Class 10 Science Chapter 12 Important Questions

Magnet and Magnetism: A substance that possesses the property of attracting certain other substance such as iron, nickel, cobalt, etc. is known as magnet.

Natural Magnet: The substance which is found in nature and has an attractive property to attract iron and some of its natural ores is known as natural magnets. It has an irregular shape. Example: lodestone, basalt. The earth itself acts as a giant natural magnet.
Artificial Magnet: They are generally made from special iron or steel alloy which are usually magnetised electrically. Therefore, the magnet produced from magnetic material is called artificial magnet. It can be shaped in any desired shape and size such as bar shape, U-shape and circular shape. Artificial magnets have a wide variety of application.

Magnetic Poles: If a bar magnet is dipped into iron filings, the filings cling to the magnet in concentration near the end of bar. These regions of concentrated magnetic strength inside the magnet just near its ends are called magnetic poles. The end of a freely suspended magnet which point towards north is called the north seeking or north pole while its end pointing toward south seeking or south pole.

(i) Every magnet attracts a small piece of magnetic substance towards it. The ends of a bar magnet where attraction is maximum are called poles of the magnet.
(ii) A freely suspended magnet aligns itself along the north direction. Geographic north seeking end of the magnet is called north pole.
(iii) Like poles repel each other and unlike poles attract each other.
(iv) Magnetic poles always exist in pair. They can never be separate that’s why magnetic monopole does not exist.

Uses of magnets:
(i) In children’s toys.
(ii) In radio and stereo speaker.
(iii) On hard disc and floppies for computer.
(iv) In refrigerator and other doors to snap them close.
(v) In audio and video cassette tapes.
(vi) In medical therapy.
(vii) An MRI scan uses a magnetic field of powerful magnets to create picture of organs and structures inside the human body.
(viii) In production of electricity.

(i) In plotting the magnetic field of a bar magnet.
(ii) As a navigational aid in ships and jet-liner.

A magnetic line of force may be defended as the curve the tangent to which at any point gives the direction of magnetic field at that point. It may also be defined as the path along with a free north pole tends to move.
Properties of lines of force:
(i) These are closed curves which start in air from the N pole and end at the S pole and then return to the N pole through the interior of the magnet.
(ii) No two magnetic lines of force can intersect each other.
(iii) They start from and end on the surface of the magnet normally.
(iv) The line of force has a tendency to contract lengthwise and expand sidewise. This explains attraction between unlike poles and repulsion between like poles.
(v) The relative closeness of the lines of force gives a measure of the strength of the magnetic field which is maximum at the poles.

The properties of magnetic field lines are:
(i) Magnetic field lines are closed continuous curves. They come out from the north pole of a bar magnet and go into its south pole. Inside the magnet they move from south pole to north pole.
(ii) A tangent drawn at any point on the magnetic field lines gives the direction of magnetic field lines gives the direction of magnetic field at the point that’s why compass needle align itself along the tangent to the lines of force at that point.
(iii) The magnetic lines of force do not intersect one another. If they do so then at the point of intersection.
(iv) The region where the magnetic field lines are crowded the field are crowded the field is stronger and where they are apart from each other the field is weaker.

(i) A natural magnet or an artificial magnet.
(ii) A current carrying straight conductor or circular conductor or solenoid
(iii) a moving charge and,
(iv) changing electric field are the various sources of magnetic field.

Magnetic field due to a current through a circular loop: The magnetic field lines of a circular wire are almost concentric circles. As we move towards the center of loop, the concentric circles become larger and larger. Near the center of the loop the arcs of these big circles appear as parallel straight lines. Thus the magnetic field is almost uniform at the center of loop. By applying right hand rule, we can see that the magnetic field lines due to all sections of the wire are in the same directions within the loop. That is why the field is stronger at the center or near the circumference of the loop.

(a) Solenoid: A long cylindrical coil of insulated copper wire of large number of circular turns is called a solenoid.

(b) When a current is passed through the solenoid, the current in each circular loop has the same direction, their magnetic effects get added up producing a strong magnetic field. Inside the solenoid, the magnetic field is almost uniform and parallel to the axis of the solenoid.
The magnetic field produced by a solenoid is very much similar to that of a bar magnet. Like a bar magnet one end of the solenoid has N-polarity while the other end has S-polarity. The polarity of any end (face) of the coil can be determined by using clock rule. For all practical purposes, the magnetic field of a solenoid and that of a bar magnet can be taken identical.

