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8.3 Induce Current

The meaning of electromagnetic induction

The production of induced current or induced e.m.f. without using the power supplies but using the relative motion between a conductor or a magnet.

What is meant by relative motion?

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Figure (a)

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Figure (b)

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Figure (c)

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Figure (d)

For Figure(a)and Figure(b):

(a) moving the magnet in a stationary conductor

(b) moving the conductor in a stationary magnet

(c) moving magnet and conductor in the opposite directions.

(d) moving magnet and conductor in the same direction with different speeds.

For Figure (c)

(a) Closing and opening switch S

(b) Adjusting rheostat R

(c) Flowing the coil P with current and moving the coil P nearer or farer the the coil Q.

(d) Replacing the battery with a.c. power supply the and closing and keeping closing S.

For Figure (d)

Oscillating the pendulum bob in the magnetic field

The mechanism of the production of induced current or induced e.m.f.

When the relative motion between a conductor or a magnet happened , the conductor cuts across the magnetic field lines or change of the magnetic flux.

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The types of induced current

(a) d.c. current

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A d.c. current is produced when the magnet is moved into or out of the solenoid.

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(a) a.c. current

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An a.c. current is produced when the magnet is moved into and out of the solenoid.

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To determine the magnitude of the induced current – Faraday’s Law

Faraday’s law state that “ The magnitude of the induced current or induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage with the solenoid or the rate at which a conductor cuts through the magnetic flux.”

Hence, based on the Faraday’s law is increased when

(a) the number of turns of the solenoid is increased

(b) the strength of the magnet is increased

(c) the speed of relative motion is increased

To determine the direction of the induced current – Lenz’s Law

Lenz’s law state that” The direction of an induced current always flows in such as a direction so to oppose the change which is causing it.”

For example:

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When the magnet is moved towards into the solenoid, there is a force of repellent exists between the solenoid and the magnet and the end P is induced to become the south pole and the direction of the current is from B to A.

When the magnet is moved away from the solenoid, there is a force of Attraction exists between the solenoid and the magnet and the end P is induced to become the North pole and the direction of the current is from A to B.

The work done is converted into Electrical

energy which creates the induced current.

To determine the direction of the induced current in the dynamo – Fleming’s Right-hand rule

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Fleming’s Right-hand Rule states” If the thumb, first finger (forefinger)and second finger of the right hand are held at right angles to each other, then if the first finger (forefinger)represents the direction of the magnetic field and the thumb represents the direction of the motion of the conductor , then the second finger will represents the direction of the induced current ”

For example:

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When coil WXYZ is rotated in anticlockwise direction the direction of the induced current is

From X to Y.

The experiment to investigate the relationship between the magnitude of an induced current and the speed of the relative motion between conductor and magnet

Hypothesis:

The magnitude of an induced current increases as the speed of the relative motion between conductor and magnet increases.

Aim of the experiment :

To investigate the relationship between the magnitude of an induced current and the speed of the relative motion between conductor and magnet

Variables in the experiment:

Manipulated variable: the magnitude of induced current

Responding variable: the speed of the relative motion between conductor and magnet.

Fixed variable: number of turns of solenoid and the strength of the magnet.

List of apparatus and materials:

Sensitive zero-centre galvanometer, solenoid, bar magnet and ruler.

Arrangement of the apparatus:

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The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

The height of the magnet above the solenoid is measured by a ruler = H

The magnet is dropped into the solenoid and the reading of the galvanometer is recorded = I

The experiment is repeated 5 times with different height of the magnet above the solenoid.

Tabulate the data:

H

           

I

           

Analysis the data:

Plot the graph I against H

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Applications of Electromagnetic Induction

(1) d.c. and a.c. generator

(2) transformer

D.c. generator.

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Initially , the coil in a horizontal plane (0o) , the cutting of the magnetic field is maximum and hence the induced current is maximum.

After a 90o rotation , the sides of the coil parallel to the magnetic fields. There is no cutting of the magnetic field lines. The induced current is zero.

After a 180o rotation, the coil is horizontal again and the maximum induced current is produced. But the commutators reverses contact with carbon brushes and the current flows in same direction as before.

After a 270o rotation , the sides of the coil parallel to the magnetic fields again and the induced current is zero again.

the magnetic fields. There is no cutting of the magnetic field lines. The induced current is zero.

The following graph shows the variation of the induced current, I against the angle turned through by the coil.

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A.c. generator.

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Initially , the coil in a horizontal plane (0o) , the cutting of the magnetic field is maximum and hence the induced current is maximum.

After a 90o rotation , the sides of the coil parallel to the magnetic fields. There is no cutting of the magnetic field lines. The induced current is zero.

After a 180o rotation, the coil is horizontal again and the maximum induced current is produced but the current flows in opposite direction as before.

After a 270o rotation , the sides of the coil parallel to the magnetic fields again and the induced current is zero again.

the magnetic fields. There is no cutting of the magnetic field lines. The induced current is zero.

The following graph shows the variation of the induced current, I against the angle turned through by the coil.

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