IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Making of an Electric Motor

## Objective

To learn about working of an electric motor.

## Introduction

The current carrying conductor in a magnetic field experiences a force. In this demo, force on a current carrying coil rotate it.

## Apparatus

A coil, two safety pins, magnet, battery

## Procedure

Take an enamelled copper wire and make a circular coil of say 10 turns and diameter of about 3 cm. You can make it by rolling the wire on a dry cell. Tie the wire at two ends of the coil in diametrically opposite direction. Completely remove the insulation from one end of the coil wire and remove it partially (half) from the other end. Make these ends straight as shown in the figure.

Fix two safety pins vertically on a stand. The round ends of the pins should be on the top and parallel to each other. The separation between the pins should be slightly greater than coil diameter. The straight end of the coil will pass through the round end of the pin and the coil will be hanging between the pins. Connect two safety pins to the two ends of a battery. Bring a magnet close to the coil. Manually give initial rotation to the coil. The coil will start rotating. You may have to practice as mechanical stability and positioning of the magnet is crucial for successful demonstration.

## Discussion

When an electric current flows through the coil, the coil becomes an electromagnet with two poles – a north pole and a south pole. The north pole of the electromagnet is attracted by the south pole of the permanent magnet and is repulsed by the south pole of the permanent magnet. This mutual attraction-repulsion makes the coil to rotate. The coil will stop ones its N and S poles align with the S and N poles of the permanent magnet. But when this situation occurs, something interesting happens. Until now, the copper part of the coil ends were in contact with the pins. But now the enamelled part of the coil end comes in contact with the pins which switch off the current through the coil. The coil completes this half of the rotation due to angular momentum. After completion of half rotation, once again the circuit is closed and the coil becomes an electromagnet. In this way, the coil continues to rotate.