Thread: Electrostatics Equilibrium Problem.

-It is required to hold four equal point charges +q each in equilibrium at the corners of a square. Find the point charge that will do this if kept at the center of the square?
I am doing the basics of coulomb's law,can anyone explain this to me?
I cannot do this problem,the problem seems to be complex to me?

Hi Turiya, welcome to the forum. Do you know the formula for electrostatic potential?

Originally Posted by Turiya Chatterjee

-It is required to hold four equal point charges +q each in equilibrium at the corners of a square. Find the point charge that will do this if kept at the center of the square?
I am doing the basics of coulomb's law,can anyone explain this to me?
I cannot do this problem,the problem seems to be complex to me?

There are four Coulomb forces acting on each of the corner charges:

-the attractive force exercised by the charge -Q in the center of the square
-the three repulsive charges due to the other 3 charges +q placed at the corners of the square

You need to sum the 4 forces vectorially in order to find -Q from the condition of equilibrium.
You should be getting

Originally Posted by Turiya Chatterjee

-It is required to hold four equal point charges +q each in equilibrium at the corners of a square. Find the point charge that will do this if kept at the center of the square?
I am doing the basics of coulomb's law,can anyone explain this to me?
I cannot do this problem,the problem seems to be complex to me?

Welcome to the forum. It's customary to show the work you've done in your attempt to solve the problem. Otherwise students will use forums to cheat and have other people do their homework for them.

Originally Posted by Physicist

Welcome to the forum. It's customary to show the work you've done in your attempt to solve the problem. Otherwise students will use forums to cheat and have other people do their homework for them.

You could have tried to help him understand the problem like I did, Peter. Is this the best you can do?

X0x, Physicist is right. It's customary for the OP to show how far they have got. Giving solutions is not helpful.
Ps Your answer is much too small.

Originally Posted by Jilan

X0x, Physicist is right. It's customary for the OP to show how far they have got. Giving solutions is not helpful.

Perhaps you could help the OP , Jilan ? Do you think you could do that? Because your trolling doesn't help him at all.

Ps Your answer is much too small.

Get stronger glasses.

The OP has not come back with a yes or no answer to my response. If it were a no, I would go the forces route (less easy but do-able ) if it were a yes we would go the potential route (easier). The question was important. Either way (I've checked both ways) your answer is too small with or without my glasses, LOL!

Originally Posted by Jilan

Either way (I've checked both ways) your answer is too small with or without my glasses, LOL!

You mean that you are unable to solve correctly a basic exercise. Instead of trolling me, you could try to learn how to solve it.
Let me help you out a little, pick a corner

-the force exerted by the Q charge is: fill in the blanks
-the forces exerted by the other three particles of charge q are : fill in the blanks
-write the condition of equilibrium: fill in the blank
-solve for Q

If you manage to do the above steps correctly, you should get the answer I posted. If you don't, try again. Or take a remedial class.

Originally Posted by Jilan

X0x, Physicist is right. It's customary for the OP to show how far they have got. Giving solutions is not helpful.

That's correct but only when it appears to be a homework problem. That's why so many forums require that as part of their forum rules such as these

Must read before posting a problem : please show your attempt! - Physics Help Forum

Guidelines for students and helpers

Look at the homework guidelines there. They are

Show us that you've thought about the problem.

Don't simply say "I have no clue," "I have no idea where to start," or "I'm completely lost."

Don't simply say "I tried for hours and didn't get anywhere."

Don't just give a vague or general description of what you tried.

Do not simply post images of the problem statement or your work.

On helping with questions
Any and all assistance given to homework assignments or textbook style exercises should be given only after the questioner has shown some effort in solving the problem. If no attempt is made then the questioner should be asked to provide one before any assistance is given. Under no circumstances should complete solutions be provided to a questioner, whether or not an attempt has been made.

These are wise guidelines which are time tested. Although I can't even find the forum rules here I doubt it mentions homework, but it should.

Originally Posted by Physicist

These are wise guidelines which are time tested. Although I can't even find the forum rules here I doubt it mentions homework, but it should.

So, what is the answer, peter? Jilan gave the wrong answer, can you help her? Do you think you could get the correct answer, ?

Thanks x0x I'm getting the right answer this morning. It can't be solved by considering the potentials.

Originally Posted by Jilan

Thanks x0x I'm getting the right answer this morning.

So, you retract the earlier claims?

It can't be solved by considering the potentials

Of course it cannot, you need to follow the steps I showed you, equilibrium is solved through force, not potential.

Originally Posted by x0x

So, you retract the earlier claims?

Yes, trying to neutralise the potentials I was getting a charge twice as big as it needed to be. Thanks for putting me on the right track.

Originally Posted by Jilan

Yes, trying to neutralise the potentials I was getting a charge twice as big as it needed to be. Thanks for putting me on the right track.

1. The potential is a scalar, you can't "neutralize" it.
2. Potential have nothing to do with equilibrium.
3. Forces are the ones that determine the conditions of equilibrium.

Originally Posted by Jilan

X0x, Physicist is right. It's customary for the OP to show how far they have got. Giving solutions is not helpful.
Ps Your answer is much too small.

I'm not only right but I found the forum policy on this. See http://www.thephysicsforum.com/annou...ing-forum.html

11. We will help you with your homework but we will not do your homework for you. When you post a question here you are expected to explain the origin of the problem (homework, test, self-study, etc.) and the nature of the class (high school, introductory college, undergraduate, graduate, etc.) and indicate the level of mathematics expected in that class (algebra, calculus, differential equations, etc.) You are also expected to show some attempt to have solved the problem yourself and present all work relative to that attempt. We will attempt to help you answer your question for yourself via leading questions, suggestions and hints.

Has the OP demonstrated any attempt to have solved the problem themselves and have they presented all the work relative to that attempt?

Originally Posted by Physicist

I'm not only right but I found the forum policy on this. See http://www.thephysicsforum.com/annou...ing-forum.html


Has the OP demonstrated any attempt to have solved the problem themselves and have they presented all the work relative to that attempt?

That's why I haven't helped in this thread. I don't help people who are violating the rules by not showing their attempt. It's not useful to them in the long wrong since doing someone's work for them rarely, if ever, helps them.

Hey Peter,

I not only helped the OP getting started, I also helped Jilan understand her errors in trying to solve this exercise. Do you need a hand as well?

Originally Posted by Turiya Chatterjee

-It is required to hold four equal point charges +q each in equilibrium at the corners of a square. Find the point charge that will do this if kept at the center of the square?
I am doing the basics of coulomb's law,can anyone explain this to me?
I cannot do this problem,the problem seems to be complex to me?

I won't try to solve that problem for you, but look for how one finds the electric field as a function of the positions of the sources and the observer, where the positions and the field are all vectors.