# Please help i dont know what to do

• flamethrower20
In summary, the conversation revolved around a question about the placement of point charges on the corners of a square to achieve a zero electric field and potential at the center. The solution involved alternating between positive and negative charges, resulting in a net potential of zero due to the superposition principle. There was also a discussion about the difference between electric potential and electric field, with the final solution involving a potential of \frac{kq}{r} with respect to infinity. The original poster expressed gratitude for the help.

#### flamethrower20

Need help with "charges" question please!

please help! this must be taught tomorrow in class, and this is my first year teaching. How can I explain it to my students?

Q: What point charges, all having the same magnitude, would you place at the corners of a square (one charge per corner), so that both the electric field and the electric potential (assuming a zero reference value at infinity) are zero at the center of the square? Account for the fact that the charge distribution gives rise to both a zero field and a zero potetial. Explain Thoroughly.

A: ?

## Homework Statement

please help! this must be taught tomorrow in class, and this is my first year teaching. How can I explain it to my students?

Q: What point charges, all having the same magnitude, would you place at the corners of a square (one charge per corner), so that both the electric field and the electric potential (assuming a zero reference value at infinity) are zero at the center of the square? Account for the fact that the charge distribution gives rise to both a zero field and a zero potetial. Explain Thoroughly.

A: ?

no eqn needed..

## The Attempt at a Solution

I assume that there would be a positive charge in top left, negative charge top right, postive charge bottom right, and negative charge bottom left..but I'm sad to say I'm not sure.

So, you have a degree in Physics but cannot explain a simple array of charges?

So, you have a degree in Physics but cannot explain a simple array of charges?

Please do not double post.

Note to Mentors; identical thread in Intro Physics

I'm sorry, I didn't know not to post twice. I just need help. I've taught Chemistry for 27 years, this year the physics teacher left. So now I'm having to balance both. Can you please help?

Well we know that electric potential is given by $$V = \frac{kq}{r}$$. As you go around the corners of the square, if you alternate between charges of +q and -q, that ought to do it. This way, the two positive charges will give a potential of $$\frac{2kq}{r}$$, and the two negative charges will give $$-\frac{2kq}{r}$$. Since potentials add as scalars, the net potential will be 0. As for the electric field, the superposition principle makes it fairly obvious that the field will be zero as well.

Anyway, someone let me know if I made a careless error. But I think this should do it.

arunma said:
Well we know that electric potential is given by $$V = \frac{kq}{r}$$. As you go around the corners of the square, if you alternate between charges of +q and -q, that ought to do it. This way, the two positive charges will give a potential of $$\frac{2kq}{r}$$, and the two negative charges will give $$-\frac{2kq}{r}$$. Since potentials add as scalars, the net potential will be 0. As for the electric field, the superposition principle makes it fairly obvious that the field will be zero as well.

Anyway, someone let me know if I made a careless error. But I think this should do it.

Seems like you attempted to give a direct answer to a homework problem. btw, how did $$\frac{1}{r^2}$$ become $$\frac{1}{r}$$ ?

ranger said:
how did $$\frac{1}{r^2}$$ become $$\frac{1}{r}$$ ?

potential energy is not force.

you mean electric field, not force since its just one charge, not 2

cowshrptrn said:
you mean electric field, not force since its just one charge, not 2

No, he's right. With the case of two charges, the electric potential will be negative if the charges have opposite sign and positive if the charges have the same sign. Therefore we can see how the energy can be lost by opposing charges, thus satisfying arunma's explanation.

Last edited:
I've merged the two threads that was cross-posted, so if it doesn't make any sense, it's not my fault.

To the original poster, please re-read the PF Guidelines.

Zz.

ranger said:
Seems like you attempted to give a direct answer to a homework problem. btw, how did $$\frac{1}{r^2}$$ become $$\frac{1}{r}$$ ?

Oh, sorry. I figured that since it's not a homework problem, but rather something he needs for teaching purposes, I could answer directly.

I used 1/r because we're discussing potentials rather than fields. Electric potential has one more dimension of length than electric field (also one can remember that electric field has SI units of volts/meter). And with respect to infinity, any single electric charge creates a potential of $$\frac{kq}{r}$$.

thanks so much, yall have been very helpful

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