Physics II
posted by Alona .
A bead with a mass of 0.085 g and a charge of 20 nC is free to slide on a vertical rod. At the base of the rod is a fixed 20 nC charge.
In equilibrium, at what height above the fixed charge does the bead rest?

Let the charge separation be x. The elecrostatic repulsion force must equal the weight.
k*(20*10^9)^2/x^2 = M g
M = 85*10^6 kg
g = 9.8 m/s^2
k is the Coulomb constant. (You may need to look it up. If you are studying elctricity, your text will say what the value is.)
Solve for x 
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