Homework Help: Charge q

1. Jun 3, 2015

mirzaicpc

four charges each equal -Q placed at the four edges of a square and a charge q is placed at the centre. if system is in equilibrium the value of q is,

the answer is q = Q/4 * ( 1 + 2(√2)

can any one please solve this, would be great

2. Jun 3, 2015

rumborak

That sounds like a homework question, you should probably take it there. Also, I don't understand the setup; you say a charge "q" is placed in the middle. So, isn't the value of q .... q?

3. Jun 3, 2015

ArmanCham

Are you sure about the answer ? It should be zero I guess net force will be zero any case so we cant tell anything about q and Q

4. Jun 3, 2015

mirzaicpc

ya i am sure abt it,

5. Jun 3, 2015

rumborak

But the problem as you describe it make no sense. You already know the value of q, why do you need to calculate it again?

6. Jun 3, 2015

ArmanCham

Q's are stable or unstable ?

7. Jun 3, 2015

mirzaicpc

this was one of the replies i got, but couldnt understand it, hope it will help

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8. Jun 3, 2015

ArmanCham

I solved it.I will send a picture I hope it will be not a problem cause its not in homework section

9. Jun 3, 2015

mirzaicpc

thank you so much,,, waiting for the pic

10. Jun 3, 2015

ArmanCham

Who moved this in homework section I will be banned now. This post first opened general physics forum.For that I send picture.I dont take any responsibility to share this picture.I dont break the rules.

11. Jun 3, 2015

mirzaicpc

thank u so much people

12. Jun 3, 2015

Staff: Mentor

Post number 2 stated it was homework, and in the post you edited, you knew it was homework, you won't be banned, don't worry, just make sure to follow the homework rules for helpers.

13. Jun 3, 2015

jbriggs444

Don't panic. You're not getting banned. The moderators are just working to help you out by moving your post into the forum where it should have been placed to start with. We're not evil overlords around here. [Though I hear that the ban stick is a fun thing to wield]

It is clear by inspection that the center charge will be in equilibrium regardless of the value of q.

So the original problem is surely intended to find the value of q for which the four equal charges at the corners are each in an equilibrium. Whether that equilibrium would be stable is a separate question that we need not delve into here.

One should be able to calculate the repulsion on each corner charge from its peers. And one should be able to calculate the attraction of the corner charge from the center charge in terms of the known value Q and the unknown value q. Find q such that the net force on a corner charge is zero and the problem is solved. I believe that is the approach that was taken in the posted answer.

Last edited: Jun 3, 2015