Chapter 19: Electromagnetic Induction

Question 1

What is Faraday’s Law?

Answer 1

Faraday’s Law states that the induced e.m.f in a circuit is proportional to the rate of change of the magentic flux through the circuit

Question 2

What are the factors that affect the magnitude of the induced e.m.f?

Answer 2

• The strength of the magnet used
• The speed at which the magnet is moved
• the number of turns of the coil
• the type of core of the coils

Question 3

What is Lenz’s Law

Answer 3

Lenz’s law states the direction of the induced current in a conductor is such that it produces a magentic field which opposes the change to the initial magnetic field

Question 4

Explain how this works

Answer 4

• When the North pole of a magnet moves nearer to the coil, the induced current in the coil will flow anticlockwise to produce a North pole facing the magnet.
• The direction of the induced current is to oppose the incrrasing amount of North pole magnetic flux
• To overcome this repulsion, the user has to apply a force to push the magnet nearer to the coil

Question 5

Explain how this works

Answer 5

• When the North pole of a magnet moves away from the coil, the induced current in the coil will flow clockwise to produce a South pole facing the magnet.
• The direction of the induced current is to oppose the decreasing amount of North pole magnetic flux.
• To overcome this attraction, the user has to apply a force to pull away the magnet.

Question 6

Explain how this works

Answer 6

• When the South pole of a magnet of a magnet moves away from the coil, the induced current in the coil will flow anticlockwise to produce a North pole facing the magnet.
• The direction of the induced current is to oppose the decreasing amount of South pole magnetic flux.
• To overcome this attraction, the user has to apply a force to pull away the magnet

Question 7

Explain how this works

Answer 7

• When the South pole of a magnet moves nearer to the coil, the induced current in the coil will flow clockwise to produce a Sotuh pole facing the magnet.
• The direction of the induced current is to oppose the increasing amount of South pole magentic flux.
• To overcome this repulsion, the suer has to apply a force to push the magnet nearer to the coil.

Question 8

How does an AC generator work, and what are the functions of the apparatus?

Answer 8

• An alternatiing current (A.C.) generator generates electricity from the rotation of a coil or magnet, based on the principle of electromagnetic induction.
• emf is induced when a conductor ‘cuts’ the magnetic flux
• As the coil rotates in a magnetic field, there is a changing magentic flux that links the coil. An emf is induced in the coil, and current flows through the external circuit.
• A slip-ring commutator connects the coil to the rest of the circuit
• Carbon brushes provide electrical contact with the external circuit. They can be easily replaced when worn off.

Question 9

What are the factors that affect the magnitude of the induced emf?

Answer 9

The magnitude of the induced emf changes with:

• the strength of the magnet
• the speed of the movement of the bar magnet
• the number of turns of the coil
• the coil wrapped around a soft iron core

Question 10

What is the relationship between the strength of the magnet affects the magnitude of the induced emf

Answer 10

• Increasing the strength of the magnetic field increases the magnitude of the induced emf
• Decreasing the strength of the magnetic field decreases the magnitude of the induced emf

Question 11

What is the relationship between the speed of the movement of the bar magnet and the magnitude of the induced emf

Answer 11

• Increasing the speed of the movement of the magnet increases the magnitude of the induced emf
• decreasing the speed of the movement of the magent decreases the magntitude of the induced emf

Question 11

What is the relationship between the speed of the movement of the bar magnet and the magnitude of the induced emf

Answer 11

• Increasing the speed of the movement of the magnet increases the magnitude of the induced emf
• Decreasing the number of turns of the coil decreases the magntitude of the induced emf

Question 12

What is the relationship between the number of turns of the coil and magnitude of the induced emf?

Answer 12

• Increasing the number of turns of the coil increases the magnitude of the induced emf
• Decreasing the number of turns of the coil decreases the magnitude of the induced emf

Question 13

Explain how this setup works

Answer 13

• When the coil is perpendicular to the magnetic field lines, segments AB and CD are moving parallel to the magnetic field lines. Hence, no emf is induced.
• As the coil rotates away from this position, segments AB and CD begin to ‘cut’ the magnetic flux. The induced emf increases.
• The induced emf is maximum when the coil has rotated 90°
• As the coil continues to rotate, the induced emf decreases until zero when the segments AB and CD are moving parallel to the magnetic field lines.
• As the coil roatets another 90°, until AB and CD are now on the opposite sides, the induced emf is maximum but the direction of the current flow in CD is now reversed.

Question 13

How does the coil wrapped around a soft iron core affect the magntiude of the induced emf

Answer 13

This increases the strength of the magnetic field as the soft iron core helps to concentrate the magnetic flux. The magnitude of the induced emf thus increases

Question 14

What is the graph of output voltage against time graph of an AC generator?

Answer 14

Question 15

What are the factors that affect the output voltage against time graph

Answer 15

• the strength of the magnet m
• the speed at which the coil is rotated
• the number of turns of the coil
• the coil wrapped around a soft iron core

Question 16

State the relationship between the strength of the magnet and the output voltage against time graph of an A.C generator

Answer 16

Increasing the strength of the magnetic field. Hence, the amplitude of the graph increases

Question 17

State the relationship between the speed at which the coil is rotated and the output voltage against time graph of an A.C generator

Answer 17

Increasing the speed of the roation increases both the frequency of rotation of the coil and magnitude of the output emf. Hence, the frequency and amplitude of the graph increases.

Question 17

State the relationship between the number of turns of the coil and the output voltage against time graph of an A.C generator

Answer 17

• Increasing the number of turns of the coil increases the magnitude of the output emf. Hence, the amplitude of the graph increases.

Question 18

How does a simple transformer work?

Answer 18

The alternating current in the primary coil produces a changing magnetic field which links the secondary coil via the soft iron core. This changing magnetic field induced an emf in the secondary coil.

Question 18

State the relationship between the coil wrapped around a soft iron core and the output voltage against time graph of an A.C generator

Answer 18

This increases the strength of the magnetic field.

Question 19

What is the formula for an ideal transformer?

Answer 19

Question 20

Explain how power loss works in a tranformer, and how to combat it

Answer 20

• In a transformer, due to the changing magnetic field produced by the current in the primary coil, a current is also indcued in the iron core. These induced. currents are called eddy currents
• Some of the energy transfer from the primary cpil to the internal (thermal) store by eddy currents. Therefore, an ideal transformer in which all the energy is transferred from the primary coil to the secondary coil, does not exist.
• To minimise unwanted energy transfer by the eddy currents, we use sheets of laminated core to construct a transformer.

Question 21

What is the formula for transformer's effciency?

Answer 21