FUN WITH MAGNETS
A mineral was discovered in the town of magnesia which was found to have a wondrous property. It could attract pieces of iron towards it.
This mineral is called magnetite. Further it was found that thin strips of magnetite always align themselves in a particular direction when
suspended freely In air. It was found that magnetite is mainly composed of oxides of iron (Fe30 4). Magnetite (Fe30.) is the world's first magnet.
It is also called natural magnet.
(a) Types of Magnet :
(i) Natural magnet :
A magnet which occurs naturally and is not made by any artificial means is called a natural magnet.
Eg : Magnetite, Which is an ore of iron [Fe3O4].
(ii) Artificial magnet :
A substance to which properties of the natural magnet are imparted by artificial means is called artificial magnet.
Eg : The magnets made from iron, steel, cobalt and nickel.
(b) Properties of a Bar Magnet :
(i) It attracts small pieces of iron towards itself.
Spread out some iron fillings over a sheet of paper. Now, move a bar magnet in the fillings taking care that all parts of the magnet moves
through iron fillings and observe how the iron fillings are distributed all over the magnet. We will notice that most of the iron fillings cling
near the ends of the magnet while there are a few iron fillings near the middle.
Repeat the experiment with a horseshoe magnet. This experiment explains the attractive property of magnets.
(ii) The magnetic pull seems to come from two points near the ends. These preferred regions of attraction are called the magnetic poles.
We will find that all magnets have maximum attractive power at their poles. Poles of a magnet remain slightly inside from the end points. .
(iii) A freely suspended magnet always align itself along the north-south direction.
The end of the magnet that points towards the north is called the north pole (N-pole) and the other end of the magnet pointing towards
the south is called the south-pole (S-pole).
(iv) Like poles of the magnets repel each other while unlike poles attract each other.
(v) Magnetic poles always exist in pair . .
(vi) Repulsion is the surest test of magnetism. Since a magnet can attract magnetic substances and magnet also but it will always
repel the magnet only.
TEMPORARY AND PERMANENT MAGNETS
(a) Temporary Magnets :
The magnets which cannot retain their magnetism for a long time are called temporary magnets. The temporary magnets are made
from the 80ft Iron.
Eg : Electromagnet
(b) Permanent Magnets :
The magnets which retain their magnetism for a very long time are called permanent magnets. The permanent magnets are generally
made from steel. More powerful permanent magnets are made from ALNICO, an alloy of aluminium, nickel and cobalt or from ferrite.
The ferrite made permanent magnets are quite strong.
TYPES OF SUBSTANCE
(a) Magnetic Substances :
Those substances that are attracted by magnets are called magnetic substances.
Eg. : Iron, Cobalt, nickel, steel etc.
(b) Non - Magnetic Substances :
Those substances that are not attracted by magnets are called non - magnetic substances.
MAKING A MAGNET
The methods by which an ordinary piece of any magnetic material, like iron or steel, can be made a magnet are
single touch method, double touch method and electrical method. Let us study each method separately.
(a) Single touch method :
A piece of any magnetic material, like iron or steel (nail, knitting needle, bolt, etc.), is taken and placed on a bench
or tale. A bar magnet is brought close to the magnetic material (say an iron piece AB). One end of the bar magnet is
stroked against it, moving from end A to end B of the iron piece.
When the bar magnet reaches and B, It is lifted
the stroke repeated from end A to end B, it is lifted and the stroke repeated from end A to B. This procedure is repeated several times, keeping two things in mind.
(i) the same pole of the bar magnet should be used every time.
(ii) Strokes should be in the same direction.
By Stroking the iron piece with the north pole of the bar magnet, end A becomes the north pole and end B the south pole.
(b) Double Touch Method :
The iron piece to be magnetised, say AB, is placed on a bench and two bar magnets of equal strength are taken. Here, the opposite poles of both the magnets are stroked, at the same time from the centre to the opposite ends of the iron piece being magnetized. This step is repeated may times.
Then, the end A of the iron piece becomes the north pole and end B becomes the south pole.
1. The chemical formula of magnetite (natural magnet)
(A) Fe2O3 (B) Fe3O4
(C) FeO (D) FeO2
2. The poles of the magnet always exist in:
(C) both (A) and (B)
(D) cant say
3. The magnet can be demagnetised by -
(C) dropping from height
(D)all of these
4. Which of the following statements is true?
(A) An electromagnet does not attract a piece of iron.
(B) An electric current flowing in a circuit deflects a magnetic needle.
(C) An electromagnet is a permanent magnet.
