Question

A soap bubble is given negative charge, its radius will:
A. Increase
B. Decrease
C. Remain unchanged
D. Fluctuate

Answer 1

We know that like charges repel each other. When we place a charge on the soap bubble, it gets uniformly distributed over the surface of the soap bubble. Recall the electrostatic force between the two charges to answer this question.

Complete step by step solution:
If we have an uncharged soap bubble, the soap bubble attains equilibrium due to the same number of positive and negative charges. Now, when we give an extra negative charge to the soap bubble, it will get distributed uniformly all over the surface of the bubble.
Now, we know that like charges repel each other. Due to these extra negative charges, the equilibrium of the soap bubble will get lost and the negative charges will start to repel each other. We can draw the situation as shown in the figure below.

Due to their repulsion, we can see that the charges will move away from each other along with the surface soap bubble. Therefore, the volume of the soap bubble will increase till the surface tension plays a role to stop the further increase in the volume of the soap bubble. The surface tension of the soap bubble will balance the electrostatic repulsion of the charges and the soap bubble will attain the equilibrium.

Thus, we can observe that the volume of the soap bubble has increased due to repulsion of the like charges. Since the volume is proportional to the cube of the radius of the soap bubble, we can say that the radius of the soap bubble also increased in this case.

So, the correct answer is “Option A”.

Note:
Even if we give extra positive charges to the soap bubble, it will get distributed uniformly on the surface of the soap bubble. The result will be the same as in the case of negative charge that we discussed above. Note that the soap bubble will not fluctuate while increasing the volume since the surface tension of the soap bubble then balances the electrostatic force.