When a charged balloon sticks to a wall, the downward gravitational force is bal
ID: 1578918 • Letter: W
Question
When a charged balloon sticks to a wall, the downward gravitational force is balanced by an upward static friction force. The normal force is provided by the electrical attraction between the charged balloon and the equal but oppositely charged polarization induced in the wall's molecules.
Part A
If the mass of a balloon is 1.2 g, its coefficient of static friction with the wall is 0.74, and the average distance between the opposite charges is 0.30 mm, what minimum amount of charge must be placed on the balloon in order for it to stick to the wall?
Express your answer to two significant figures and include appropriate units
Explanation / Answer
Normal force is balanced by electric force between the charges
N = Fc
N = k q^2/ r^2
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upward static frction force is balanced by weight
fs = mg
us N = mg
us k q^2/ r^2 = mg
q = sqrt mgr^2/ k us
= sqrt ( 1.2* 10^-3) (9.8) ( 0.3* 10^-3)/9.0* 10^9 (0.74)
=2.3* 10^-8 C
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