# Find the magnitude and sign of the point charge at the origin

1. Mar 30, 2008

### bastige

1. The problem statement, all variables and given/known data
The value of the Electric field at a distance of 82.8 m from a point charge is 75.3 N/C. Its direction is radially in toward the charge. The Coulomb constant is 8.98755 E9.
Find the magnitude and sign of the point charge at the origin. Answer in units C.

2. Relevant equations
I don't think I have all the necessary formulas. But this is what I do have
E=(KE*Q)/r^2 <---equation 1

F=[KE(Q1)(Q2)]/r^2 <--equation 2

3. The attempt at a solution

I am basically physics illiterate...so bare me some slack...
I used equation 1 to determine Q = 5.743E-5 .
How do I finish this problem? i feel like I'm missing a formula, or need some extra information.

2. Mar 30, 2008

### neu

Coulomb's Law

$$\mid \underline{E} \mid=\frac{1}{4 \pi \epsilon_{0}}\frac{Q}{r^2}=\left[\frac{1}{C^2N^{-1}{m^-2}}\frac{C}{m^2}\right]$$

So that gives you the magnitude of Q, and the direction of the electric field lines tells you the sign of the charge.

It's a point charge so its that simple