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:

image f96d3eceb1164622a5b17bde1f4d77c6

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 a greater elevation, because they will have greater charges so the force of electrical repulsion between them will be greater.

B.
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.

C.
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.

D.
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.

Answer 1

Answer: A.

Touching the balloons with a metal rod lets any previous charge redistribute/neutralize so you start fresh. Rubbing each balloon with fur transfers the same sign of charge to both; doing it 10 times will put more charge on each balloon than 5 rubs did. By Coulomb’s law the repulsive force increases with the amount of charge, so the balloons will repel more strongly and hang farther apart (larger elevation/angle).

(B, C and D are incorrect: repulsion does depend on charge magnitude; more rubbing does not remove charge; rubbing both with the same material gives like, not opposite, charges.)