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A-Level Physics - OCR A > M6, C3 Electromagnetism > Flashcards

M6, C3 Electromagnetism Flashcards

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1
Q

Which way do field lines go on a magnetic field

A

from north to south

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2
Q

the closer the field lines on a magnetic field, the _______ the field

A

stronger

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3
Q

how could you determine the direction of the magnetic field around a current-carrying wire

A

When a current flows through a wire, a magnetic field is created around it.
Use the right-hand rule.
Point your thumb in the direction of the current through the wire.
Your curled fingers will then show the direction of the field.

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4
Q

what can magnetic fields be caused by

A

moving charges
or
permanent magnets

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5
Q

draw the magnetic field pattern of a flat coil

A

page 147 of year 2 textbook

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6
Q

draw the magnetic field pattern of a long solenoid

A

page 147 of year 2 textbook

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7
Q

what is Fleming’s left-hand rule

A

You can use your left hand to find the direction of the current, the direction of the external magnetic field or the direction of the force on the wire.
Point your thumb up, middle finger to the right and index finger forward.

First finger = direction of the uniform magnetic field
Second finger = direction of the conventional current
Thumb = direction of the force (motion)

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8
Q

define magnetic flux density

A

the force on one metre of wire carrying a current of one amp at right angles to the magnetic field

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9
Q

is magnetic flux density a scalar or vector quantity

A

vector

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10
Q

what are the units of magnetic flux density

A

teslas (T)

One tesla is equal to one newton per amp per metre

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11
Q

What equation can you use to calculate the force on a current-carrying wire which is perpendicular to a magnetic field

A

F = BIL

B = magnetic flux density
I = current through wire
L = length of the wire
F = force on the current-carrying wire
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12
Q

what happens to the magnetic field when you put a current-carrying wire in between 2 magnets?

A

page 147 of the year 2 textbook

The 2 fields are added together making a resultant field.
Lines closer together show where the magnetic field is stronger.

The size of the force depends on the component of the magnetic field that is perpendicular to the current. Use Fleming’s left-hand rule.

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13
Q

What does the equation F = BILsinθ mean

A

For a wire that is at an angle to the field, the force is given by this equation.

F = force on a current-carrying wire
B = magnetic flux density
I = current through the wire
L = length of the wire
θ = angle between wire and field
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14
Q

Outline an experiment you can do to measure the magnetic flux density

A

Use F=BIL
page 151 of year 2 textbook shows diagram

a current-carrying wire is placed between two magnets. the mass is measured, it will change because a force will act upwards or downwards

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15
Q

An electrons travels perpendicular to a magnetic field of flux density 0.15T. Calculate the acceleration of the electron given its speed is 5X10^6 ms^-1.

A

F = BQv = Bev
= 0.15 X 1.6X10^-19 X 5X10^6
= 1.2X10-13 N

F = ma
a = 1.2X10^-13 / 9.11X10^-31
= 1.3X10^17 ms^-2

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16
Q

Charged particles travelling perpendicular to a magnetic field travel in a __________.

A

Circular path

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17
Q

Derive an equation you can use to calculate the radius of circular path of charged particles.

A 
F = mv^2 / r
F = BQv

mv^2 /r = BQv

r = mv / BQ

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18
Q

What happens to the radius of the circular path, if the mass or velocity of the particles increases

A

increases

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19
Q

what happens to the radius of the circular path, if the strength of the magnetic field or charge on the particle increases

A

decreases

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20
Q

what are velocity selectors used for

A

to separate out charged particles of a certain velocity from a stream of accelerated charged particles moving at a range of speeds

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21
Q

how do velocity selectors work

A

they apply both a magnetic and electric field at the same time perpendicular to each other, while a stream of particles is fired perpendicularly to both fields at a device with a narrow gap called a collimator

opposing forces are experienced from the magnetic and electric fields

magnetic field tries to deflect the particles upwards F=BQv
electric field tries to deflect the particles downwards F=EQ

particles won’t deflect if the forces are balanced
BQv = EQ
Bv = E

So only particles with velocity v = E/B will travel in a straight line to pass through the gap in the collimator

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22
Q

what is magnetic flux

A

The total magnetic flux passing through an area, perpendicular to a magnetic field is defined as:
Φ = BA
You can think of flux as the number of field lines in an area.

