Magnetism and Electricity for standard 10 GSEB Course

Review

An electric current passing through a conductor produces a magnetic field around it. This effect of electric current is called magnetic effect.

When a cell is not connected to a load, the potential difference between the poles of the cell is called its electro motive force (emf).

The S,I. unit of potential difference and ‘electro motive’ force is ‘volt’ and the device used to measure them is ‘voltmeter’.

1.1 INTRODUCTION

As you know, a current flowing through a conductor produces a magnetic field around it.

Can we have reverse effect of this?

Can a magnetic field produce an electric field?

Michael Faraday, a British scientist, answered this question through a series of experiments.

1.2 FARADAY’S EXPERIMENTS

Faraday wound a long copper wire, on a cardboard cylinder. Between the turns, he wound twine and between the layers he placed calico-cloth. The ends of the wire were connected to a galvanometer.

Faraday thrust a pole of a bar magnet quickly into the coil. The galvanometer showed the presence of an electric current. He pulled the magnet out of coil. The pointer of the galvanometer deflected, showing the presence of an electric current; but this time the pointer moved in the opposite direction. The amount of deflection was found to increase with the increase in speed of the magnet. He found that when the magnet was at rest inside the coil, no electricity was produced. He repeated the experiment in a different way, moving the coil and keeping the magnet still. The result was the same. Thus, he discovered how magnetism could produce electricity. Relative motion between the conductor and the magnet produces electricity in the conductor.

Food for thought

1.                 Why did Faraday wind twine in between the turns of copper wire?

2.                 Why did he place calico-cloth in between layers?

Faraday's Experiment

Faraday's Experiment Presentation.

When south pole of a magnet approaches the coil the direction of induced current in the coil is clockwise.

When the south pole of a magnet recedes from the coil, the direction of induced  current in the coil is anticlockwise.

MICHAEL FARADAY

(1791 – 1867)

Faraday was born in a poor blacksmith’s family. He has made remarkable discoveries in both physics and chemistry. He was an assistant to Sir Humphery Davy. In 1831, he liquified chlorine and he discovered banzene in 1825. in 1831, he conducted his experiments on electro-magnetic induction. He has formulated the laws of electrolysis also.

Activity

i.Conduct the above experiment by using magnets of different strengths and a coil of insulated copper wire.

ii.Keep the magnet stationary and move the coil.

iii.Increase the number of turns of the coil and repeat the experiment.

iv.Observe the deflections for different speeds of the magnet.

Record your observations in all the situations. What conclusion can you draw out of your observation?

Recall

What is galvanometer?

For what purpose can it be used?

Electromagnetic induction – Meaning :

“When the magnetic field linked with a circuit changes, an electromotive force (emf) will be induced in the circuit. This phenomenon is called electromagnetic induction”.

In a closed circuit an electric current will be produced.

It is called induced current.

 1.3 FACTORS INFLUENCING INDUCED EMF.

Experiments show that the induced e.m.f. in a coil increases with the increase in

       i.            The number of turns of the coil and

     ii.            The rate of change of magnetic field linking with the coil.

1.4 FARADAY’S LAWS OF ELECTROMAGNETIC INDUCTION.

Faraday enunciated the following laws on the basis of his experiments carried out by him.

       i.             First law : -  A changing magnetic field linking a conductor induces an electro motive force in the conductor.

     ii.            Second law : - The induced electromotive force is proportional to the rate of change of magnetic field linking the conductor.

1.5 FLEMING’S RIGHT HAND RULE (DYNAMO RULE)

Fleming’s right hand rule gives the relationship between the directions of magnetic field, induced current and the direction of motion of the coil.

“Arrange the main finger(thumb), the fore finger and the centre finger of the right hand at right angles to each other such that the fore finger (2) indicates the magnetic field and the main finger (1) indicates the direction of motion of the conductor, then the centre finger (3) indicates the current induced.”

FLEMING’S RIGHT HAND RULE (DYNAMO RULE)

1.6 AC DYNAMO

A device that converts mechanical energy into electrical energy using the principle of electromagnetic induction is called a dynamo.

John Ambrose Fleming

(1849 -  1945)

Fleming was a British electrical engineer. He invented diode, which was used as a rectifier in Radio, TV, Telegraph, Radar and other devices. This paved the way for a new branch of physics. He worked as a professor in a university college in London. He is remembered for his contribution in the field of electronics.

A schematic representation of a simple dynamo is shown. A dynamo consists of  a rectangular coil of insulated copper wire ABCD mounted between the poles N and S of  a powerful magnet. The free ends of the copper wire are connected to two copper rings R₁ and R₂. Two carbon brushes B₁ and B₂ are touching the rings R₁ and R₂ respectively.

These brushes are connected to load L in the external circuit. The coil along with the rings is called armature.

A C Dynamo

When the armature is made to rotate, say in clockwise direction, the magnetic field linked with the coil changes. This induces an electric current in the coil ABCD. During the first half of the rotation, the current flows along ABCD. D is connected to R₁ which is in contact with B₁ . Therefore the current flows in the external circuits from B₂ to B₁.

A C Dynamo Graph

AC Dynamo Graph full

During  the second half of the rotation of the armature, current is induced in the coil along DCBA. Therefore, in the external circuit current flows from B₁ to B₂ . In all these cases the direction of induced current is found by applying Fleming’s right hand rule. It is found that the direction of induced current in the external circuit keeps reversing every half a cycle. Such a current is called ‘alternating current’ or ‘AC’. A dynamo that produces an alternating current is called ‘alternating current dynamo’ or ‘AC dynamo’.

