Finding electric flux through a surface, simple problem, whats wrong?

[mr_coffee]

At each point on the surface of the cube shown in Figure 23-27, the electric field is parallel to the z axis. The length of each edge of the cube is 4.0 m. On the top face of the cube E = -40 k N/C, and on the bottom face of the cube E = +29 k N/C. Determine the net charge contained within the cube.
Image: here

Flux = (29E3 N/C)(4.0)^2;
Flux = 464000 k going up
Flux = (-40E3 N/C)(4.0)^2;
Flux = -640000 k going down
Net Flux = -640000 + 464000 = -176000

I submitted it and was inncorrect,
So i tried added them together and i got 1.104E6, that's the flux, now I found an equation that says, Flux = Qtotal/Eo, so I took Flux*Eo = Qtotal and got 9.7704E-6 and it is still wrong!

 

[Doc Al]

When finding the flux through a surface, think in terms of inward (-) and outward (+).

Flux = (29E3 N/C)(4.0)^2;
Flux = 464000 k going up

That should be negative, since it points inward.

Flux = (-40E3 N/C)(4.0)^2;
Flux = -640000 k going down

This is also inward, so negative.

Net Flux = -640000 + 464000 = -176000

Redo this, since both are negative.

I submitted it and was inncorrect,
So i tried added them together and i got 1.104E6, that's the flux, now I found an equation that says, Flux = Qtotal/Eo, so I took Flux*Eo = Qtotal and got 9.7704E-6 and it is still wrong!

Signs matter! The flux is negative.

 

[mr_coffee]

So i got -1.104E-6 for the Flux. So Q = -1.104E-6*8.85E-12 = -9.7704E-6? I have only one more chance so i have to make sure I'm doing this one right, thank you!

 

[Doc Al]

Well... does that follow from what I said? And do you think what I said is accurate?

 

[mr_coffee]

ahh it was wrong, what did i mess up?

 

[Doc Al]

ahh it was wrong, what did i mess up?

Nothing that I can see. But those on-line systems can be picky. (Units, significant figures, etc.)

 

[mr_coffee]

yeah i hate this system, i'll ask the professor tomarrow what's up, thanks!