Electric fields of charged plates

[swinfen]

Homework Statement

Find the magnitude of the electric field of a large plate with surface charge density of 300Cm2 at a point 2.00 m away from the surface?

Relevant Equations

E = σ/2e0

I thought it might be the case that the "2m away" wasn't applicable as the electric field doesn't change if the point away is less than the length of the plate, so I thought I should use the equation listed. All examples I can find talk about two charged plates, or the effect on cylinders through or around a charged plate.

 

[BvU]

Hello @swinfen,

$\qquad$ !​

You have a relevant equation.
The cylinders are involved in the derivation of the equation.
What stops you from applying it ?

$\$

 

[swinfen]

Thanks for your response BvU. Firstly, am I correct that the '2m away' is not relevant? If this is then the only equation to use, then I was marked wrong by the professor (they do not provide any feedback on wrong answers). The answer I gave was:

E = σ/2e0
=> E = 300/2*(8.854*10^-12)
=> E = 1.694*10^13

 

[BvU]

Perhaps you were punished for omitting the units ?
I can't think of anything else.
The distance should be << plate dimensions, but that is what the problem statement says.

$\$

 

[Gordianus]

Let me speculate...
The charge density of 300 C/m² is too large and so is the electric field. As for the distance, it is irrelevant because the plate is large. Many times "large" means infinitely big.

 

[swinfen]

Thanks for the feedback. Yes I realized that I missed the units on this one (only this one!) but even my one of the professors (who I finally managed to get in touch with and had tried to solve the problem) couldn't get the same answer as the answer key. So I think we must be right and the answer key must be wrong!
Now just need to understand the other problem I posted, if any of you would like to help with that one? :)

 

[Gordianus]

I can't see the other problem.