NCERT Solutions for Class 10 Science Chapter 12 Magnetic Effect of Electric Current

Law Of Magnetic Poles: It states that like poles repel while unlike poles of magnets attracts each other. Thus, two N-poles repels, two S-poles repel while N-pole attracts S-pole.

Some basic properties of magnets are as follows:
(i) Attractive property: a magnet attracts small pieces of iron, cobalt, nickel, etc.
(ii) Directive property: a freely suspended magnet aligns itself nearly in the north-south direction.
(iii) Law of magnetic poles: Like magnetic poles repel and unlike magnetic poles attract each other.
(iv)Magnetic poles exist in pairs: If we break a magnet into two pieces, we always get two small dipoles magnets. It is not possible to obtain an isolated N-pole or S-pole.

Magnetic lines of force:
A magnetic line of force may be defined as the curve the tangent to which at any point gives the direction of the magnetic field at that point. It may also be defined as the path along with a free north pole tends to move.
Properties of line of force:
(i) These are closed curves which start in air from 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 interest each other.
(iii) They start from and end on the surface of the magnet normally.
(iv)The lines of force have 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.

Uses of magnets:
(i) Magnets are used in radio and stereo speakers.
(ii) They are used in video and audio cassette tapes, on the hard discs and floppies for computers.
(iii) They are used in almirah and refrigerator doors to snap them closed.
(iv) In children’s toys.
(v) In medicine, the magnetic resonance image (MRI) scanners expose the inner parts of the patient’s body for detailed examination by doctors.

A magnetic field line may be defined as the path along with a unit north pole tends to move. The direction of magnetic field is determined by placing a small compass needle. The direction of deflection of N-pole of the compass needle gives the direction of the magnetic field at that point.

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

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.

Earthing: Earthing of an electrical appliance means connecting the metallic body of the high powered appliance (electric iron, toaster, refrigerator, oven, etc.,) to the earth through the earth wire in the domestic circuit. The earth wire has green insulation cover and is connected to a metal plate deep in the earth near the house and hence it is at zero potential.
Function and importance of earthing: If, by chance, the live wire touches the metallic body of the electrical appliance, its current flows to the earth through the earth wire, which provides a low resistance conducting path for the current. Thus, the earth wire (or earthing) is used as a safety measure which ensures any leakage of current to the metallic body of the appliance keeps its potential equal to that of the earth (zero volt) and the user may not get a severe electric shock.

Three methods of producing magnetic field are as follows: Magnetic field can be produced by placing a permanent bar magnet or a horse-shoe magnet at the place, where magnetic field is required. Magnetic field is produced around a current-carrying straight conductor or a current carrying circular coil. A very good method to produce magnetic field is due to flow of current in a solenoid.

The force experienced by a current-carrying conductor placed in a magnetic field is largest when the current-carrying conductor is placed in a direction perpendicular to that of magnetic field.

Electric motors are used in all such devices where we want to convert electrical energy into mechanical energy so as to drive that machine. In our houses, electric motors are being fitted in electric fans, coolers, air conditioners, mixer grinders, washing machines, refrigerators, juicers, computers etc. In factories, motors are used in almost all machines.

A current induces in a solenoid if a bar magnet is moved relative to it. This is the principle of electromagnetic induction. (i) When a bar magnet is pushed into a coil of insulated copper wire, a current is induced momentarily in the coil. As a result, the needle of the galvanometer deflects momentarily in a particular direction. (ii) When the bar magnet is withdrawn from inside the coil of the insulated copper wire, a current is again induced momentarily in the coil in the opposite direction. As a result, the needle of the galvanometer deflects momentarily in the opposite direction. (iii) When a bar magnet is held stationary inside the coil, no current will be induced in the coil. Hence, galvanometer will show no deflection.

Yes, a current is induced in the coil B. When the current in the coil A is changed, the magnetic field associated with it also changes. As coil B is placed close to A, hence magnetic field lines around this coil also change. Due to change in magnetic field lines associated with coil B, an induced current is also induced in it.

If either the insulation of wires used in an electrical circuit is damaged or there is a fault in the appliance, live wire and neutral wire may come in direct contact. As a result, the current in the circuit abruptly rises and short-circuiting occurs.

The metallic body of electric appliances is connected to the earth by means of earth wire so that any leakage of electric current is transferred to the ground. This prevents any electric shock to the user. That is why earthing of the electrical appliances is necessary.

Magnetic compass needle and bar magnet both have magnetic field. When they brought near to each other, these magnetic fields interact each other giving deflection in needle.

No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.

According to Fleming’s left-hand rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

The working principle of an electric motor is based on the magnetic effect of current. A current-carrying loop experiences a force and rotates when placed in a magnetic field. The direction of rotation of the loop is given by the Fleming’s left-hand rule.

The split ring in the electric motor acts as a commutator. The commutator reverses the direction of current flowing through the coil after each half rotation of the coil. Due to this reversal of the current, the coil continues to rotate in the same direction.

An electric generator is based on the principle of electromagnetic induction. When a rectangular coil is rotated in a uniform magnetic field, an induced voltage is generated between the ends of the coil.

Some sources of direct current are a cell, a battery and a D.C. generator.

A.C. generator and invertors (used in house for emergency power supply) produces alternating current.

The precautions that should be taken to avoid the overloading of domestic circuits are as follows: Too many appliances should not be connected to a single socket. Too many appliances should not be used at the same time. Faulty appliances should not be connected in the circuit. Fuse should be connected in the circuit.