The Physics of Faraday Cages (2024)

The world relies on electromagnetic waves for communications: Wi-Fi, Bluetooth, 5G, even radio waves. But suppose you want to prevent a device from communicating—or interfering—with the rest of the world. You can't block EM waves, but you can cancel them by surrounding the device with an electrically conducting material. We call this a Faraday cage, and here’s how it works.

What Is an Electromagnetic Wave?

An electric charge (like a proton) creates an electric field in the region around it. This field points away from positive charges and decreases in strength as it gets farther away from the charge. Here’s a visualization of the electric field, showing a positive charge (the red sphere) along with arrows at different locations representing the electric field:

But there's actually another way to create an electric field—with a magnetic field. As you might guess, a magnet makes a magnetic field. If you move this magnet around, the magnetic field will change, and that change creates an electric field.

If you think that's weird, it turns out that changing an electric field also creates a magnetic field. That means we can have a situation in which a changing electric field creates a changing magnetic field—which then creates another electric field. This is one of the key ideas in Maxwell's equations, which show the relationship between electric and magnetic fields. These four equations, published in the 19th century by physicist James Clerk Maxwell, show the mathematical possibility of electromagnetic waves. (He’s also the inventor of the famous “Maxwell’s Demon” thought experiment.)

If you could see the electric and magnetic fields in a wave, it might look something like this:

If the wavelength of this electromagnetic wave is very long (greater than 10 meters), we call it a radio wave. For shorter waves, in the range of 1 millimeter to 1 meter, that would be a microwave. Your eyes can detect shorter wavelengths in the range of 400 to 700 nanometers—that's visible light. We group these EM waves into the electromagnetic spectrum.

There's one more important concept: the superposition principle. It says that when there is more than one field created by more than one charge, the net field is the vector sum of the individual fields.

Consider the following example: Suppose you have two electric charges in the same region of space. How do you find the electric field at a location near these charges?