A soap bubble is given a negative charge, then its radius:
A.)Decrease
B.)Increase
C.)Remains unchanged
D.)Can’t say

Hint: When extra charge is given to a soap bubble, all the extra charge will be distributed uniformly on the surface. This uniform distribution of charge causes repulsion on the surface of the bubble and this tends to increase the surface area of the bubble.

Complete step by step answer:
In an uncharged soap bubble, there is equilibrium between the positive and negative charges and it remains stable. However, if extra charge is somehow given to the soap bubble, all this extra charge will accumulate on the surface and be uniformly distributed over it. This will cause repulsion between the distributed charges on the surface since all the charges are of a similar nature and like charges repel.

This force of repulsion tends to stretch the bubble and increase the surface area so that the charges on the surface can be as far apart as possible. Hence, the radius and ultimately the surface area of the bubble start increasing. The opposing force to this is the surface tension of the bubble which tries to shrink the surface and make it smaller. When the surface tension is able to completely balance this electrostatic repulsive force, then a new equilibrium is reached.

Hence, the radius of the bubble will increase when it is given a negative charge. Therefore, the correct option is B) increase.

Note: Students might think that when extra charge is added to the soap bubble, it will contract since there will be a greater electrostatic force of attraction between the opposite charges. However, this is not true and they must understand that if a particular type of charge is given, it becomes in excess and then the electrostatic force of repulsion between these like charges in excess outweighs the electrostatic forces of attraction between the opposite charges.