Location where the electric potential is zero between charges

[songoku]

Homework Statement

Two charges, -2Q and -Q, are located along x axis and separated by distance r.
a. Find the location of a point where it is neutral
b. Sketch the electric potential between the two charges against x axis

Relevant Equations

V = kQ/r

a)
I take "a point where it is neutral" as the electric potential at that point is zero. Is this correct?

And because the two charges are both negative, there can not be any point where V = 0? Am I wrong or maybe one of the charge should be positive?

Thanks

 

[haruspex]

I take "a point where it is neutral" as the electric potential at that point is zero.

I assume the question means where the field is neutral. And no, that has nothing to do with the potential being zero.
Potentials are always relative to an arbitrary zero, e.g. gravitational potential relative to ground level. In electrostatics, points at infinity are often taken to be at zero potential, but that is just a convention.

The field is the gradient of the potential. If the field is zero at some point, what does that tell you about how the potential changes if you move a small distance from that point?

 

[songoku]

I assume the question means where the field is neutral. And no, that has nothing to do with the potential being zero.
Potentials are always relative to an arbitrary zero, e.g. gravitational potential relative to ground level. In electrostatics, points at infinity are often taken to be at zero potential, but that is just a convention.

Does "neutral" always mean electric field = 0? Because if one charge is positive, there will be a point between the two charges where V = 0

The field is the gradient of the potential. If the field is zero at some point, what does that tell you about how the potential changes if you move a small distance from that point?

The potential is constant so the potential change is zero. But I don't understand the relation of this to the question asked. The question does not ask about 'change'

Thanks

 

[haruspex]

Does "neutral" always mean electric field = 0?

The question is unclear because it says "where it is neutral". What is "it"?
Saying that a potential is neutral has no meaning, so the only sense I can make of it is that "it" is the field.
Have you quoted the question word for word, as given to you? Is the question a translation?

The question does not ask about 'change'

No, but if it means where the field is neutral then that is the same as locally unchanging (local min or max of the) potential.

 

[songoku]

The question is unclear because it says "where it is neutral". What is "it"?
Saying that a potential is neutral has no meaning, so the only sense I can make of it is that "it" is the field.
Have you quoted the question word for word, as given to you? Is the question a translation?

I quoted the question word for word, that is what I got but I am not sure whether the question has been translated or not

And by "it", I interpret it as "point" so the question becomes "Find the location of a point where that point is neutral". I also take reference from electric circuit where neutral point is usually taken as reference point where the potential is zero.
[/quote]

No, but if it means where the field is neutral then that is the same as locally unchanging (local min or max of the) potential.

Oh I see.

I just googled and found most of the result refer "neutral point" as point where the resultant force or field is zero so my assumption is wrong. I will search more about what you said previously (Potentials are always relative to an arbitrary zero, e.g. gravitational potential relative to ground level. In electrostatics, points at infinity are often taken to be at zero potential, but that is just a convention. ) to have better understanding about this.

Thank you very much for the help haruspex