It's so annoying! Static electricity always occurs in winter. Is there anyone who can take care of it?

I was shocked when I opened the door and got shocked by the metal handle...

Take off a sweater at night and sparks and lightning will fly all the way...

Even petting a cat makes crackling sounds...

It's that time again when our old friend, static electricity, comes to visit. Speaking of static electricity, we usually think of two things first - one is the electrification caused by friction, and the other is that static electricity is really annoying.

However, static electricity is actually more than just frictional charging. In some situations, the harm caused by static electricity is far more than just annoying. But if used properly, static electricity can serve mankind. Let's talk about it in detail below.

Frictional electrification is not simple

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The most common static electricity in life is frictional charging, which is also the one that tortures us the most. From the crackling sound of handshakes to the rumble and crackle of lightning, it is all due to it.

Historically, this was one of the earliest electrical phenomena noticed and recorded by humans: in Greek, ἤλεκτρον (pronounced electron) is a synonym for amber, and amber is easily electrified and dusty, which was recorded by the ancient Greek philosopher Thales in the 6th century BC.

Although frictional electrification is well known, scientists have not yet fully understood this phenomenon... because its behavior is quite unstable and its causes are complex and varied. For example, the general impression we get when we go to school is that whoever can grab more electrons will be negatively charged. If we only consider the materials, there should be an ultimate boss who wins the game of grabbing electrons.

But in reality, for some materials, the friction-generated electron-grabbing competition is not a game of the strongest taking all. It is more like playing rock-paper-scissors: silk loses to paper, paper loses to cotton, cotton loses to glass, glass loses to zinc, and zinc loses to silk in the first place!

It can be seen that the properties of materials are not necessarily the only factor. After research, some of the currently recognized influencing factors include: the properties of the material itself, surface structure and direction, local roughness, friction intensity, environmental humidity, etc.

Static electricity is not just caused by rubbing

Friction can generate static electricity, but it is not just friction that can generate static electricity. Another type of static electricity is piezoelectricity, represented by quartz and tourmaline (also known as tourmaline).

What is the principle of piezoelectricity? To use an inaccurate analogy, it is like a bus. When there are not many passengers, people stand facing all directions. As the bus becomes more and more crowded, people will involuntarily stand in one direction. From one side, all faces are seen, and from the other side, all backs are seen. This phenomenon is called polarization, which will cause positive charges to appear at one end of the piezoelectric material and negative charges to appear at the other end. Because heat will also cause stress in the lattice, some piezoelectric materials also have pyroelectric effect.

There is another type of static electricity that is more likely to occur in space. When high-energy ultraviolet rays and X-rays in sunlight shine on objects, they will drive out electrons and leave a layer of positive charge in their original position. For example, on the surface of the moon, dry and fine lunar soil particles carry positive charges, which will repel each other and rise from the ground, suspended several meters high, forming a layer of lunar dust "atmosphere" (not gas), and once the lunar dust encounters foreign objects, it is easy to absorb them.

Readers who have opened foam plastic packaging must have experienced this. Electrostatically suspended lunar dust has a great impact on lunar exploration equipment. Our first Yutu rover had just been up there for more than a month (the second day on the moon) when it had a power problem, which was probably caused by lunar dust entering the interior. The dust raised by the Apollo lunar rover did not fall for a long time because of the same principle, and it was not the effect of a ground studio.

Yutu lunar rover from Wikipedia

How harmful is static electricity?

Most static electricity is harmful and annoying. Although there are many causes of static electricity, the main theme is still "static". If the charge is generated but does not flow, it may accumulate and cause disasters.

In the winter in the north, the room is heated and the environment is dry. If you don't use a humidifier (although it may not be enough), the static electricity when walking or changing clothes is difficult to be discharged, so it accumulates on the body surface and waits for an opportunity to vent. For example, the small sparks that usually make our fingers numb can reach a voltage of tens of thousands of volts. This number sounds scary, but the physical formula behind it is as follows:

Charge = Capacitance × Voltage

It can be seen that capacitance and voltage are inversely proportional, the smaller one is, the larger the other is. When we shake hands, there is a very small capacitance between the two hands, so only a small charge is needed to generate a high voltage to break through the air and form the small arc that makes us tremble. But don't be afraid, it is the current rather than the voltage that is really lethal. The charge of static electricity in daily life is very small, so the current is also very weak and will not cause death or disability.

