# Gauss's law to solve for electric field.

1. Sep 20, 2014

### playoff

An infinitely long line charge with a linear density +λ inside an infinitely long cylinder of radius R and area density -λ/(2pi*R).

So if I set up a cylindrical Gaussian surface with length L, the positive charge inside the surface would be λL and negative charge inside the surface would be area density multiplied by area so -λ/(2pi*R)*(2pi*RL) => -λL, cancelling each other out no matter what.

But the problem is that electric field has to exist, because I am supposed to compare this derived electric field with experimental data.

Did I miss out on something?

2. Sep 20, 2014

### Simon Bridge

What does the experimental data tell you?

3. Sep 20, 2014

### playoff

Well there definitely seems to be an existing electric field according to my data. I should have added this: I am supposed to find the size of lambda that would generate the electric field recorded from the lab. But since electrical field is always 0, there can't be a lambda that would fit my experimental data.

Could someone evaluate whether I have used Gauss's law correctly and came up with a correct answer?

4. Sep 21, 2014

### Simon Bridge

If the coaxial model you are using is a good fit to the experiment, then the electric field should be very small outside the coax.
That is why I asked you what the data is telling you. Is the electric field almost zero?

It is unlikely that you actually have an infinitely long bit of coax in the lab though ... so you will not get an exactly zero field.

The answer to the question as you wrote it down is, as you have pointed out, there is no value of lambda that can give the measured field.
If you have missed something, it was in something you have not told us yet.

5. Sep 21, 2014

### rude man

Hey, how did you get that lambda into your post? I mean the Greek letter.

Last edited: Sep 21, 2014
6. Sep 21, 2014

### rude man

Are you sure you were'nt supposed to find the E field between the inner wire and the outer shield?

7. Sep 21, 2014

### Simon Bridge

You mean $\small{\lambda}$? Same way as always - LaTeX.
But it looks like the first post may have used the "insert unicode" trick.

8. Sep 21, 2014

### Orodruin

Staff Emeritus
I would like to second this question. Where exactly did you measure the field?

9. Sep 21, 2014

### Simon Bridge

It's my bet for the missing information - yes.