1. A negative charge –Q is located between two positive charges, +4Q and
+16Q. The distance between +4Q and –Q is d1 and the distance between
+16Q and –Q is d2.
All charges lie along the same line and d1+d2 = 9 cm.

Calculate d1 and d2 which will provide a stable equilibrium position for
the charge –Q, i.e., the location where the total force acting on –Q due
to +4Q and +16Q is zero.

So far I have F1 + F2 = Ftotal = 0 since it specifies a stable equilibrium position. However, I can't seem to get the next step.

Any help would be much appreciated.

2. Originally Posted by WilsonWilson
However, I can't seem to get the next step.

Any help would be much appreciated.
The next step would be to express the forces in terms of the charges and distances.

3. Originally Posted by KJW
The next step would be to express the forces in terms of the charges and distances.
Hey,

I have them both in the forms Q1Q2/4.pi.epsilon.r^2 and then tried to work r out from them both. The problem is doing this step.

4. Originally Posted by WilsonWilson
Hey,

I have them both in the forms Q1Q2/4.pi.epsilon.r^2 and then tried to work r out from them both. The problem is doing this step.
Why is this a problem? (Show us where the problem is)

5. Originally Posted by KJW
Why is this a problem? (Show us where the problem is)
http://i.imgur.com/akL45re.jpg

I have made various attempts at working it out from here. All with ridiculous answers. I have either done it wrong so far or am missing something very obvious right now. Any ideas?

Thank's.

Also ignore the random numbers on the right.

6. Your third lines with the Qs in is nearly right but there should be a minus sign in the middle. Also the r's should be different variables. R1 + R2 = 9.

7. Hey, yeah so my problem with it after that is just getting an R on its own. If I get one R on its own it will still be in terms of the other R if you get me?

8. You have two equations and two unknowns so you can solve for both of them by substitution. R1= 9 - R2.