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Electricity is the lifeblood that supports the smooth operation of human society. Without volts and amps, many of our technological innovations would not exist. Without the rapid transfer of electric charge through our cells, even our lives would not be sustainable.

But success also Xiao He, defeat also Xiao He. Although electricity is vital to our lives, it only works in moderation. Excessive electricity can electrocute people and render modern electronic equipment and machinery useless. Thanks to Michael Faraday, a brilliant 19th-century scientist, and one of his namesake inventions, the Faraday cage, we humans have developed many ways to control electricity based on it, making computers, cars, and ourselves safer.

Faraday cages protect objects inside the cage from electromagnetic radiation. Normally, when an electromagnetic field is applied to an electrically conductive object, the charge stays outside the conductor and does not move inward. Faraday cages, as their name suggests, look like cages, and some are as simple as barbed wire or ice buckets. Others use fine metal mesh.

We are surrounded by all kinds of electromagnetic radiation. It is present in visible and ultraviolet light, in microwaves used to cook food, and even in FM and AM radio waves used to play music on the radio. But sometimes, the radiation is not welcome, and can even be seriously damaging. In this case, it is the Faraday cage's turn.

Under the action of the applied electric field, Faraday cage can distribute the charge or radiation to the outside of the cage, forming an electric field equal to and opposite to the applied electric field, thus canceling the charge or radiation generated outside the cage, so that an equipotential body is formed inside the cage. In other words, a Faraday cage is a hollow conductor, with charges concentrated on the outer surface of the cage. This feature makes Faraday cages useful in an electron-rich, technologically advanced world. Although Faraday would eventually have his own time, his background to invention actually dates back to an earlier era. So, whose idea was this super-useful cage?

Benjamin Franklin and Michael Faraday Although Faraday cages are named after Michael Faraday, many of the ideas behind Faraday cages were inspired by Benjamin Franklin. Franklin is known for trying to attract lightning by flying a kite in a thunderstorm at some point during his illustrious career, and so he had some understanding of the vagaries of electricity and the concept of electricity. In 1755 Franklin began to play with electricity in a very new way. He electrified a silver pint can, and placed an uncharged cork ball into it with an electrically nonconductive thread until the cork ball touched the bottom. Franklin wrote to a colleague: "If the cork ball is attracted to the outside of the jar and not to the inside, although it touches the bottom of the jar, it is not charged by contact with the bottom as it touches the outside of the jar when it is pulled out. This is a very strange phenomenon. "He was puzzled by the interaction of electric currents with charged and uncharged objects and admitted it: " You wonder why, but I don't know. Maybe you'll find out why, and then you'll be very kind to tell me."

British chemist and physicist Michael Faraday (1791-1867) was the inventor of Faraday cages. Decades later, a British physicist and chemist named Michael Faraday came up with another related idea-he realized that conductive objects, such as metal cages, when charged, only show charges on their surfaces and have no effect on the interior of the conductor.

Faraday confirmed this observation again by laying metal foil in a room and then charging it with an electrostatic generator. He placed an electroscope (a device for detecting electric charge) in the room and, as he expected, the electroscope showed that there was no charge in the room. The charge simply moved along the surface of the foil and did not penetrate the room at all. Faraday further examined this phenomenon with his famous ice-bucket experiment. In this experiment, he essentially repeated Franklin's idea by placing an electrically charged brass ball in a metal cup. As expected, his results were identical to Franklin's.

This concept has all sorts of magical applications, one of which is relevant to everyone who has ever flown. Imagine what would happen if your plane was suddenly struck by lightning? This is not uncommon, and even happens frequently, but neither the aircraft nor the passengers are affected. This is because the aluminum fuselage of the aircraft forms a Faraday cage, and the charge in the lightning only passes through the surface of the aircraft, without harming the equipment or people inside. This isn't fun, it's real science. So how does this clever cage design work?

What does Faraday Cage do? To understand how Faraday cages work, one must have a basic understanding of how electricity works in conductors. The process is simple: metallic objects such as aluminum mesh are conductors in which electrons (negatively charged particles) move. When it is not charged, the number of positive and negative particles in a conductor is roughly the same. If a charged external object approaches the conductor, the positive and negative particles will separate. Electrons with an opposite charge to the external object are attracted to the vicinity of the external object, while electrons with the same charge as the external object are repelled and moved away from the object. This rearrangement of charge is called electrostatic induction.

Faraday cage experiment: When a metal cage is charged, it only shows charge on the surface of the cage, and the objects in the cage will not be affected by external charges and electromagnetic radiation. When an external charged object exists, the positive and negative particles of the conductor will disperse on both sides of the conductor, generating an electric field equal to and opposite to the external electric field formed by the external charged object, thus canceling the electric field effect of the external object inside the metal conductor. Therefore, the net charge inside the aluminum mesh is zero.

