A pith ball (a small sphere of tree bark) has mass m = 60mg = 6 × 10 5 kg and re

Question

A pith ball (a small sphere of tree bark) has mass m = 60mg = 6 × 10 5 kg and receives a charge of +1nC = 1 × 10 9 C when zapped with the electrophorous. Our electrostatic generator (the Van de Graaff) has a spherical surface with radius 12 . 5cm . When in operation a substantial surface charge is transferred to the sphere. If the pith ball is placed d irectly on top of the Van de Graaff generator (without transferring charge), how much total charge must be placed on t he Van de Graaff to lift the pith ball against the force of gravity

Explanation / Answer

to lift the ball

electrostatic repulsive force=mg

electrostatic repulsive force=k*q*1*10^(-9)/(0.125^2)

where q is the charge of the generator

mg=6*10^(-5) *9.8

equating both and finding for q

we get q=1.02*10^(-6) C=1.02uC

and it should be positive

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