What happens when some charge is placed on a soap bubble?

The answer is A: the bubble's radius increases. When charge is placed on a soap bubble, it spreads over the film, and like charges repel each other. This repulsion produces an outward electrostatic pressure on the surface. Surface tension pulls the film inward, giving an excess pressure of about 4T/r inside. Adding charge adds an outward push that partly opposes surface tension, so the film settles at a larger radius.

Why the others fail:

• B: A shrinking radius would need an inward force. Like charges on the same surface repel, so they cannot pull the film inward.
• C: Collapse happens when the inward pressure wins. Charge does the opposite and supports the film, so it does not collapse under ordinary charging.
• D: Rejected, because A is correct.

The core concept is electrostatic pressure on a charged conductor surface, which is related to the field just outside the surface and is covered in the electrostatics unit of Class XII physics. It also links to the surface tension and pressure ideas in the General Science part of the syllabus.

Trap: Aspirants often assume that a charged body is always pulled inward or that charge makes things contract. Here the repulsion between like charges on a curved surface acts outward.