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23
Q

What is electromagnetic induction

A

If there’s relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force which cause them to accumulate at one end of the rod.
This induces an emf across the ends of the rod exactly as connecting a battery to it would. This is called electromagnetic induction.

24
Q

Explain electromagnetic induction

A

An emf is induced when the conductor cuts the magnetic flux. The conductor can move and the magnetic field stay still or the other way round.

25
Q

what is magnetic flux linkage

A

the magnetic flux multiplied by the number of turns on the coil cutting the flux

magnetic flux linkage = NΦ

26
Q

what are the units of magnetic flux

A

Weber (Wb)

27
Q

what are the units of magnetic flux linkage

A

Weber (Wb)

28
Q

what does this equation mean

Φ = BAcosθ

A

magnetic flux = magnetic flux density X area X cos(angle to the normal)

29
Q

what equation could you use for flux linkage if there’s an angle between the magnetic flux and normal of the plane

A

flux linkage = NΦ = BANcosθ

30
Q

what is Faraday’s law

A

The induced emf is directly proportional to the rate of change of flux linkage.

31
Q

what does this equation mean

ε= -∆(NΦ) / ∆t

A

also written as
ε= -N∆Φ / ∆t

ε= induced emf
N = number of turns in the coil cutting the flux
∆Φ = change in the magnetic flux
∆t = time taken for flux linkage to change
32
Q

For a coil, what does induced emf depend on

A

the number of turns and how fast flux through the coil is changing

33
Q

define weber in terms of induced emf

A

a change in flux linkage of one weber per second will induce an emf of 1 volt in a loop of wire

34
Q

On a flux linkage against time graph, what does the gradient equal

A

emf

35
Q

on a time against emf graph, what does the area under the graph equal

A

flux linkage change

36
Q

What is Lenz’s law

A

The induced emf is always in such a direction as to oppose the change that caused it

37
Q

A copper bar in a uniform magnetic field of 0.11T is moved perpendicular to the field for 0.63s. The total area that is covered by the bar in this time is 42cm^2.
Calculate the emf induced across the bar.

A 
ε= -∆(NΦ) / ∆t
Φ = BA
so
ε= -∆(BAN) / ∆t
= -0.11 X 42X10^-4 / 0.63
= -0.73 mV
38
Q

Outline an experiment you can do to investigate magnetic flux

A

1) Place 2 bar magnets a small distance apart with opposite poles facing each other (close enough to give a uniform field).
2) Get a search coil (a small coil of wire with a known number of turns and a known area.
3) Connect it to a data recorder and set the recorder to measure the induced emf with a very small interval between intervals.
4) Place the search coil in the middle of the magnetic field so that the area of the coil is parallel to the surface of the magnets. Start the data recorder.
5) 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 linkage changing from max to 0.
6) Plot a graph of induced emf against time. Area under graph = total flux linkage change.
7) Repeat this experiment several times and find the mean of your values of Φ.

39
Q

Describe what Lenz’s Law actually means?

A
  • If the original magnetic field is getting stronger, the current will be induced in the direction that generates a magnetic field in the opposite direction to the external field, to try to weaken it.
  • If the original magnetic field is getting weaker (collapsing), the current will be induced in the direction that generates a magnetic field in the same direction as the external field, to try to maintain it.
40
Q

Why is electricity generated when a conductor rotates in a magnetic field

A

the coil cuts the flux and an alternating emf is induced

41
Q

Explain what’s happening as a coil rotates in a magnetic field

A

The angle is changing so the flux linkage varies sinusoidally between +BAN and -BAN.
The rate of change of flux linkage is always changing so the induced emf is also changing and varies sinusoidally.

42
Q

A coil is rotating in a magnetic field.
When is emf 0?
When is emf at its maximum?

