# Parallel plate capacitor electric field problem

megr_ftw

## 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

## Answers and Replies

Homework Helper
Gold Member
2021 Award
Look at your second equation. What can you find from it?

myxomatosii
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.

myxomatosii
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.

Oh well, computers give grades.

myxomatosii
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.

Homework Helper
Gold Member
2021 Award
Look at your second equation.

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:
myxomatosii
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.

megr_ftw
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...

Homework Helper
Gold Member
2021 Award
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.