Electric fields in a plane, :smile:

[JFonseka]

Homework Statement

Charge of a uniform density (8.0 nC/m2) is distributed over the entire xy plane. A charge of uniform density (5.0 nC/m2) is distributed over the parallel plane defined by z = 2.0 m. Determine the magnitude of the electric field for any point with z = 1.0 m.

Homework Equations

E = F/q

The Attempt at a Solution

Calculate the electric field at z = 1 due to the first plane, and then calculate the electric field at z = 1 due to the second plane. And then subtract the two?

I however don't know how to get the force, since the charge is given over a metre^2, what do I do?

 

[Astronuc]

Determine the E-fields of both planes separately, then superimpose them at the midpoint (1 m). They will point in opposite directions.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elesht.html

 

[JFonseka]

Ok I calculated the electric fields for the two planes. But those equations for calculating electric field don't have anything to do with the distance...

So if there is no distance in the equation how can the electric field be affected by distance.

 

[Astronuc]

Is the field constant between the plates? If so, then it is the same at 1 m as anywhere else. Think about the field inside parallel plates.

The electric field is not affected by distance (i.e. it's constant), but the potential is.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elewor.html

 

[JFonseka]

WHat do you mean 'between' the planes? The electric field for the first one will be higher because it's 8nC/m^2 than the second with 5nC/m^2 right?

I calculated the first one to have an electric field of 451.7N/C and the second to have 282.4
N/C

So i subtracted them and got 169N/C

That doesn't look right.

 

[JFonseka]

Aha, hang on. The answer is 733N/C instead. So therefore I have to add them, why do I have to add them? They are pointing in the opposite directions aren't they? So one is positive and the other is negative. Hmm...

 

[JFonseka]

Thanks for your help Astronuc! I will figure out the latter bit soon, but I understand this stuff better now.