Where to place -2q for equilibrium with charges q and 3q

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briteliner
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Homework Statement



a charge of q is fixed on a plane at the origin of an xy coordinate system and a charge 3q is fixed at (3cm, -3cm). where must a charge of -2q be placed at rest for it to be in equilibrium so that it remains at rest? is the equilibrium stable?

Homework Equations



coulomb's law

The Attempt at a Solution

 

Discussion

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briteliner said:
a charge of q is fixed on a plane at the origin of an xy coordinate system and a charge 3q is fixed at (3cm, -3cm). where must a charge of -2q be placed at rest for it to be in equilibrium so that it remains at rest? is the equilibrium stable?

Hi briteliner! Welcome to PF! :smile:

How far have you got?

Can you work out what line it must be on, and whether it's between or outside the charges? :smile:
 
i don't really know where to start, all i know is that since the -2q charge is negative it should be in between the charges. i think?
 
briteliner said:
i don't really know where to start, all i know is that since the -2q charge is negative it should be in between the charges. i think?

Hi briteliner! :smile:

Yes, it is repelled by both the other charges, and so it must be between them.

ok … there's a line joining the two other charges … which side of that line must the third charge be on? :smile:
 
why would it make a difference which side it is on?