# A and ϕ inside a Faraday cage?

• I
• cuallito
In summary, the potentials A and ϕ inside a Faraday cage are not always zero. Their values are only physical up to gauge transformations, so it doesn't make sense to discuss their specific values without specifying a gauge fixing. However, if the electromagnetic field is zero, the potentials can be expressed in terms of an arbitrary scalar field, and the observable electromagnetic field has a specific physical meaning. Overall, Maxwell's electrodynamics is a gauge theory, where the potentials do not have a unique physical interpretation.
cuallito
Are A and ϕ always zero inside a Faraday cage, like E and B are?

If not, can its design be modified to accomplish that? If not, is there an analogous mechanism that'd always have A=0 and ϕ=0 inside?

Last edited:
No. Same answer as your other thread. The potentials are only physical up to gauge transformations. It therefore makes little sense to talk about the value of the potentials (unless you fully specify the gauge fixing).

If the em. field is zero somewhere, i.e., ##\vec{E}=\vec{B}=0##, then ##\Phi=\partial_t \chi/c## and ##vec{A}=-\vec{\nabla} \chi## for an arbitrary scalar field ##\chi##. The gauge potentials are not uniquely defined by the equations of motion describing a physical situation. That's, because Maxwell's electrodynamics is a gauge theory. The potentials do not have a specific physical meaning but the observable electromagnetic field ##\vec{E}=-\partial_t \vec{A}/c - \vec{\nabla} \chi##, ##\vec{B}=\vec{\nabla} \times \vec{A}##, which is a gauge-invariant quantity, has.

## 1. What is a Faraday cage?

A Faraday cage is an enclosure made of conductive material, such as metal, that is designed to block electromagnetic fields. It works by redirecting the electric charge on the surface of the cage, thus preventing any external electromagnetic fields from entering the cage.

## 2. How does a Faraday cage protect against electromagnetic interference (EMI)?

A Faraday cage blocks electromagnetic radiation by creating a barrier that absorbs or reflects the energy. This prevents the radiation from reaching the objects inside the cage, effectively protecting them from EMI.

## 3. What is the purpose of having A and ϕ inside a Faraday cage?

A and ϕ are variables used to represent the electric and magnetic fields inside a Faraday cage. These fields are important to understand because they determine the effectiveness of the cage in blocking electromagnetic radiation.

## 4. Can a Faraday cage protect against all types of electromagnetic radiation?

No, a Faraday cage is not 100% effective in blocking all types of electromagnetic radiation. It is most effective against low frequency radiation, but may not be as effective against high frequency radiation such as X-rays or gamma rays.

## 5. Are there any limitations to using a Faraday cage?

Yes, there are some limitations to using a Faraday cage. It is important to ensure that the cage is properly grounded and has no gaps or openings that could allow electromagnetic radiation to enter. Additionally, the strength and frequency of the radiation being blocked can also affect the effectiveness of the cage.

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