(c) Factors on which the strength of the magnetic field produced by a current carrying solenoid depends:
(i) Number of turns in the solenoid: The larger the number of turns in the solenoid, stronger is the magnetic field produced.
(ii) Strength of the current: The larger the number passed through the solenoid, stronger is the magnetic field produced.
(iii) nature of the core materials: By winding the coil over a soft iron cylinder, called core, the magnetic field can be increased several thousand times.

Electromagnet: A soft iron core placed inside a solenoid behaves like a powerful magnet when a current is passed through the solenoid. This device is called an electromagnet. When the current is switched off, the iron core loses its magnetism and so it is no longer an electromagnet. Thus, electromagnets are temporary magnets.
Factors on which the strength of an electromagnet depends:
(i) Number of turns in the coil: The larger the number of turns in the coil, greater is the strength of the electromagnet.
(ii) Strength of the current: The larger the current passed through the solenoid more powerful is the electromagnet.
(iii) Nature of the core material: The core of the magnetic material like soft iron increases the strength of the electromagnet.

Permanent magnets are usually made of following materials:
(i) Carbon steel, chromium steel, cobalt and tungsten steel.
(ii) Nipermag and alnico are two important alloys whose permanent magnets are used in industry. Nipermag is an alloy of iron, nickel, aluminum and titanium while alnico is an aluminum-nickel-cobalt alloy of iron.

An electromagnet has following advantages over a permanent magnet:
(i) An electromagnet can have produced a very strong magnetic field.
(ii) The strength of the magnetic field of an electromagnet can be increased/decreased by increasing/decreasing the strength of current or the number of turns in the solenoid.
(iii) The polarity of an electromagnet can be reserved by sending the current in the reverse direction.

Electromagnet:
(i) It is a temporary magnet. It shows magnetism only as long as the current is passing through its coil.
(ii) It can produce very strong magnetic field.
(iii) The strength of an electromagnet can be easily varied by changing the strength of current or number of turns of coil.
(iv) The polarity of an electromagnet can be reserved by sending the current in reverse direction.

Permanent magnet:
(i) It retains magnetism for a long time even after the removal of the magnetising field (or current).
(ii) it produces a much weaker field than an electromagnet.
(iii) Its strength cannot be changed.
(iv) The polarity of a permanent magnet cannot be changed.

Uses of permanent magnets:
(i) In microphones, loudspeakers and electric clocks.
(ii) In devices like ammeters, voltmeters and speedometers.
(iii) In electric generators and motors.
(iv) On video and audio cassette tapes.

Uses of electromagnets:
(i) Uses of electromagnets:
(i) Electromagnet are used in factories and cranes to lift heavy iron ingots and steel scraps from one place to another.
(ii) They are used in hospitals to remove iron splinters from the eyes of the particles.
(iii) They are used electric bells, relays, electric switches, etc.

We know that an electric current flowing through a conductor produce a magnetic field. This field exerts a force on a magnet placed near the conductor. In accordance with Newton’s Third law, the magnet must also exert an equal and opposite force on the current carrying conductor. Thus a magnetic field exerts a force on a force on current carrying conductor. Such a force was first suggested and demonstrated experimentally by French Scientist Andre Marie Ampere in 1820.

(i) When the current carrying is held perpendicular to the direction of the magnetic field the force exerted on it is maximum.
(ii) When the current carrying conductor is held parallel to the direction of the magnetic field, the force exerted on it is minimum or zero.

Name and state the rule to determine the direction of a force experienced by a straight conductor carrying current placed in a magnetic field which is perpendicular to it.

(i) Displacement of the conductor towards left increases.
(ii)Displacement of the conductor towards left increases.
(iii) The conductor is displaced towards right.

(a) Force on the conductor increases, as F∝ I.
(b) Force on the conductor increases, as F∝L.
(c) Force on the conductor remains same both in magnitude and direction.
(d) The direction of force on the conductor is removed.

Electric motor: An electric motor is a rotating device which converts electric energy into mechanical energy. An electric motor brings about rotational motion in domestic appliances such as electric fans, washing machines, mixers, grinders, blenders, MP3 players, etc.

In an electric motor, electrical energy is converted into mechanical energy.
Electric motor is used as an essential component in electric fan, washing machine etc.

