# Homework Help: Electrostatic exercise, Electrical charge calculation

1. Jan 11, 2012

### nicolas.ard

Hello, i have to solve the following problem, i have the result, but i can get the way to arrive to it.
This is the problem:
There are 2 spheres of copper, separated at 1 meter, each sphere have a weight of 1 Kg initially.

The problem want's to know how many electrons i need to transfer from one sphere to the other one, to get a attractive force between the both spheres of 1x10^4 Newtons.

The result it's 6.59x10^15 electrons

I planted the Coulomb equation, but it's don't work. (B it's the number of electrons to transfer)
$Q_{1} = 2.7502*10^{26}electrons - B$
$Q_{2} = 2.7502*10^{26}electrons + B$

$1*10^{4}= \frac{Q_{1}*(-1.6)*10^19*Q_{2}*(-1.6)*10^19}{1^{2}}$

Greets from Argentina!
Nicolas

2. Jan 11, 2012

### SammyS

Staff Emeritus
First things first.

Use Coulomb's law to find how much charge needs to be transferred. Of course, Q1 = -Q2 , so I suggest letting Q = Q1 and Q2 = - Q.

After finding Q, then find out how many electrons need to be transferred.