The Physics of Faraday Cages

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?

The electric field at any point is just the vector sum of the fields due to each charge. Here’s what that would look like with two charges (the red spheres) that create electric fields (the white arrows). The resulting total field at that point is represented by the yellow arrow.

If the two charges make electric fields in the same direction, the resultant field will be larger. However, if the two fields are in opposing directions, then the field will be smaller. It will be zero if they cancel each other perfectly.

Electric Fields in Conductors

This is exactly what a Faraday cage does: It cancels an EM field by creating a second one in the opposite direction. Thanks to superposition, the two fields cancel and create a net field of zero. With a zero electric field, you no longer have an electromagnetic wave. But it’s important to remember that the cage isn’t blocking the electric fields, it is canceling them.

Typically, the “cage” is a spherical shell that encloses an object—like a cell phone—and is made of some type of electrically conductive metal. The conductivity of this material allows electrical charges in the cage material to move along its surface and create a second electric field that cancels the EM wave coming from the phone. So if the phone inside the shell pings out a signal, you won’t be able to detect it outside of the Faraday cage.

This also works the other way: Incoming electromagnetic waves will get canceled by the moving charges in the Faraday cage. Your phone won’t know that it’s getting a text message or call.

Let’s focus for a minute on why the cage’s materials are important. A Faraday cage is made from an electrical conductor, metals like copper, aluminum, and steel. In a conducting material, atoms are able to share one of their electrons with neighboring atoms. This means that an electron is mostly free to move from one atom to the next. That’s not the case for an insulator, a material like wood, plastic, or glass. For an insulator, these electrons are stuck with their original atoms and can not move around.

Because conductors can let charges move, some cool stuff can happen. Namely, when an electric field encounters a conducting material, it will move charges so that the net electric field is zero.

Here’s a thought experiment: Imagine that I have a sphere made of a conducting metal and I add some extra electrons. (These extra charges could come from anywhere, but the most common real-life example is from an electrostatic interaction, like what happens when you rub a balloon on your hair: Electrons move from your hair to the balloon. This interaction is also what gives you a shock when you take your socks out of the dryer, what makes your hair stick up in the winter, what makes an N95 mask work, and what makes a Leyden jar glow.)

Let’s say I add 100 electrons to my sphere by touching it to some electrically charged socks straight from the dryer. These electrons all create electric fields that push on the other electrons. As a result, they all get pushed apart and end up on the surface of the sphere. (They can’t just jump off the sphere.) Here’s what it would look like:

But here is the very important part: Now these electrons are arranged on the surface of the sphere in such a way that the total electric field at any point inside the sphere is zero. (It has to be zero. If the field wasn’t zero, then it would push on the free electrons, and any charge that can move would move toward the surface of the sphere.) With a zero electric field, you can no longer have an electromagnetic wave. The sphere is now a Faraday cage.

What about the magnetic field—does that get canceled too? Not in the same way as the electric field. The problem is that there’s no such thing as a magnetic charge. This means you can’t get a separation of magnetic charges to cancel the magnetic field inside the conductor. But don’t worry, remember that an electromagnetic wave needs both a changing electric field and a changing magnetic field. If you cancel the electric field, you won’t have an electromagnetic wave.

Real Faraday Cages

A Faraday cage doesn’t have to be a sphere. It can pretty much be any shape with a hollow interior. (Since the charges end up on the surface of the shape, it doesn’t matter if it’s hollow.) But in practice, you can’t just cover your phone with any electrical conductor and expect it to act as a Faraday cage. There are two factors that are also important: the thickness of the material and its solidity. Let’s start with the thickness.

One parameter of a Faraday cage is its “skin depth.” This is a way to calculate the minimum thickness of a material so that it can effectively cancel EM waves. The skin depth depends on the resistivity of the material (how difficult it is for the electrons to move), the frequency of the EM wave, and also the magnetic properties of the material. This means that for longer wavelengths (like radio waves) you would need thicker material in your cage.

Suppose you wrap your phone with a single layer of aluminum foil. Aluminum foil is indeed an electrical conductor, but it’s also very thin. There aren’t many electrons that you can move around, and they can’t get very far apart (because the foil is thin). In the end, they can’t perfectly cancel the electric field inside. So maybe one layer of aluminum foil won’t be quite enough.

You don’t have to take my word on this: Take your phone and wrap it in one layer of aluminum foil. Now try to call your phone. (You will, of course, need another phone for this.) If your phone rings, your Faraday cage isn’t thick enough. Keep adding layers of aluminum foil until it stops receiving a call. That’s when you’ve acquired enough skin depth for your Faraday cage to work.

A Faraday cage can also be a mesh material, rather than perfectly solid. It’s a complicated calculation, but in general if the diameter of the holes in the mesh are smaller than the wavelength of the EM wave, it should work fine.

Imagine you have an FM radio tuned to a station at 100 MHz. The wavelength of this radio wave would be 3 meters. So as long as the diameter of the holes in the mesh is smaller than 3 meters, it would still cancel EM waves in the radio spectrum. (That means you could make a Faraday cage with holes big enough for a person to squeeze through.)

A 5G signal from your phone has a much smaller wave. These have frequencies around 30 GHz, which means a wavelength of around 1 centimeter. A mesh wiring Faraday cage would still block phone signals, as long as the holes were smaller than 1 centimeter in diameter.

Of course, if you want to really get off the grid and prevent people from finding your phone, there’s an easier solution: Just turn it off.