Question

A soap bubble is given a negative charge, then its radius
(A) Decreases
(B) Increases
(C) Remains unchanged
(D) None of these

Answer 1

Hint : From the principle of electrostatics, negative charges will repel one another. On a surface, these negative charges tend to maximize the distance between each other.

Complete step by step answer
A soap bubble made normally, which has a particular radius, has neutral charge. Now imagine we have such a soap bubble, and we have allowed it to become negatively charged, say by a stream of electrons placed on its surface. The electrons being negatively charged, by the law of electrostatics, repel each other and tend to have the maximum distance apart from one another. As the electron seems to spread out from one another on the surface area of the soap bubble, they reach a maximum horizontal displacement (displacement parallel to surface of soap bubble). Now as the forces still continue to act on each other, to increase their displacement, the surface area of the soap bubble then increases. Since the surface area can only increase by the radius increasing then, we can conclude that the radius of a soap bubble increases when given a negative charge.
Hence, the correct answer is option B.

Note
For clarity, the electrons possess a maximum horizontal displacement from each other because all the electrons are pushing each other away from one another, i.e. when one electron pushes one to the left another at the left pushes that one to the right, which then creates a horizontal equilibrium. However, since the soap bubble is a sphere, hence, are not exactly on the same plane, the forces thus possess a component in the vertical (actually radial) direction. However, these radial components will soon be balanced by the surface tension of the bubble or burst when it can’t.