EMI

Question 1

Magnitude of induced current increases when (3)

Answer 1

– magnet is moved at faster speed relative to the coil
– stronger magnet is used
– number of turns per unit length in solenoid is increased

Question 2

Faraday Law of EMI

Answer 2

the magnitude of
the induced e.m.f. is proportional to the rate of change of magnetic flux
linkage or the rate at which the magnetic field lines are cut

Question 3

Lenz Law

Answer 3

the direction of the induced e.m.f., and hence the
induced current in a closed circuit, is always such that its magnetic effect
opposes the motion or change producing it

Question 4

A bar magnet with North pole facing solenoid is pushed towards the solenoid

Using Lenz’s law, explain how the direction of the induced current in the solenoid is
determined

Answer 4

To oppose the motion of incoming magnet, direction of induced current in
solenoid is such that there is North pole at left end to repel North pole of bar magnet.

Question 5

Why current reverse in ac generator

Answer 5

When the coil rotates 180°, sides AB and CD change places.
• The induced e.m.f. reverses, hence current also reverses.
• As current changes direction, it is called an alternating current

Question 6

When does the coil induce a min emf

Answer 6

vertical as rate at which coil cuts magnetic field lines is least

Question 7

When does the coil induce a max emf

Answer 7

horizontal as rate at which coil cuts magnetic field lines is greatest

Question 8

How increase induced emf on generator (4)

Answer 8

increasing speed of rotation of coil
• increasing the number of turns of coil
• winding coil on soft iron core
• using stronger magnets

Question 9

What happen when speed of rotation of coil doubled

Answer 9

emf increase

frequency of graph increase

Question 10

Describe the process in a iron cored trnasformer

Answer 10

• When the switch is closed, the current in the primary coil increases from zero to a
maximum.
• This produces an increasing magnetic field in the core.
• The increasing magnetic flux linkage in the secondary coil induces an
electromotive force (e.m.f.) and hence a current in the secondary coil.
• This lights up the lamp momentarily.
• Once the current in the primary coil is steady, there is no change in the magnetic
flux linkage.
• There is no induced e.m.f. in the secondary coil.
• The lamp no longer lights up

Question 11

How increase efficiency of trnasformer (4)

Answer 11

a special core design to ensure the magnetic field produced by the primary coil is
completely linked to the secondary coil,
– a core made of soft magnetic material so that it is easier to magnetise and
demagnetise,
– a laminated core to reduce eddy currents,
– use of thick copper wires to reduce resistance