# Magnitude of q ~ What am I doing wrong?

Magnitude of "q" ~ What am I doing wrong?

## Homework Statement

This is my homework problem:
"Two equal charges are separated by 3.7 x 10^-10 m.
The force between the charges has a magnitude of 2.37 x 10^-3 N. What is the magnitude of q on the charges?"

## Homework Equations

F(electricity) = k(Coulomb)(q1 • q2)/r^2
E = F(e)/q0
E = k(C) • q/r^2
...I can't really seem to think of any other relevant equations for this particular problem, though I have a lot of others that go along with this electricity unit in Physics. If you can think of some that apply that I missed, please let me know. :)

## The Attempt at a Solution

Here's my insanity work below, hahaha. I tried two approaches and got the same answer, but I still don't think it's correct:

(a)
F = kc(q1q2/r^2)

2.37 x 10^-3 = 8.99 x 10^9 • q1q2 / (3.7 x 10^-10)^2

F • r^2 = kc • q1q2
F • r^2 / kc = q1q2
2.37 x 10^-3 • (3.7 x 10^-10)^2 / 8.99 x 10^9 = q1q2

q1q2 = 3.61 x 10^-32

(b)
E = kc • q/r^2

2.37 x 10^-3 = 8.99 x 10^9 • q/(3.7 x 10^-10)^2
2.37 x 10^-3 / 8.99 x 10^9 = q/(3.7 x 10^-10)^2
2.37 x 10^-3 / 8.99 x 10^9 • (3.7 x 10^-10)^2 = q

q = 3.61 x 10^-32

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Please help me. I don't really know what I'm doing wrong, or if I'm doing anything wrong. I have a feeling I got this incorrect, however. Also, I don't exactly understand the concept of "q". I know what q1 and q2 are, but if anyone could explain it just one more way (I've already heard it three different approaches, but I still don't comprehend), I'd be much obliged for any input as soon as possible. Thank you! :)

## Answers and Replies

Related Introductory Physics Homework Help News on Phys.org
Cyosis
Homework Helper

The problem statement gives you one very important hint, two EQUAL charges. Therefore q1=q2=q and the equation for the force between two charges becomes, $$F=k \frac{q^2}{r^2}$$. What they mean with the magnitude of q on the charges is, how much charge do q1 and q2 have,ignoring the sign.

Thanks!

So basically, I calculated for q^2? I just take the square-root of my answer for q?

Cyosis
Homework Helper

Yep, you can then check your answer by calculating what the force would be between q1 and q2 given q=q1=q2.

Yay! The sun came up and suddenly Physics isn't half as evil as I thought! ;)