Concepts of Physics

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

A water bottle with a hole in bottom

Introduction

If you have a bottle filled with water with a small hole in the bottom and cap closed, water does not come out. This is a widely used demonstration to show the effect of atmospheric pressure. But what is missing from the discussion is the question ``Why air does not enter through the hole?''. Indeed if the hole is too big, air enters and water falls. This experiment addresses this issue.

Apparatus

A plastic box (in place of bottle), 3-4 caps with different sizes of hole, a polythene bag with water in it, a pin, water, a tub

Procedure

  1. Fill water in the box and screw the cap with smallest hole. Hold the box and invert over the tub. Does water come out? Do few drops come out? Explain your observations.
  2. Repeat step-1 with other caps of bigger holes. Write your observations.
  3. Use the cap with the smallest hole. Fill water, put the cap and invert. Keep holding in your hand. When everything is settled and water does not come out, laugh vigorously as in Hasyasan. You may find that with every burst of laugh, some water comes out. If not, use the cap with next bigger hole. Explain why water comes out when you laugh and does not come out other wise.
  4. Now take the polythene bag filled with water and closed tightly. Hold it over the tub and make a small hole in the bottom using the pin. Why does water keep coming out even with such a small hole? As the water comes out, does the air pressure in the polythene bag decrease?