Faraday Cage
Charge inside/outside a cage. Builds on Electrostatic Potential.
A hollow metal sphere is placed in a strong, uniform external electric field. The sphere has no net charge of its own. You want to know what happens to the electric field inside the hollow interior of the sphere.
What is the electric field inside the hollow metal sphere?
The electric field inside is EXACTLY zero — not approximately, not ‘very small,’ but precisely zero everywhere inside the hollow! When the external field pushes on the free electrons in the metal, they redistribute across the surface. Negative charges pile up on one side, leaving positive charges on the other. This surface charge creates its own electric field that perfectly cancels the external field inside the cavity. It’s a perfect cancellation, guaranteed by the physics of conductors.
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Watch the external field (arrows outside) try to penetrate the conductor. Toggle the surface charges to see how electrons rearrange. Increase the external field strength — no matter how intense, the interior stays perfectly field-free. Try placing a charge INSIDE the cage to see what happens!
Eᵢₙₛᵢ₂ₑ = 0
If any electric field existed inside a conductor, free electrons would move to cancel it. Equilibrium is reached only when the internal field is exactly zero. No exceptions.
Your car’s metal body acts as a Faraday cage. If lightning strikes, the charge flows around the outside. You’re safe inside — not because of the rubber tires, but because of the metal shell.
Sensitive electronics are enclosed in metal cases to block external electromagnetic interference. MRI rooms are Faraday cages to keep stray radio signals from corrupting the medical images.
Government Sensitive Compartmented Information Facilities are essentially giant Faraday cages. No electromagnetic signals get in or out — preventing electronic eavesdropping.
Airplanes are struck by lightning roughly once per year. The aluminum fuselage conducts the charge around the outside. Passengers inside the ‘Faraday cage’ feel nothing.
“A hollow conductor is the ultimate shield. No matter how intense the field outside, the inside is a sanctuary of perfect calm — exactly zero field, guaranteed by the laws of physics.”
Faraday Cage Lab
Build and test your own Faraday cage. Place external charges and watch the cage’s surface charges rearrange to protect the interior. Try different cage shapes, mesh densities, and field strengths. Then break the rules: place a charge INSIDE the cage and see what happens to the outside world.
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Start with a solid spherical cage in a uniform external field. Toggle the field view inside — it’s zero. Now crank the external field to maximum: still zero inside. Switch to a mesh cage and reduce the number of wires — watch how even a coarse mesh provides excellent (though not perfect) shielding. Now place a positive charge INSIDE the cage. The interior field is no longer zero, and an equal charge appears on the outside surface — you can’t hide charge! Finally, try an open cage (a gap in the mesh) and see how fields leak in through the opening.
A Faraday cage doesn’t ‘absorb’ or ‘block’ the field — its free charges actively rearrange to create a counter-field that exactly cancels the external one. It’s a dynamic equilibrium, not a passive barrier. And it works for any shape of conductor, any field strength, and any configuration of external charges.