Electrostatics question

[studyq]

Two charges, +Q and -2Q, are fixed at a distance d to the left and right of the origin along the x axis as shown above. The potential infinitely far from the charges shown is equal to zero. Find the position on the x axis CLOSEST TO THE ORIGIN where the potential is also zero. (the picture is an x axis with two charges Q and -2Q on the left and right respectively both at a distance d from the origin)

1. d/3 to the left of the origin
   
   
2. 2d/3 to the left of the origin
   
   
3. d/3 to the right of the origin
   
   
4. 2df3 to the right of the origin

I don't understand the basic principle behind this question. My understanding is that potential is the potential energy per charge. How could the potential energy between two charges of opposite magnitude be zero. Wouldn't the potential always be directed to the negative charge?

I think of it as being similar to gravity, how could the potential energy due to gravity be zero when the object is above the ground? Unless it is infinitely far away because g because infinitely small. The solutions say to set the sum of the potentials equal to zero and you can set the magnitudes equal to zero but if you ever were to put a positive test charge in between the two charges it would go to the negative charge. The answer given is A. Any help would be appreciated.

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[DrknoSDN]

http://dev.physicslab.org/Document.aspx?doctype=3&filename=Electrostatics_PointChargesVoltage.xml
The bottom of the page has some great diagrams.

Here your confusing Electric Potential (voltage) with electric field strength. The electric potential can be zero while at the same point the field could exert a force on a test charge.

Think of Electric potential near a positive charge. It would have a positive voltage relative to if there was no charge present.
Repeat with a negative charge. A point near a negative charge would have a negative voltage relative to if there was no source charge present.

Because of that the point directly between +q and -q has zero potential voltage (relative to if neither source charge was there) because they directly counter each other.
If you drop a positive test charge between them it would still move toward -q because of the field (like a gravitational field)

You could visualize +q as creating a positive distortion of voltage (a hill) and -q creating a negative distortion (a valley), the slope of the line (hill to valley) is the field strength, and zero potential is where the line crosses the horizontal axis.

That is an awful explanation, maybe just google "Electric potential vs Electric Field"