Magnetic Field

Question 1

In terms of north and south, which way do Fields lines act away from and act towards

Answer 1

Act away from north poles and towards south poles

Question 2

In a magnetic field what does it mean if Fields lines are closer together

Answer 2

Magnetic flux density is greater

Question 3

Define a Tesla

Answer 3

The magnetic flux density that produces a force of newton per metre on a current of one ampere flowing in a direction perpendicular to the magnetic field

Question 4

Define magnetic flux density and give equation related

Answer 4

Magnetic flux density at a point is equal to the force per unit length on a conductor carrying a unit current in a direction perpendicular to the field
F= B x I x L

Question 5

Here current is not perpendicular, what magnetic flux density equation should you use

Answer 5

Question 6

Describe practical of measuring magnetic force on a current carrying conductor

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Question 7

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Question 8

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Question 9

Consider a charge in a magnetic field, how do we work out the radius of the orbit the charge is moving in (hint you need to derive an equation)

Answer 9

Question 10

In a magnetic field, why is the work done on a moving charge equal to zero and explain why the charge moves at a constant speed in a magnetic field

Answer 10

Question 11

Discuss process that occurs when a proton is inserted between plates of a cyclotron

Answer 11

Process continues until it reaches the edge where it leaves a cyclotron
In the next Dee, it travels in a bigger semi-circle as velocity has increased (r=mv/BQ)

Question 12

Why are dees in a cyclotron connected to an AC current

Answer 12

To create an electric field

Question 13

What is SI unit of flux

Answer 13

Question 14

Draw graph of electric field strengh against seperation (radius)

Answer 14

Question 15

How to keep electron moving straight in between parallel plates

Answer 15

Fe = EQ, Not Fb

Question 16

Derive an equation for time in a circular in a semi-circular path

Answer 16

Pi x r / v comes from distance / speed

Question 17

What happens when a magnet is pushed into a coil

Answer 17

Question 18

Define induced emf

Answer 18

The electrical work done on a unit charge flowing once around the circuit

Question 19

What is faraday’s law

Answer 19

The magnitude of emf is proportional to rate of change of flux linkage

Question 20

What is Lenz’s law

Answer 20

The direction of an emf tends to produce a current which opposes the change causing it

Question 21

Why must Lenz’s law exist

Answer 21

Question 22

What is the flux here

Answer 22

Zero as it is parallel to lines of flux

Question 23

What is the total flux here

Answer 23

Question 24

Draw two graphs of flux linkage against time and emf against time as you move a magnet in and out of a coil

Answer 24

As the actual equation for magnitude of induced emf is e = - Flux linkage / change in time
Note that is not taking the negative into account in formula sheet

Question 25

Answer 25

Missing info from end : This means potential difference is reversed and current flows the other way

Question 26

What is An equation to do with faraday's law that is not provided

Answer 26

e= BLv e= induced emf

Question 27

How to calculate emf induced here

Answer 27

Question 28

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Question 29

Draw a graph to represent this

Answer 29

Question 30

What are the graphs of flux linkage against time and emf against time

Answer 30

Flux = cos graph emf = negative sin graph

Question 31

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Question 32

What is everything you can measure from this graph

Answer 32

Question 33

Difference between alternating current and direct current

Answer 33

Direct current can only flow in one Direction (from positive to negative) Alternating can flow in any direction

Question 34

State what is meant by Vrms and Irms

Answer 34

Vrms - value of DC voltage that would that would provide the same average power per cycle as the AC supply Irms - is same but just swap word voltage for current

Question 35

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Question 36

How does resistance in cables lead to heat loss

Answer 36

Question 37

Provide solutions for these heat loss methods in transformers

Answer 37

Question 38

How is heat lost to Eddy currents in transformers

Answer 38

Question 39

How is heat lost to hysterical losses in transformers

Answer 39

Question 40

How is heat lost to flux losses in transformers

Answer 40

Question 41

What would happen if transformer was used with a direct current

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Question 42

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Question 43

On graph draw a horizontal line to show the DC voltage that gives the same energy dissapation

Answer 43

Question 44

An oscilloscope has a screen of eight vertical and ten horizontal divisions. Describe how you would use oscilloscope to provide alternating waveform in the graph so two cycles are available

Answer 44

Question 45

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Question 46

Answer 46

B

Question 47

What is meant by root mean square voltage

Answer 47

Square root of the mean of the square of all the values of the voltages in one cycle

Question 48

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Question 49

Answer 49

C

Question 50

Answer 50

Vertical line formed Trace just moves up and down

Question 51

Unit for flux linkage

Answer 51

Wb - webner

Question 52

Flux linkage is 6.05 x 10^-2. Coil is rotated through 90 degrees in a time of 0.50 seconds. Determine mean emf of the coil

