Concepts of Physics

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

Visualize Force of Surface Tension

Objective

To show the pull by a soap solution film.

Introduction

Soap solution films are easy to make and they stays for quite some time. If you place a thread on such a film, the thread can stay in the arbitrary shape you start with. This is because the film is on all sides of the thread and hence the surface tension forces on the thread gives zero resultant force. However if the film is only one side of the thread, the resultant force on the thread is not zero and it shows up very clearly in this demo.

Apparatus

soap solution, a ring

Procedure

Take water in a plate and put some soap or detergent in it. Bend a wire, say copper enamelled wire, in circular form to make a ring. Tie a thread at the diametrically opposite points of the ring. The thread should be larger than the diameter.

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Put the ring in the soap solution for a while and gently lift it. You will get a film on the ring. If not, try again. Air currents from the fan often disturb formation of the film. Look at the thread. It is lying on the film in an irregular shape. At any small portion of the thread there are forces from the film portion on the both sides of it and it stays in equilibrium.

Now puncture the film on one side of the thread. All you have to do is touch the film on one side gently with your finger. Look at the thread. It has taken the shape of a perfect circular arc.

Discussion

The film is only on one side of the thread. So it pulls the thread towards itself. The minimum surface area of the film is formed when the thread takes the shape of a circular arc. That is why the thread takes this shape.