# Finding the E-field between two infinite plane charged sheets

## Homework Statement

Two infinite-plane non-conducting, thin sheets of uniform surface charge p1 = 13.35 uC/m2 and p2 = -8.65 uC/m2) are parallel to each other and d = 0.245 m apart. (As shown in the diagram below.) What is the electric field between the sheets? (Note: the field is positive if it is parallel to the vector x). ## Homework Equations

Sigma/2*epsilon_not

Epsilon not = 8.85e-12 (SI Units)

## The Attempt at a Solution

The distance between the sheets should not matter in calculating the E-field between them because the E-field between them is uniform. We are given two sigmas and since the net charge on the field is toward the negative plane we just sum the two over 2*epsilon_not, and Epsilon not is a constant.

sigma1=13.35e-6 C/m^2
sigma2=-8.65e-6 C/m^2

(sigma1 + sigma2)/(2*epsilon_not) = E-field
(13.35e-6 + 8.65e-6)/(2*epsilon_not) = 1.2e6 N/C
Than since the E-field is perpendicular to the planes, but pointing in the opposite direction of positive X the answer becomes negative.
Answer: -1.2e6 N/C (unfortunately I tried this and did not get the problem right)

What did I get wrong?

Related Introductory Physics Homework Help News on Phys.org
SammyS
Staff Emeritus
Homework Helper
Gold Member
In the region between the sheets:
What is the direction of the E field due to the σ1 sheet?

What is the direction of the E field due to the σ2 sheet?​

The E-field on sigma1 is going -x and so is sigma2 right?

gneill
Mentor
Is the marking software fussy about significant figures?

SammyS
Staff Emeritus
Homework Helper
Gold Member
The E-field [STRIKE]on[/STRIKE] due to sigma1 is going -x and so is sigma2 right?
Right, so you should have a negative plus a negative.

Right but that doesn't change the answer numerically am I forgetting a step?

Doc Al
Mentor
Right but that doesn't change the answer numerically am I forgetting a step?
Why did you express your answer to only 2 sig figs? (To repeat what gneill already pointed out.)

More sig figs worked thank you everyone!

SammyS
Staff Emeritus
Homework Helper
Gold Member
Right but that doesn't change the answer numerically am I forgetting a step?
You have:
E-field due to sigma1=13.35e-6 C/m2
E-field due to sigma2=-8.65e-6 C/m2

But, if they're both pointing left (as vectors), then they should both be negative !

E1 + E2 = -13.35×10-6 + -8.65×10-6 = ____ ?

yes thats the way I ended up doing it -13.35e-6+-8.65e-6

SammyS
Staff Emeritus