CBSE NOTES CLASS 10 SCIENCE CHAPTER 13

MAGNETIC EFFECTS OF ELECTRIC CURRENT

Properties of magnet

• A bar magnet has two poles, North Pole (N) and South Pole (S).

• A free suspended bar magnet always points towards north and south direction.

• The pole of a magnet which points toward north direction is called North Pole.

• The pole of a magnet which points toward south direction is called South Pole.

• Like poles of a magnet repel each other while unlike poles of magnets attract each other.

Magnetic Field

The influence of a magnet in the area surrounding a magnet is called magnetic field.

The region surrounding a magnet, in which the force of the magnet can be detected, is said to have a magnetic field.

Magnetic field is a vector quantity, i.e. it has both direction and magnitude.

Magnetic Field Lines

The imaginary lines of magnetic field around a magnet are called field lines or magnetic field lines.

When iron fillings are allowed to settle around a bar magnet, they get arranged in a pattern which mimicks the magnetic field lines.

Drawing a magnetic field line with the help of a compass needle

• Take a small compass and a bar magnet.

• Place the magnet on a sheet of white paper fixed on a drawing board, using some adhesive material.

• Mark the boundary of the magnet.

• Place the compass near the north pole of the magnet. The south pole of the needle points towards the north pole of the magnet. The north pole of the compass is directed away from the north pole of the magnet.

• Mark the position of two ends of the needle.

• Now move the needle to a new position such that its south pole occupies the position previously occupied by its north pole.

• Repeat the process till you reach the south pole of the magnet.

• Join the points marked on the paper by a smooth curve. This curve represents a field line.

Direction of Field Lines

• Outside the magnet, the direction of magnetic field line is taken from North Pole to South Pole. Inside the magnet, the direction of magnetic field lines is taken from South Pole to North Pole. Hence the magnetic fields lines are closed curves.

• The tangent to the magnetic field line at any point gives the direction of magnetic field at that point.

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

Strength of magnetic field

• The closeness of field lines shows the relative strength of magnetic field. Closer the lines, stronger the magnetic field and vice-versa. Crowded field lines near the poles of magnet show more strength.

Magnetic field due to a current carrying conductor

Electric current produces magnetic effect. The magnetic effect of electric current is known as electromagnetic effect.

It is observed that when a compass is brought near a current carrying conductor; the needle of compass gets deflected because of flow of current through the conductor.

This shows that electric current through a conductor produces a magnetic effect.

Activity to show that the direction of magnetic field produced depends upon the direction of current.

• Take a long straight copper wire, two or three cells of 1.5 V each, and a plug key. Connect all of them in series as shown in first figure.

• Place the straight wire parallel to and over a compass needle.

• Plug the key in the circuit.

• It is observed that, if the current flows from north to south, the north pole of the compass needle moves towards the east.

• Now replace the cell connections in the circuit as shown in second figure , so that the current flows from south to north.

• The needle, now, moves in opposite direction, that is, towards the west.

• We can thus conclude that the direction of magnetic field due to straight current carrying conductor depends upon the direction of current through the wire.

Magnetic field due to current through a straight conductor

A current carrying straight conductor has magnetic field lines in the form of concentric circles around it.

Magnetic field of current carrying straight conductor can be shown by magnetic field lines, which can be drawn using a compass or iron fillings.

Drawing the magnetic field lines around a straight current carrying conductor

• Take a battery (12 V), a variable resistance (or a rheostat), an ammeter (0–5 A), a plug key, and a long straight thick copper wire.

• Insert the thick wire through the centre, normal to the plane of a rectangular cardboard. Take care that the cardboard is fixed and does not slide up or down.

• Connect the copper wire vertically between the points X and Y, as shown, in series with the battery, a plug and key.

• Gently tap the cardboard a few times. Observe the pattern of the iron filings. We see that the iron filings align themselves showing a pattern of concentric circles around the copper wire

• Place a compass at a point (say P) on one of the circles.

• The direction of the north pole of the compass needle would give the direction of the field lines produced by the electric current through the straight wire at point P. The direction of magnetic field gets reversed in case of a change in the direction of electric current.

• If we increase/decrease the current through the conductor, the deflection in the compass needle also increases/decreases

• We also observe that the deflection of compass needle decreases as we move the compass away from the conductor

• We can thus conclude,

• The magnitude; of magnetic field increases with increase in electric current and decreases with decrease in electric current.

• The magnitude of magnetic field produced by electric current; decreases with increase in distance and vice-versa.

• The direction of magnetic field due to a current carrying conductor depends upon the direction of flow of electric current.

Right Hand Thumb Rule

The direction of magnetic field due to electric current through a straight conductor can be given by using the Right Hand Thumb Rule. It is also known as Maxwell’s Corkscrew Rule.

If a current carrying conductor is held by right hand keeping the thumb straight and if the direction of electric current is in the direction of thumb, then the direction of wrapping of other fingers will show the direction of magnetic field.

As per Maxwell’s corkscrew rule, if the direction of forward movement of screw shows the direction of current, then the direction of rotation of the screw shows the direction of magnetic field.

Magnetic field due to current through a circular loop

In case of a circular current carrying conductor, the magnetic field is produced in the same manner as it is in case of a straight current carrying conductor.

The magnetic field lines would be in the form of concentric circles around every part of the periphery of the conductor.

