Magnetic Field Flashcards
In terms of north and south, which way do Fields lines act away from and act towards
Act away from north poles and towards south poles
In a magnetic field what does it mean if Fields lines are closer together
Magnetic flux density is greater
Define a Tesla
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
Define magnetic flux density and give equation related
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
Here current is not perpendicular, what magnetic flux density equation should you use
Describe practical of measuring magnetic force on a current carrying conductor
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)
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
Discuss process that occurs when a proton is inserted between plates of a cyclotron
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)
Why are dees in a cyclotron connected to an AC current
To create an electric field
What is SI unit of flux
Draw graph of electric field strengh against seperation (radius)
How to keep electron moving straight in between parallel plates
Fe = EQ, Not Fb
Derive an equation for time in a circular in a semi-circular path
Pi x r / v comes from distance / speed
What happens when a magnet is pushed into a coil
Define induced emf
The electrical work done on a unit charge flowing once around the circuit
What is faraday’s law
The magnitude of emf is proportional to rate of change of flux linkage
What is Lenz’s law
The direction of an emf tends to produce a current which opposes the change causing it
Why must Lenz’s law exist
What is the flux here
Zero as it is parallel to lines of flux
What is the total flux here
Draw two graphs of flux linkage against time and emf against time as you move a magnet in and out of a coil
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
Missing info from end :
This means potential difference is reversed and current flows the other way
What is An equation to do with faraday’s law that is not provided
e= BLv
e= induced emf
How to calculate emf induced here
Draw a graph to represent this
What are the graphs of flux linkage against time and emf against time
Flux = cos graph
emf = negative sin graph
What is everything you can measure from this graph
Difference between alternating current and direct current
Direct current can only flow in one Direction (from positive to negative)
Alternating can flow in any direction
State what is meant by Vrms and Irms
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
How does resistance in cables lead to heat loss
Provide solutions for these heat loss methods in transformers
How is heat lost to Eddy currents in transformers
How is heat lost to hysterical losses in transformers
How is heat lost to flux losses in transformers
What would happen if transformer was used with a direct current
On graph draw a horizontal line to show the DC voltage that gives the same energy dissapation
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
B
What is meant by root mean square voltage
Square root of the mean of the square of all the values of the voltages in one cycle
C
Vertical line formed
Trace just moves up and down
Unit for flux linkage
Wb - webner
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
C
D
D
A
A
C
B
A
B
A
D
A
Current travels in opposite direction too charge
A
C
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
C
Force acts towards left and opposite direction of field lines as charge is negatige
Two moments where a charged particle experience no magnetic force in a magnetic field
- when particle is at rest
- when particle moves parallel to the field
When given radius, how do you calculate lenghth of a semi-circle
Pi x radius
What is meant by magnetic flux
Product of Flux and number of turns
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
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
Explain why the ions travel in a semicircular path whilst in the magnetic field.
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
Outline the essential features of a step-down transformer when in operation.
primary coil with more turns than secondary coil
Input is AC (alternating current)
The transformer circuit includes a fuse. Explain why this is necessary
fuse prevents transformer from overheating [or prevents transformer from supplying excessive currents
Why is the fuse placed in the primary circuit rather than in the secondary circuit?
all of transformer is disconnected from supply when fuse fails or fuse in secondary circuit would leave primary circuit live
State Lenz’s law
direction of induced emf or current
opposes change of magnetic flux that produces it
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
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
When a coil is rotating, when is emf the greatest and why
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
The equation F = BQv may be used to calculate magnetic forces.
State the condition under which this equation applies
magnetic fields must be at right angles to the velocity
Explain why the ions have no overall deflection when a magnetic field of the required strength has been applied
magnetic force = electric force or BQv = EQ
these forces act in opposite directions, are balanced and resultant vertical force is zero
Explain how a step-down transformer differs in construction from a step-up transformer
primary coils must have more turns than secondary coils
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.
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
State two situations in which a charged particle will experience no magnetic force when placed in a magnetic field.
- when charged particle is at rest / not ,moving relative to the field
- when charged particle moves parallel to the relative field
State, in words, the two laws of electromagnetic induction.
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
advantage and disadvantage of electromagnetic induction compared to conventional brake
advantage : nothing needs replacing
disadvantage : ineffective at low speed, requires an electrical circuit
