Class 10 Science Chapter 12 Magnetic Effects of Electric Current NCERT Notes

Chapter 12 Magnetic Effects of Electric Current Class 10 Science CBSE NCERT Notes

Magnet

A piece of iron or other material which has its component atoms so ordered that the material exhibits properties of magnetism and aligning itself in an external magnetic field.

Properties of a magnet

Attracts material like iron, nickel and cobalt.
Attraction is maximum at its poles.
Always aligns itself in north-south direction.

In 1820, Christian Oersted discovered that a compass needle get deflected when a current carrying metallic conductor is placed nearby it.

He concluded that the deflection of compass needle was due to the magnetic field produced by the electric current. Hence, it was deduced that electricity and magnetism are related to each other.

The SI unit of magnetic field is Tesla.

Magnetic Field

The space surrounding a magnet in which the force of attraction and repulsion is exerted is called a magnetic field.

Magnetic Field Lines

The magnetic field lines are the lines drawn in a magnetic field along which a north magnetic pole would move. These are also known as magnetic lines of forces.

Properties of Magnetic Field Lines

A magnetic field lines originate from north pole and end at its south pole.
A magnetic field line is a closed and continuous curve.
The magnetic field lines are closer near the poles of a magnet where the magnetic field is strong and farther apart where the magnetic field is weak.
The magnetic field lines never intersect each other.
A uniform magnetic field is represented by parallel and equidistant field lines.

Magnetic field due to a straight conductor carrying current

The magnetic field due to a straight conductor carrying current is in the form of concentric magnetic lines of force, whose centre lies on the conductor. These magnetic lines of force lie in a plane perpendicular to the plane of linear conductor.

Strength of magnetic field produced by a straight current-carrying wire at a given point is

directly proportional to the current passing through it.
inversely proportional to the distance of that point from the wire.

Right Hand Thumb Rule

When a current-carrying straight conductor is holding in right hand such that the thumb points towards the direction of current. Then fingers will wrap around the conductor in the direction of the field lines of the magnetic field, as shown in below figure. This is known as the right-hand thumb rule.

Thumb-points in the direction of current then direction of fingers encircle the wire give the direction of magnetic field around the wire.

Magnetic Field due to a current through a circular loop

The magnetic field lines are circular near the current carrying loop. As we move away, the concentric circles becomes bigger and bigger. At the centre, the lines are straight.

At the centre, all the magnetic field lines are in the same direction due to which the strength of magnetic field increase.

The magnetic of magnetic field produced by a current carrying circular loop at its centre is

directly proportional to the current passing
inversely proportional to the radius of the circular loop

The strength of magnetic field produced by a circular coil carrying current is directly proportional to both number of turns(n) and current(I) but inversely proportional to its radius(r).

Magnetic Field due to a Current in a Solenoid

The insulated copper wire wound on a cylindrical tube such that its length is greater than its diameter is called a solenoid. The solenoid is from greek word for channel.

The solenoid is a long coil containing a large number of close turns of insulated copper wire.

The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet.

The current in each turn of a current carrying solenoid flows in the same direction due to which the magnetic field produced by each turn of the solenoid ads up, giving a strong magnetic field inside the solenoid.

The strong magnetic field produced inside a current-carrying solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the solenoid. The magnet thus formed is called an electromagnet. So, a solenoid is used for making electromagnets.

The strength of magnetic field produced by a carrying current solenoid depends on

number of turns(n)
strength of current(1)
nature of core material used in solenoid – use of soft iron as core in a solenoid produces the strongest magnetism.

Force on a current-carrying conductor in a magnetic field

When a current carrying conductor is placed in a magnetic field it experiences a force, except when it is placed parallel to the magnetic field.

The force acting on a current carrying conductor in a magnetic field is due to interaction between:

Magnetic force due to current-carrying conductor and
External magnetic field in which the conductor is placed.