Electrical motor: We describe here a d.c. motor operates on direct current obtained from a battery.
Principle: An electric motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force, the direction of force is given is given by Fleming’s left and rule.
Construction: An electric motor consists of the following main parts:
1.Field magnet: It is a strong horse shoe type magnet with concave poles.
2. Armature: It is a rectangular coil ABCD having a large number of turns of thin insulated copper wire wound over a soft iron core. The armature is placed between the poles of the field magnet and it can be rotated about an axis perpendicular to the magnetic field lines.
3. Spilt ring commutator: It consists of a cylindrical metal ring spilt into two halves S₁ and S₂. The two ends A and D of the armature coil are connected to the spilt rings S₁ and S₂ respectively. As the coil rotates, the spilt rings also rotate about the same axis of rotation. The function of the spilt ring commutator is to reverse the direction of current in the coil after every half rotation.
4. Brushes: Two graphite or flexible metal rods maintain a sliding contact with spilt rings S₁ and S₂ alternately.
5. Battery: A battery of few cells is connected to the brushes. The current from the battery flows to the armature coil through the brushes and the spilt rings.
Working: Initially, suppose the plane of the coil is horizontal. The spilt ring S₁ touches the brush B₁ and spilt ring S₂ touches the brush B₂. The current in coil flows in the direction ABCD.
Clearly, the currents in arms AB and CD are in opposite directions. On applying Fleming’s left hand rule for the direction of force on a current carrying conductor in a magnetic field, we find that the force acting on arm AB pushes it downwards while the force acting on the arm CD pushes it upwards. Thus the armature coil along with the axle rotates anticlockwise. After half a rotation, the spilt ring S₁ comes in contact with brush B₂ and S₂ in contact with brush B₁. Therefore, the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flows of current through a circuit is called commutator. In electric motors the spilt ring acts as a commutator. Thus the arm AB is ow pushed up and the arm CD is pushed down. Therefore, the coil and the axle rotate half a turn more rise to a continuous rotation of the coil and to the axle.

The speed of rotation of the armature coil can be increased by increasing:
(i) the strength of the magnetic field.
(ii) the number of turns in the coil.
(iii) the current in the coil.

Electromagnetic induction: Whenever the magnetic lines of force passing through a closed circuit, a voltage and hence a current is induced in it. This phenomenon is called electromagnetic induction. The voltage so produced is called induced electromotive force and the current is called induced current. This phenomenon was discovered in 1831 by Michael Faraday in England.

The magnitude of the induced current can be decreased by rapidly increasing or decreasing the number of magnetic field lines passing through a closed coil.

The magnitude of the induced current can be decreased by rapidly increasing or decreasing the number of magnetic field lines passing through a cloud coil.

(i) The term electromagnet induction means inducing electricity by magnetism. Whenever the magnetic flux linked with a closed circuit changes an induced and hence an induced current is set up in it. This phenomenon is called electromagnetic induction.
(ii) The value of current induced in a circuit depends on the rate of change of magnetic flux linked with the closed circuit.
(iii) Fleming’s right hand rule: This rule gives the direction of current induced in a conductor moving perpendicular to a magnetic field. Stretch the thumb, forefinger and the central finger of the right hand naturally perpendicular to each other. If the forefinger points in the direction of the direction of motion of the conductor, then the central fingers points in the direction of current induced in the conductor.
(iv) Electric generator is an important practical application based on electromagnetic induction.

(i) When the coil is moved towards the bar magnet the galvanometer shows deflection ion one direction.
Reason: When the coil is moved towards the stationary bar magnet, the magnetic flux linked with it increases and a current is induced in it as is shown by deflection in the galvanometer.
(ii) When the coil is moved away from the coil the galvanometer shows deflection in opposite direction. `Reason: When the coil is moved away from the magnet, the magnetic flux linked with it decreases and a current is induced in the coil in the direction, as shown by deflection in the galvanometer in opposite direction.

Electric generator: An electric generator is a device which convert mechanical energy into electric energy.

Principle: It works on the principle of electromagnetic induction. When a closed coil is rotate in a uniform magnetic field with its axis perpendicular to the magnetic field the magnet field lines passing through the coil change and an induced and hence a current is set up in it.
Construction: It consist of the following main parts:
1. Field magnet: It is a strong horse shoe type permanent magnet with concave poles.
2. Armature: ABCD is a rectangle armature coil. It consists of a large number of turns of insulated copper wire wound on a soft cylindrical core. It can be rotated about an axis perpendicular to the magnetic field of the field magnet.
3. Slip rings: These are two brass rings S₁ and S₂ rigidly connected to the two ends of the armature coil. As the coil rotates slip rings also rotate about the same axis of rotation.
4. Brushes: These are two graphite rods B₁ and B₂ which are kept pressed against the slip rings S₁ and S₂. Through these brushes, the current induced in the armature coil is sent to the external circuit.

