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SAJID OCR A-Level Physics > 6.3 Electromagnetism > Flashcards

6.3 Electromagnetism Flashcards

1
Q

what is a magnetic field?

A

a magnetic field is the region around a permanent magnet or a moving charge in a current-carrying conductor in which another body with magnetic properties will feel a force

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2
Q

what do magnetic field lines show?

A
  • the shape of the field

- the direction of the field

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3
Q

How is a magnetic field of a bar magnet created?

A

It is, created by the e-‘s whizzing around the iron nuclei. You can visualise the iron atoms as tiny magnets, all aligned in the same direction

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4
Q

magnetic field lines always go from…

A

NORTH to SOUTH

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5
Q

State two methods of producing a uniform magnetic field

A

Use two opposite magnetic poles. The magnetic field is uniform in space between the opposite poles of a magnet

Use a current-carrying solenoid. The magnetic field is uniform at the centre of a solenoid carrying a current.

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6
Q

what is the symbol for current going into the page?

A

a circle with a cross, think arrowhead moving away from you

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7
Q

what is the symbol for current coming out the page?

A

a circle with a dot, think arrowhead coming towards you

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8
Q

what do you always get around a wire carrying an electric current?

A

a magnetic field

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9
Q

what is a solenoid?

A

a coil of wire

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10
Q

what rule can be used to work out the magnetic field around a current carrying wire? and what are the two differences in the rule for a simple wire and a solenoid?

A

you use the right hand rule where…
thumb = current
fingers = field direction
FOR A STRAIGHT CONDUCTOR^

thumb = field direction
fingers = current (curl your fingers)
FOR A SOLENOID^

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11
Q

when you have a current perpendicular to a magnetic field what happens?

A

a force is induced
(this force comes from the interaction between the magnetic field of the wire and the magnetic field of the external magnets)

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12
Q

what rule can you use to work out induced force from a current carrying wire being perpendicular to a magnetic field?

A

Fleming’s left hand rule, where…
thumb = force
index = field
middle = conventional current (positive to negative)

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13
Q

what is the equation for the induced force on a current carrying wire in a magnetic field?

A 
F = BILsinϴ
where F = induced force
B = magnetic flux density
I = current in the wire
L = length of current carrying wire in the field
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14
Q

when is the induced force from a current carrying wire in a magnetic field at its maximum?

A

when the current carrying wire and magnetic field are perpendicular, i.e F = BILsinϴ when ϴ = 90

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15
Q

outline an experiment to investigate flux density (calculate B)

A
  • place magnets on a top-pan balance. The magnetic field between them is almost uniform
  • a copper wire is held perpendicular to the magnetic field between the two poles
  • length of wire is measured w a ruler
  • using crocodile clips, connect the wire to a circuit to produce a current by having a section of the wire connect to an ammeter and a variable power supply in series
  • the power supply should be connected to a variable resistor to be able to alter the current,
  • zero the balance when there is no current so the mass reading is only due to the electromagnetic force not the weight force, then turn on the power supply
  • note the mass and current, then change the current by altering the variable resistor and record the new mass and current, do this for a range of currents and repeat to get averages
  • convert the mass into force using w = mg, plot a graph of F against I and the gradient will be equal to B x L (because of F = BIL)
  • measure gradient and divide by length, L, of wire in the field to get B, magnetic flux density
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16
Q

Why does the mass reading change when the current is changing

A

Because when there is a current, the wire experiences a vertical upward force.

Now according to Newton’s 3rd law or motion, the magnets experience an equal downward force F, which can be calculated from the changed in the mass reading, m, using F = mg

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17
Q

Explain why a current-carrying wire experiences a force when placed close to a magnet

A

A current-carrying sure is surrounded by its own magnetic field as well as the magnet. And when these fields come in close proximity they each interact with each other. As a result it produces a force on the wire

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18
Q

what is the equation for magnetic flux when the area cuts through the field at 90 degrees?

A 
Φ = B x A
magnetic flux (Φ) = magnetic flux density (B) x area at right angles to the flux (A)
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19
Q

what is magnetic flux density measured in?

A

tesla, T

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20
Q

what is magnetic flux measured in?

A

weber, Wb

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21
Q

what is magnetic flux density defined as? (in words)

A

magnetic flux density is a measure of the strength of the magnetic field and is defined by the equation for the force on a current-carrying conductor in a magnetic field, F = BILsinϴ

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22
Q

What is magnetic flux density

A

is 1T when a current conductor is carrying a current of 1A, placed perpendicular to the magnetic experiences a force of 1N per metre of its length

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23
Q

is tesla a large or small unit?

A

a very LARGE unit, the Earth’s magnetic field is roughly equal to about 60 micro tesla

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24
Q

what is the equation for the induced force on a single charged particle in a magnetic field?

