# Electric Field (multi choice)

1. Feb 22, 2009

### roflcopter

1. The problem statement, all variables and given/known data
Charge is distributed uniformly on the surface of a large flat plate. The electric field 2cm from the plate is 30 N/C. The electric field 4cm from the plate is:

A. 120 N/C
B. 80 N/C
C. 30 N/C
D. 15 N/C
E. 7.5 N/C

2. Relevant equations

E=kq/r^2 (possibly)

3. The attempt at a solution

Well, the answer I got was E, 7.5 N/C. I got that by using the above equation, plugging in 30 for E and 2 for r and solving for q. Then, I used that same value of q and the new r to solve for E and got 7.5 N/C. Supposedly the correct answer is C, 30 N/C. I don't understand how they got that though. Why is the E field still the same? Shouldn't it be decreasing with the greater distance?

Anyways, I think I might be wrong because I remembered afterwards that E=kq/r^2 is the E field due to a point charge and not a plate....maybe that's why I got the wrong answer?

2. Feb 22, 2009

### LowlyPion

I think you are on to something.

What is the field of a sheet of charge?

3. Feb 22, 2009

### roflcopter

Oh. I believe it's in here: http://230nsc1.phy-astr.gsu.edu/hbase/electric/elesht.html

So the eq looks like E=charge density/2*epsilon. So this is independent of distance. I see.

4. Feb 22, 2009

### LowlyPion

5. Feb 22, 2009

### roflcopter

Yep. Thanks for the help!

6. Feb 22, 2009

### LowlyPion

Actually you solved it yourself.

The best outcome of all.