The concentric circles representing the magnetic field around it would become larger and larger as we move away from the wire. At the centre, the arcs of big circles would appear as straight lines.

Every point on the wire carrying current gives rise to the magnetic field appearing as straight lines at the center of the loop. By applying the right hand rule, every section of the wire contributes to the magnetic field lines in the same direction within the loop.

Clock Face Rule

A current carrying loop works like a disc magnet. The polarity of this magnet can be understood with the help of clock face rule.

If the current is flowing in anti-clockwise direction, then the face of the loop shows North Pole. On the other hand, if the current is flowing in clockwise direction, then the face of the loop shows South Pole.

Magnetic field and number of turns of coil

Magnitude of magnetic field gets summed up with increase in the number of turns of coil. If there are ‘n’ turns of coil, magnitude of magnetic field will be ‘n’ times of magnetic field in case of a single turn of coil.

Magnetic Field due to a current in a Solenoid

A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.

• A current carrying solenoid produces similar pattern of magnetic field as a bar magnet. One end of solenoid behaves as the North Pole and the other end behaves as the South Pole.

• Magnetic field lines are parallel inside the solenoid which means that magnetic field is same at all points inside the solenoid.

Electromagnets

A strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic material, like soft iron, when placed inside the coil.

Magnet formed by producing magnetic field inside a current carrying solenoid is called electromagnet.

The strength of magnetic field depends on,

1. Number of turns

2. Magnitude and direction of the current

3. Nature of core

4. Distance from the solenoid

Force on a current carrying conductor in a magnetic field

A current carrying conductor exerts a force when a magnet is placed in its vicinity. Marie Ampere suggested that a magnet also exerts equal and opposite force on the current carrying conductor.

The direction of force on the conductor gets reversed with the change in direction of flow of electric current.

The magnitude of force is highest when the direction of current is at right angles to the magnetic field.

Fleming’s Left Hand Rule

If direction of electric current is perpendicular to the magnetic field, The direction of force is also perpendicular to both the electric current and magnetic field and is given by Fleming’s Left Hand Rule.

According to this rule, stretch the thuMb, First finger 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 (Central) finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

The directions of electric current, magnetic field and force are similar to three mutually perpendicular axes, i.e. x, y and z axes.

Many devices, such as electric motor, electric generator, loudspeaker, etc. work on the principal of force on the current carrying conductor on a magnetic field.

Electric Motor

An electric motor is a rotating device that converts electrical energy to mechanical energy. Electric motor is used as an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players etc.

Principle of DC Motor

The principle of working of a DC motor is that "whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force".

Structure of Electric Motor

An electric motor consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field.

The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle.

The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively.

Working of Electric Motor

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, 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, 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.

Commutator is a device which reverses the direction of flow of electric current through a circuit. The split ring acts as a commutator here.

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.

The commercial motors use

(i) an electromagnet in place of permanent magnet

(ii) large number of turns of the conducting wire in the current carrying coil and

(iii) 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

Michael Faraday, an English Physicist studied the generation of electric current using magnetic field and a conductor.

When a conductor is brought in relative motion vis-à-vis a magnetic field, a potential difference is induced in it. This is known as electromagnetic induction.

This can be brought about by,

• Moving a conductor inside a magnetic field.

• Changing a magnetic field around a conductor.

We can also use a solenoid or electromagnet instead of the permanent magnet.

Consider the setup of two coils.

A potential difference is induced in the coil-2 whenever the electric current through the coil–1 is changing (starting or stopping).

Coil-1 is called the primary coil and coil-2 is called the secondary coil. As the current in the first coil changes, the magnetic field associated with it also changes. Thus the magnetic field lines around the secondary coil also change.

The process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction.

The field due to a coil-1 passing through coil-2 can be changed by,

1. Changing the current through the coil-1

2. Changing the number of turns in coil-1

3. Rotating or moving coil-2 with respect to magnetic field.

Note

• It is convenient in most situations to move the coil in a magnetic field.

• The induced current is found to be the highest when the direction of motion of the coil is at right angles to the magnetic field. In this situation, we can use a simple rule to know the direction of the induced current.

Fleming’s Right hand Rule

Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other, then if the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of conductor, then the middle finger will show the direction of induced current.

The directions of movement of conductor, magnetic field and induced current can be compared to three mutually perpendicular axes, i.e. x, y and z axes.

• Electromagnetic induction is used in the conversion of kinetic energy into electrical energy.

AC Generator

An electrical Generator is a device which converts mechanical energy (or power) into electrical energy (or power).

Principle of AC Generator

The phenomenon of electromagnetic induction is the principle behind the electric generator.

The essential parts of an electric generator are

• A magnetic field and

• A conductor or conductors which can move so that the magnetic field passing through it is continuosly changing.

Structure of AC Generator

An electric generator 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 sides 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 is 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 of AC Generator

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.

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.

The current in the external circuit flows from B2 to B1.

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 changes.

AC Current

The current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC).

Since the current generated changes direction periodically, this device is called an AC generator.

In India, power is generated and distributed as alternate current (AC). The direction of current changes after every 1/100 second i.e. the frequency of AC in India is 50 Hz.

AC is transmitted upto a long distance without much loss of energy.

DC – Direct current

Current that flows in one direction only is called direct current. Electrochemical cells produce direct current.

DC 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.