A

emf is 0 when the plane of the coil is perpendicular to the lines of flux

emf is maximum when the plane of the coil is parallel to the lines of flux because that’s when there is maximum rate of change of flux linkage

(graph on pg 168 of year 2 textbook)

43
Q

how does increasing the speed of rotation of a coil in a magnetic field, affect the induced emf

A

Increasing the speed of rotation will increase the frequency of rotation
this causes an increase in the rate of change of flux linkage and therefore increases the maximum emf

44
Q

how does increasing the magnetic flux density of a rotating coil in a magnetic field affect the induced emf

A

it will increase the maximum emf but have no effect on the frequency of rotation

45
Q

How do simple a.c. generators work

A

Generators convert kinetic energy into electrical energy - they induce an electric current by rotating a coil in a magnetic field.
An a.c. generator has slip rings and brushes to connect the coil to an external circuit. The output voltage and current change direction with every half rotation of the coil, producing an alternating current.

46
Q

define transformers

A

devices that make use of electromagnetic induction to change the size of the voltage for an alternating current

47
Q

how does a simple laminated iron-cored transformer work

A

An a.c. flowing in the primary coil produces an alternating magnetic field, causing the core to magnetise, demagnetise and remagnetise continuously in opposite directions.
The changing magnetic field is passed through the iron core to the secondary coil, where the rapidly changing flux linkage through the coil induces an alternating voltage due to Faraday’s law. The emf produced is of the same frequency as the input voltage.

48
Q

What is the difference between step-up transformers and step-down transformers

A

Step-up transformers increase the voltage by having more turns on the secondary coil than the primary coil.

Step-down transformers reduce the voltage by having fewer turns on the secondary coil.

49
Q

How is power loss in transformers reduced

A

The core is laminated.

This involves having layers of the core separated out by thin layers of insulator, so a current can’t flow.

50
Q

What do all the parts of this equation mean:

n_s / n_p = V_s / V_p = I_p / I_s

A

For transformers:

n_s = number of turns on secondary coil
n_p = number of turns on primary coil
V_s = voltage across secondary coil
V_p = voltage across primary coil
I_p = current in primary coil
I_s = current in secondary coil
51
Q

The secondary coil of a step-up transformer has 95 turns. Calculate the number of turns on the primary coil if the current across the transformer decreases from 22A to 4A.

A

n_p = (n_s X I_s) / I_p
= 95 X 4 / 22
= 17.27…

= 17 turns

52
Q

There is a 1.2m long copper wire moving upwards at a velocity of 0.5ms^-1 through a perpendicular magnetic field of flux density 5.4mT.
Calculate the magnitude of the emf induced in the wire.

A 
s = vt
So A = Lvt
Φ = BA = BLvt
ε= -∆(NΦ) / ∆t
ε= -∆(NBLvt) / ∆t
N=1
ε= -BLv

= -5.4X10^-3 X 1.2 X 0.5 = -0.00324V
so magnitude of emf
= 0.0032 V

53
Q

why are transformers an important part of the national grid

A

They allow us to step-up and step-down voltage so when travelling through wire there is a high voltage but when going into homes, there is a small voltage.

54
Q

Why does there need to be a step-up transformer in the national grid

A

This is because a high current causes greater energy losses due to heating in the cables.
The power losses due to the resistance of the cables is equal to P=I^2R so if you double the transmitted current, you quadruple the power lost. Using cables with the lowest possible resistance can also reduce energy loss.

P= VI
A low current means a high voltage for the same amount of power transmitted. Step-up transformers increase the voltage.

55
Q

where else could transformers be used other than the national grid

A

Household devices
laptops, mobiles, monitors and speakers
the chargers for these contain the transformers

56
Q

In London, the Earth’s magnetic field makes an angle of 66 degrees with the horizontal and has flux density 4.9 X10^-5 T. Estimate the magnetic flux for a small coin (radius 1cm) lying on flat ground.

A

The angle has to be to the vertical so use sin66 or cos24.
Φ = BAcosθ
= 4.9X10^-5 X (π(1X10^-2)^2) X cos24

= 1.4X10^-8 Wb

A-Level Physics - OCR A (20 decks)

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