# Calculating the electric field

1. Oct 5, 2007

### aliaze1

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

What are the strength and direction of the electric field at the position indicated by the dot in the figure?

2. Relevant equations

Edipole = ~ [1/(4πε0)] * [2p/r3 ]
on the axis of an electric dipole

Edipole = ~ [-1/(4πε0)] * [p/r3 ]
in the plane perpendicular to an electric dipole

3. The attempt at a solution

Which equation should I use??

Thanks

Last edited: Oct 5, 2007
2. Oct 5, 2007

### Avodyne

Neither. Your equations only apply if the two charges in the dipole are much closer together than the distance r from the dipole.

3. Oct 6, 2007

### aliaze1

what formula would i use?

4. Oct 6, 2007

### aliaze1

my textbook uses these forumulas...so i tried the problem using them

so here is my process:

p = qs

s=0.1
q=1*10-9
p=1*10-10

1/(4πε0) = 9*109

r=0.05m

plugging everything into the second equation {[-1/(4πε0)] * [p/r3 ]}, i get -7200, but this is incorrect

5. Oct 6, 2007

### nicksauce

Why not just use coulomb's law, and superimpose the two electric fields?

6. Oct 6, 2007

### aliaze1

good idea..this was my original approach, which didn't work for some reason

E=[1/(4??0)]*[q/r2]

using the two charges:

q1=1*109
q2=-1*109

and their respective distances:

r1=0.05
r2=0.01250.5 (square root)

and doing all calculations, and then adding the two charges (3600 and -720) gives me 2880, which is incorrect...

7. Oct 7, 2007

### aliaze1

lol so yea....umm...any help?