Answer 52

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Question 56

Answer 56

C

Question 57

Answer 57

D

Question 58

Answer 58

D

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A

Question 63

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A

Question 64

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C

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Question 69

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B

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A

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B

Question 75

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A

Question 76

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D

Question 77

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Question 78

Answer 78

A

Question 79

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Question 80

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Current travels in opposite direction too charge

Question 81

Answer 81

A

Question 85

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Question 86

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C

Question 87

Answer 87

D is correct answer Thumb goes down, magnetic flux goes to right and current goes into page Use left hand rule and just flip your hand so thumb is going downwards

Question 88

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Question 89

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C

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Question 94

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Force acts towards left and opposite direction of field lines as charge is negatige

Question 95

Two moments where a charged particle experience no magnetic force in a magnetic field

Answer 95

- when particle is at rest - when particle moves parallel to the field

Question 96

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Question 97

When given radius, how do you calculate lenghth of a semi-circle

Answer 97

Pi x radius

Question 98

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Question 99

What is meant by magnetic flux

Answer 99

Product of Flux and number of turns

Question 100

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Question 103

if magnetic field is acting upwards, what direction is force acting on a particle when in the field b) particle travelling horizontally, then enters uniform magnetic field, describe motion of particle c) state effect of path if magnetic field strength is doubled

Answer 103

Using flemings left hand rule, index finger acting upwards so thumb is pointing to you, coming out of the page b) circular path, out of the plane of the diagram c) radius is halved

Question 104

Explain why the ions travel in a semicircular path whilst in the magnetic field.

Answer 104

magnetic field is perpendicular to velocity force is perpendicular to both magnetic field and velocity (or Fleming's left hand rule) (hence) force acts perpendicular to velocity force changes direction of velocity but not its magnitude force remains perpendicular to velocity as direction changes

Question 105

Outline the essential features of a step-down transformer when in operation.

Answer 105

primary coil with more turns than secondary coil Input is AC (alternating current)

Question 106

The transformer circuit includes a fuse. Explain why this is necessary

Answer 106

fuse prevents transformer from overheating [or prevents transformer from supplying excessive currents

Question 107

Why is the fuse placed in the primary circuit rather than in the secondary circuit?

Answer 107

all of transformer is disconnected from supply when fuse fails or fuse in secondary circuit would leave primary circuit live

Question 108

Answer 108

Question 109

State Lenz’s law

Answer 109

direction of induced emf or current opposes change of magnetic flux that produces it

Question 110

The steel cylinder Q is a strong permanent magnet. It passes through a copper tube. Explain why you would expect and emf to be induced as it passes through

Answer 110

there is a change in flux linkage as Q passes through electrons in copper are acted on by magnetic field of Q so an emf is induced

Question 111

When a coil is rotating, when is emf the greatest and why

Answer 111

induced emf is proportional to rate of change of flux linkage flux linkage changes through the coil as it is rotated biggest change is from maximum at 0 or 180 dgerees, to minimum at 90 or 270 degrees

Question 112

The equation F = BQv may be used to calculate magnetic forces. State the condition under which this equation applies

Answer 112

magnetic fields must be at right angles to the velocity

Question 113

Explain why the ions have no overall deflection when a magnetic field of the required strength has been applied

Answer 113

magnetic force = electric force or BQv = EQ these forces act in opposite directions, are balanced and resultant vertical force is zero

Question 114

Explain how a step-down transformer differs in construction from a step-up transformer

Answer 114

primary coils must have more turns than secondary coils

Question 115

Discuss the principles involved in high voltage transmission systems, explaining why a.c. is used in preference to d.c. and how the energy losses are minimised.

Answer 115

voltages are changed using transformers, which work with ac but not with dc ac generation and transmission is therefore essential current in cables causes joule heating (or IR^2 loss) resistance of cables should be as low as possible losses are reduced if current in cables can be reduced current can be reduced (for same power I V) if voltage is increased the higher the voltage, the smaller the proportion of the input power that is wasted high voltage introduces insulation problems and raises safety issues voltage must be reduced as the supply reaches its consumers this is done in stages as the supply is moved from overhead cables to underground wires transformers cause energy losses because they are not perfectly efficient features are incorporated in the design of transformers to reduce losses from them

Question 116

State two situations in which a charged particle will experience no magnetic force when placed in a magnetic field.

Answer 116

- when charged particle is at rest / not ,moving relative to the field - when charged particle moves parallel to the relative field

Question 117

State, in words, the two laws of electromagnetic induction.

Answer 117

Faradays law : induced emf is proportional to the rate of change of flux linkage Lenz's law : direction of induced emf (or current) is opposite to the change in flux producing it

Question 118

advantage and disadvantage of electromagnetic induction compared to conventional brake

Answer 118

advantage : nothing needs replacing disadvantage : ineffective at low speed, requires an electrical circuit

Question 119

Derive emf = BLv

Answer 119

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Question 129

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B

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Question 131

Only complete 4.3, data from 4.1 needed

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C

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B

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A

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