# Homework Help: Electric Field and Potential difference

1. Jul 24, 2015

### hellojojo

Hey guys, I have a problem that I really have no idea what to do.

It was discussed in class that we do not need to know how to integrate or use the dot product-- any of the complex stuff.

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

The electric field in a given region in space is given by (3 i - 1 j) x 10^5 N/C. Find the potential difference between points a (3, 1) and b (7, -1). The coordinates are in mm

2. Relevant equations

All I have is V =kq/r but I'm not sure what to do with this

I also know that V=-E*r = kq/r2 - kq/r1 because the field is not uniform.

3. The attempt at a solution
I drew out a chart and plotted a and b and the marked out where the unit vectors ultimately lead to..
I don't actually know what the question is asking and where to start.

Last edited: Jul 24, 2015
2. Jul 24, 2015

### Orodruin

Staff Emeritus
This is not applicable to your problem. It is the potential from a point charge q and you are dealing with a homogeneous electric field.
The field you have been given is uniform. Also, there are not charges involved in this problem.

3. Jul 24, 2015

### hellojojo

Ok so charges aren't involved.
And if the field is uniform I would use: (Delta)V=-E*(delta)r
But how do i get the field E with all these unit vectors?
Would I just plug in the differences of x and y coordinates of point a and b into (3 i - 1 j) x 10^5 N/C to find the field?

4. Jul 24, 2015

### tommyxu3

The true relation between $V$ and $E$ should be $\Delta V=-\int E\cdot dl.$ In this case, for $E$ is a constant vector, as you said, $\Delta V = -E\cdot r.$
So you have to find out the displacement between the two points, of course it should be the vector parallel to the electric field $E.$