ICSE Class 8 Physics Chapter 13 Electromagnet and Electromagnetic Induction

Chapter 13: Electromagnet And Electromagnetic Induction

An Electromagnet

An electromagnet consists of a soft iron piece on which an insulated copper wire is wound or an electromagnet is a current carrying coil of insulated wires wrapped around a piece of iron.

The arrangement behaves like a magnet when an electric current is passed through the insulated wire and it loses its magnetism when current is stopped.

An electromagnet is a temporary magnet. The soft iron piece used in it is called a core. An insulated wire is an ordinary wire coated with some insulating material such as: varnish, rubber, plastic, etc. The insulation of the wire prevents the different turns of the wire from coming in contact with each other.

Making Electromagnets

In general, the electromagnets are of two shapes:

1. Bar magnet

2. U-shaped or horse-shoe type magnet.

Bar Magnet

In a bar shaped electromagnet, an insulated copper wire is wound over a soft iron bar. The two ends of the wire used are connected to a source of electricity through a key as shown in Fig 13.2.

When key is pressed, current flows through the wire and the system starts behaving like a magnet. Here, the soft iron bar behaves like a bar magnet. The end of this electromagnet at which current is in the clockwise direction behaves as a south pole and the other end of it at which the current is in the anticlockwise direction behaves as a north pole.

When key is opened, it stops the flow of current through the coil, with the result, the electromagnet demagnetises.

U-shaped Magnet

In a U-shaped electromagnet, an insulated copper wire is wound over a U-shaped soft iron core. The two ends of the wire used are connected to a source of electricity through a key as shown in Fig. 13.3.

When key is pressed, current flows through the coil and it starts behaving like a magnet. On stopping the current, it will get demagnetised.

In case of a U-shaped magnet also, the end of the electromagnet at which current is in the clockwise direction behaves as a south pole and the other end of it at which current is flowing in the anticlockwise direction behaves as a north pole.

The strength of an electromagnet depends on:

(i) The Number Of Turns In Its Coil: A stronger electromagnet will be obtained on increasing the number of turns in its coil and on decreasing the number of turns, a weaker electromagnet is obtained.

(ii) The Amount Of Current Passed Through The Coil: On increasing the amount of current in the coil, a stronger electromagnet is obtained and on decreasing the amount of current in the coil, a weaker electromagnet is obtained.

Teacher's Note

Electromagnets are used in everyday devices like electric bells and door locks, demonstrating how electricity and magnetism work together to control mechanical motion.

Activity 1

To make an electromagnet.

- Take an iron nail of about 6-10 cm in length and wind an insulated copper wire on it as shown in Fig 13.4.

- Now connect the ends of the copper wire to the two terminals of a dry cell via a switch.

- Switch on the current through the circuit and bring a few pins near the wounded nail.

- Now switch off the current and see what happens.

The iron nail behaves like a magnet as long as current flows through the circuit. The pins cling to the nail when the switch is 'on' while they drop as soon as the switch disconnects the electric circuit.

You can see an enhanced magnetic field (more pins cling to the nail) if you use a battery in place of a cell.

Uses Of Electromagnets

Electromagnets are used at different places in our daily life.

- In electrical appliances such as electric bell, electric fan, electric motors, etc.

- In lifting heavy loads of iron scrap.

- To remove tiny particles of iron from wound.

- In loading furnaces with iron.

- In the separation of iron ores (magnetic substances) from impurities (non-magnetic substances).

- Electromagnets are also used in medical science to cure certain ailments.

- They are used for the preparation of strong, permanent magnets.

Teacher's Note

From medical devices to recycling centers, electromagnets enable modern technology to manipulate materials and power essential services we depend on daily.

Difference Between Permanent Magnet And Temporary Magnet/Electromagnet

An electromagnet is not a permanent magnet. The soft iron core demagnetises as soon as electric current stops.

Several turns of wire around a nail makes a coil; when electric current passes through the coil, a magnetic field is induced in the coil.

Electric Bell

An electric bell involves one of the most common use of an electromagnet.

