# Homework Help: Net electrical flux on a cylindrical Gaussian Surface

1. Feb 12, 2013

### Broem

1. The problem statement, all variables and given/known data
a cylindrical solid of charge q, radius R, and length H. The Gaussian surface S is a cylindrical shell of radius r and length h, with r < R. Determine the net electric flux given that q = -48Q, R = 4L, H = 3L, r = 2L, and h = 2L (type the integer value, along with the sign, without typing units Q/e0):

2. Relevant equations
Inet = Qenclosed / ε = δ A / ε
Vsphere = ∏r^2 h

3. The attempt at a solution

I've tried using my V in substitute for the A so:
-48Q*(∏2^2 * 2) / ε
But I am unable to find the correct solution. I'm really lost when it comes to this in theory. Also, the results in form Q/ε....I understand sort of. Do I use ε as 8.85E-12 or does it remain as just the known variable?
Please let me know how I can approach this and also how to ask this question better :)
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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• ###### gauss_cyl_in.jpg
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2. Feb 12, 2013

### Redbelly98

Staff Emeritus
Looks like you are multiplying the total charge (-48Q) of the large cylinder times the volume of the smaller Gaussian surface. Instead, you need to multiply the charge density by the Gaussian volume. Only that way will the units work out for the enclosed charge:
(charge density)·(volume) → (C/m3)·(m3) = C → charge units, as desired

So ... start by finding the charge density, that's the charge per volume, for the larger cylinder of charge q=-48Q.

I think they mean, if the answer comes out to be -19Q/ε, then you'd just enter -19 as your answer.
The best way to ask is exactly how it is asked in your textbook or wherever you got the problem from. I am assuming that the charge is uniformly distributed throughout the large cylinder. You didn't say so in your post, but I am wondering if the actual problem statement says something along those lines?

p.s. Welcome to Physics Forums!

3. Feb 12, 2013

### Broem

Thank you soooooo much Redbelly98!!! I can't believe it was that elementary, I had been stuck on this problem for far too long. I really appreciate the great direction.
Thanks for the warm welcome as well!!