Maharashtra Board Class 7 Science Chapter 19 Properties of a Magnetic Field PDF Download

Properties Of A Magnetic Field

Let's Recall

1. Where and how are magnets used in our houses and our surroundings?

2. In which direction does a freely suspended magnet settle?

3. What are the names given to the two ends of a magnet? Why are they named thus?

4. Which metals are used for making magnets?

5. What are the characteristics of magnets?

Magnets are made from alloys of iron, cobalt and nickel. Nipermag, an alloy of iron, nickel, aluminium and titanium is used to make magnets. We have also learnt that alnico is a magnetic alloy of aluminium, nickel and cobalt.

Magnetism

Try This

Apparatus: Steel bar, bar magnet, iron fillings, thread, etc.

Procedure: Place a steel bar AB, on a table. Take a bar magnet. Place its 'N' pole on the 'A' end of the steel bar and drag it towards the 'B' end. Lift the bar magnet and drag its 'N' pole from the end A to the B of the steel bar again. Repeat this 15 to 20 times. Now take the steel bar near some iron filings and observe what happens. Hang the bar freely by a thread and observe.

The steel bar will be seen to have developed magnetism. This method of magnetisation is called the single touch method. The magnetism created by this method is of low strength and lasts for a short time.

Procedure: Place a steel bar on a table. Take two bar magnets. Place two opposite poles of the two bar magnets at the centre of the steel bar. Drag these poles apart, one to the 'A' end of the steel bar, and the other to the 'B' end.

Repeat this 15 to 20 times. Now take the steel bar near iron filings and observe. Hang the steel bar freely and observe.

This method of magnetisation is called the double-touch method. The magnetism generated by this method lasts longer compared to that generated by the single touch method.

Teacher's Note

Magnets are very useful in our daily life. Your mother uses magnets to close the cupboard door or refrigerator door easily.

Exam Trick

Remember: Single touch method = weak magnet that loses power quickly. Double touch method = strong magnet that lasts longer. Always use double touch method to make better magnets.

Points to Remember

Magnets are made from iron, cobalt, and nickel alloys.
Single touch method creates weak magnetism that does not last long.
Double touch method creates stronger magnetism that lasts longer.
Both methods magnetise a steel bar by making its atoms align in one direction.

Can You Tell?

Why does a freely suspended magnet always settle in the north-south direction?

Earth: A Gigantic Magnet

The scientist William Gilbert gave a scientific explanation, based on experiment, of the observation that a freely suspended magnet always settles in the north-south direction only.

He gave a round shape to a naturally occurring magnetic rock. He suspended this spherical magnet so that it could turn freely and brought the north pole of a bar magnet near it. The south pole of the magnetic sphere was attracted towards it.

The north pole of a freely suspended magnet settles in the direction of the geographic north pole of the earth. It means that the south pole of some gigantic magnet must be near the geographic north pole of the earth and the north pole of that magnet, near the geographic south pole of the earth. Gilbert inferred from this that the earth itself is a gigantic magnet. However, the south pole of this magnet must be near the geographic north pole of the earth while the magnetic north pole is near the geographic south pole.

Can You Tell?

1. Which magnetic poles attract each other?

2. Which pole of a spherical magnet will get attracted towards the south pole of the bar magnet?

Teacher's Note

The Earth itself acts like a big magnet. This is why your compass always points north. India is between the magnetic north and south of the Earth.

Exam Trick

Remember: Earth is a magnet with south pole near geographic north and north pole near geographic south. This is opposite! The word "opposite" helps you remember this.

Points to Remember

Earth is a giant magnet.
The south pole of Earth's magnet is near the geographic north pole.
The north pole of Earth's magnet is near the geographic south pole.
A freely suspended magnet always points north-south because of Earth's magnetism.

Use Your Brain Power!

Which direction will a magnetic needle show on the geographic north pole?

Magnetic Needle

Take a square cardboard and mark the directions on it. Place a pot filled with water at the centre of the cardboard. Take a magnetized needle. Stick it to a small piece of a cardboard by means of a sticking tape. Place the piece of cardboard with the needle, on the surface of water in the pot. In which direction does the magnetized needle point?

Find Out

In any place, why does the magnetic needle of a compass not settle parallel to the ground but at a an angle to it?

Magnetic Field

Try This

Apparatus: A bar magnet, pins, cardboard, iron filings, plastic bottle, bucket, water.

Procedure: Take a bar magnet and some pins. Place them at such a distance from each other that they do not stick to each other. Now slowly move the magnet towards the pins. Observe the pins as they get pulled to the magnet.

The magnet attracts the needles from afar. In other words, a magnet has an effect even at a distance.

Procedure: Take a small cardboard. Place a bar magnet at its centre. Sprinkle iron filings on the cardboard around the magnet. Tap the cardboard gently. Observe the iron filings.

What is the inference from the above experiments? The British researcher Michael Faraday named these lines, going from one end of the bar magnet to the other, 'magnetic lines of force'. The region around a magnet where the magnetic force acts on an object is called a magnetic field. The magnetic field around a magnet can be shown by means of magnetic lines of force. The intensity of the magnetic field at a place can be guaged by the number of lines of force that pass through a unit area at that place, perpendicular to that area. Michael Faraday, imagined that there might be invisible lines of force going from one pole of a magnet to the other, and that magnetic attraction or repulsion might be taking place through the medium of these lines of force. If Faraday's idea is accepted, the intensity of the magnetic field can be obtained from the number of lines of force, as explained above.

The intensity of a magnetic field is low where the lines of force are sparse, and the intensity is high where the lines of force are concentrated.

Use Your Brain Power!

Is magnetic force a vector or a scalar quantity?

Properties Of Magnetic Lines Of Force

While proposing the concept of lines of force, Michael Faraday argued that, if all observed effects are to be explained satisfactorily, then the lines of force must have certain properties.

Teacher's Note

Magnetic field lines are invisible but they are very real. You can see their effect when iron filings arrange in a pattern around a magnet, just like how the wind moves leaves on the ground.

Exam Trick

Remember: Lines of force always go from North to South pole, like water flowing downhill. They never cross each other, just like two roads cannot cross at the same point.

Points to Remember

Magnetic lines of force are imaginary lines showing the path of magnetism.
These lines always run from north pole to south pole.
The lines are closer together where the magnetic field is strong.
Magnetic lines of force never cross or intersect each other.

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