The Faraday cage

[hobz]

I have a few questions concerning the Faraday cage.

 

As I understand it, the electric field inside a cage build from a conducting material, is zero. Does this mean that the sum of electric forces acting inside the cage is also zero, or are there simply no forces at work?

 

The conducting material opposes outside electric fields, by aligning the electrons in such a way, that the electric field inside is zero.

But what if the total number of electrons in the cage, are not "enough" to oppose the outside field? That is, if a very strong outside field is applied, how can a limited number of electrons align to oppose the field?

 

And finally. The cage can be a grid, or bars. But is it also a Faraday cage to build a cube of conducting material, and stand inside the cube, or is there some minimum of what constitutes a cage. What determines such a minimum?

 

Thanks!

[Edtharan]

A Faraday cage is made of metal and charge on a conductor spreads it self out. Also, like charge repels.

 

These together mean that any electric field creates a charge on the Faraday Cage and that this charge th4en covers the entire cage. As all this charge is the same polarity it repels all the charges elsewhere on the cage.

 

Therefore net charge inside must be 0.

 

It helps if you think of it as a sphere (other shapes are possible, it is just that the mathematics are more complicated but they still cancel to 0 inside the cage).

 

Because of this, if you have a metal bodied car, it is quite protected during a lightning strike (from electricity, but the sound of the thunder would be quite loud and you might get shattered windscreens).

[hobz]

Thanks for posting, but it doesn't really answer any of my questions.

[tvp45]

Inside an ideal Faraday cage, the electric field due to static sources outside the cage is zero.

 

Inside a real Faraday cage, the electric field due to static outside sources will be greatly diminished, but may not be exactly zero everywhere.

 

Building a cube and standing inside is a good idea. Just don't touch the conductor if you can help it. A grid is OK, but bars??? Bars don't sound like a good idea.

[hobz]

I know that the electric field is zero. But does that mean that the net sum of electric FORCE is zero? That is, if I put one test charge inside the sphere (if you like), would all the forces acting on that charge sum up to zero, or would there simply BE no forces?

 

Check out

where the guys inside the cage demonstrates a Faraday cage where he touches different stuff.

 

If I was informed to be exposed to an electric field of high strength, how little metal would is require to protect my self?

 

I understand that the electrons on the outside of the cage, would align themselves in a manner that would be related to the strength of the field. Suppose my field was so strong, that it would require all free electrons to be at exactly the same position in space, because the field was so strong that it compressed the electrons into one point.

According to QM, this would not be possible. How can we "prevent" this from happening by imposing some law on the phenomenon? It would require infinite energy?

[tvp45]

I know that the electric field is zero. But does that mean that the net sum of electric FORCE is zero? That is, if I put one test charge inside the sphere (if you like), would all the forces acting on that charge sum up to zero, or would there simply BE no forces?

 

What do you know about the relationship between electric field and force due to that field?

 

Your last question is about a difference without a distinction. It's like asking whether someone is broke or just has no money.

[swansont]

I understand that the electrons on the outside of the cage, would align themselves in a manner that would be related to the strength of the field. Suppose my field was so strong, that it would require all free electrons to be at exactly the same position in space, because the field was so strong that it compressed the electrons into one point.

According to QM, this would not be possible. How can we "prevent" this from happening by imposing some law on the phenomenon? It would require infinite energy?

 

This happens with gravitational forces — there's a point at which you overcome the electron degeneracy, which is how neutron stars are formed. I'm not seeing a situation where the field would require this, however. Electric fields have a divergence related to the charge, so I think (it's early yet) the only way to get the field you want is to already have assembled opposite charges via some other force.

[imp]

hobz, here is how I have always pictured this question: knowing that electric charge always tends to distribute itself on the OUTSIDE of a conductor, and that a Faraday cage represents a conductor, the charge distributes itself on the OUTSIDE of the cage, with none existing on the INSIDE.

 

Simplistic, yes. But, for me, understandable in everyday terms. imp