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8.1 Electromagnet

Ferromagnetic materials

Ferromagnetic materials are the materials attracted to the magnet. The examples of the ferromagnetic materials such as iron, nickel and cobalt.

Electromagnet

An electromagnet is a device in which magnetism is produced by an electric current.

An electromagnet acts as a temporary magnet.

Magnetic field

A magnetic field is a region in which a magnetic material experiences a force as the result of the present of a magnet or a electromagnet.

The direction of a magnetic field is from north pole to south pole.

Magnetic Effects of a current –carrying Conductor.

The pattern of magnetic field produced by a current – carrying conductor depends on the shape of the conductor used.

(a) Current in a straight wire

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The magnetic field lines form a pattern of concentric circles round the wire carrying the current.

The direction of the field around a wire can use two rules:

Maxwell’s screw rule.

If a right -handed screw is turned so that it moves forwards in the same direction as a electric current, its direction of rotation gives the direction of the magnetic field due to the current.

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The right -hand grip rule.

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If a wire carrying a current is gripped with the right hand and with the thumb pointing along the wire in the direction of the current, the fingers point in the direction of the magnetic field around the wire.

The map of the magnetic field due to an electric current flowing in a vertical wire through centre of a horizontal card.

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The direction of the electric field and the plotting compass

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(b) Current in a single coil

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The pattern of the magnetic field produced at the centre of the coil is in the form of straight line.

The line in the coils are more closed spaced tab the outside.

The field lines around the two wires are in opposite directions.

The Maxwell’s screw rule or The right -hand grip rule can also be used to both sides of the coil to determine the direction of the magnetic field.

(c) Current in a solenoid

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The pattern of the magnetic field produced by a solenoid is similar to that of a bar magnet.

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The pattern in the centre of the solenoid is in the form of straight lines.

However , the field lines are uneven and more widely spaced outside the solenoid.

We can use two methods to determine the direction of the magnetic field:

The right -hand grip rule.

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If a solenoid carrying a current is gripped with the right hand and with the thumb pointing along the solenoid so that the fingers curling round the solenoid in the direction of the current and the thumb then points towards the north pole.

By determine the poles of the solenoid.

Looking at the direction of the current from each end of the of the solenoid.

If the direction of the current is clockwise, the viewed end of the solenoid is the south pole.

If the direction of the current is anticlockwise , the viewed end is the north pole.

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Factors Affecting the strength of the magnetic field of an electromagnet

(1) Current

When the current is increased , the strength of the magnetic field increases.

(2) Number of turns of wire

When the number of turns of wire is increased , the strength of the magnetic field increases.

(3) Type of the material of the core

Different material of the core have the different strength of the magnetic field.

Soft iron core has the strongest magnetic field.

A soft iron core is able to concentrate magnetic field lines through it.

(4) The shape of the iron core

U-shaped iron core has more stronger magnetic field than the straight iron core because U-shaped iron core has two opposite pole s adjacent to each other and as a result the magnetic field lines are concentrated in space between the poles.

Comparison soft iron core and steel core.


Soft iron core

Steel core

Easily magnetised and demagnetised

Hardly magnetised and demagnetised

Suitable to make a temporary magnet (electromagnet)

Suitable to make a permanent magnet

The experiment to investigate the relationship between the strength of an electromagnet and the current.

Hypothesis:

The strength of an electromagnet increases as the current increases.

Aim of the experiment :

To investigate the relationship between the strength of an electromagnet and the current.

Variables in the experiment:

Manipulated variable: the current

Responding variable: the strength of an

electromagnet

Fixed variable: number of turns of solenoid, type of core.

List of apparatus and materials:

Ammeter, connection wires, rheostat, retort stand, ,switch, d.c. supply , soft iron core, solenoid, small iron nails and plastic container.

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 switch is closed.

The reading of the ammeter is recorded = I

The end of the solenoid is dipped into the plastic container full of small iron nails.

The plastic container is removed and the number of nails attached to the electromagnet is counted = N

The experiment is repeated 5 times with different value of current by adjusting the rheostat.

Tabulate the data:

I

           

N

           

Analysis the data:

Plot the graph N against l

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The experiment to investigate the relationship between the strength of an electromagnet and the number of turns of solenoid

Hypothesis:

The strength of an electromagnet increases as the number of turns of solenoid increases.

Aim of the experiment :

To investigate the relationship between the strength of an electromagnet and the number of turns of solenoid.

Variables in the experiment:

Manipulated variable: the number of turns of solenoid

Responding variable: the strength of an

electromagnet

Fixed variable: the current, type of core.

List of apparatus and materials:

Ammeter, connection wires, rheostat, retort stand, ,switch, d.c. supply , soft iron core, solenoid, small iron nails and plastic container.

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 number of turns of solenoid is recorded = M

The switch is closed.

The end of the solenoid is dipped into the plastic container full of small iron nails.

The plastic container is removed and the number of nails attached to the electromagnet is counted = N

The experiment is repeated 5 times with different value of the number of turns of solenoid

Tabulate the data:

n

           

N

           

Analysis the data:

Plot the graph N against l

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Uses of electromagnets

The electric bell

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When the switch is pressed, the current flows through the solenoid and the iron core is magnetized.

The soft iron armature attracted to the electromagnet.

The hammer strikes the gong ,thus ringing the bell.

The spring contact moves away from the contact adjusting screw and breaks the circuit.

The iron core loses its electromagnetism .

The spring contact touches the screw again, remakes the circuit and the switches on the current.

The cycle repeats rapidly as long as the switch is pressed.

The circuit breaker

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A circuit breaker which does the same job as fuse wire designed to disconnect the power supply when there is too large current flowing through a circuit.

When the current get too high, the magnetic fields force become strong to pull the soft iron armature and release the catch. The contacts are separated and the current stopped.

The spring P keeps the contacts apart.

When the reset button is pressed , spring Q pulls the soft iron armature back to its original position.

The electromagnetic relay

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An electromagnetic relay is a switching device is used to control (switch on or off) another powerful circuit (high –voltage or dangerous supply).

When the switch is on, the small current flows through the solenoid and the soft iron core is magnetised.

The soft iron armature attracted to the electromagnet.

This action makes the other end of the soft iron armature to push and closes the contact.

When the switch is off , no current flows and the iron core loses its magnetism. The soft iron armature returns to its original position and the powerful circuit is open.

The telephone ear-pieces

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When a person speaks through a telephone mouthpiece , sound energy is changed to a varying electric current.

When the varying electric current flows the soft iron core is magnetized with varying strength.

The electromagnet pull on the diaphragm.

The diaphragm vibrates with the same frequency as the varying current.

The vibration of the diaphragm causes a series of compressions and rarefactions of the air.

The sound waves is produced with the same frequency as the original sound

The other uses of electromagnet

1. Electromagnetic lifting machine

2. To remove steel splinters which have accidentally entered a patient’s eye.

3. Electronic card

4. Parking machine

5. Magnetic Levitated Train (MAGLEV train)

6. Tape recorder

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