Magnitude of electric field at a point

[Peewee]

Homework Statement

A force of 5.7 N. is exerted on a 8.8 microCoulomb charge in a downward direction. What is the magnitude of the electric field (in N./C.) at this point?

Homework Equations

E = (kel * abs(q)) / r^2\
E = Fel / q

The Attempt at a Solution

8.8 microcoulombs --> 8.8e-6 Coulombs

At first I tried using:
E = (8.99x9)(8.8e-6) / r^2

But I didn't understand how I was supposed to find the radius. If it were a square, rectangle, triangle etc. and given some more information I could use trigonometry to determine the radius. The equation is for a point charge, though.

I then used E = Fel / q and simply plugged and chugged to get the right answer.

My question, though, is when do I use the E = Fel / q and when do I use the E = kq/r^2 equation?

Is the point charge for when two points are being compared to each other and the E = Fel/q is for one item?

 

[Doc Al]

My question, though, is when do I use the E = Fel / q and when do I use the E = kq/r^2 equation?

The first equation (F_el = Eq) is true in general--it relates the field to the force produced by that field on a particular charge. The second equation is for finding the field due to a point charge. (Which wasn't relevant here, as you were not given any information about the source of the field, only the force it produced on a given charge.)