Wireless charging technology has been applied to electric toothbrushes, razors, and mobile phones, and preliminary experiments have also been conducted on wireless charging for electric vehicles. If wireless charging can be achieved, can we eliminate high-voltage transmission lines to achieve wireless transmission, and even receive electricity from space solar cells, completely solving human transmission and energy consumption problems? Today, the editor tried to talk about whether wireless transmission is reliable.

Wireless energy transmission

Before solving the problem of wireless transmission, let's first expand the scope of wireless transmission - "wireless energy transmission". The concept of energy encompasses a wider range, with sunlight being the most common form of "wireless energy transmission", which spans 150 million kilometers and lasts for 4.5 billion years.

First, let's take a look at the energy forms available, and then see if they can transmit wirelessly.

We are already familiar with the forms of energy in the world, and we have learned four types of physics in high school: force, heat, light, and electricity. Adding nuclear and chemical energy, there are roughly six forms of energy (where force refers to mechanical energy and electricity refers to electromagnetic energy). The atomic and chemical bond forces from which nuclear and chemical energy originate cannot be transmitted over long distances, so we only consider the four types of energy transmission: mechanical energy, thermal energy, light energy, and electromagnetic energy.

Firstly, let's talk about the four common ways of wired transmission: mechanical energy transmission corresponds to rigid rod transmission, and steel rod is the carrier of mechanical energy, such as the transmission shaft of an automobile; The transmission of light energy corresponds to optical fibers, which are the carriers of light; Thermal energy corresponds to good conductors of heat, such as various metals; Electromagnetic energy corresponds to wires or iron cores, which are good conductors of electricity or magnetism. The characteristic of wired transmission is that the resistance of this channel is very low: steel rods have no loss of mechanical energy, optical fibers have little loss of optical energy, good conductors have little thermal resistance, wires have little resistance, and iron cores have little magnetic resistance. This way, we can harvest energy with minimal loss at the far end.

Which energy can be transmitted wirelessly

Wireless transmission no longer has these "low resistance channels", only air or a vacuum with nothing. What is the effect of the four types of energy propagating in air or vacuum? The first is mechanical energy and thermal energy, which must have a medium to propagate. Vacuum cannot propagate, while both types of energy in the air will diffuse around, resulting in poor propagation efficiency.

Light energy propagation does not require a medium, and humans can use lasers for high-power light energy transmission, and lasers can even be made into laser cannons. But lasers have high requirements for terrain and weather. Light can only travel in a straight line and cannot be diffusely reflected by particles in the air, otherwise the loss will be too great. Even if transmission is possible, the efficiency of converting from electricity to light and from light to electricity is low at both the transmitting and receiving ends, but the cost is high. Perhaps in the future, only space energy can be transmitted to the ground through lasers with certain feasibility.

There is only electrical energy left. The electrical energy we are usually exposed to is only 50 or 60 Hz, which is a very low frequency. The wavelength of electromagnetic waves can reach 5000 to 6000 kilometers, which is not characteristic of electromagnetic waves for general transmission distances (about 40000 kilometers from the equator of the Earth). Therefore, electromagnetic induction is the main way of energy transmission. The wireless charging technology we are currently applying, as well as the wireless transmission technology in the laboratory, all utilize the principle of electromagnetic induction. Appendix 1 explains the differences between electromagnetic induction and electromagnetic waves.

The essence of wireless energy transmission

Since air or vacuum is a "high resistance channel" for energy transmission, there is only one way to achieve smooth energy transmission without being scattered and diffused due to "high resistance" - resonance!

Laser is a resonant light source, and radios rely on resonance to receive signals. Similarly, power transmission also requires the assistance of resonance. For details on resonance, please refer to Appendix 2 below, which provides a detailed explanation of the concept of resonance and examples of resonance in various energy sources.

Wireless transmission of electrical energy

We need to establish a circuit with a resonant frequency of 50Hz so that electrical energy can be transmitted. Although that is the case, it is not easy to implement. The real environment is too complex. Electric and magnetic fields will encounter various obstacles in the air, and when they encounter iron or steel in buildings, the electromagnetic field will be absorbed. The professional term is "shielding" (similar to putting a phone in a closed iron box and there is no signal, all signals are absorbed by the iron box). It is too difficult for us to fully consider the capacitance and inductance composed of all the steel along the road when designing the circuit parameters. (Capacitance and inductance are the most common but also the most difficult to understand concepts in electricity. If there is an opportunity, the editor will write an introduction to the physical essence of capacitance and inductance.)

At present, there are also application cases of wireless transmission, such as toothbrushes, mobile phones, and cars with wireless charging. One common feature of these cases is short distance. The toothbrush and phone are both placed on the charging stand, and the chassis of the wireless charging car is also very close to the ground (within 30cm). The purpose of being close is only to reduce the influence of stray capacitance and inductance in the surrounding environment, so that the circuit we design can transmit energy to the maximum extent.

Of course, from another perspective, this is also to improve the efficiency of energy transmission. The longer the distance, the more stray capacitors and inductors they absorb, and the less energy they can transmit to their destination, resulting in low energy transfer efficiency.

So from the perspective of the influence of stray parameters, it is almost impossible to complete long-distance wireless transmission, as the influence of stray parameters and various shielding can hardly be removed.

Is wireless transmission feasible through electromagnetic waves?

