Concepts of Physics

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

The Rotating Straw

Objective

To see attraction and repulsion of charges

Introduction

As we know, by rubbing or induction electric charge can be produced. By combing your hair you must have tried to attract small pieces of paper. You must also be knowing that charges are of two types - positive charge and negative charge. Like charges repel while unlike charges attract each other. We shall see this with the help of an interesting experiment.

Apparatus

A plastic bottle with a smooth cap, two cylindrical straws

Procedure

Take a plastic bottle whose cap is plane. The friction between the cap and the straw should be low, so that the straw is free to rotate on the cap.

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Take two plastic straws and by holding them from one end rub them well two or three times with the help of a handkerchief. Now, place one of the straws on the top of the bottle cap in such a manner that it remains parallel to the ground and its middle portion sits on the top of the cap. Now take the other straw, holding it from one end bring it near to the first straw. As soon as the second straw is brought near to the straw resting on the bottle, the latter moves away from the former by rotating on its resting point. If we keep moving the hand held straw in a circular manner towards the straw on the bottle cap, the latter also continuously keeps getting away. While doing so it also rotates presenting a very interesting view of repulsion. Now, instead of the second straw bring the handkerchief which was used for rubbing the straws, towards the first straw. The first straw now starts getting attracted towards the handkerchief. Moving the handkerchief away from the straw in a circular manner, the straw also keeps moving towards the handkerchief while making a circular rotation.

Now, rub only one of the straw with handkerchief and place it over the cap. Now bring neutral straw close to the charged straw. The straws start attracting each other.

Discussion

When both the straws are rubbed with a handkerchief, the same kind of charge is developed in them while the opposite charge is developed in the handkerchief. In the first case, as both the straws hold the same kind of charge they repel each other and so the straw resting on the bottle moves away from the hand held straw. In the second case as the charge on the handkerchief has a sign opposite to that on the straw, the handkerchief and the straw attract each other, therefore, the straw on the bottle moves towards the handkerchief.

The attraction between charged straw and neutral straw is caused by induced charges. The charges in neutral straw gets re-distributed in such a way that opposite charges (opposite to charge on the charged straw) comes close to the charged straw. The attraction is caused by attractive force between the induced charges and charges on the charged straw.

During winter people are often found rubbing their hands. When you rub your hands against each other do they develop charges? Devise an experiment which can answer this question.

Note

It is difficult to do this experiment in humid environment e.g., in rainy season.