# What is energy?

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Energy exists in various forms: mechanical, potential or kinetic energy, electrical, chemical, nuclear and finally heat. Energy is essential for all living beings, and first and foremost for plants, which convert solar energy into oxygen and nutrients for the benefit of animals. Among these, men, with their machines, consume much more energy than others. And this poses many problems, linked in particular to carbon dioxide pollution, the risk of accidents and, finally, the depletion of resources in the more or less long term. Some hope to find the solution to the problem in renewable energies and energy savings. Others believe that the massive use of nuclear energy, fission or fusion, is inevitable.

## 1. Energy in all its forms

### 1.1. nothing is lost, everything is transformed

A major principle of Physics is that a certain amount, called energy, is constant. Energy takes various forms that we will successively review: potential energy, kinetic energy, heat, etc. Energy passes from one form to another without creation or disappearance: it is the principle of energy conservation.

Let’s start with the potential energy associated with a force (see “The Laws of Dynamics” for an introduction). The potential gravity energy of an object at height h above the ground is (with one additive constant) the work of the weight P of this object when it falls from height h, either Wpot = Ph. The weight is proportional to the mass m, or P = mg, where g is the acceleration of gravity (or gravity). If the object is an elevator, it requires electrical energy at least equal to mgh to bring it to a higher floor, which is actually higher because the elevator wastes energy (i.e. it also converts part of the electrical energy into heat). If the object is now an apple perched in its tree at height h0, it may happen that the apple comes off and falls. It then loses height, but acquires a velocity v(t) = -dh/dt which depends on time t, and consequently a kinetic energy Wcin = mv²(t)/2. The principle of energy conservation tells us that the total energy mgh(t)+mv²(t)/2 is constant. Its derivative with respect to time is therefore nil, i.e. mgv(t) = mv(t) dv(t)/dt and by simplifying: dv(t)/dt = g. The constant g is therefore an acceleration.

In the 17th century, it is said, the Englishman Isaac Newton saw such an apple fall (Figure 1) while meditating on the movement of the planets, which the German Johann Kepler had solved a few years earlier. He then had a brilliant idea: weren’t the fall of the apple and the ellipse described by the planets around the Sun two aspects of a universal phenomenon? The calculation confirmed Newton’s intuition that two objects of masses m and m’ at a distance r attract each other with a force F equal to Gmm’/r², where G is a universal constant, the same if the two objects are the Sun and the Earth, or the Earth and the Moon, or an apple and a piece of Earth. This phenomenon is called gravitation. To this force F corresponds a potential energy Wpot = -Gmm’/r, which is the mechanical work of the force F when the two objects are brought closer together, initially at an infinite distance from each other. In other words, F = -dWpot/dr. The gravitational force is attractive. A general rule is that force tends to decrease the potential energy.

The fall of the apple illustrates the conservation of the total energy Wtot = Wpot + Wcin, called mechanical energy. Another less ephemeral example is the oscillation of a swing (Figure 2), which allows us to observe alternately the growth of kinetic energy at the expense of potential energy, and then the opposite phenomenon. Similar oscillations can be achieved by attaching a mass (not too heavy) to a coil spring. Let’s pull a little on the spring and let go: it contracts, then stretches, then contracts, etc. Here again, the sum of potential and kinetic energies is constant, or at least would be if the spring did not absorb the energy (i.e. it converts part of the energy into heat). The potential energy Wpot here is elastic in nature, proportional to the square of the difference δz between the spring length and its equilibrium value: Wpot = γδz².

Thus the energy is expressed by different formulas depending on the case. Often we prefer to express the power supplied or absorbed, i.e. the energy per unit of time. Thus, if we connect an appliance (our elevator for example) to a current source with a voltage V = 220 V, and a current I flows through it, the power expended is W = VI. If our elevator takes a time t = 20 seconds to climb, the energy that will have to be paid to the electricity supplier will be VIt.

### 1.2. The heat

But the energy provided by this supplier is not only used to run engines. It allows us to heat ourselves, because we can transform it into heat. The heat stored by air is essentially the sum of the kinetic energies of the molecules, (mv²/2 ñ for a molecule of velocity v ñ and mass m). Since it is not a question of knowing the speeds of the millions of billions of billions of molecules contained in a room, we prefer to say that the heat stored in a body is a function of its temperature and pressure, a function that can be calculated at least approximately (read “Pressure, temperature and heat“).

This is an opportunity to return to the examples mentioned above, and to ask ourselves why the spring stops oscillating quickly enough, as well as the swing, if we do not maintain its movement. It’s that there are frictions and they generate heat, even if we don’t notice it. The energy is constant, but some of it is dissipated as heat.

### 1.3. Matter, reservoir of electrical and nuclear energy

A century after Newton discovered the law of gravity, the French physicist Charles Coulomb established a similar law for the interaction between two electrical charges q and q’. The force is thus proportional to the product qq‘ and inversely proportional to the square of the distance [1]. However, unlike mass, the charge can be either positive or negative: the force is then attractive if the charges are of opposite signs, but repellent if they are of the same sign.

In an electric battery or other generator, positive sign electric charges are distributed on the positive pole while negative sign electric charges are distributed on the opposite pole. An electron or any other charge q’ moving in the electric field thus produced has a potential energy that it can convert into heat (in a resistance) or mechanical energy (in a motor). The electrical potential is obtained by dividing this potential energy by the charge q’. This is the equivalent of the gh product for the gravity field. For a potential difference V, the available energy is therefore Vq’ and the power expended VI, where the current I is the charge per unit of time travelled the circuit between the two poles.

