6 MAGNETISM I. Tick ( ) the most appropriate answer. 1. When a bar magnet is suspended freely, it comes to rest : (a) east-west direction (b) north-east direction (c) north-south direction (d) north-west direction 2. A bar magnet is cut into four pieces. Each piece is : (a) complete magnet (b) some pieces have only north pole (c) some pieces have only south pole (d) all pieces lose magnetism 3. A bar of steel can be permanently magnetised by : (a) rubbing a bar magnet with it along the length (b) rubbing a bar magnet at its ends (c) rubbing a bar magnet at its centre (d) none of these 4. Similar poles of bar magnets : (a) attract each other (b) repel each other (c) neither attract nor repel each other (d) none of these 5. Lodestone is an example of : (a) artificial magnet (b) electromagnet (c) natural magnet (d) none of these 6. An artificial magnet which is used for finding geographical directions is : (a) bar magnet (b) electromagnet (c) horseshoe magnet (d) magnetic compass 7. The surest test for magnetism is : (a) attraction (b) repulsion (c) both attraction and repulsion (d) none of these 8. Very strong magnets are : (a) natural magnets (b) permanent magnets (c) electromagnets (d) magnetic needle 9. Making a nail behave like a magnet by bringing a magnet near it but not touching it is : (a) single-touch method (b) magnetic induction method (c) double-touch method (d) none of these 1
10. The attraction of iron filings by the poles of a magnet is : (a) maximum (b) zero (c) medium (d) minimum 11. In a magnet : (a) north pole is stronger than south pole (b) south pole is stronger than north pole (c) both the poles are of equal strength (d) none of these 12. The earth behaves like a huge bar magnet with its magnetic north pole situated near the : (a) geographical north pole (b) geographical west pole (c) geographical south pole (d) geographical east pole 13. Ferrite magnets are made up of a mixture of : (a) iron oxide and aluminium oxide (b) iron oxide and barium oxide (c) iron oxide and calcium oxide (d) potassium oxide and barium oxide 14. Which of the following is an example of a natural magnet? (a) Lodestone (b) Iron (c) Steel (d) Cbalt 15. Artificial magnet used in the laboratory to detect the direction is a : (a) horseshoe magnet (b) U-shaped magnet (c) bar magnet (d) magnetic needle 16. In an unmagnetized substance, the molecular magnet forms : (a) open chains (b) straight chains (c) closed chains (d) none of these 17. A permanent magnet : (a) attracts all substances (b) attract only magnetic substances (c) repels all substances (d) none of these 18. Which of the following is a magnetic substance? (a) Cobalt (b) Aluminium (c) Copper (d) Wood 19. Temporary magnets are made up of : (a) steel (b) copper (c) soft iron (d) aluminium 20. Permanent magnets are made up of : (a) steel (b) copper (c) soft iron (d) aluminium 21. Presence of magnetism in a magnet, is due to : (a) basic structural property 2 2
(b) due to molecular arrangement in the form of a closed chain (c) due to molecular arrangement in a particular order (d) none of these 22. Effective length of a magnet is : (a) equal to its geometric length (b) greater than its geometric length (c) smaller than its geometric length (d) either greater or smaller depending on the nature of the material 23. Which of the following is a non-magnetic substance? (a) Iron (b) Steel (c) Nickel (d) Copper Ans. 1. (c) 2. (a) 3. (a) 4. (b) 5. (c) 6. (d) 7. (b) 8. (c) 9. (b) 10. (a) 11. (a) 12. (c) 13. (a) 14. (a) 15. (d) 16. (c) 17. (b) 18. (a) 19. (c) 20. (a) 21. (c) 22. (c) 23. (d). II. Fill in the blank spaces using correct words from the list given below [List: magnetised, natural, broken, powerful, magnet, surest magnetic, electromagnets, telegraph.] 1. Artificial magnets are preferred to magnets because they are more. 2. A substance which is strongly attracted by a is called substance. 3. are used in telephones and. 4. Repulsion is the test of magnetism. 5. A piece of iron can be by rubbing a bar magnet over it. 6. When a magnet is every piece is a complete magnet. Ans. 1. natural, powerful 2. magnet, magnetic 3. Electromagnets, telegraph 4. surest 5. magnetised 6. broken. III. Fill in the blanks. 1. The Chinese sailors used natural magnets as. 2. Magnet is a term derived from. 3. Hammering a magnet violently destroys its property. 4. Temporary magnets are usually made of. 5. Magnets are used in the construction of certain toys to give like effect. 3
