Concepts of Physics

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

Drinking water through the Punctured Straw

Objective

To experience the role of pressure while drinking through the straw.

Introduction

Air pressure always plays an important role to push the water above the surface.

Apparatus

two straws, two glasses of drinking water

Procedure

Make a hole below the end of one of the straw. Take drinking water in the two glasses. The end of the straw which has the hole should be above water. Let two students try to drink water from the glasses with the straw. You will observe that the student who is using the straw with the hole cannot drink water, but other student can drink water easily.

Discussion

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There is air in the straw between the top of water and the top of straw. When sucked, a partial vacuum is created in the space above the water in the straw. The difference between the pressure on the surface of water in the glass and the reduced pressure inside the straw causes the water to rise in the straw. In the straw with a hole, partial vacuum cannot be created by sucking as the inside of the straw is always connected with the outside atmosphere. Hence there is no pressure difference to move the water up.