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

In summary, Feynman misstated the effects of a grounded conducting shell. He now admits that he was wrong and that a grounded conducting shell should not shield both ways.
  • #1
Barloud
17
0
Hi everyone,

Well everything is in the title. Answers are not clear on the net and I cannot figure it out myself.
 
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  • #2
Controversial? Weird...

Answer is: no. Ex definitione - Faraday's cage is a structure totally blocking any elecrostatic interaction between its outside and inside.
 
  • #3
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
xts said:
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.
 
  • #5
Shouldn't the Faraday's cage be grounded? Maybe that's my misunderstanding...
In such case it shields both ways.
 
  • #6
xts said:
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.
 
  • #7
chrisbaird said:
That's true that if the conducting shell is grounded

Ok, thanks. That's exactly what was confusing me.
 
  • #8
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.”"
 

1. Does a charge located in a Faraday's cage create an electric field outside the cage?

Yes, a charge located inside a Faraday's cage will create an electric field outside the cage. However, the electric field outside the cage will be significantly weaker compared to the electric field outside the cage if the charge were located outside the cage.

2. How does a Faraday's cage prevent external electric fields from entering the cage?

A Faraday's cage works by conducting any external electric fields around the cage and not allowing them to enter the enclosed space. This is because the metal material of the cage acts as a shield and redistributes the electric charges evenly to cancel out any external electric fields.

3. Can a Faraday's cage block all types of electromagnetic radiation?

A Faraday's cage can block most types of electromagnetic radiation, including radio waves, microwaves, and visible light. However, some types of electromagnetic radiation, such as extremely low-frequency waves and very high-frequency gamma rays, may still be able to penetrate through the cage.

4. Is it possible for a Faraday's cage to fail in shielding against external electric fields?

Yes, it is possible for a Faraday's cage to fail in shielding against external electric fields. This can happen if there are gaps or holes in the cage's metal material, allowing the external electric fields to enter the cage. Additionally, if the external electric fields are too strong, they may be able to overcome the shielding effect of the cage.

5. Can a Faraday's cage be used to protect against lightning strikes?

Yes, a Faraday's cage can be used to protect against lightning strikes. The metal material of the cage can conduct the electrical charges from the lightning strike around the cage and into the ground, preventing any damage or harm to the objects or people inside the cage.

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