1. Magnet And Types Of Magnet -
ü Material which attract materials like iron, cobalt and nickel are called as a magnet.
ü The Greek shepherd was the first person to discover mineral which act as magnet.
ü It is a naturally occurring mineral named magnetite.
ü The magnets are obtained naturally from a Magnetite rock, are called as the natural magnets.
ü And those magnets prepared by the combination of certain mineral ores are called as the artificial magnets.
ü Iron, nickel and cobalt are some examples of magnetic materials. These metallic materials easily attract a magnet.
ü Aluminium, zinc, wood, and rubber are called the non-magnetic materials. These materials are not attracted towards the magnet at all.
ü There are different types of magnets and are classified based on their shapes. The different types of magnets include – bar magnet, dumb-bell shaped magnet, horseshoe magnet, cylindrical magnet, etc.
ü Magnets that retain their magnetic properties only for a short period of time are called temporary magnets.
ü Magnets that retain their magnetic properties for a long period of time are called permanent magnets.
2. Magnets For Finding Direction -
ü Lode stone- It is a stone used by sailors in old days to identify directions in sea.
ü Compass- This is a small glass case containing a magnetized needle pivoted on a nail. The needle can rotate freely.Wherever it is kept, its needle always rests in north-south direction. Normally the north-pole of the needle is painted red or some other indication is given to identify north and south-poles. So using this needle, north and south can be identified.
3. Poles Of a Magnet -
ü When magnetic materials (like iron filings) are brought close to a magnet, they do not stick evenly to all parts of the magnet.
ü They stick more on certain parts of the magnet. These are called the poles of the magnet.
ü Magnetic forces are the strongest at the poles. For example, the two ends of a bar magnet are its poles.
ü There are two types of poles in every magnet, irrespective of its shape. These are, by convention, called the North Pole (N) and the South Pole (S).
ü North pole: When suspended freely, one pole of the magnet always points towards north. This is known as north-pole.
ü South pole: When suspended freely, the end of a magnet points towards south. This is known as south-pole.
ü The two poles cannot exist independently. That is, they always come in pairs.
ü If we break a bar magnet in the middle, we would get two pieces, each having a North Pole and a South Pole.
4. Attraction And Repulsion -
ü When two magnets are brought close to each other, they are either pulled towards each other, or pushed away from each other.
ü When the magnets are pulled towards each other, they are said to attract each other.
ü When they are pushed away from each other, they are said to repel each other.
ü Whether the magnets attract or repel depends on which poles of the magnets are facing each other.
ü When like poles of the magnets (N-N or S-S) are brought close to each other, they repel. This is called repulsion.
ü When unlike poles of the magnets (N-S or S-N) are brought close to each other, they attract. This is called attraction.
5. Care Of Magnets -
ü A magnet can lose its properties due to the following activities - Dropping from a height, hitting with a hammer, applying heat and improper storage can also cause loss of magnetic properties.
6. Uses of Magnet -
ü Credit cards, ATM cards, and identity cards have a strip of magnetic material that stores information.
ü Television and computer monitors use magnets.
ü Computer hard discs and audio and video cassettes have magnetic material that store information.
ü Magnets are used in picking up substances made of iron from scrapyard.
NCERT PRACTICE QUESTIONS
1. Fill in the blanks in the following -
(i) Artificial magnets are made in different shapes such as __________, __________ and ____________.
(ii) The Materials which are attracted towards a magnet are called________.
(iii) Paper is not a ______ material.
(iv) In olden days, sailors used to find direction by suspending a piece of ___________.
(v) A magnet always has __________ poles.
Ans. - (i) bar magnet, horse shoe and cylindrical. (ii) magnetic. (iii) magnetic (iv) magnet. (v) two
2. State whether the following statements are true or false -
(i) A cylindrical magnet has only one pole.
(ii) Artificial magnets were discovered in Greece.
(iii) Similar poles of a magnet repel each other.
(iv) Maximum iron filings stick in the middle of a bar magnet when it is brought near them.
(v) Bar magnets always point towards North-South direction.
(vi) A compass can be used to find East-West direction at any place.
(vii) Rubber is a magnetic material.
Ans. - i) False ii) False iii) True iv) False v) True vi) True vii) False
3. It was observed that a pencil sharpener gets attracted by both the poles of a magnet although its body is made of plastic. Name a material that might have been used to make some part of it.
Ans. - Iron, nickel or cobalt might have been used to make some part of it like the blade or screw that holds the blade.
5. Write any two properties of a magnet.
Ans. - It attracts material made of Iron, Nickel and Cobalt. Freely suspended magnet points in north - south direction.
6. Where are poles of a bar magnet located?
Ans. - On two ends of the bar magnet.
7. A bar magnet has no markings to indicate its poles. How would you find out near which end is its north pole is located?
Ans. - A bar magnet is hanged in the air with a thread and the end pointing to the geographical north is the north pole of the magnet.
8. You are given an iron strip. How will you make it into a magnet?
Ans. - Take a bar magnet and keep in contact with one of its poles with one edge of the bar of iron. Without lifting the bar magnet, move it along the length of the iron bar till you reach the other end.Lift the magnet and bring the pole (the same pole you started with) to the same point of the iron bar from which we began. Move the magnet again along the iron bar in the same direction as you did before. Repeat this process for about 30-40 times.
9. How is a compass used to find directions?
Ans. - Compass - This is a small glass case containing a magnetized needle pivoted on a nail. The needle can rotate freely.Wherever it is kept, its needle always rests in north-south direction. Normally the north-pole of the needle is painted red or some other indication is given to identify north and south-poles. So using this needle, north and south can be identified.
10. A magnet was brought from different directions towards a toy boat that has been floating in water in a tub. Affect observed in each case is stated in Column I. Possible reasons for the observed effects are mentioned in Column II. Match the statements given in Column I with those in Column II.
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