Since the method of electromagnetic induction is not optimistic, is it possible to use electromagnetic waves? Someone has also proposed microwave wireless transmission. The difference between electromagnetic induction and electromagnetic waves lies only in the ratio of frequency, or wavelength to transmission distance. If the wavelength is longer than the transmission distance, it is the principle of electromagnetic induction, otherwise it is the transmission principle of electromagnetic waves.

The propagation mode of electromagnetic waves is the same as that of broadcast signals, which are scattered and diffuse. As long as they are diffuse, they lack efficiency. Radio and television signals can diffuse into space, but because only signals are transmitted, the energy is low and the efficiency is low; But if high-power electrical energy permeates the space, not only does it require high-power transmitting and receiving devices, but also high-power electromagnetic waves permeate the space, making people feel like they are living in a microwave oven. At present, the electromagnetic environment issues around power transmission lines have aroused people's doubts. I don't know how you would feel if there were invisible high-power electromagnetic waves surrounding you.

In summary, the editor holds a cautious and pessimistic attitude towards future long-distance transmission.

Appendix 1: Electromagnetic Waves and Induction

The difference between electromagnetic waves and electromagnetic induction lies only in the relationship between wavelength and propagation distance. If the wavelength is much shorter than the propagation distance, there is a changing magnetic field (or electric field) around the transmitting end, which then extends outward to induce the changing electric field (or magnetic field), and then extends outward... From the receiving end, it is the alternating electromagnetic field that spreads over here, reflecting the nature of "waves".

If the receiving end is very close to the transmitting end and much smaller than the wavelength, any slight changes in the electric and magnetic fields at the transmitting end will quickly transmit to the receiving end, causing the same changes. From the perspective of the receiving end, there is no wave property, only induction.

Appendix 2: Resonance and Electromagnetic Induction

The changing electric field generates a magnetic field, and the changing magnetic field generates an electric field, which is electromagnetic induction. Before discussing electromagnetic induction resonance, let's give an example: how can a child swing a swing higher and higher when pushing it? It must have been pushed up as the swing reached its highest point and was falling, and it had to be done at the same time every time. This time interval can be directly calculated by a formula, which is the pendulum formula: time=2 π times the root sign (the length of the pendulum rope L divided by the gravitational acceleration g), discovered by Galileo.

T＝2π×√(L/g) 

T is only related to the length of the swing rope and is a constant. That is to say, no matter who sits on the swing and how high the swing is, the time for a round trip is fixed. This principle is also the principle of a pendulum, no matter how long the pendulum is or how high it is, a swing back and forth is one second.

The person pushing must take action at this time interval to make the swing swing higher and higher. Frequency and time are reciprocal to each other, in other words, only when a person exerts force at this frequency can their power be fully transmitted to the swing. This frequency can be called the natural frequency of the swing.

Natural frequency, also known as resonant frequency or resonant frequency, is a reciprocating motion; Resonance is vibration together, and resonance is a "harmonious" vibration of something that is consistent with the natural frequency of the object. Two vibrate together. So the three words are universal and are only used in different fields based on habits and history.

In mechanics, this frequency is widely used. We have heard examples of soldiers walking in unison when a bridge collapses. This is where the frequency of soldiers walking is consistent with the resonance frequency of the bridge, and the maximum force of the steps is transmitted to the bridge. The bridge receives more and more force (energy) (similar to the principle of swinging higher and higher), and finally collapses. There are also examples in the United States where a gentle breeze blows over a bridge and eventually collapses, and the frequency of the breeze is equal to the resonance frequency of the bridge. Architects also need to consider how to prevent the resonance frequency of the building from being equal to the frequency of seismic waves when designing buildings.

The resonant frequency in optics is also the natural frequency. For example, an optical resonant cavity is an essential component of a laser emitter, where light resonates and the energy gathered becomes greater and greater, ultimately becoming a high-energy laser.

Whether it is mechanics or optics, the natural frequency has a characteristic: when work is done at this frequency, the loss in the entire process is minimized and the energy transmitted is the highest.

In electricity, it is also called resonant frequency. At this frequency, the resistance in the entire circuit is the smallest and the energy transmitted is the largest. The resonant frequency is determined by the inductance and capacitance of the transmission circuit:

f=1/（2×π×√LC），either or T=2π×√LC

Is it similar to the simple pendulum formula above, it's all 2 pi times something under a root sign. The frequency f is the reciprocal of the resonant period T. L is the inductance and C is the capacitance.

When we were young, we made radios that had two semicircular metal plates inside. When we turned the knob, the relative position of the two metal plates would change, the area would change, the capacitance value C would change, and thus the resonant frequency would change. The frequency modulation of 98.5 MHz or medium wave 792 kHz that we usually hear in broadcasting refers to the magnitude of the resonant frequency.

The air is already filled with various radio waves. When we tune the radio to a certain resonant frequency, the energy transmission path at that frequency has the smallest loss and the largest energy transmission, so we listen to this radio station. Rotate the button a little more, and it will gradually become unclear because the resonance frequency of the radio has changed, resulting in greater resistance to the transmission path and less energy being transmitted. (There is a general concept here, called quality factor, which describes the ratio of energy transmitted at resonant frequency to energy transmitted at non resonant frequency. The concept comes from broadcasting. We listen very clearly to a program at a certain radio frequency, but a slight movement of the knob makes it unclear. This indicates that the energy transmission effect at this frequency is very good, and if we hear clearly, the "quality" is high.)

The same applies to the transmission of electrical energy using the principle of electromagnetic induction. We need to establish a circuit that can resonate at 50Hz in order to maximize the reception of electrical energy.