A

F = BQV

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25
Q

A proton is travelling at 4.0 x 10^6 ms-1 describes a circular path in a uniform magnetic field of flux density 800 mY. Calculate:

a) the radius of the path;
b) the period of the proton;
c) show that the period of the proton is independent of its speed

A 
r = 0.053 
T = 2pier/v = 8.3 x 10^-8 s

T = 2pie r / v

r = mv/BQ

T = 2 pie m/ BQ

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26
Q

how do you derive F = BQV?

A 
F = BIL
I = Q / t
V = L / t so L = Vt
F = B x Q/t x Vt
F = BQv
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27
Q

how do charged particles move in a magnetic field? and why?

A

they are deflected in a circular path (circular motion) because the force is acting at right angles to the direction of motion, no net force so particles have constant speed

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28
Q

how can you come to an equation for the radius of curvature of a charged particle in a magnetic field?

A

circular motion, centripetal force F = mv^2 / r
force on a charged particle F = BQV
BQV = mv^2 / r
r = mv / BQ

29
Q

what are velocity selectors used for?

A

velocity selectors are often used in mass spectrometers to ensure that the accelerated particles entering the magnetic field have the same velocity.

30
Q

what’s the device called that produces the accelerated particles

A

with a device called a collimator

31
Q

How do velocity selector work?

A

velocity selectors use a magnetic and electric field perpendicular to each other, a stream of particles is fired at right angles to both these fields (with a range of speeds) with a device called a collimator

They have two parallel horizontal plates connected to a power supply. They produce a uniform electric field of field strength E between the plates. A uniform magnetic field of flux density B is also applied perpendicular to the electric field. The charged particles travelling at different speeds will then travel through the first slit. After that stage the magnetic and electric field will deflect them in opposite dog reactions. Only the particles with a specific speed v will not be deflected as the electric and magnetic field cancel out. As a result, particle will travel in a straight line and emerge from the second narrow

For an undeflected particle:
EQ = BQv

v = E/B

32
Q

The parallel plates of velocity selector are connected to a 1.3kV supply and have a separation of 2.5cm. It selects particles of speed 4.0 x 10^5 ms-1. Calculate the magnetic flux density of the magnetic field used in the velocity selector

A

B = 0.13T

33
Q

A particle of charge +2e describes a circle of radius 2.5cm in a uniform magnetic field of flux density 130mT.
Calculate its momentum

A

p = 1.04 x 10-21 kg ms-1

34
Q

what happens to the undeflected ions coming out a velocity selector?

A

they are passed through just a single magnetic field and will follow a curved path, then the mass can be determined using r = mv / BQ

35
Q

what is the equation for magnetic flux when the area that has been cut is not at 90 degrees to each other?

A

Φ = BAcosϴ

36
Q

what is the formula for magnetic flux linkage? (for N number of loops for a coil)

A

NΦ or BANcosϴ (because Φ = BAcosϴ)

where ϴ is the angle between the field and coil

37
Q

what is the definition for electromagnetic induction?

A

electromagnetic induction is the process of inducing an e.m.f in a conductor when there is a change in magnetic flux linkage across the conductor

38
Q

what are the two things you can do to induce an emf in a flat coil or solenoid?

A
  • moving the coil towards or away from the poles of the magnet
  • moving a magnet towards or away from the coil

in both cases, the emf is caused by the magnetic field (or magnetic flux) which is constantly changing that passes through the coil, a current is produced when the circuit is complete

39
Q

what is Fleming’s left hand rule used for and what is Fleming’s right hand rule used for?

A

left hand rule —> MOTOR EFFECT, force, field and current
right hand rule —> GENERATORS, motion, field and current
(in both cases the current is conventional)

40
Q

what is Faraday’s Law of electromagnetic induction?

A

Faraday’s Law of electromagnetic induction states that the magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linkage

41
Q

what is the equation linked to Faraday’s Law of electromagnetic induction?

A 
ε = -△NΦ / △t or 
ε = -△NBA / △t
42
Q

what does the minus sign in ε = -△NΦ / △t account for?

A

Lenz’s law and it represents the conservation of energy

43
Q

what is Lenz’s law?

A

Lenz’s law states that the direction of any induced emf or induced current is always in a direction that opposes the flux change that causes it (this law came about because of conservation of energy)

44
Q

if you’ve got a complete circuit and an emf is induced what direction will the current be in?

A

the same direction as the emf

45
Q

outline an experiment to investigate magnetic flux density

2nd method

A
  • place two bar magnets a small distance apart with opposite poles facing each other, they should be far enough apart not to snap together, but otherwise as close as possible to give a uniform field
  • get a search coil (this is a small coil of wire with a known number of turns, N, and a known area, A), connect it to a data recorder and set the recorder to measure the induced emf with a very small time interval between readings
  • place the search coil in the middle of the magnetic field so that the area (A) of the coil is parallel to the surface of the magnets, start the data recorder, keeping the coil in the same orientation immediately move the coil out of the field
  • an emf will be induced due to the magnetic flux density through the coil changing from max to zero as you remove the coil from the field
  • use your data to plot a graph of induced emf against time
  • using Faraday’s and Lenz’s law, estimating the area under the graph gives you an estimate for the total flux linkage
  • flux linkage = NΦ or BAN so to find B, divide the total area by the N X A
  • repeat this experiment several times and find the mean of your values for B
46
Q

how is a constantly changing induced emf produced in the coil of a generator?

