Homework Help: Electric Fields of two plates

1. Feb 7, 2010

Physics197

1. The problem statement, all variables and given/known data
1. If two plates, 1.0 cm by 1.0 cm, with charges of −2.30 µC and +2.30 µC, what is the magnitude of the electric field at b?
+++++++++++++
.........b
-----------------

2. A uniform electric field of strength E = 4.80×10^4 N/C is observed in the region adjacent to a large, uniformly charged plate. What is the charge per unit area on the plate?
ll---->
ll---->
ll---->
ll---->
bar ll E field -->

2. Relevant equations

E=F/Q
E= sum kq1/(d1)^2
density = E*$$\epsilon$$o
$$\epsilon$$ = 8.85x10^-12

3. The attempt at a solution

Not sure how to solve 1, due to the distance from the plate to B is not given.

for 2, I thought it would just be (8.85x10^-12)(4.80x10^4) = 4.25x10^-7 C/m^2

2. Feb 7, 2010

thebigstar25

for part (1):

have a look at this figure which shows the electric field produced from the plates:

http://img189.imageshack.us/img189/2665/29517379.jpg [Broken]

the electric field between two oppositely charged parallel plates is uniform in any region between them , so what that tells you?

for part (2):

did you write the whole question?

Last edited by a moderator: May 4, 2017
3. Feb 7, 2010

Physics197

So for 1, the distance has no effect on the electric field.

and for #2, I copied and pasted the question exactly, I too felt like something was missing.

4. Feb 8, 2010

thebigstar25

for question(1) .. You are correct the distance doesnt matter .. And as you can notice the electric field between the plates is just the vector addition of the electric field of each one which is simply 2E (here E is the electric field from one plate since it is the same for both) ..

For question (2) im sorry i dont understand the drawing you made .. Can you please scan the figure from the book or make a better drawing and attach it , maybe then i would be able to help. .