In the above figure, a current-carrying rod, AB, experiences a force perpendicular to its length and the magnetic field.

The displacement of the rod in the above activity suggests that a force is exerted on the current-carrying aluminium rod when it is placed in a magnetic field. It also suggests that the direction of force is also reversed when the direction of current through the conductor is reversed.

Now change the direction of field to vertically downwards by interchanging the two poles of the magnet. It is once again observed that the direction of force acting on the current-carrying rod gets reversed.

It shows that the direction of the force on the conductor depends upon the direction of current and the direction of the magnetic field. We considered the direction of the current and that of the magnetic field perpendicular to each other and found that the force is perpendicular to both of them.

Fleming’s left hand rule

Fleming’s left hand rule (for electric motors) shows the direction of the thrust on a conductor carrying a current in a magnetic field. The left hand is held with the thumb, index finger and middle finger mutually at right angles.

The First finger represents the direction of the magnetic Field. (north to south)
The Second finger represents the direction of the Current (the direction of the current is the direction of conventional current; from positive to negative).
The Thumb represents the direction of the Thrust or resultant Motion.

Electric Motor

It is a rotating device used for converting electric energy into mechanical energy. It is based on the principle that when a rectangular coil is placed in a magnetic field and current is passed through it, two end equal and opposite forces act on the coil which rotate it continuously.

Construction

It consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two rings R1 and R2. The inner side of these rings are made insulated. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively. The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.

Working

Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y.

The current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB.

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 arm CD pushes it upwards. Thus the coil and the axle O, mounted free to turn about an axis, rotate anti- clockwise.

At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator.

The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

Uses of electric motor

An electromagnet in place of permanent magnet
Large number of turns of the conducting wire in the current-carrying coil
A soft iron core on which the coil is wound. The soft iron core, on which the coil is wound, plus the coils, is called an armature. This enhances the power of the motor.

Electromagnetic Induction

The phenomenon is which electric current is generated by varying magnetic fields is called electromagnetic induction.

We can induce current in a circuit by moving magnet in the coil:

Current can be induced in coil either by moving it in a magnetic field or by changing the magnetic
field around it as indicated by deflection in galvanometer needle.

Fleming’s right hand rule

Fleming’s right hand rule (for generators) shows the direction of induced current when a conductor moves in a magnetic field.

The right hand is held with the thumb, first finger and second finger mutually perpendicular to each other at right angles, as shown in the diagram.

The Thumb represents the direction of Motion of the conductor. The First finger represents the direction of the Field. (north to south)

The Second finger represents the direction of the induced or generated Current (the direction of the induced current will be the direction of conventional current; from positive to negative).

Electric Generator

In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.

Principle

Whenever in a closed circuit, the magnetic field lines change, an induced current is produced.

Construction

An electric generator, as shown in the below figure, consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two rings R1 and R2. The inner side of these rings are made insulated.

The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively. The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.

Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.

Working

When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say the coil ABCD is rotated clockwise in the arrangement shown in the above figure.

By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from B2 to Bl.

After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA.

The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes. Such a current, which changes direction after equal intervals of time, is called an alternating current (AC). This device is called an AC generator.

To get a direct current (DC, which does not change its direction with time), a split- ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down. We have seen the working of a split ring commutator in the case of an electric motor Thus a unidirectional current is produced. The generator is thus called a DC generator. The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.

Domestic Electric Circuits

If the resistance of an electric circuit becomes very low, then the current flowing through the circuit becomes very high. This is caused by connecting too many appliances to a single socket or connecting high power rating appliances to the light circuits. This results in a short circuit.

When the insulation of live and neutral wires undergoes wear and tear and then touches each other, the current flowing in the circuit increases abruptly. Hence, a short circuit 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.

Electric Fuse consists of a piece of wire made of a metal or an alloy of appropriate melting point, for example aluminium, copper, iron, lead etc. If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit.

Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits. The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unduly high electric current. The fuses used for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc.