Is the Unit Vector Always in the Direction from the Charge to the Point?

[FocusedWolf]

I did this practice exam problem and i keep on getting an answer that doesn't match with the answer key provided.

http://focusedwolf.googlepages.com/problem.jpg
http://focusedwolf.googlepages.com/work.jpg

The answer sheet says "E = 3.05e13 N/C direction = to the left"

 

[Doc Al]

Your answer and work look OK to me.

 

[FocusedWolf]

Thx, my confidence doesn't feel so shattered now :P

 

[Doc Al]

Though your work was just fine, you could save yourself some effort by using a symmetry argument.

 

[FocusedWolf]

Teacher just got back to me... turns the side of the square is A...somehow looking at how they depicted A in that drawing made me think A was the distance from center... even though it says in the instructions that A is the whole side... zzz

using distance from center = a/2 then problem comes out to their answer 3.05e13

 

[Doc Al]

D'oh! Sorry about that... I must have been sleeping also. (I just looked at the diagram and ASSumed that "a" was the distance from the origin.)

 

[FocusedWolf]

It's ok...this problem was booby trapped!

 

[FocusedWolf]

o quick question...

is the unit vector R always in direction from the charge to the point..QP

 

[Doc Al]

is the unit vector R always in direction from the charge to the point..QP

Yes, if you mean in the expression for the field at point P due to charge Q:
$$\vec{E} = \frac{k Q}{R^2} \hat{R}$$