect the wire to a battery a current flows
-one side of the wire goes upwards and the other goes downwards
-however this motor would not spin very well
-the coil will start to rotate back the other way as soon as it passes vertical position

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18
Q

How does an electric motor work?

A

-you need a way to keep the current moving in the right and out on the left all the time, but still allowing the coil to spin.
-you can do this with a split ring commutator

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19
Q

What does the split ring commutator do?

A

The split ring commutator
-enables the current to flow the same way from the battery, but change to different halves of the coil as it spins.
-this makes sure that the force on the left hand side of the coil is always upwards, and the force of the right hand side of the coil is always downwards

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20
Q

How do you change the speed of the motor?

A

By changing the:
-magnitude of the current flowing in the coil
-the strength of the magnetic field
-the number of coils of wire
-the length of coil

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21
Q

What is electromagnetic field induction?

A

Electromagnetic induction is one of the most important discoveries in the history of science
-you produce an induced potential difference across the ends of a wire
If the wire is in a changing magnetic field so that it cuts field lines, or flux lines.
-this can happen when you move a conductor in magnetic field or make a magnetic field change around a conductor
-one way of producing an induced potential difference is if there is a relative motion between the wire, or coil, and the magnets
-if the wire does not cut field lines there is no induced potential difference
-the wire cuts field lines as you move it
-the potential difference that you induce depends on the length of wire in the field, and the rate at which you cut field lines

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22
Q

How can you induce a potential difference across a coil?

A

you can induce a potential difference across a coil by moving a magnet

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23
Q

How do you increase the induced potential difference?

A

You can increase the reading in the voltmeter by:
-moving the wire faster
-using a stronger magnetic field
-using more wire (more loops/coils)

Moving the wire faster and using stronger magnets means that you are cutting more field lines, or flux lines per second

Using more loops means that you induce a potential difference in each loop, so the potential difference increases.

24
Q

How do you know the direction of an induced potential difference?

A

If you connect a wire to a circuit, a current will flow
-when a current flows in a conductor it produces a magnetic field
-if it is in a magnetic field then a force acts on it
-if the force acting on the wire is the same direction as the force applied, then the wire would “fly” off
-you would get energy from nowhere
-this cannot happen because energy is conserved so the magnetic force produced is in the opposite direction to the field that produces the potential difference

25
Q

Why does a magnet take longer to fall in a metal tube?

A

The magnet takes longer to fall because of the current in the metal tube
-the changing magnetic field induced a potential difference in the metal tube , and a current flows because it is a complete circuit
-you might have a remote charger for your mobile phone
-this uses the principle of electromagnetic induction, so do induction hobs in kitchens

26
Q

How does an alternator work?

A

-you know that you need a changing magnetic field to induce a potential difference
-if you move a magnet in and out if a coil you induce a potential difference
-the output of the coil is a potential difference that changes direction
-it is an alternating potential difference
-you could get the same potential difference by shaking a magnet in a tube with wire around it
-if you connect the a bulb to a wire, you have a torch
-it makes sense that if the magnetic field is changing then the induced potential difference changes the same way
-in an alternator, which is an alternating current (a.c) generator, it is the coil of wire that spins between the poles of a magnet.
-this is equivalent to moving the magnet in and out
-the brushes in an a.c generator are not attached to the slip rings
-they brush against the slip rings so that the voltmeter is always connected to the ends of the coil, but the coil does not become tangled

27
Q

What does a potential difference against time graph in an alternator show?

A

The potential difference against time graph shows that the potential difference changes direction from positive (above the line) to negative (below the line)
-in a commercial generator in a lower station, there is a rotating electromagnet inside a coil of wire.
-the generator spins at a frequency of 50Hz

28
Q

What is a dynamo?

A

A dynamo is a direct current (d.c generator. The potential difference produced by a dynamo that drives a current does not change direction. You can use the same arrangement that you used for a motor to produce a direct potential difference

29
Q

How does a dynamo work?

A

The potential difference does not change in magnitude, but it does not change direction because the coil is connected to a split-ring commutator

30
Q

How can the output of alternators and dynamos be increased?

A

-in both alternators and dynamos the output can be increased by:
-using a stronger magnetic field
-using more turns on the coil
-spinning the coil faster

31
Q

What is a transformer?

A

A transformer can be used to increase or decrease a potential difference
-the potential difference (p.d) is induced when the magnetic field lines cutting a coil change

32
Q

What are the three ways you can change a the field lines that cut the coil?

A

-you change the field lines that cut the coil in three ways:
-moving the magnet or the coil
-using another coil and turning the current in it on and off
-using another coils that has an alternating current in it

33
Q

How does a transformer work?

