# Parallel plate capacitor electric field problem

## Homework Statement

Two circular disks spaced 0.50 mm apart form a parallel-plate capacitor. Transferring 1.8*10^9 electrons from one disk to the other causes the electric field strength to be 1.3* 10^5 N/C. What are the diameters of the disks?

## Homework Equations

F=k(q1)(q2)/r^2
E_capacitor= Q/(epsilon_0)*A

## The Attempt at a Solution

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kuruman
Homework Helper
Gold Member
Look at your second equation. What can you find from it?

I'm doing the same problem.

All I can find from the second equation is that Q = Epsilon * A * E

Which doesn't really help because even if I break down A into pi * r^2 I still don't know r.. because if I knew r I would know d, which is what we're looking for.

I don't get how that helps.

Well I have another equation I was solving for earlier.

r = sqrt(Q/pi*eta)

I suppose I could substitute that into the equation set equal to Q and solve for Q.

Then maybe I could plug that Q from Q=Epsilon*A*E into the r = sqrt(Q/pi*eta) equation to find r.

Hmm. I can't stand this because of the precision required for these stupid online assignments, I get zero credit or full credit, none in between. Rewarded none for the effort I've put forth the same as someone who did nothing.

And now I end up with

Q=(Epsilon*E*Q) / eta

that makes no sense because if I go any further the Q's will cancel out *sigh*.. so I know I did something wrong.

Back to the drawing board.

kuruman
Homework Helper
Gold Member

You know Q, you know ε0 and you know E. Can you find A? Once you know A can you find the diameter?

Last edited:
What is Q?

N electrons * Charge of electron?

I'm going back and re-reading the entire chapter leading up to this one in cased I missed something.

I'm completely lost when it comes to electricity and thats not like me.

thanks for the help. now if only webassign would stop lagging all the time and I could see if my answer is correct or not...

kuruman
Homework Helper
Gold Member
What is Q?

N electrons * Charge of electron?

I'm going back and re-reading the entire chapter leading up to this one in cased I missed something.

I'm completely lost when it comes to electricity and thats not like me.
Yes, Q is the total charge of the N electrons.