To understand what will happen when some charge is placed on a soap bubble, we have to understand how charges interact with other charges. A charge or a charged object produces a magnetic field around it, which is a space around the charge where its effect can be felt by other charges or charged objects. Charges have this nature where they repel similar charges and attractive opposite charges. To make this more clear, in simple words positive charges (or positively charged bodies) repel other positive charges (or positively charged objects), negative charges (or negatively charged bodies) repel other negative charges (or negatively charged objects) and positive charges attract negative charges (and negative charges positive charges). So, we know a soap bubble is somewhat spherical and is formed when air is trapped in a mixture of soap and water. Now if we place a charge (no matter positive or negative) the charge would spread through the entire surface of the soap bubble. So now the whole surface of the soap bubble is covered with a charge of the same nature and we discussed above that the same nature of charges repels each other. So the now charged molecules that make up the soap bubble repel each other and this causes an increase in the size of the soap bubble. Hence, the radius of the soap bubble increases.