# (Electric Charge and Electric Field)

1. Jan 24, 2005

### Axeman2k

Consider point a which is 70 cm north of a -3.6 µC point charge, and point b which is 84 cm west of the charge (Fig. 17-23).

(a) Determine Vba = Vb - Va.
V = ____
(b) Determine Eb - Ea.
Magnitude
N/C = ____
Direction = _____
° (counterclockwise from east is positive)

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2. Jan 24, 2005

### Axeman2k

At first I was thinking the first part had to do with the E = (KQ1)/r2 equation. You take that for both point a and b then subtract b-a to get the final velocity. That didnt seem to work, and I have been looking frantically for the past 30 min through my notes and have not been able to find an equation to start off this problem. I think I may be confusing test charges with normal charges.

3. Jan 24, 2005

### dextercioby

That is not velocity,that is ELECTRIC POTENTIAL.And you shouldn't be using the formula for the electric field,but the formula for the electric potential...

Daniel.

4. Jan 24, 2005

### Axeman2k

Hmmm.... tried the equation V=Ed for potential but that didnt work. Am I using the right equation?

5. Jan 24, 2005

### dextercioby

No,u should be using the electric potential created by an electric charge...

$$V(r)=\frac{1}{4\pi\epsilon_{0}\epsilon_{r}} \frac{q}{r}$$

Daniel.

6. Jan 24, 2005

### Axeman2k

Haven't had a chance to use that equation... can you explain where the e sub 0 and r come from please?

7. Jan 24, 2005

### dextercioby

Those are electric permitivities.The one with the subscript "0" is the electric permitivity of vacuum.The one with the "r" subscript is the relative electric permitivity of the medium wrt the vacuum and is dimensionless...

Irrelevant into discussion,really.U got a problem to solve...

Daniel.

8. Jan 24, 2005

### Cantari

I don't think it's irrelevant, I like to know all the values and what they are if I am going to use an equation. I just find it hard to believe, Axeman, you dont know what $$\epsilon_{0}$$ is since it seems you are in an electricity class.

9. Jan 25, 2005

### dextercioby

What do you mean...??He SHOULD KNOW what $\epsilon_{0}$ is.Even if he's in High School...

Daniel.