6. A freely floating magnet points in the direction. 7. In a magnet, have the greatest magnetic force. 8. is a sure test of magnetism. 9. There is a very large around the earth. 10. Artificial magnets can be made into different. 11. Effective length of a bar magnet is always than geometric length. 12. A freely suspended bar magnet always points towards and. 13. Similar poles each other. 14. Lodestone is a magnet. 15. Electromagnet is an magnet. 16. The imaginary line joining north and south pole is called magnetic. Ans. 1. compass 2. magnetite 3. magnetic 4. soft iron 5. magic 6. north-south 7. poles 8. Repulsion 9. magnetic field 10. shapes and sizes 11. less 12. north and south 13. repel 14. natural 15. artificial 16. lines. IV. State whether the following statements are true or false (T/F). 1. If a substance is attracted by a magnet, then the substance is also a magnet. 2. Natural magnets are not used because they have weak magnetic force. 3. Magnetic force can easily pass through non-magnetic substances. 4. A mixture of iron and brass powder can be separated with magnet. 5. Electromagnets can be prepared by passing current around any metal. 6. Repulsion is the surest test of magnetism. Ans. 1. F 2. T 3. T 4. T 5. F 6. T. V. Classify the following into magnetic and non-magnetic substances: Wood, iron, copper, air, brass spoon, plastic ruler, soap cake, water paper, button, cobalt, nickel, aluminium. 4 4
Ans. Magnetic substances: Iron, cobalt, nickel. Non-magnetic substances: Wood, copper, air, brass spoon, plastic ruler, soap cake, water, paper, button, aluminium. VI. Statements given below are incorrect. Write the correct statements. 1. Natural magnets are stronger than artificial magnets. Ans. Artificial magnets are stronger than natural magnets. 2. A mixture of iron and aluminium powder can not be separated with a magnet. Ans. A mixture of iron and aluminium powder can be separated with a magnet. 3. Attraction is the surest test of magnetism. Ans. Repulsion is the surest test of magnetism. 4. Alnico is used in the coils of transistors. Ans. Magnadur is used in the coils of transistors. 5. For magnetising a magnetic substance, single-touch method is better than double-touch method. Ans. For magnetising a magnetic substance, double touch method is better than single-touch method. 6. A magnet has no effect even if it is heated to a very high temperature. Ans. A magnet gets demagnetised if it is heated to a very high temperature. 7. Permanent magnets lose their magnetism after the magnetising force is removed. Ans. Temporary magnets lose their magnetism after the magnetising force is removed. 8. Magnetic poles always occur in single. Ans. Magnetic poles always occur in pairs. 9. The north pole of a freely suspended bar magnet will always face towards geographic south. Ans. The north pole of a freely suspended bar magnet will always face towards geographic north. 10. Iron is a non-magnetic substance. Ans. Iron is a magnetic substance. 5
11. Wood is a magnetic substance. Ans. Wood is a non-magnetic substance. 12. Similar poles attract each other. Ans. Similar poles repel each other. 13. An electromagnet is a permanent magnet. Ans. An electromagnet is a temporary magnet. 14. Aluminium is a magnetic substance. Ans. Aluminium is a non-magnetic substance. 15. Steel is a non-magnetic substance. Ans. Steel is a magnetic substance. 16. Unlike poles of magnets repel each other. Ans. Unlike poles of magnets attract each other. 17. Electromagnets are used for removing brass pieces from a scrap of brass and copper. Ans. Electromagnets are used for removing iron pieces from a scrap of iron and copper. 18. When a soft iron rod is placed in coil carrying current, it changes to permanent magnet. Ans. When a soft iron rod is placed in coil carrying current, it changes to temporary magnet. VII. Match the following Column A Column B 1. Iron, cobalt, nickel (a) Natural magnet 2. Wood, air, water (b) Electromagnet 3. Used in large computers (c) Non-magnetic substances 4. Used in electric bell (d) Magnetic substances 5. Lodestone (e) Ceramic magnet 6. Soft iron (f) Magnetic needle 7. Steel (g) Alnico 8. Magnet made from a mixture (h) Ferrite magnet of iron oxide and barium oxide 9. Magnet made from a mixture (i) Magnetic toys of aluminium, nickel and cobalt Ans. 1. (d) 2. (c) 3. (e) 4. (b) 5. (a) 6. (f) 7. (i) 8. (h) 9. (g). 6 6