Figure 13.5 shows the essential components of an electric bell. It works on the principle of magnetic effect of current. When the switch is pressed, the circuit gets closed and the current starts flowing through the U-shaped electromagnet. The core turns into an electromagnet and attracts the armature made of iron. Due to the movement of the armature towards the electromagnet, the hammer strikes the gong and the bell rings. As the armature moves towards the electromagnet, its contact with adjustment screw is broken which breaks the circuit and stops the current flowing through the coil of the electromagnet. The electromagnetism is lost by the core and hence due to its spring nature, the armature returns back to its original position. This completes the circuit once again and the action is repeated. This making and breaking of the circuit of the electromagnet continues as long as the bell switch remains pressed.

Magnetic Field Associated With A Straight Current Carrying Conductor

In 1820, a Dutch scientist, Hans Christian Oersted discovered that if an electric current is passed through a conductor, a magnetic field is developed around it. This brought the concept that electricity and magnetism are not separate entities but are closely associated with each other. When electricity flows through a conductor, a magnetic field is produced around it.

The study of magnetic effects produced due to electric current is known as electromagnetism.

Following are the two experiments to show the magnetic effects of current.

Experiment - 1

Take an insulated copper wire and place it in such a way that the wire length AB of it is along the North-South direction. Connect the two ends of the wire to a battery through a key. Now, if we place a compass needle close and parallel to the wire AB, we see that the magnetic needle rests in north-south direction only (Fig. 13.6(a)). But on closing the key, we find that the magnetic needle gets deflected from its North-South direction (Fig. 13.6(b)).

When the key is open, no current flows through the wire and hence the needle rests in its original position in the north-south direction. But on closing the key, the magnetic needle gets deflected from its original position. This experiment shows that flow of electricity produces magnetic field around it. The magnetic field so produced is represented by a well defined pattern of magnetic lines of forces which can be observed in the following experiment.

Experiment - 2

Take a plane cardboard and fix it in a horizontal position. Make a small hole at its centre. Through the hole, pass a vertical wire. Connect a battery and a key to the wire. Spread some iron filings on the cardboard and also place a small compass needle any where on the board.

Press the key and tap the board. You will notice two things. Firstly, the iron filings will get set into a definite pattern. Secondly, the compass needle is deflected in a particular direction.

Now reverse the terminal connections of the battery. You will notice that the direction shown by the magnetic needle of the compass gets reversed. From this, we conclude that:

- An electric field is set up around a wire as long as current flows through it.

- If electric current flows downwards, the direction of magnetic field is clockwise (Fig. 13.7(a)) from above.

- If electric current flows upwards, the direction of magnetic field is anticlockwise (Fig. 13.7(b)) from above.

- Magnetic lines of forces are more concentrated near the current carrying wire.

It is important to use electrical appliances with ISI mark on them. ISI mark ensures the safety of the appliance and also minimum electrical energy wastage.

Methods To Find The Direction Of Magnetic Field

The direction of magnetic field due to a straight current carrying conductor is obtained by any of the two right hand rules shown below:

(1) Right Hand Thumb Rule: When we hold current carrying straight conductor in the right hand in such a way that the thumb points in the direction of the current, then the direction of fingers holding that conductor gives the direction of magnetic lines as shown in Fig. 13.8. In this case, the direction of current in the conductor is upwards. The magnetic lines around the conductor are anticlockwise (from above) around the conductor as shown in the figure.

(2) Right Hand Cork Screw Rule: Hold a cork screw in your right hand and rotate its handle in such a way that the screw moves in the direction of current flowing in the straight conductor. The direction in which the thumb rotates gives the direction of magnetic lines of forces, as shown in Fig. 13.9. The direction of current in the conductor is towards the right and the magnetic field lines are clockwise, as observed from the side of holding the screw.

Michael Faraday (1791-1861) was an English chemist and physicist who contributed to the field of electromagnetism and electrochemistry.

Solenoid

A solenoid is a long coil made of insulated copper wire wound on a cylindrical card board or a soft iron core. The diameter of the cylindrical card board is smaller as compared to its length. It infact is the same as an electromagnet, without current.

Magnetic Field Due To A Current Carrying Solenoid

The magnetic field developed due to current in the coil of a solenoid is same as in the case of an electromagnet. The magnetic polarities at the ends of a solenoid depend on the direction of the current in its coil and is determined by the clock rule described below:

Clock Rule: The end of the solenoid where the direction of current is anticlockwise, becomes the north pole and the end where the direction of current is clockwise, becomes the south pole. Figure 13.10 will help in understanding the flow of direction.

In order to understand this concept with more clarity, see Fig. 13.2 and Fig. 13.3.

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