Electricity & Magnetism: Potential Field

[Cmertin]

Homework Statement

The lines show the equipotential contours in the plane of three point charges, Q1, Q2, and Q3. The values of the potentials are in kV as indicated for the +5, 0, and -5 kV contours. The positions of the charges are indicated by the dots.
[PLAIN]http://img824.imageshack.us/img824/4397/stupidpic.gif

A. Calculate the work required to move a charge of -0.63×10-12C from i to b.
(Answer: 3.78×10^-9 J)

B. Calculate the size of the electric field at g.

C. Calculate the size of the force on a charge of 4.80×10^-19 C at k.

D. Calculate the size of Q3. The magnitudes of the three charges are in the exact ratios of 1 to 2 to 3.

Homework Equations

W=q∆V
E≅-∆V/∆l
F=q(-∆V/∆l)

The Attempt at a Solution

B. I measured from the center of the Q3 charge to be 2.8 cm (.028m).
The change in voltage is 1KV/line so it is 5000V
E=-5000V/.028m=-1.79×10^5N/C --- It says that this is the wrong answer.

 

[jbsteele]

C. I measured from the center of the Q3 charge to be 1.7cm (.017m) The change in voltage is 0.5KV/line so it is 2500VF=4.8×10^-19*(-2500V/.017m)= -3.53×10^-15N --- It says that this is the wrong answer. D. I assumed the magnitudes of the three charges are in the exact ratios of 1 to 2 to 3Q1=1×10^-12C Q2=2×10^-12C Q3=3×10^-12C ---- It says that this is the wrong answer. I would really appreciate if someone could point out where I went wrong.