But everyone should be careful, if this small arc that can break through the air is placed in a dangerous place, it may cause a disaster. For example, if the oil tanker is not fully filled, the oil sloshing in the tank may cause friction and electricity to cause an explosion. Early oil tankers were equipped with an iron chain hanging down to the ground at the rear of the vehicle, which was to send static electricity to the ground in time.

Today, there are more and more reliable anti-static measures: iron chains are replaced with conductive rubber to ensure that they are always pressed against the ground without being intermittent or producing sparks. The tire material is also made of conductive rubber. Oil separators are placed in oil tanks to reduce the impact and turbulence of the oil.

Gas stations also have many measures to prevent static electricity, from loading and unloading oil products to refueling car owners, such as discharging electricity before refueling, etc. In addition to open flames, industries with serious dust, such as flour mills, also strictly prevent static electricity, because static electricity may cause terrible dust explosions and affect the normal production of flour.

Those who like to assemble their own computers know that if you touch the computer motherboard or memory chip with your bare hands, the machine may become a "brick". The reason is the same as the above formula, the high voltage generated by static electricity breaks down the chip. Therefore, the electronic equipment sent by the manufacturer is usually wrapped in a bag that can slowly release or conduct away static electricity.

When we disassemble or assemble these devices, we also have to touch the wall first, or hold the metal shell of the large machine with one hand. The technicians who specialize in circuit maintenance always have a grounded conductive rubber on their desks and conductive "handcuffs" on their wrists.

Anti-static packaging of electronic equipment, taken by the author

In the semiconductor industry, the upstream of electronic equipment production, static electricity is a serious threat. Cleanroom clothing is not only used to cover the whole body to prevent dandruff from falling off, but also has conductive filaments woven into the clothing and carbon conductive rubber mixed into the soles. Even when walking in the "gray area" outside the workshop, anti-static slippers must be worn.

I have worn these slippers for several years. They are soft and hard, durable, and can be used for petting cats. However, wearing these slippers will greatly increase the risk of electric shock! (I will leave it to the readers to think about why.) We have to weigh the pros and cons and endure the crackling noise.

Friends who often fly may notice that there are many small spikes on the wings. They are not communication antennas, nor can they increase power, but they are used to release static electricity, and are called static electricity brushes. The high-speed friction between the aircraft and the air will inevitably generate static electricity, but there is no way to use mopping equipment like tank trucks, so these small spikes are installed based on the principle that static electricity will be released into the air at sharp-shaped parts.

The largest-scale electrostatic discharge phenomenon we can experience in daily life is probably lightning, but this topic is too broad to elaborate on.

Make good use of static electricity

Although most static electricity is harmful and useless, it can create value if it is guided properly and made to act according to our will.

The most successful use of static electricity is probably in the copy machine. It first fills the semiconductor with static electricity, then uses light to illuminate the area where there should be no words, turning the semiconductor into a conductor, eliminating the static electricity there, and leaving the un-irradiated static electricity area to absorb toner.

By introducing a large amount of electric charge and confining it at a position designed by us, a strong electric field is formed, which can accelerate charged particles and study basic physics and many related disciplines. Compared with cyclotron accelerators, this electrostatic accelerator has low cost and small footprint, and is widely used in the world.

Electrostatic accelerator from Wikipedia

By making the static electricity generated by the piezoelectric effect flow, we can make a quartz resonator, which is the core component of a quartz watch and has brought great convenience to people's lives (although with the popularity of mobile phones, watches seem to be just clothing).

In the spraying industry, the principle of electrostatic repulsion can be used to evenly disperse the droplets. When they approach the spraying target, they can be adsorbed by induction static electricity. This scene is actually very familiar. It is basically the principle of moon dust destroying the lunar rover, but when used in the right place, it has a different effect.

Therefore, it seems to be an irrefutable truth that more understanding can turn enemies into friends!

This article is a work of Science Popularization China-Starry Sky Project

Produced by: Science Popularization Department of China Association for Science and Technology

Producer｜China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

Author: Qu Jiong Popular Science Writer

Review丨Liu Qian, Researcher at Beijing Planetarium