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

# Detecting Static Electricity by Electroscope

## Introduction

When we rub a balloon over our hair, we can make it stick to the walls. The act of rubbing makes the balloon charged. In this demonstration we find out more about this type of charging with the help of a home made electroscope.

## Apparatus

Straws, cloth, aluminium leaf electroscope, and a plastic bottle. An electroscope can be made using the mosquito repellent `All-out' box by hanging aluminium leaves inside it with the help of a paper clip.

## Procedure

1. Hold the two straws together and charge them by rubbing on a silky cloth.
2. Place one of the straw on the bottle and bring the other straw near it. What do you see?
3. Now bring the cloth near to the straw. What do you see?
4. Bring one of the charged straws and touch it to the aluminium ball of the electroscope. Do the leaves show divergence? Why?
5. Bring the charged straw near the electroscope without touching. Do the leaves again show divergence? Why?

## Discussion

Electrons get transferred from one body to the other on rubbing, making the bodies oppositely charged. When the two straws are rubbed together with a silk cloth, both acquire the same charge while the silk cloth acquires an opposite charge.

Charge can be detected using an electroscope. When we touch the charged straw to the aluminium ball of the electroscope, some charges from the straw get transferred to the electroscope. Aluminium being a good conductor, these charges get distributed and the leaves diverge as both acquire the same charge. This is charging by conduction method.

When a charged straw is brought near the electroscope, the leaves again show divergence. By bringing the charged straw near the electroscope the electrons get re-distributed in the aluminium ball and its leaves acquire the same charge as the straw.