Calculating Magnitude of two identical charges (Coulomb's Law)

[Integrator741]

Good Evening,

I was struggling with this question for about 30 min, I tried using Coulomb's Law Calculators, youtube and etc. Nothing helped. This question is from "Foundation Science For Engineers" book by Keith L. Watson, Chapter 26: Electric Charge;

Two positive charges of equal magnitude experience a repulsive force of 0.133 N between them when they are separated by a distance of 13 mm in air. (E0 = 8.85*10^-12).
?) What is the magnitude of each charge (A= 50nC)

I tried solving for Q1Q2 using (F=Q1*Q2/4*(pi)*E*r^2) - Did not get anything near 50nC.
Then I watched a video on youtube, where the guy used quadratic formula, but I didn't really get that.

Please help me to find a correct method of solving this question.

Thank you.

 

[(*steve*)]

show us your workings where you try to solve that equation.

 

[Arouse1973]

He you go.


Edit: This only proves the formula, it doesn't tell you how to get the answer
Adam

 

[Integrator741]

He you go.
View attachment 25887

Edit: This only proves the formula, it doesn't tell you how to get the answer
Adam

Yeap, that's one of the things I did first, because I thought that the answer might be wrong :/

 

[Integrator741]

show us your workings where you try to solve that equation.

Sorry mate for being so unorganized and messy, hopefully you can see that I have been trying.

 

[Arouse1973]

I have to go out now, for roast dinner . One of the other guys will be along to help.
Cheers
Adam

 

[Arouse1973]

Sorry mate for being so unorganized and messy, hopefully you can see that I have been trying.

Messy, god you wana see my desk.

 

[Integrator741]

Messy, god you wana see my desk.

heh

 

[(*steve*)]

OK, so the first step is to check wikipedia for the correct formula. As a hint, Adam used it.

The formula as you have written it is so poorly written that I am not sure what Q1.Q2 is divided by. I suspect you've combined the correct formula (which uses ke) with the formula for determining ke from E0.

Do it step by step, and do it in a way that doesn't require me to view half a dozen pages of scribble. It should really only require a few lines.

 

[Integrator741]

OK, so the first step is to check wikipedia for the correct formula. As a hint, Adam used it.

The formula as you have written it is so poorly written that I am not sure what Q1.Q2 is divided by. I suspect you've combined the correct formula (which uses ke) with the formula for determining ke from E0.

Do it step by step, and do it in a way that doesn't require me to view half a dozen pages of scribble. It should really only require a few lines.

So I assume there is no difference if I use "f=(1/4 (pi)*8.85*10^-12)*(q1.q2/r^2)" or the one that Adam used? Because essentially they are the same? So what I am trying to do is solve for q1.q2 right?

 

[(*steve*)]

yes

 

[Integrator741]

yes

I don't want to give up, but.... UGH!! What am I doing wrong?

 

[Arouse1973]

Because you know the two charges are the same, how about this?
Adam

 

[Integrator741]

Because you know the two charges are the same, how about this?
Adam
View attachment 25908

Thank You!

 

[Arouse1973]

Your welcome! However if the two charges are different and of different sign, then that's a whole different thing.
Adam