Question

student hangs two inflated balloons from 1-meter strings so that they touch. The student then holds the balloons apart and rubs each balloon 25 times with animal fur. The student then releases the balloons. The balloons now hang a large 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 5 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 smaller number of rubs with fur will give them less force of electrical repulsion.

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

C.

Touching with a metal rod discharges the balloons (or lets them share/ground any residual charge), so they start neutral. Rubbing them only 5 times gives them much less net charge than 25 rubs did, so the Coulomb repulsive force between them is much smaller. With little or no charge they will hang straight down or much closer together.