A

the rotation of the coil within a magnetic field produces a constantly changing flux linkage through the coil, this in turn produces a constantly changing induced emf in the coil

47
Q

A coil connected to a voltmeter is places next to one end of a long current-carrying solenoid. The voltmeter reads zero. When the current in the solenoid is switched off, the voltmeter shows a reading for a very short interval of time and the goes back to zero. Explain these observations.

A

The initial reading of the voltmeter is zero, because there is no change in the flux linkage.
When the current is switched off, the magnetic field collapses in a very short interval of time, hence a large e.m.f. is induced.
Explanation justified in terms of e.m.f. = dt (N x flux linkage) / dt (t)

48
Q

what is AC current?

A

electrical current that reverses its direction with a constant frequency

49
Q

what is the main difference between a motor and a generator

A 
motor = putting in electrical energy to get kinetic energy
generator = putting in kinetic energy to get electrical energy
50
Q

what does the graph look like for induced emf-time?

A 
a sine graph (when a coil rotates at a constant frequency), parallel to field = O emf
perpendicular to field = max emf
parallel to field = O emf
perpendicular field (but other way round) = - max emf
parallel to field = O emf
51
Q

what is a transformer?

A

a transformer is a device that uses electromagnetic induction to either increase or decrease the size of a alternating voltage with little loss of power

52
Q

what is a transformer made up of?

A

consists of two coils of wire wrapped around an iron core

53
Q

how does a transformer work?

A

an alternating current flowing in the primary coil (or input coil) produces a changing magnetic field in the iron core, the changing magnetic field is passed through the iron core to the secondary (or output) coil, where it induces an alternating voltage of the same frequency as the input voltage

54
Q

where are transformers used?

A

the National Grid

55
Q

what do step up transformers do to the voltage?

A

INCREASE the voltage

56
Q

what do step down transformers do to the voltage?

A

DECREASE the voltage

57
Q

what is the turns ratio on a step up transformer?

A

more turns on the secondary coil

58
Q

what is the turns ratio on a step down transformer?

A

more turns on the primary coil

59
Q

are transformers 100% efficient? & how can we increase efficiency?

A

no, they are not 100% efficient - some power is always lost however by using a laminated core (made up in layers) reduced power losses by reducing the size of eddy currents which reduces power loss

60
Q

what is the equation for an ideal transformer?

A 
Ns / Np = Vs / Vp = Ip / Is
where N = number of turns
V = voltage
I = current
(comes from power in = power out, IV = IV)
61
Q

why do you want a small current in transformers?

A

we want a small current to reduce power loss due to the resistance of the cables

P = IV

62
Q

outline an experiment to investigate the number of turns and voltage of a transformer

A
  • set up a transformer with coils of wire around the iron core with a low voltage ac power supply connected to the first coil and a voltmeter on both primary and secondary coils
  • begin with five tuns on the primary and 20 on secondary (use ratio 1:2)
  • turn on the ac supply to the primary coil, use a low voltage (remember transformers increase voltage, keep it at a safe voltage), record the voltage across each coil
  • keeping Vp the same so its a fair test, repeat the experiment with different ratio of turns,
  • you should find for each ratio of tuns Ns / Np = Vs / Vp
63
Q

outline an experiment to investigate the number of turns, voltage and current of a transformer

A
  • set up a transformer with coil of wire around the iron core with a voltmeter and ammeter connected to each coil on either side and a variable resistor in the primary coil circuit
  • turn on the power supply and record the current through the voltage across each coil
  • leaving the number of turns constant, adjust the variable resistor to change the input current, record the current and voltage for each coil then repeat this process for a range of input currents
  • you should find for each current Ns / Np = Vs / Vp = Ip / Is
64
Q

Explain why a large current-carrying foil can produce dangerously high ‘back’ e.m.f. when the current is suddenly switched off

A

A large coil is linked by its own magnetic field. When the current is switched off, this magnetic field collapses in a very short interval of time and induces a very large e.m.f. In the opposite direction

Explanation justified in terms of ε = -△NΦ / △t

65
Q

Why will transformers not work with direct current

A

because there is no changing magnetic flux

66
Q

What’s the equation used to measure or compare the efficiency of a transformer between a primary and a secondary coil

A

Psecondary = Pprimary

VsIs=VpIp

67
Q

Explain the purpose of the iron core in a transformer

A

The iron core ensures that all the magnetic flux created by the primary coil links the secondary coil

68
Q

Why do we increase the voltage in a transformer

A

To decrease the current so therefore reducing the heating effect in the wires

Reducing the power lost

SAJID OCR A-Level Physics (25 decks)

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