1.7 DC DYNAMO

The construction of a DC dynamo is similar to that of an AC dynamo. But instead of full rings as in AC dynamo, two halves S₁ and S₂ of a copper ring, are used in DC dynamo. This is called ‘split ring’

When the armature is made to rotate in clock wise direction, the magnetic field linked with the coil changes. This induces an electric current in the coil ABCD. During the first half of the rotation of the

armature, the current flows along ABCD. As D is connected to S₂ and S₂ to B₂, the current flows through the load L in the external circuit from B₂ to B₁.

D C Dynamo

During the second half of the rotation of the coil, the current is induced in the coil in the direction DCBA. As the split half S₂ now comes in contact with B₁, the current flows in the external circuit in the same direction. This type of current is called ‘direct current’ or DC and the dynamo is called ‘direct current dynamo’ or ‘DC dynamo’.

D C Dynamo Graph

Do you know

The electric current supplied to homes and industries is alternating current.

Discuss with your teacher.

Observe the structure of a dynamo used in a bicycle with the help of a mechanic. There instead of a single coil, a number of coils are used. They are arranged in different planes, but in series, why? What is the reason ? Discuss with your teacher.

  

1.8 FLEMING’S LEFT HAND RULE (MOTOR RULE)

A conductor carrying current when kept in a magnetic field experiences a mechanical force. As a result, the conductor moves or tends to move in the direction of the mechanical force. Mechanical force is highest when the direction of current is at right angles to the direction of magnetic field. Fleming’s left hand rule gives the relation between the directions of electric current, magnetic field and the mechanical force acting on the conductor.

FLEMING’S LEFT HAND RULE (MOTOR RULE)

“Arrange the main finger (Thumb), forefinger and the centre finger of the left hand in such a way that they are perpendicular to one another. If the forefinger indicates the magnetic field and the centre finger indicates the direction of electric current, then the main finger indicates the direction of mechanical force acting on the conductor. (and direction of motion, if the conductor is free to move)”.

Do it your self

Arrange an aluminum rod AB of about 1 or 2 mm thick in between the two poles N and S of a powerful magnet as shown in the fig, with a switch and a DC source connected in the circuit. What do you notice when you switch on? Note the directions of magnetic field, electric current and the mechanical force on th rod. Apply Fleming’s left hand rule to find the relation between these directions. If the current is reversed, the deflection of the rod is also reversed.

Magnetic field direction

1.9 ELECTRIC MOTOR – DC MOTOR.

Electric motor is a device that converts electrical energy into mechanical energy. A motor works on the principle that a conductor carrying current in a magnetic field experiences a mechanical force.

A simple DC motor consists of a rectangular coil of insulted copper wire(ABCD) mounted in between the poles N and S of a powerful magnet. The free ends of the coil are connected to two halves S₁ an S₂

of a split ring,. The conducting brushes B₁ and B₂ are in contact with S₁ and S₂ respectively. These brushes are connected to poles of a battery.

D C Motor

Let an electric current be passed through the coil, say, in the direction ABCD. Mechanical force acts on its limbs in opposite directions. By applying motor rule, the direction of mechanical force can be found. The forces acting on AB and CD, form a couple. Therefore, the coil begins to rotate about its axis. When the coil completes the first half of its rotation, S₁ comes in contact with B₂ and S₂ comes in contact with B₁.

Now, electric current flows in the coil in the direction DCBA. Due to the effect of the coupled formed, the coil continues to rotate in the same direction. In the same way, a motor converts electrical energy into mechanical energy. Since it works on direct current, it is called ‘DC motor’.

Motor is used in electric fans, cranes, electric trains, textile machines, and in many other devices.

Remember

Two equal and parallel forces acting on a body at two different points in opposite directions, constitute a couple.

A couple acting on a body tends to rotate the body or actually rotates the body.

In a motor, the coil of wire together with split rings is called ‘armature’. Brushes and the connecting wires together with load form ‘external circuit’.

Activity

Can a DC motor work on AC?

What changes are to be made in it so that it works on AC?

Discuss this with  your teacher

POINTS TO REMEMBER

When the magnetic field linked with a circuit changes, an e.m.f. will be induced in the circuit. This phenomenon is called electromagnetic induction.

The induced emf in a coil increases with the increase in the number of turns of the coil and the rate of change of magnetic field linking with the coil.

A changing magnetic field linking a conductor induces an e.m.f. in the conductor.

The induced emf is proportional to the rate of change of magnetic field linking the conductor.

Dynamo Rule:

Arrange the main finger (thumb), the fore finger and the centre finger of the right hand at right angles to each other such that the fore finger indicates the magnetic field and main finger indicates the direction of motion of the conductor, then th centre finger indicates the current induced.

Dynamo:

A device that converts mechanical energy into electrical energy using the principle of electromagnetic induction is called a dynamo.

Motor Rule:

Arrange the main finger(thumb), fore finger and the centre finger in such a way that they are perpendicular to one another. If the fore finger indicates the magnetic field and the centre finger indicates the electric current, then th main finger indicates the direction of mechanical force acting on the conductor.

Electric motor (DC motor) is a device that converts electrical energy into mechanical energy.