Concepts of Physics

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

Why water not going to the bottle through a funnel?

Objective

To learn that pressure of the air increases when it is compressed.

Introduction

It is generally observed that funnel is kept slightly above the bottle when it is filled with a liquid.

Apparatus

a transparent bottle, funnel, flour dough

Procedure

Place the funnel on the bottle and fill the bottle with water. Observe that water enters the empty bottle. Place the funnel again and put flour dough in the space between the funnel stem and the bottle mouth in such a way that air cannot go inside the bottle or come out of it. Now, try to fill the bottle with water. You will observe that after a little water has gone in, no more water goes in. Make a hole in the dough. Observe that water starts filling again into the bottle.

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Discussion

There was air in the bottle before it was filled with the water. When water is poured without the dough, the air inside the bottle was connected with the air outside it through the gap between the funnel and bottle mouth. Hence, when water is poured into the bottle, the air could come out through the gap. But when the dough was put, the air could not come out. After a little water had entered the bottle, the air inside the bottle get compressed and the pressure increased (Boyle's law). This increased pressure does not allow water to enter the bottle.

When a hole was made in the dough, a connection was established between the outside and the inside of the bottle. Hence, as water enters into the bottle, the air comes out through the hole.