Here's the real point: Although there is no charge inside the conductor, the opposing electric field has an important function-it shields external static charges as well as electromagnetic radiation, such as radio waves and microwaves. This is the true value of Faraday cages.

Different Faraday cage structures have different shielding effects. Changes in the electrical conductivity of different metals, such as copper or aluminum, affect the function of Faraday cages. The mesh size of the shield or grid also changes the function of the Faraday cage, which can be adjusted according to the frequency and wavelength of electromagnetic radiation you want to shield inside the cage.

Faraday cages sometimes have other names, including Faraday shielding, RF (radio frequency) cages, or EMF (electromotive force) cages. Whatever the name, Faraday cages are most commonly used in scientific laboratories for experiments or product development.

What Michael Didn't Expect When Faraday built his first cage, he probably didn't expect it to be used by thieves. But police often catch thieves laying aluminum foil inside shopping bags because the foil interferes with RFID security tags on expensive items in retail stores, rendering them useless.

Faraday cages are used for a variety of purposes, sometimes in esoteric laboratory environments and sometimes in everyday objects. For example, your car is basically a Faraday cage. In the event of lightning strikes nearby, it is the Faraday cage effect that protects you, not the rubber tire. Although it may be a coincidence, many buildings can also function as Faraday cages. Plaster or concrete walls, for example, are interspersed with metal rebar or barbed wire, which form a Faraday cage and steal wireless and cell phone signals without your knowledge.

Microwave ovens are a common commodity that uses Faraday cages. Instead of shutting out microwaves, microwaves confine them to a small space and "spot blast" your food. But shielding benefits humans more often. Microwave ovens can reverse this effect, trapping radio waves in cages and cooking food quickly. Shielded cable on TV reduces external interference to maintain clear images. Line workers at power companies often wear special protective clothing made using the Faraday cage concept. Wearing these protective suits, power line workers can work on high-voltage power lines, greatly reducing the risk of electric shock. Governments can protect vital telecommunications equipment from lightning strikes and other electromagnetic interference by building Faraday cages around them. Scientific laboratories in universities and corporations create a completely electrically-neutral testing environment for all kinds of experiments and product development by using more advanced Faraday cages that completely exclude all external charges and electromagnetic radiation.

Making your own Faraday cage doesn't cost millions or a physics degree. You can find instructions on the Internet for making simple Faraday cages from common household products such as aluminum foil and plastic wrap.

A Faraday cage is what a magnetic resonance imaging (MRI) room in a hospital is. MRI relies on powerful magnetic fields for medical scanning of the human body, so the MRI room must be electromagnetically shielded to prevent stray electromagnetic fields from affecting the patient's diagnostic image.

Faraday cages also have many political and military uses. Politicians may choose to discuss sensitive issues only in shielded rooms, which prevent eavesdropping. All modern armed forces 'communications and weapons systems rely on electronics, but there is a problem-these systems are vulnerable to electromagnetic pulses (EMPs), which can be solar storms or man-made EMPs. To protect critical systems, the military sometimes uses shielded shelters and shielded vehicles.

Faraday cages have become a hot topic among doomsayers for the same reason. These people preach self-sufficiency, distrust government responses in the face of man-made or natural disasters, and believe that the use of homemade Faraday cages can shield all important electronic equipment. Once the end of the world comes, they can still have shortwave radios and other high-tech tools, which may be lifesaving.

Even if you don't care about doomsday survival, Faraday cages are probably already integrated into your daily life. These Faraday cages harness the fundamentals of physics to make our lives safer, more luxurious, and more convenient, and to facilitate many exciting technological developments.

Frequently Asked Questions about Faraday Cage What is Faraday Cage? How does it work? A Faraday cage is a container or shield made of conductive material that blocks electromagnetic radiation from the outside of the cage and protects the contents from any static or non-static charge or radiation.

What can penetrate Faraday cages? Faraday cages can largely protect the interior from external charges or electromagnetic radiation, especially if the thickness of the Faraday cage is large enough. Faraday cages, however, cannot block stable or slowly changing magnetic fields. For example, Earth's magnetic field can penetrate Faraday cages, so compasses can still work.

Can aluminum foil be used as a Faraday cage? Aluminum foil can be used as a Faraday cage as long as there are no cracks in the foil and there is an insulating layer between its surface and the object to be protected inside.

by Chris Pollette & Nathan Chandler

Translator: Unit 7

Reviser: small line

How Faraday Cages Work

This article comes from Weixin Official Accounts: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop), by Chris Pollette

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