# Homework Help: Electric Potential Difference diagram

1. Apr 19, 2015

### sugz

1. The problem statement, all variables and given/known data
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 **

2. Relevant equations
V = ke (q/r)

3. 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!

#### Attached Files:

• ###### Q18.PNG
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2. Apr 19, 2015

### Orodruin

Staff Emeritus
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?

3. Apr 19, 2015

### 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 :/

4. Apr 19, 2015

### Orodruin

Staff Emeritus
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?

5. Apr 19, 2015

### sugz

what do you mean by the upper charge?

6. Apr 19, 2015

### Orodruin

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

7. Apr 19, 2015

### sugz

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

8. Apr 20, 2015

### Orodruin

Staff Emeritus
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?

9. Apr 20, 2015

### 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?

10. Apr 20, 2015

### Orodruin

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

11. Apr 20, 2015

### 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?

12. Apr 20, 2015

### Orodruin

Staff Emeritus
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.

13. Apr 20, 2015

### sugz

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