VIII. Find the odd-one out. Give reasons for your choice. 1. Plastic, brass, iron, copper, zinc Ans. Iron. Only iron is magnetic substance but other are nonmagnetic substance. 2. Iron, nickel, cobalt, aluminium Ans. Aluminium. Only aluminium is non-magnetic substance but other are magnetic substance. 3. Magnetic induction, magnetic separation, single-touch method, double-touch method. Ans. Magnetic separation. Except-magnetic separation, other are methods for magnetising a magnetic substance. 4. Horseshoe magnet, U-shaped magnet, bar magnet, lodestone, cylindrical magnet. Ans. Lodestone. Only lodestone is a natural magnet but other are artificial magnets. 5. Steel, copper, aluminium, brass. Ans. Steel. Only steel is magnetic substance but other are nonmagnetic substance. IX. Give reasons for the following. 1. Artificial magnets are more used than natural magnets. Ans. As artificial magnets are more powerful and can be made in various shapes and sizes hence, artificial magnets are more used than natural magnets. 2. A freely suspended magnet comes to rest in the north-south direction. Ans. It is because the earth itself behaves like a magnet such that the north pole of the earth s magnet is pointing at the geographic south and the south pole pointing at the geographic north. 3. It is impossible to obtain a piece of magnet having only one magnetic pole. Ans. As the magnetic poles always occur in pairs, north pole and south pole hence, it is impossible to obtain a piece of magnet having only one magnetic pole. 7
4. Most of the iron filings cling near the ends of the magnet. Ans. Because the ends of the magnet have greater magnetic strength than the centre of the magnet. So most of the iron filing cling near the ends of the magnet. 5. When a magnet is heated strongly, it gets demagnetised. Ans. Because the kinetic energy of the molecules increases when magnet are heated strongly. Then the straight chains of the molecules get disrupted and the molecules go back to the closed chain arrangement. 6. Repulsion is the surest test of magnetism. Ans. Attraction can be caused either by opposite poles of magnets or between one pole of magnet and an unmagnetised bar of iron whereas repulsion is possible only between the two similar poles of magnets. Hence repulsion is the surest test of magnetism. X. Define the following: 1. Pole of a magnet 2. Effective length of a bar magnet 3. Magnetic induction 4. Magnetic axis 5. Magnetic meridian 6. Magnetic keeper Ans. 1. Pole of a magnet. The regions near the ends of a bar magnet where the attractive power is most pronounced are called the poles of the magnet. 2. Effective length of a bar magnet. It is the distance between the magnetic north pole and magnetic south pole. 3. Magnetic induction. The phenomenon due to which temporary magnetism is produced in a magnetic substance due to the presence of a magnet is called magnetic induction. 4. Magnetic axis. The imaginary line passing through the magnetic north (N) and south (S) poles of a bar magnet is called magnetic axis. 5. Magnetic meridian. It is an imaginary vertical line passing through the magnetic axis of a freely suspended magnet. 6. Magnetic keeper. It is a piece of soft iron. Two such soft iron bars are placed at the either end of the pair of bar magnets. It is used to avoid self-demgnetisation of magnets. 8 8
