# Does a charge located in a Faraday's cage creates a field outside the cage?

1. Oct 17, 2011

### Barloud

Hi everyone,

Well everything is in the title. Answers are not clear on the net and I cannot figure it out myself.

Last edited: Oct 17, 2011
2. Oct 17, 2011

### xts

Controversial? Weird...

Answer is: no. Ex definitione - Faraday's cage is a structure totally blocking any elecrostatic interaction between its outside and inside.

3. Oct 17, 2011

### chrisbaird

Yes. Conducting shells (for example, Faraday cages) do shield their insides from external charges, but do not totally shield the outside from internal charges. You can prove this very easily using Gauss's law in integral form and putting a Gaussian surface around the conducing shell. Note that the conducting shell does destroy the internal field pattern, so that if we are outside a conducting shell, all we can know is the total charge inside, but not the distribution of that charge.

4. Oct 17, 2011

### chrisbaird

I would not say it is controversial (which implies unsettled science), just misunderstood as you have demonstrated. See my comment above.

5. Oct 17, 2011

### xts

Shouldn't the Faraday's cage be grounded? Maybe that's my misunderstanding...
In such case it shields both ways.

6. Oct 17, 2011

### chrisbaird

That's true that if the conducting shell is grounded, then the induced charge on the shell will drain away, and Faraday cages are usually grounded. But I just wanted to point out that that is not always the case.

7. Oct 17, 2011

### Barloud

Ok, thanks. That's exactly what was confusing me.

8. Oct 17, 2011

### atyy

http://www.feynmanlectures.info/flp_errata.html

"Volume II, page 5-9 now says “…no static distribution of charges inside a closed grounded conductor can produce any [electric] fields outside” (the word grounded was omitted in previous editions). This second error was pointed out to Feynman by a number of readers, including Beulah Elizabeth Cox, a student at The College of William and Mary, who had relied on Feynman’s erroneous passage in an exam. To Ms. Cox, Feynman wrote in 1975,[1] “Your instructor was right not to give you any points, for your answer was wrong, as he demonstrated using Gauss’s law. You should, in science, believe logic and arguments, carefully drawn, and not authorities. You also read the book correctly and understood it. I made a mistake, so the book is wrong. I probably was thinking of a grounded conducting sphere, or else of the fact that moving the charges around in different places inside does not affect things on the outside. I am not sure how I did it, but I goofed. And you goofed, too, for believing me.”"