Concepts of Physics

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

Keep the Paper Dry in Water

Apparatus

glass, handkerchief of pieces of paper, a pot of water

Procedure

Stuff a large handkerchief or some crumpled newspaper into an empty glass. Make sure the handkerchief won't fall when you turn the glass upside down. Then, fill a pot with water. Holding the glass so that its rim is down, put the glass deep into the pot of water and hold it there. After a minute or two, pull the glass out of the water and remove the handkerchief. You will see that handkerchief is still dry.

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Discussion

Water cannot enter the glass because the glass is already filled with air. The `empty' glass is full of air, a gas. It has no size or shape of its own but fill every space it can. Actually, water rises in the glass to a small height. The rise of water inside the glass reduces the volume of air trapped inside the glass. By Boyle's law, the pressure of the air increases. The increases in air pressure increases the force on water surface which stops the rise of water inside the glass.