(D) An electromagnet can be used to separate plastic bags from a garbage heap
5. If there are 3 bar magnets, the total number of poles will be :
(A) 2 (B) 6
(C) 3 (D) 4
6. Which scientist discovered that current carrying wire behave as magnet?
(B) Hans Christian oersted
7. Coloured part of the needle of magnetic compass represents
(A) north (B) south
(C) east (D) west
8. ALNICO is an example of _________ magnet (A) permanent
(C) both (A) and (B)
(D) None of these
9. Electromagnet is an example of __________ magnet
(A) permanent (B) temporary
(C) both A and B (D) None of these
10. A freely suspended magnet always align itself along the (A) north - south direction.
(A) north - east direction
(B) south - east direction
(C) south - east direction
(D) east - west direction
11. Which one of these is temporary magnet?
(A) Bar magnet
(C) U shaped magnet
(D) Horse - shoe magnet
12. Which of the following is/are non magnetic materials :
(A) plastic (B) glass
(C) rubber (D) All of these
1. Discuss the any one method for making a magnet.
2. Distinguish between the temporary and permanent magnet.
3. What are the methods of demagnetising a permanent magnet?
4. What is electromagnet?
5. Discuss the properties of bar magnet.
1. B 2. A 3. D 4. B 5. B 6. B
7. A 8. A 9. B 10. A 11. B 12. D
1. A freely suspended magnet takes its
(a) north-south (b) north-east
(c) east-west (d) south-east
2. Magnetic poles always exist as :
(a) dipole (b) monopole
(c) no-pole (d) none of these
3. A magnetic field can be produced by :
(a) a moving charge
(b) a changing electric field
(c) None of these
(d) Both of these
4. The phenomenon of electromagnetic induction was discovered by :
(a) Faraday (b) Lenz
(c) Maxwell (d) Fleming
5. The magnetic strength is maximum :
(a) At poles of the magnet
(b) In middle of the magnet
(c) uniform at all places
(d) none of these
6. The magnetic lines of forces :
(a) intersect each other
(b) do not intersect each other
(c) parallel to each other
(d) perpendicular to each other
7. Which one of these is temporary magnet ?
(a) bar magnet
(c) magnetic needle
(d) horse-shoe magnet
8. A current carrying wire in the neighbourhood produces :
(a) No field
(b) electric field only
(c) magnetic field only
(d) electric and magnetic field
9. A generator or dynamo works on the
(a) magnetic affect of electric current
(b) electromagnetic induction
(c) chemical effect of electric current
(d) heating affect of electric current
10. The magnetic field lines due to a bar magnet are correctly shown in :
11. The materials suitable for making electromagnets should have :
(a) low retentivity and high coercivity
(b) low retentivity and low coercivity
(c) high retentivity and low coercivity
(d) high retentivity and high coercivity
12. Magnetic effect of current was discovered
(a) Faraday (b) Oersted
(c) Ampere (d) Bohr
13. The attraction of iron filling in a magnet is maximum at :
(a) poles of the magnet
(b) middle part of the magnet
(c) all places in the magnet
(d) none of these
14. The direction of magnetic lines of forces close to a straight conductor carrying current will be:
(a) Along the length of the conductor
(b) Radially outward
(c) Circular in a plane perpendicular to the conductor
15. The magnetic substance among the following is :
(a) copper (b) silver
(c) iron (d) aluminium
16. A magnet attracts from the following :
(a) Iron pieces (b) aluminium pieces
(c) copper pieces (d) glass pieces
17. Choose the correct option (s)
The magnetic field inside a long straight solenoid-carrying current
(a) is zero
(b) decreases as we move towards its end
(c) increases as we move towards its end
(d) is the same at all points
18. Magnetic poles :
(a) exist separately
(b) do not exist separately
(c) mayor may not exist separately
(d) none of these
19. Like poles of two magnet
(a) attract each other
(b) repel each other
(c) no effect for each other
(d) none of these
20. Which of the following correctly describes the magnetic field near a long straight wire :
(a) the field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.
21. The phenomenon of electromagnetic induction is :
(a) the process of charging a body
(b) The process of generating magnetic field due to a current passing through a coil.
(c) producing induced current in a coil due to relative motion between a magnet and the coil.
(d) the process of rotating a coil of an electric motor.
22. The device used for producing electric current is called as :
(a) generator (b) galvanometer
(c) ammeter (d) motor
23. The essential difference between an AC
generator and a DC generator is that :
(a) AC generator has an electromagnet while a DC generator has permanent magnet
(b) DC generator will generate a higher voltage
(c) AC generator will generate a higher voltage
(d) AC generator has slip rings while the DC generator has a commutator.
