Negative charge between two positive?

[gmmstr827]

In my Physics class tonight (Physics for Scientists and Engineers 2), we have a quiz about the following topic:
"Where to place a negative charge between two positive charges so that the negative charge has an equal force with each of the positive charges."

I was hoping that someone could give me tips on how to solve such a problem, so that I might be able to do well on the quiz. Oddly enough, the professor also said that we will be working with our lab groups to do this quiz, so at least I won't be alone on this one.

It's vital that I have any formulas that I might need, since we have to make our own "formula cheat sheets" for every quiz/exam. If my lab partners and I don't have the right formulas prior to the quiz, we won't do well. As for what formulas I think I would need, I'm considering writing down Coulomb's Law. Would I need anything else?

Thank you!

 

[Doc Al]

Coulomb's law is what you need.

How would you go about solving the problem?

 

[gmmstr827]

Coulomb's law is what you need.

How would you go about solving the problem?

Okay, good, that's what I thought. Would I need the formula for an electric field as well? E=F/Q?

That's what I just asked you. I realize you don't want to give any answers away, but you wouldn't be. You're giving a method away. It's up to me to apply it.

 

[Doc Al]

Would I need the formula for an electric field as well? E=F/Q?

If you have Coulomb's law, you can easily divide by Q to get the field if needed.

You're giving a method away.

I want you to think of your own method for solving this kind of problem. What would you do with Coulomb's law or the electric field?

 

[Doc Al]

Here's a push. You were wise enough to know that you'll need Coulomb's law. If the charges were arranged as

Q1(+) ... Q2(-) ... Q3(+)

, you'd use Coulomb's law to find the force between what charges?