• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Gauss's law to solve for electric field.

  • Thread starter playoff
  • Start date
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?
 

Simon Bridge

Science Advisor
Homework Helper
17,823
1,636
What does the experimental data tell you?
 
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?
 

Simon Bridge

Science Advisor
Homework Helper
17,823
1,636
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.
 

rude man

Homework Helper
Insights Author
Gold Member
7,433
641
Hey, how did you get that lambda into your post? I mean the Greek letter.
 
Last edited:

rude man

Homework Helper
Insights Author
Gold Member
7,433
641
Are you sure you were'nt supposed to find the E field between the inner wire and the outer shield?
 

Simon Bridge

Science Advisor
Homework Helper
17,823
1,636
You mean ##\small{\lambda}##? Same way as always - LaTeX.
But it looks like the first post may have used the "insert unicode" trick.
 

Orodruin

Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
2018 Award
15,284
5,444
Are you sure you were'nt supposed to find the E field between the inner wire and the outer shield?
I would like to second this question. Where exactly did you measure the field?
 

Simon Bridge

Science Advisor
Homework Helper
17,823
1,636
It's my bet for the missing information - yes.
 

Want to reply to this thread?

"Gauss's law to solve for electric field." You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top