A

When a current flows in a coil it produces a magnetic field.
-if you turn the coil on and off you change the number of field lines cutting the second coil.
-if you use alternating current (a.c) then the current flows in one direction, then changes to the opposite direction.
-the field lines around the coil continually change

34
Q

How do you make a transformer?

A

If you make a loop of iron with two coils you have made a transformer
The magnetic field is “trapped” inside the iron core
-an alternating p.d. across the primary coil produces an alternating current in the primary coil
-an alternating current in the primary coil produces a magnetic field in the iron core that is always changing
-this induces a changing p.d in the secondary coil

35
Q

How does the output of a transformer depend on the number of turns on the coil?

A

There is a relationship between p.d.s across the coils and the turns on the coils. You need to be able to apply this equation:

Pd across primary coil(V)
/ Pd across secondary coil(v)

=

Number of turns in primary coil
/ number of turns in secondary coil

36
Q

How can you make the output (the pd across the secondary coil) the subject of the equation?

A

Secondary p.d = turns in secondary coil / turns in primary coil x primary pd

37
Q

What does a step up transformer do?

A

This transformer increases pd

38
Q

What is the link between magnetic field, force, and potential difference?

A

-motors and generators are similar as they both use magnets that produce a magnetic field
-in a generator, movement produces a potential difference in a wire (and a current flows if there is a circuit)
-in a motor, a current in the wire (that flows because of a potential difference across they ends of the wire) produces a movement
-you can see similarities by comparing loud speakers and microphones

39
Q

How does a dynamic microphone work?

A

A dynamic microphone is like a generator.
A sound wave is a pressure wave
-as a sound wave hits the diaphragm if a microphone, areas of high pressure, compressions, push the diaphragm in and areas of low pressure, rarefactions, pull it out

40
Q

How is an electrical signal produced

A

As the diaphragm moves in and out so does the coil, so a potential difference is induced across the ends of the wire.
-this is the electrical signal that a microphone produces

41
Q

What other types of microphones are there?

A

-in a carbon microphone there are carbon granules behind the diaphragm.
As the sound wave hits the diaphragm it changes the resistance of the carbon. A current passing through the carbon granules increases or decreases as the resistance changes

42
Q

How does a loud speaker work!?

A

A loudspeaker, or one of your headphones is a bit like a motor.
-you can make a speaker with a paper cone, a piece of wire and a pair of magnets
-if you connect the ends of the wire to a changing potential difference of a suitable frequency you will hear a sound.

43
Q

How does a loud speaker work?

A

The changing p.d produces a changing current
-this current produces a force in a coil of wire because it is a magnetic field.
The cone does not spin but moves in and out.
-this produces a sound wave

44
Q

What is a magnetic material

A

Any material that can be influenced by magnetic fields and has the potential to become a magnet

45
Q

What is an actual magnet

A

An actual magnet is any object that creates a magnetic field

46
Q

What are permanent magnets

A

Magnets that produce their own magnetic field all of the time

47
Q

Induced/ temporary magnets

A

Magnets that only have a magnetic field temporarily

48
Q

What are magnetically hard materials?

A

Materials like steel that lose their magnetic field slowly

49
Q

What are magnetically soft materials?

A

Materials that lose their magnetic field very quickly such as nickel or iron

50
Q

What is the motor effect?

A

The idea that a current carrying wire in the presence of a magnetic field will experience a force

51
Q

What direction does your first finger point to in Flemings left hand rule?

A

Your first finger points to the direction of the magnetic field so from the north to the south

52
Q

What direction does your second finger point to in Flemings left hand rule?

A

Your second finger points to the direction of current

53
Q

What direction does your thumb point to in Flemings left hand rule?

A

Your thumb points to the direction of the force

54
Q

How do you calculate the strength of the force acting on a wire in a magnetic field

A

F (N) = magnetic field strength (T) x current (amps) x length of wire (m)

55
Q

What does a split ring commutator do in an electric motor?

A

Swaps the positive and negative connections eve try half turn so the direction of current swaps every half turn so forces acting on the coil will be always be acting in the same direction

56
Q

How do you find out if the cone in a loudspeaker is moving in or out?

A

If you take a short section of the coil and apply Flemings left hand rule you can work out whether you he coil is moving in or out

57
Q

What does the movement of the coils depend on in a loudspeaker?

A

The movement of the coil depends on the size of the potential difference producing the current in the coil

-you can amplify the changing potential difference from a microphone and send it to loudspeakers to produce a loud sound

Physics Revision (5 decks)

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