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


by Barloud
Tags: cage, charge, creates, faraday, field, located
Barloud
Barloud is offline
#1
Oct17-11, 12:53 PM
P: 17
Hi everyone,

Well everything is in the title. Answers are not clear on the net and I cannot figure it out myself.
Phys.Org News Partner Physics news on Phys.org
Information storage for the next generation of plastic computers
Scientists capture ultrafast snapshots of light-driven superconductivity
Progress in the fight against quantum dissipation
xts
xts is offline
#2
Oct17-11, 12:58 PM
P: 882
Controversial? Weird...

Answer is: no. Ex definitione - Faraday's cage is a structure totally blocking any elecrostatic interaction between its outside and inside.
chrisbaird
chrisbaird is offline
#3
Oct17-11, 01:02 PM
P: 617
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.

chrisbaird
chrisbaird is offline
#4
Oct17-11, 01:07 PM
P: 617

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


Quote Quote by xts View Post
Controversial? Weird...

Answer is: no. Ex definitione - Faraday's cage is a structure totally blocking any elecrostatic interaction between its outside and inside.
I would not say it is controversial (which implies unsettled science), just misunderstood as you have demonstrated. See my comment above.
xts
xts is offline
#5
Oct17-11, 01:25 PM
P: 882
Shouldn't the Faraday's cage be grounded? Maybe that's my misunderstanding...
In such case it shields both ways.
chrisbaird
chrisbaird is offline
#6
Oct17-11, 01:34 PM
P: 617
Quote Quote by xts View Post
Shouldn't the Faraday's cage be grounded? Maybe that's my misunderstanding...
In such case it shields both ways.
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.
Barloud
Barloud is offline
#7
Oct17-11, 02:21 PM
P: 17
Quote Quote by chrisbaird View Post
That's true that if the conducting shell is grounded
Ok, thanks. That's exactly what was confusing me.
atyy
atyy is offline
#8
Oct17-11, 05:56 PM
Sci Advisor
P: 8,004
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.”"


Register to reply

Related Discussions
faraday cage General Physics 10
Faraday's cage Classical Physics 8
charge.E field and Faraday's cage Classical Physics 19
Faraday Cage Introductory Physics Homework 6
Faraday cage General Physics 3