Q. 29

A wire forming a loop is dipped into soap solution and taken out so that a film of soap solution is formed. A loop of 6.28 cm long thread is gently put on the film and the film is pricked with a needle inside the loop. The thread loop takes the shape of a circle. Find the tension in the thread. Surface tension of soap solution = 0.030 N m-1.

Answer :

3 × 10-4 N


After pricking, the loop will take a circular shape as shown.


Hence, the total force is given by .


Surface tension of soap solution,


Given, length of the thread,


Hence,



Let be the length of contact surface.


We know,


From the figure, .


Hence,



For small angles,




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