Electric Potentials.. graphs

[vsage]

The electric potential along the x-axis (in kv) is plotted versus the value of x, (in meters). Evaluate the x-component of the electrical force (in Newtons)on a charge of 5.10 micro-C located on the x-axis at x=2.8 m.

http://www.geocities.com/vsage3/p.bmp

I tried finding the value of kV at x = 2.8 so I would have this:

dV/dx = E = F/q

q = 5.1e-06C

Hint: Use graphical techniques to evaluate the electric field, i.e., the x component of the electric field is the negative of the change of the potential with respect to x. Careful with units of potential (given in kV) and of charge (micro-C). In order to check the sign, remember in which direction the positive charge moves when located at the given position.

(this was given with the problem)

Any ideas? I have to get the value within 3% of what the computer says so that might be why I'm having such a hard time.

Edit: here is the value I got the time I tried it

V = Electric field * distance

V = Force / charge * distance

-2500V = F / 5.1e-06 * 2.8

F = -0.00455N but it's wrong according to the computer.

Edit thanks but I got the answer wrong too many times and I can't correct it. I heeded what you said but apparently I don't have a good enough grasp on the subject to apply it :(

[robphy]

E= -dV/dx, that is, minus the slope of the V-vs-x graph.

V=Ed only when E is uniform.