# Finding the Electric Field as a Result of Two Point Charges

1. Oct 2, 2014

### Jefferson

1. The problem statement, all variables and given/known data
There is a −3 µC point charge at (0,0,4) and a +3 µC point charge at (0,0,-4). What is the magnitude and direction of the electric field at point (0,0,8)? What is the magnitude and direction of the electric field at point (0,0,8)?

2. Relevant equations
$E = \frac{ k_e q }{ r^2 }$

3. The attempt at a solution
It should be as simple as just adding the electric field caused by each point charge.
$(0,0,8)$:
$(0,0,8) - (0,0,4) = (0,0,4)$
$(0,0,8) - (0,0,-4) = (0,0,12)$
$\frac{ k_e −3 \mu C (0,0,4) }{ 4^3} + \frac{ k_e 3 \mu C (0,0,12) }{ 12^3 }$

-1685.625 + 187.29166 = -1498.33334
However, the answer I should be getting (apparently) is 609

(4,4,0):
(4,4,0) - (0,0,4) = (4,4,-4)
(4,4,0) - (0,0,-4) = (4,4,4)

ke −3 µC (4,4,-4) / (4^2 + 4^2 + 4^2)^(3/2) + ke 3 µC (4,4,4) / (4^2 + 4^2 + 4^2)^(3/2)

ke −3 µC (4,4,-4) / (48)^(3/2) + ke 3 µC (4,4,4) / (48)^(3/2)

(-324.4, -324.4, 324.4) + (324.4,324.4,324.4) = (0,0, 648.8)
However, the answer I should be getting is 487

Am I doing something wrong?
(and why is it ignoring my LaTex tags?)

Last edited: Oct 2, 2014
2. Oct 2, 2014

### Jefferson

Oops, sorry, didn't mean to double post.