# How does a Faraday Cage block electric fields?

• solzonmars
In summary, a Faraday Cage works by using polarisation to cancel out the electric field created by charges placed outside of the conducting shell. This can be proven using Gauss' Law. The spherical shape of the shell also allows for the cancellation of the field. Additionally, the electric field within the shell affects the movement of electrons, causing them to accumulate on the inside surface and balancing out the external charge. This is why Faraday cages are commonly used in products like computers and smart TVs to prevent high frequency electric fields from escaping.
solzonmars
Why does a Faraday Cage work? (or more generally, why does the inside of a conducting shell have no electric field if there are charges placed outside it?)

I understand that this is the result of polarisation, but why does the polarisation happen to exactly cancel out the field? Could there be a way to prove this with Gauss' Law?

Have you looked up Wikipedia? There is a section Operation, you might want to read that up first.

Ah, so it doesn't actually cancel out fully. I was intending to use a Faraday cage as an example of a charged conductor (in the Falstad stimulation I had actually used a spherical shell). Any idea why a spherical shell would cancel out the field though?

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solzonmars said:
why does the polarisation happen to exactly cancel out the field?
If it isn't balancing then charge will flow until it is. A Faraday cage is metallic (a good conductor) to allow charges to move as necessary.

Simplistically... Units of an electric field are volts/meter. Can you can't have a voltage gradient across an ideal conductor?

Nor with a 'real' conductor when the current has stopped. (Static case)

CWatters
@sophiecentaur I understand what you mean for a solid conducting object, but for a shell does the electric field in the centre cavity actually affect the electrons in the conductor?

solzonmars said:
@sophiecentaur I understand what you mean for a solid conducting object, but for a shell does the electric field in the centre cavity actually affect the electrons in the conductor?
It does. If there is charge inside the shell, an E field exists within the shell. Opposite charge (which is electrons moving either towards the inside surface or away from it depending on the sign of the inside charge) will move to the inside surface of the shell due to electrostatic induction. The opposite charge will be equal in magnitude (total coulombs) to the internal charge so that the net E field within the metal is everywhere zero. And charge equal in magnitude and sign to the internal charge will accumulate on the outside of the shell.

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Some engineers spend a lot of time trying to prevent high frequency electric fields escaping from products like computers and smart TV. Using a Faraday cage is one tool in their armoury.

## 1. How does a Faraday Cage block electromagnetic fields?

A Faraday Cage works by creating a barrier that prevents electromagnetic fields from passing through. This is achieved by using a conductive material, such as metal, to surround the area that needs to be protected. When an electromagnetic field hits the Faraday Cage, the conductive material absorbs and redistributes the energy, effectively blocking it from entering the protected space.

## 2. Why is a Faraday Cage important in electronics?

In electronics, a Faraday Cage is used to protect sensitive equipment from outside electromagnetic interference. This is crucial in industries such as telecommunications, where even small amounts of interference can disrupt signals and cause malfunctions. By shielding the equipment with a Faraday Cage, it ensures that the equipment can function properly without any outside interference.

## 3. Can a Faraday Cage protect against all types of electromagnetic radiation?

No, a Faraday Cage can only protect against certain types of electromagnetic radiation, specifically those in the radio frequency and microwave range. It cannot protect against lower frequency electromagnetic fields, such as those from static electricity, or higher frequency fields, such as X-rays or gamma rays.

## 4. Is a Faraday Cage 100% effective in blocking electromagnetic fields?

While a Faraday Cage is highly effective in blocking most electromagnetic fields, it is not 100% effective. There are certain scenarios where electromagnetic fields can still penetrate the cage, such as if there are gaps or holes in the conductive material. Additionally, extremely strong electromagnetic fields, such as those from lightning strikes, may be able to penetrate a Faraday Cage.

## 5. Can a Faraday Cage be used to protect against EMPs?

Yes, a Faraday Cage is commonly used to protect against electromagnetic pulses (EMPs). EMPs are high-intensity bursts of electromagnetic energy that can damage electronic equipment. By surrounding the equipment with a Faraday Cage, it can shield it from the effects of an EMP and prevent any damage from occurring.

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