Concepts of Physics

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

Playing with Syringe and Atmospheric Pressure

Introduction

Air exerts pressure. The force on a surface due to air pressure depends on the orientation of the surface exposed to air. The force is always perpendicular to the surface and is of push type. If you rotate the surface, the force on it due to air also rotates. So air can exert force from any direction. The force is proportional to the area of the surface. A disposable syringe is very effective to demonstrate these facts.

Apparatus

Two disposable syringes of different sizes

Procedure

Remove the needles of the syringes. Hold a syringe in one hand. The syringe has two parts, a piston and a barrel. At one end of the barrel there is a small opening in which the needle fits. At the other end there are two protruding portions. %You have to hold the syringe as shown in figure. Close the opening in the barrel by your thumb and put your fore finger and the middle finger on the farther faces of the protruding parts. Go to a student with the syringe vertical, your thumb below the syringe and the fingers above. Ask the student to pull the piston up. The student has to apply some force to pull it up. Now ask the student to leave the piston. When the student leaves the piston, the piston goes back in the barrel. The movement will be from top to bottom. Stress on the question why did the piston go back when you left it. Who pushed in back? Air pushed it back. What is the direction of the force exerted by the air? It is the direction in which the piston moved, that the opening goes to the top and the protruded parts come down. Hold the syringe in the same fashion. Now the thumb is above and fingers are below. Again go to a student and ask him/her to pull the piston and leave. When the student leaves it, the piston goes back but this time the direction of the movement is upwards. So air pushed the piston in upward direction. Previously, it was downward direction. Next time rotate the syringe to make it horizontal and repeat the action. This will show that air is pushing the piston in horizontal direction. You can change the orientation of the piston in different direction, and show that air can exert force from all directions. The direction depends on the orientation of the surface on which the force acts.

Now hold one syringe in one hand and the other syringe in the other hand, in the same fashion as described above. Go to a students and ask him/her to pull the two pistons one by one. Bigger syringe needs greater force to pull. So the force by air on a surface increases with the area of the surface.

Discussion

The experiment is very convincing in showing that air exerts force on the surface. It only needs a qualitative idea of Newton's law of motion that to start a motion of a body in a particular direction an unbalanced force is needed in that direction. These ideas are developed from experience even before the age of 3 years.