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## Homework Statement

During a collision between two initially uncharged polystyrene balls of radius 2 mm, some electrons are transferred from one of the balls to the other. In what direction does the resulting electrostatic force between the balls act?

The magnitude of the force between the balls is 0.02 N when their centres are separated by 7 mm. Assuming that the charge is uniformly distributed over the surface of each of the balls, determine the number of electrons that was transferred from one ball to the other in the collision.

As polystyrene does not conduct electricity, it is likely that the charge is in fact concentrated at the points on the two balls which came into contact in the collision. Determine the number of electrons transferred in the collision under this assumption, assuming also that the balls are free to rotate.

## Homework Equations

F=(1/4(pi)e

_{0})*(q

_{1}q

_{2}/r

^{2})

## The Attempt at a Solution

For the first part I put the resulting electrostatic force between the balls is acting inward since one ball is now positively charged after losing an electron whereas the other is negatively charged as its gained an electron. Is this correct?

For the 2nd part I used the equation above to get:

0.02=(8.99*10

^{9})(q

^{2}/(7*10

^{-3})

^{2})

q

^{2}=1.09*10

^{-16}

q=1.04*10

^{-8}

no of electrons=(1.04*10

^{-8})/(1.6*10

^{-19})=6.53*10

^{10}

is this correct?

Finally for the 3rd part I havent got a clue where to start so any hints on how to begin it would be grateful, thanks.