Question

A student hangs two inflated balloons from 1-meter strings so that they touch. The student then holds the balloons apart and rubs each balloon 5 times with animal fur. The student then releases the balloons. The balloons now hang a small distance apart from one another:

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The next step in the student's procedure is to touch the balloons with a metal rod and allow them to settle, and then to rub them 10 times with animal fur. Which result should the student expect, and why?

A.
The balloons will hang at the same elevation, because electrical repulsion is an unvarying phenomenon which results from the existence of charge and not its quantity.

B.
The balloons will hang at a smaller elevation, because more thoroughly rubbing them will more completely remove their charges, reducing the electrical repulsion between them.

C.
The balloons will hang straight down or more closely together, because a larger number of rubs with fur will give them opposite rather than like electrical charges.

D.
The balloons will hang at a greater elevation, because they will have greater charges so the force of electrical repulsion between them will be greater.

Answer 1

D.

Touching with a metal rod (and letting them settle) discharges and equalizes the balloons so they start neutral. Rubbing each balloon with fur gives them the same sign of charge; rubbing more times deposits more charge. By Coulomb’s law the repulsive force between like charges increases with the product of the charges, so the balloons repel each other more strongly and hang farther apart (greater elevation of the strings).