# Homework Help: Electric field at a point on a contour line

1. Feb 17, 2009

### maxsthekat

Hi guys!

In this question I am asked to "Calculate the size of the electric field at k". See image at: http://capa-new.colorado.edu/msuphysicslib/Graphics/Gtype54/prob04a_threeqcontour.gif

Note: although the picture states "Volts", everything is actually measured in kV.

2. Relevant equations
∆V = Ed

3. The attempt at a solution
I tried to set V = -5kV, and solving for E by measuring the distance from point k to Q1, but this doesn't seem to work. Will I have to do this for all three charges? Or, am I totally off base and should be approaching it from a different angle?

Thanks for your help!

-Max

2. Feb 17, 2009

### AEM

As I figure it the difference between each contour line is 1 KV. Since

$$E = -\frac{\Delta V}{d}$$

you want to measure the perpendicular distance from k to the nearest contour line and express E as so many volts per meter (or what ever distance unit you desire). Note the minus sign in my equation. That implies something about the direction of E.

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