24. In an electric motor, conversion takes place of :
(a) chemical enrgy into electrical energy
(b) electrical energy into mechanical energy
(c) Electrical energy into light
(d) Electric energy into chemical energy
25. Which one of the following substances is the magnetic substances :
(a) Mercury (b) Iron
(c) Gold (d) Silver
26. Magnetic lines do not intersect one-another because :
(a) they are at a distance
(b) they are in the same direction
(c) they are parallel to another
(d) at the point intersection there will be two direction of the magnetic force which is impossible
27. Instrument can be shielded from outside magnetic effects by surrounding them with :
(a) Rubber shield (b) Glass shield
(c) Iron shield (d) Brass shield
28. The vertical plane which passes through the magnetic axis ofa freely suspended magnetic at rest is called :
(a) magnetic meridian
(b) Geographical meridian
(c) North meridian
(d) South meridian
29. By removing the inducing magnet, the induced magnetism is :
(a) Finished after some time
(b) Finished just after
(c) Not finished for a long time
(d) Not charged
30. The similar magnets of steel are ______than the magnets of soft iron :
(b) of equal strength
(d) None of the above
31. The magnetism is a magnet is mainly due to :
(a) The orbital motion of the electrons
(b) the spinm motion of the electrons
(c) the nuclear charge
(d) None of the above
32. When the north pole ofa strong magnet is brought near to the north pole of a weak magnet then they will :
(a) Attract each other
(b) repel each other
(c) first attract and the repel
(d) first repel and then attract
33. A magnet can be demagnetised by :
(a) Hammering the magnet
(b) Putting it in the water
(c) Cooling it
(d) Putting in contact with iron
34. The effective length of the magnet is :
(a) The complete length of the magnet
(b) the distance between the two poles of the magnet
(c) the half of the length of the magnet
(d) the square of the length of the magnet
35. When the bars of bismuth are placed between the magnetic poles they set their length :
(a) perpendicular to the lines of force
(b) along the lines of force
(c) neither perpendicular nor along the lines of force
(d) In any direction
36. Two bars of soft iron exactly alike are given. One of them is a magnet. Without using any thing more, how would you find which is a magnet.
(a) By bringing two bars near and noting which one is attracting. The attracting one is a magnet.
(b) by bringing two bars near and noting which one is repelling. One which repells is an ordinary iron.
(c) By rubbing one bar with the other and noting which becomes magnet. The bar which is magnetised is an ordinary iron.
(d) One bar is placed flat horizontal on the table and the other bar is held vertical with its one end on the middle of first bar. If there is attraction between the two, the vertical bar is magnet otherwise ordinary iron.
37. Magnetic storms are due to :
(a) the rotation of the earth
(b) the revolution of the earth
(c) the rainy season
(d) the appearance of the spots
38. Magnetic field lines start :
(a) on N-poles
(b) on S-poles
(c) on current carrying wires
39. Magnetic field lines form circles in the space :
(a) near a permanent magnet
(b) around a current-carrying wire
(c) inside a solenoid
(d) inside a current-carrying loop
40. A transformer used to reduce the alternating voltage is :
(a) step-up transformer
(b) step-down transformer
(c) both step-up and step-down transformers
(d) none of these
41. A tesla is equivalent to a :
(a) newton per coulomb
(b) newto per ampere-meter
(c) ampere per newton
(d) newton per ampere-second
42. A vertical wire carries a current upwards. The magnetic field north of the wire will be
(a) upward (b) eastward
(c) westward (d) northward
43. The current in the armature of a motor is reversed every half cycle due to the action of
(a) armature (b) field coil
(c) brush (d) commutator
44. For dynamo which one of the following statements is correct :
(a) it converts the electrical energy into light energy
(b) it converts the kinetic energy into heat energy
(c) it converts the mechanical energy into electrical energy
(d) it converts the electrical energy into mechanical energy
45. If a bar magnet is cut length wise into 3 parts, the total number of poles will be :
(a) 2 (b) 6
(c) 3 (d) 4
46. A soft iron bar is introduced inside a current carrying solenoid. The magnetic field inside the solenoid :
(a) will becomes zero
(b) will decrease
(c) will increase
(d) will remain unaffected
47. The magnetic lines of force, inside a current carrying solenoid are :
(a) along the axis and are parallel to each other
(b) perpendicular to the axis and equidistant from each other
(c) circular and they do not intersect each other
(d) circular at the ends but they are parallel to the axis inside the solenoid.
48. For making a strong electromagnet, the material of the core should be :
(a) soft iron
(d) laminated steel strips
49. Wrist watches are made antimagnetic by shielding their machinery with :
(a) plastic sheets
(b) a metal of high conductivity
(c) a magnetic substance of low permeability
(d) a magnetic substance of high permeability
50. When a bar magnet is broken into two
(a) we will have a single pole on each piece
(b) each piece will have two like poles
(c) each piece will have two unlike poles
(d) each piece will lose magnetism
1. A 2. A 3. D 4. A 5. A 6. B 7. B 8. C 9. B 10. D 11. C 12. B 13. A 14. C 15. C 16. A 17. D 18. B 19. B 20. D
21.C 22.A 23.D 24.B 25.B 26.D 27.C 28.A 29.B 30.C 31.B 32.A 33.A 34.B 35.A 36.D 37. D 38. D
39.B 40.B 41.B 42.C 43.D 44.C 45.B 46.C 47.A 48.A 49.D 50. D