Question

A thread is tied slightly loose to a wire frame as in figure and the frame is dipped into a soap solution and taken out. The frame is completely covered with the film. When the portion A is punctured with a pin, the thread:

A) Becomes concave towards A
B) Becomes convex towards A
C) Either A or B depending on the size of A with respect to B.
D) Remains in the initial position

Answer 1

Hint: Here the entire frame is covered by a soap film which is divided into two sections - A and B. On puncturing section A, the surface tension at the boundary of the two sections A and B will no longer be balanced and it will be towards B.

Complete step by step answer:
Step 1: Sketch a figure describing the change in the surface tension at the boundary of the two sections A and B.
In this problem, the wire frame gets dipped in a soap solution and when it is taken out of the solution it gets covered by a layer of soap. The soap film consists of two portions A and B separated by the thread tied to the frame. This is shown in the figure below.

So initially the surface tension of the film at the boundary of the two portions A and B was balanced. But when portion A got punctured with a pin, the surface tension will be directed towards section B. This causes the thread to move towards B i.e., to take a concave shape towards A. This is shown in the figure below.

Hence the correct option is A.

Note:
The soap film constitutes a lens. A lens is said to be concave if at least one of its spherical surfaces is curved inward and a lens is said to be convex if the two surfaces that make up the lens are curved outward.

Given above are the figures of a concave lens and a concave mirror. Here the thread curves inwards to section B and so it is concave towards A.