# Coulomb's Law and electric force

1. Mar 5, 2008

### soul5

1. The problem statement, all variables and given/known data
A point charge of -0.35nC is fixed at the origin. Where must an electron be placed in order for the electric force acting on it to be exactly at it's weight.

2. Relevant equations
Fe=Kqq/d^2

3. The attempt at a solution
All I know is that n is an exponential value of -9
so -0.35*10^-9C that's all I know plz help.

2. Mar 5, 2008

### Staff: Mentor

Better write that as:
$$F_e = \frac{k q_1 q_2}{d^2}$$

That's q_1. The other charge will be the electron. What's the charge and mass of an electron? (Look it up!)

3. Mar 5, 2008

### Tedjn

What must the total force on the point charge be when the electric force balances the weight (gravitational force)? You wrote down the formula for electric force, which comes from Coulomb's Law. Do you know how to use Coulomb's Law? If you do, what is the electric force on the point charge from the electron if the it is a distance d away from the point charge, and what is the direction of that force? What is the weight of the point charge, and in what direction is that gravitational force on the point charge?

4. Mar 6, 2008

### soul5

The charge of an electron is 1.6 *10^-19 C and I have the mass of it too what do I do it that?

5. Mar 6, 2008

### Staff: Mentor

Follow Tedjn's advice! Analyze the forces acting on the electron: its weight (which way does that act?); the electric force due to the other charge (which way does that act?). Where must the electron be placed (with respect to the first charge) so that its weight is balanced by the electric force?

What's the weight of the electron? Figure out the distance "d" such that the electric force on the electron equals its weight. Hint: Set it up an equation symbolically before plugging in numbers.