Electric Potential Difference diagram

[sugz]

Homework Statement

Point charges q and Q are positioned as shown. If q= +1.0 nC, Q = -2.0 nC, a = 3.0m, and b = 4.0m, what is the electric potential difference, Va - Vb? **The question with the diagram is also attached **

Homework Equations

V = ke (q/r)

The Attempt at a Solution

V = (8.99x10^9)[(1.0x10^-9)/4) = 2.2475 V.

The anser is 3.6V though, so I don't understand what I am doing wrong!

 

[Orodruin]

You are computing the electric potential at the -2 nC charge resulting from the other. This is not what is being asked for.

How would you compute the potentials at the points A and B?

 

[sugz]

How would you suggest to go about it? Because I tried it with the -2.0nC as well and i still didn't get the right answer :/

 

[Orodruin]

What you are trying is not what is being asked for.

How would you compute the potential at the point A resulting from the upper charge?

 

[sugz]

what do you mean by the upper charge?

 

[Orodruin]

The upper charge in your image is the 1 nC charge.

 

[sugz]

The potential at that point would be V=ke(q/r), where q is the 1 nC charge and r is a?

 

[Orodruin]

Yes. So what would be the potential at that same point from the other charge?

In the same fashion, what would be the potential at B from both charges?

 

[sugz]

So the potential at A is V= (8.99e9)(1e-9)/3 = 2.9967 V. The potential at B is V= (8.99e9)(-2e-9)/4 = -4.495 V?

 

[Orodruin]

No, you need to compute the potential at both points using both charges and the correct distances.

 

[sugz]

I'm sorry but I am quite confused right now and I really don't understand how to go about this question! Do you mind explaining conceptually why i have to find the potential at each point for both charges?

 

[Orodruin]

Because both charges contribute to the potential at both points and you are interested in the potential difference between the points. The situation is not that of a single point charge.

 

[sugz]

Oh that's what I was forgetting, thank you! It makes so much more sense now! :)