XI. Differentiate between the following. 1. Ferrite magnets and alloy magnets. Ans. Ferrite magnets Alloy Magnets (1) They are made from a (1) They are made of mixture powder of iron oxide and of aluminium, nickel, cobalt barium oxide copper and iron. (2) They are brittle and have (2) They are very strong and the properties of ceramics. do not lose magnetism with normal treatment. (3) It is known under the trade (3) It is known under the trade name magnadur. names alnico and alcomax. 2. Iron and wood. Ans. Iron is a magnetic material but wood is a non-magnetic material. 3. Magnetic induction and single-touch method. Ans. Magnetic induction is the phenomenon by which an ordinary piece of iron acquires magnetic properties temporarily due to the presence of another magnet near it. In single touch method, the magnetic substance is magnetised by rubbing with a magnet. 4. Soft iron magnet and alnico magnet. Ans. Soft iron magnet are temporary magnets which can retain their magnetism only for a short duration. They lose their magnetism after the magnetising force is removed. Alnico magnets are permanent magnets i.e. they are able to keep their magnetism for a long time even when the magnetising force is removed. 5. Length of a magnet and effective length of a magnet. Ans. Length of a magnet is the distance between the centre of the magnet and one of the magnetic poles. Effective length of a magnet is the distance between the magnetic north pole and magnetic south pole. 6. Natural and artificial magnet. Ans. Natural magnets are naturally occurring substances like lodestone which possesses the property of attracting magnetic substances. 9
Artificial magnets are magnets made by humans using artificial methods. 7. Permanent and temporary magnet. Ans. Permanent magnets are the magnets which are able to keep their magnetism for a long time even when the magnetising force is removed. Temporary magnets are magnets which lose their magnetism after the magnetising force is removed. 8. Steel and plastic. Ans. Steel is a magnetic material but plastic is a non-magnetic material. 9. Single touch method and double touch method. Ans. Single touch method is a method of magnetising a magnetic substance by rubbing with a magnet. Double touch method is a method of magnetising a magnetic substance by rubbing with a pair of magnets. XII. Answer these questions: 1. (a) What is a natural magnet? (b) How is artificial magnet prepared from a natural magnet? Ans. (a) A material which occurs naturally and has magnetic properties is called natural magnet. (b) Artificial magnets are prepared by imparting the properties of natural magnet to the magnetic substances by rubbing with magnet or by process of induction. 2. (a) What are artificial magnets? (b) Draw diagrams of four artificial magnets. Ans. (a) Artificial magnets are the magnets made of iron, steel, cobalt or nickel. (b) (i) Bar Magnet (ii) Cylindrical magnet (iii) Dumb-bell magnet. 10 10
(iv) U-shaped magnet. 3. Why are artificial magnets preferred to natural magnets? Ans. Artificial magnets are preferred to natural magnets because (i) they can be made very powerful for using in different devices. (ii) they can be made in any desired shape and size. 4. What are (a) magnetic substances (b) non-magnetic substances? Give four examples of each kind. Ans. (a) Magnetic substances are the substances which are strongly attracted by magnet or can be converted into magnet artificially. For examples, iron, cobalt, nickel and steel. (b) Non-magnetic substances are the substances which are neither attracted by magnet nor can be converted into artificial magnets. For examples, stones, glass, wood, aluminium etc. 5. (a) You have a steel nail 10 cm long. How will you magnetise it, using a bar magnet? (b) How will you test the poles of magnet? Ans. (a) We can magnetise a steel nail of 10 cm long by rubbing a permanent bar magnet over it. (b) The poles of magnet can be tested by bringing ends of magnet near the north pole of freely suspended magnetic needle. If the north pole of freely suspended magnetic needle gets repelled, then that end of the magnet is the north pole of the magnet and if it is attracted then the end of the magnet is south pole. 11
