Having trouble understanding "Faraday cage"

In summary: This is why a Faraday cage is still electrostatic even if there is a electric field from outer source. All metallic conductors are electrostatic by default.
  • #1
betelgeuse91
32
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Homework Statement


I am having trouble understanding how a Faraday cage works.

Homework Equations


$$\oint \vec{E}\cdot\,d\vec{A} = \dfrac{Q_{encl}}{\epsilon_{0}}$$

The Attempt at a Solution


It says that Faraday cage is a hollow metallic conductor and hence, inside the cage, $$\vec{E} = 0$$
I am aware that it holds for a eletrostatic metallic conductor, since if there is any electric field within the conductor, then the charges will move around by the field and it would be no longer electrostatic. Is Faraday cage a eletrostatic conductor as well? Is that why this still holds for Faraday cage? If it is, why is a Faraday cage electrostatic even if there is a electric field from outer source? Furthermore, I am wondering if all metallic conductors are naturally electrostatic and if they are, why. Thank you very much for your help.
 
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  • #2
So why the charges on the surface of the cage move around to cancel the electric field from outside?
 
  • #3
A Faraday cage is basically a conducting shell.
Suppose you have 3 of these shells (spherical for simplicity).
One has all of its excess charge on the outside.
Another has an equal distribution of its excess charges on both its inner and outer surfaces.
The third has all of its charge on its inner surface.
Which of the three has the lowest electrical potential?
Since charge is free to move about within a conductor it will assume the lowest energy configuration.
 
  • #4
betelgeuse91 said:
So why the charges on the surface of the cage move around to cancel the electric field from outside?
You seem to be making some distinction between a conductor and an electrostatic conductor. (The metallic aspect is clearly irrelevant: it's a conductor.) Electrostasis is a circumstance, not a property of the material. If you drop a charge onto one spot on a conductor it will not immediately be electrostatic, it will take some fraction of a second for the charge to redistribute. (In fact, if it is a superconductor a current will continue to flow. It only settles down because of resistance.) Once redistributed, it will be electrostatic again.
If there is a field within the material of the conductor, it will push the free electrons around. They will only stop moving when there is no field.
 

FAQ: Having trouble understanding "Faraday cage"

What is a Faraday cage and how does it work?

A Faraday cage, also known as a Faraday shield, is an enclosure made of conductive material that blocks electromagnetic fields. This is achieved by the principle of electrostatic shielding, which redistributes the electric charge on the surface of the cage, canceling out the external electric field.

Why would someone have trouble understanding Faraday cages?

Faraday cages can be a complex topic, as they involve concepts from physics and engineering. Without a strong background in these fields, it can be difficult to grasp the principles behind how Faraday cages work.

What are the practical applications of Faraday cages?

Faraday cages are commonly used in electronics and telecommunications to protect sensitive equipment from electromagnetic interference. They are also used in medical settings to shield patients from external electromagnetic fields during MRI scans.

Can any material be used to make a Faraday cage?

No, the material used for a Faraday cage must be conductive, meaning it allows electric charges to flow through it easily. Materials like copper, aluminum, and steel are commonly used for Faraday cages.

Do Faraday cages provide 100% protection from electromagnetic fields?

No, Faraday cages can only block electromagnetic fields up to a certain frequency. If the frequency of the field is too high, it can penetrate through the cage. Additionally, Faraday cages must be constructed properly and have no gaps in order to provide effective protection.

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