# Dont Understand how to Figure out this columbs law question.

1. Aug 25, 2006

### Papak

Um hello, im new to this forum and still dont understand everything very well but i need some help. I've been doing revision over the new topic we've been learning in high school physics about eletrical forces and fields and we've been spending most of the time doing questions about columbs law and ive encountered a few which i couldn't do because i have no idea where to start.

The questions says Two point charges are placed on the x axis: one at 0cm with a -4uC and the other at 80cm with a +3uC charge. It asks where in the vicinity of these two charges can a +5uC charge be placed to experice no resultant force.

all id like to know is how to start it, i know that you need two equation, also i dont know which one to use a q1 q2 and q3 to actually set up any sort of equation.

Last edited: Aug 25, 2006
2. Aug 26, 2006

### quasar987

The electric force on a charge Q is related to the electric field at the location of the charge by F=QE, right? So F is zero if and only if E = 0, right?

Additionally, the field in the viscinity of 2 charges is the sum of the field produced by each charge separately.

So, find an expression of the field produced by these two charges at each point (x,y,z) of space and solve algebraically for the point(s) (x,y,z) for which E=0.

Hint: At the point (x,y,z), the square of the distance from the negative charge is x²+y²+z² and the square of the distance from the positive charge is (x-0.80)²+y²+z²

(Are you sure you're not just looking for where on the x-axis you must put the charge? This problem seems to me to be just a notch too high in difficulty to be high school level.)

Last edited: Aug 26, 2006
3. Aug 26, 2006

### Papak

Hey thanks for the help, the book just asked for where on the x axis, but as soon as i posted it i went and tried to get a fresh look at is and i figured it out . But thank you anyway