6. How will you convert an iron nail about 8 cm long into a magnet with the help of electric current? Ans. Wind the iron nail with the insulated copper wire over it. Bare the ends of copper wire from insulation, then fixed each end on the terminals of dry cell. Then, the iron nail will change into magnet when current flows around it. 7. State six important properties of a magnet. Ans. Six important properties of magnet are: (a) it attracts magnetic substances. (b) it points north-south direction when suspended freely. (c) repulsion is the surest test of magnetism. (d) poles of magnets are always in pair. (e) similar poles of magnets repel each other. (f) magnetic force can easily pass through non-magnetic substances. 8. A box contains nails of brass and iron. How will you separate them by using a magnet? Ans. When we bring a magnet near a box containing nails of brass and iron, the iron nails will cling at the magnet. Then we can separate them easily. 9. What are magnetic keepers? How Magnetic keepers are used to store magnets. Ans. Magnetic keepers are soft iron bars placed at the either end of the pair of magnets. It is used to avoid self-demagnetisation of magnets. The magnets are arranged in pairs, with opposite poles facing each other. Two soft iron bars are placed at the either end of the pair of magnets. 10. How do you show the directive properties of a bar magnet? Ans. A string is tied at the centre of the bar magnet. When, we suspend it so that it can hangs freely. Then, the magnet will always point in the north-south direction, when it comes to rest. This show the directive property of a bar magnet. 11. Classify magnets on the basis of retention of magnetism. Ans. On the basis of retention of magnetism, we can classify magnets into two categories. (i) temporary magnet, and(ii) permanent magnet. 12 12
12. What are the important uses of magnets. Ans. Uses of magnets: (i) it is used in magnetic compass, (ii) it is used in pencil boxes, soap stands, refrigerator etc. (iii) ceramic magnets are used in large computers, (iv) it is used in bicycle dynamos, (v) it is also used in toys to give a magical effect. 13. How do you prove that earth itself is a magnet? Ans. A freely suspended magnet always points in the north-south direction. This shows that our earth itself is like a huge bar magnet with its magnetic poles near the geographical poles. The north poles of earth s magnet is near the geographical south pole and the south pole of the earth s magnet is near the geographical north pole. 14. How will you show that the magnet shows attractive property? Ans. To show the attractive property of a magnet, spread iron filings, over a sheet of paper. Then, roll the magnet on the filings so that all parts touch the filings. Iron filings will cling to the magnet when magnet is picked up. 15. Define magnetic induction. How is an unmagnetised piece of iron magnetised by this phenomenon? Ans. The phenomenon by which an ordinary piece of iron (or any magnetic substance) acquires magnetic properties temporarily due to the presence of another magnet near it is called magnetic induction. An unmagnetised piece of iron can be magnetised by bringing a magnet near the piece of iron. 16. What happens when a magnet is rubbed on a needle with a magnet in only one direction (from one end of the needle to the other)? Ans. When a magnet is rubbed on a needle with a magnet in only one direction, then the needle will magnetised temporarily. 17. Explain the ways by which the magnetic properties of a magnet get destroyed. 13
Ans. The magnetic properties of a magnet get destroyed (i) by rough handling. (ii) by hammering it violently several times. (iii) by dropping it on the floor many times, (iv) by heating it to a very high temperature. 18. Explain how a pair of bar magnets can be stored. Draw a labelled diagram showing the arrangement of the magnets and the keepers. Ans. A pair of bar magnet can be stored by arranging such that their opposite poles lie close together and a keeper is placed on the two different poles belonging to two different magnets which attract it. 19. A horseshoe magnet is to be stored. Draw a labelled diagram showing the arrangement of the magnet and keepers. Ans. This shows the arrangement of horseshoe magnet and the keeper. 20. Describe an experiment to show that like poles repel each other. Ans. Experiment to show that like poles repel each other: Suspend a magnet freely from a wooden stand with a thread. When it comes to rest in north-south direction, bring the north pole of another magnet near the north pole of the suspended magnet. Then the north pole of freely suspended magnet will repelled. Repeat the same process with south poles, then also south pole will repel. This shows that like poles repel each other. 14 14
