# Homework Help: Finding the mass of a particle, when its charge is known

1. Jan 16, 2012

### bobsmith76

1. The problem statement, all variables and given/known data

A small latex sphere experiences an electric force of 3.6 × 10^−14 N when suspended halfway between a pair of large metal plates, which are separated by 48.0 mm. There is just enough electric force to balance the force of gravity on the sphere.
(a) What is the mass of the sphere?

2. Relevant equations

q = (mgΔd)/V

F (electrostatic force) = kq/r^2

3. The attempt at a solution

I have two unknowns in my first equation, q and V. My attempt to find q fails because I get a number that is around 10^-27. So if my number for q is wrong, then my number for V will also be wrong.

2. Jan 16, 2012

### Curious3141

I assume that's just the first part of the question, because it's actually very simple. You're given the electric force and told that it exactly balances the gravitational force (which is the weight). So in essence, you're told the weight of the sphere. How do you relate weight to the mass?

3. Jan 16, 2012

### bobsmith76

Ok, I multiplied 3.7 * 10^-15 by 9.8 and got the correct answer. Thanks.

4. Jan 16, 2012

### Curious3141

I repeat (from my earlier post):

So in essence, you're told the weight of the sphere. How do you relate weight to the mass?

5. Jan 16, 2012

### Curious3141

Sure, but to go the "right way" (to get the mass given the weight), you must divide by $g$ (which is close to 9.8$ms^{-2}$.