DC generator is a device which convert mechanical energy into electrical energy of direct form. The principle and construction and of DC generator are similar to that of AC generator expect that a DC generator has split rings commutator instead of slip rings.
Split ring commutator: It consist of two semi-cylindrical brass rings S₁ and S₂ rigidly attached to the two ends of the armature coil. As the armature coil rotates the two split rings also rotate about the same axis of rotation.
Working: Suppose initially the plane of the coil is horizontal. The split ring S₁ touches the brush B₁ and S₂ touches B₂. When coil ABCD is rotated clockwise magnetic field lines passing through it change. The arm AB moves upwards and arm CD moves downwards. According to Fleming’s right hand thumb rule the induced current in the armature coil flows in the direction ABCD. In the external circuit current flows from B₂ to B₁. As the coil crosses its moves downwards an arm CD moves upwards. The induced current in the armature flows along DCBA. But in the external circuit the current still flow from B₂ to B₁. Hence a unidirectional current flows in the external circuit.

Direct current: A direct current is that current which flows with constant magnitude in the same direction.
Alternating current: An alternating current is that current whose magnitude changes continuously with time whose direction reverses after equal intervals of time.

Advantage of Ac over DC: Only alternating voltage can be stopped up or steeped down by using a transformer. This make AC more suitable than DC for transmission of electric power over long distance without much loss of energy.

Advantage of ac over dc:
1. The generation of ac is more economical than dc.
2. The alternating voltage ca be easily steeped up or steeped down by using a transformer.
3. The alternating current can be transmitted to distant places without any significant line loss.

When there is a current I in a transmission wire of resistance R the amount of power lost in the form of heat is I2R watt. For a transmission line of given resistance R the current I should be as small as possible for the power loss to the minimum. With the help of a step up transformer the voltage at the generating station is first steeped up and fed into the transmission line. When voltage is steeped up the current I decrease and thus the power loss in the transmission line is reduced. At the consumer’s end the voltage is steeped down to the desired value by a step down transformer.

Different electric appliances have different power ratings. So they draw different currents when connected to the mains. Some appliances need smaller currents, while some other need heavy currents. So we need two lines of current rating 5A and 15A.

It has the following advantages:
(i) If one line is overloaded the fuse in this circuit only will be blown off. Other distribution lines are protected.
(ii) There is a constant potential differences across each line.
(iii) If more lines are added in the circuit it makes no differences to the other line is maintained at a constant value.

Electrical fuse: It is a safety device used to protected an electric circuit from overloading or short circuiting. It is small piece of wire of a material of having low melting point and high resistance. A fuse wire is made of pure tin or of an alloy or copper and tin.

The working of an electric fuse is based on the heating effect of current. A fuse wire has low melting point and high resistance. It is connected in series with the live wire. When an excessive current flows thorough it the wire melts and the circuit is broken. The electric current stops flowing immediately. Electric appliances are thus prevented from getting damaged.

(i) The electric supply from the mains is delivered to our homes using a three core cable. On wire with red insulation is called live wire which brings in current. The second wire with lack insulation is called neutral wire which provide the return path for the current. The third wire is the earth wire. The potential difference between live and neutral wire is 220 volts in India.
(ii) The earth wire is either thick uncovered copper wire or a copper wire with green insulation. It is usually connected to a metal plate deep in the earth near the house. It is used as safety measure which does not affect the supply in any way.
(iii) The wires from the supply line pass into watt hour meter through a man fuse. Then through a main switch these wires enter a distribution box.
(iv) From the distribution box several independent distribution circuits are drawn which go into different parts of the house. The live wire of each circuit passes through a separate fuse so that during short circuiting only the electricity to that part of the house is cut off and others are not affected.
(v) In any part of the house all appliances are connected in parallel to each other across the live and neutral wire. This ensure full voltage across each appliances from the mains. Each appliances is provided with a separate switch in its live wire for its operation.
(vi) Two separate circuits are used one of 15A for appliances with high powers ratings and the other of 5A for bulbs etc.

Short Circuiting: If due to defective or damaged wiring the live and neutral wires come in direct contact the resistance of the circuit becomes almost zero and an extremely large current flows through the circuit. This is called short circuiting. It results of short circuit and cause fire. Short circuiting can be prevented by using an electric fuse in live wire.