21. Describe an experiment to detect whether a given iron piece is a magnet or not. Ans. To check whether a given iron piece is a magnet or not. First keep the given iron piece on a wooden table. Then bring one pole say N-pole of a magnet close to the ends of the iron piece one by one. If one end is attracted and other repelled then the iron piece is a magnet. If both ends are attracted then the iron piece is not a magnet. 22. Describe an experiment to show that magnetic poles always occur in pairs. Ans. If a bar magnet is broken into two pieces each piece, when freely suspended will point in the north-south direction. This shows that if a bar magnet is broken into two or more smaller pieces, then even the smallest piece is a complete magnet with two poles. This shows that magnetic poles always occur in pairs. 23. Explain the molecular theory of magnetism. Ans. Molecular theory of magnetism states that, (i) each molecule of a magnetic substance is an independent magnet. (ii) in an unmagnetised substance, the molecular magnets are randomly oriented forming a closed chain. (iii) in a magnetised substance the molecular magnets are arranged such that the north pole of each molecular magnet points in one direction and the south pole in the opposite direction and the bar has no magnetic property in its centre. (iv) when a piece of iron is magnetised the molecular magnets start setting themselves along straight chains. (v) when a magnet is strongly heated or hammered the magnet gets demagnetised. It is because the kinetic energy of the molecules increases and straight chains get disrupted. 24. On the basis of the molecular theory of magnetism, explain the following. (a) Magnetisation of iron (b) Demagnetisation of a magnetic material Ans. (a) When a piece of iron is magnetised the molecular magnets start setting themselves along straight chain. As a result the magnetism of the piece increases. 15
(n) When a magnet is strongly heated or hammered the kinetic energy of the molecules increases and therefore straight chains get disrupted. As a result the molecules go back to the closed chain arrangement. Thus, the magnet gets demagnetised. 25. Every iron atom is a tiny magnet, why all iron bars are not magnets? Ans. In an iron bar, iron atoms are randomly oriented. They form a closed chain so that they neutralise the effect of each other. So, all iron bars are not magnet even though every iron atom is a tiny magnet. 26. What are the characteristic features of the magnetic lines of force? Ans. Characteristic features of the magnetic lines of force are, (i) each line is a closed and continuous curve, (ii) normally they originate at the N-pole and terminate at the S-pole of a magnet, (iii) the lines are crowded near the poles where the magnetic field is strong and are far apart where the field is weak. (iv) they do not intersect each other. 27. Describe an experiment to prove that maximum magnetic force acts at the poles of a magnet. Ans. Take a bar magnet and place a steel pin at some distance. Nothing happens. Then, bring the steel pin near the pole of the bar magnet. It will stick to the magnet. This experiment shows that maximum magnetic force acts at the poles of a magnet. 28. With the help of a labelled diagram show the difference between the magnetic lines of force in a bar magnet and a horseshoe magnet. Ans. (a) Magnetic lines of force in a bar magnet. 16 16
(b) Magnetic lines of force in a (U-shaped magnet) horseshoe magnet. 29. With the help of a labelled diagram show the magnetic lines of force formed when the unlike poles of two bar magnets face each other. Ans. Magnetic lines of force when the unlike poles of two bar magnets face each other. XIII. Numerical Problems: 1. A long magnet is broken into 10 pieces. How many north poles will be there in (a) 10 pieces? (b) one piece? (a) 10 north poles in 10 pieces. (b) 1 north pole for 1 piece. 2. If the length of a magnet is 2 cm, what is its effective length? Ans. Ans. Length of magnet = Length of one of the magnetic pole from the centre = 2 cm... Effective length = Distance between two poles = 2 cm + 2 cm = 4 cm. **** 17