# Homework Help: Location where voltage is 0

1. Feb 7, 2013

### phunphysics2

A charge of -2.350 uC is located at (2.620 m, 4.495 m). and a charge of 1.670 uC is located at (-2.602 m, 0 m). There is one point on the line connecting these two charges where the potential is zero. Find this point.

V=U/q and U=qV

V=kq/r

I was wondering if someone could please help me set up this problem. I am not sure where to begin. Here is my attempt (I honestly don't think I am at all on the right path)

Ua-Ub=Kqoq/ra - kqoq/rb

k(-2.350 uC)(1.67)/2.602 - k(-2.350 uC)(1.670)/5.203

*Yes I know that uC needs to be in C. I just wrote that for short hand.

Could someone please help me set up this problem? That is where I am struggling the most. I have the answers (-0.433, 1.867 m) but I do not know how to arrive at them

Thank you for any help, comments, suggestions, etc

2. Feb 8, 2013

### Staff: Mentor

I would like to help you, but the equations that you wrote do not make sense to me.

Can you list the Relevant Equation for solving this type of problem? Can you also post a sketch of the problem, showing the line between the charges?

3. Feb 8, 2013

### phunphysics2

Greetings Berkeman,

Unfortunately there is no drawing to the problem. There are only words.

In addition, I am not exactly sure what equations need to be used for this problem.

4. Feb 8, 2013

### phunphysics2

I am not sure if Coulumbs law should be used or not....

5. Feb 8, 2013

### Staff: Mentor

Yes it should. Find the point where the net force is zero, and that will be the point where the E-field is zero...

http://en.wikipedia.org/wiki/Coulombs_Law

.

6. Feb 8, 2013

### Staff: Mentor

BTW, the point on the line may not be between the two charges...

7. Feb 8, 2013

### phunphysics2

thanks.

I am still not exactly sure on how to set up the rest of the problem though....

8. Feb 8, 2013

### phunphysics2

I do not know where the net force is 0, nor do I know how to begin to set up a situation where I can find the net force...

9. Feb 8, 2013

### Staff: Mentor

Write Coulomb's equation for each of the two charges. One equation for the force from the first charge as a function of distance, and the second equation for the force from the second charge as a function of distance. You probably should choose an origin somewhere that makes the problem eaiser. Either choose the origin at one of the charges, or choose it half-way between the charges.

Add the two forces and find the distance from the origin where the forces add to zero...

10. Feb 8, 2013

### phunphysics2

Thank you!
Let me work on that and post back in a while with my work....

11. Feb 8, 2013

### ehild

Berkeman: the problem asks the point where the potential is zero.

ehild