In addition to its manifestations in electricity, this “Coulombian” interaction is responsible for the stability of the material. The nuclei, with a positive electrical charge, attract negative electrons, which leads them to form atoms that themselves attract each other. In addition, when a chemical reaction occurs, it results in a reorganization of nuclei and electrons and a modification of Coulomb energy. This is called chemical energy. A fuel such as coal, gasoline or hydrogen, is a reservoir of chemical energy, but this energy is nothing more than Coulomb energy. The elastic energy of the spring mentioned above is also a consequence of the Coulomb interaction.

An interesting exercise is to compare the gravitational force Fg = Gmm’/r² and the Coulombic force Fel = qq’/(4πε0r²), between an electron and a proton at a distance of 0.1 nm. The gravitational constant is G = 6.67 x 10-11 N.m2.kg-2. The masses are m = 1.67 x 10-27 kg and m = 0.91 x 10-30 kg. From this we deduce Fg = 10-47 N. In addition, 1/(4πε0) = 9 x 109 N. m2/C2, the load being expressed in coulomb (C). The charge of the proton, opposite to that of the electron, is q =- q’ = 1.6 x 10-19 C so that Fel = 2.3 x 10-8 N. We see that the gravitational force is totally negligible, in a ratio of 2.3 x 1039!

Within atomic nuclei, there are also nuclear interactions, which are very short-range and therefore only important within these nuclei. They bind nucleons together, i.e. protons and neutrons. We can thus release enormous energy by combining light nuclei (which is what we do in an H-bomb by nuclear fusion). Huge energy is also obtained by splitting heavy nuclei such as uranium, which is done in an A-bomb or nuclear reactor by nuclear fission. It is then the electrical force of repulsion between protons that takes over and releases the Coulomb energy (see “Radioactivity and nuclear reactions“). In both cases, the potential energy, nuclear or coulombic, is converted into kinetic energy of the nuclei and then into heat.

### 1.4. Light energy

There is yet another form of energy: that carried by light and more generally by electromagnetic radiation. It is subject to the strange laws of quantum mechanics and relativity. Quantum mechanics requires that light energy can only be absorbed in finite quantities or photons, each photon having an energy, where ν is the frequency (related to color) and h is the Planck constant. Relativity makes it possible to understand how photons can have both an energy and a zero mass m. If the kinetic energy is mv2/2, and if m =0, the photon should have zero energy even if its velocity c= 300,000 km/s is not zero. The solution to the mystery, found by Einstein, is that the formula Wcin=mv2/2 is only an approximation valid for small v speeds compared to that of light. The correct (relativistic) formula leads to an infinite increase in energy when approaching the speed of light, so that a particle with zero mass like the photon has non-zero energy at the speed of light.

Another Einstein discovery is that a mass particle m has an energy even at rest (if v=0), equal to W=mc2. This famous Einstein formula[2] is confirmed by nuclear physics: if two light nuclei combine to form a heavier one with energy emission, the large nucleus is a little lighter than the sum of the masses of the two small nuclei, and the difference is equal to the energy emitted. That’s the secret of the H-bomb.

## 2. Units and orders of magnitude

What are the energy units? In the international system it is the joule. It is worth one kg.m2/s2. Another important unit of the international system is the watt (symbol W), a unit of power. The power supplied or expended is the energy supplied or expended per unit of time. One watt is therefore equal to one joule/second.

For example, to move a mass elevator m = 200 kg up to the third floor at height h = 10 m, it is necessary (assuming that the elevator does not waste any energy at all) to have an energy W = mgh, with g = 9.81 m/s2. That’s about 20,000 joules.

The annual global energy consumption is about 0.6 x 1021 J, which represents 1/10,000 of the total energy radiated by the Sun on Earth, but nearly 1/5 of the total energy of photosynthesis, the source of all terrestrial life. As for the kinetic energy of rotation of the Earth on itself, it is in the order of 1029 J.

For more or less justifiable reasons, physicists, chemists and engineers readily use various units other than the joule. For example, electricity suppliers charge in kilowatt-hours, kWh. The conversion is quite simple: 1 hour = 3600 s, so 1 kWh = 3 600 000 watt.second = 3 600 000 joules, and our 20 000 joules mentioned above are therefore (2/360) kWh, or a little more than 0.005 kWh. Economists often use the tonne of oil equivalent (toe): it is the heat produced on average by the combustion of one tonne of oil, estimated at 42 x 109 joules.

Physicists, on the other hand, readily use the electron-volt (eV) and its multiples (KeV, MeV, GeV) or submultiples (milli-eV or meV). The electron-volt is the variation in energy of an electron that passes through a potential difference of one volt. The volt (V) is the unit of electrical potential, and therefore corresponds to a joule/coulomb. The charge of an electron being 1.6 x 10-19 C, it results in 1 eV = 1.6 x 10-19 coulomb-volt = 1.6 x 10-19 J.

But some scientists use other units as well, for example:

• Calorie, which is often used to measure heat. It is worth 4.18 J.
• The Kelvin. It is then necessary to multiply by the Boltzmann constant to obtain joules (see “Pressure, temperature and heat“).

## 3. No life without energy

### 3.1. The contribution of plants

The appearance of life on Earth is a miracle that was only possible thanks to an extraordinary combination of favourable conditions (see “The biosphere, a major geological actor“), in which the Sun’s energy played an essential role. This energy is nuclear and results from the fusion of light nuclei, but it comes to us in the form of light radiation. Its role is first and foremost to maintain an appropriate temperature, but this is obviously not enough. An extraordinary effect of sunlight is photosynthesis, thanks to which plants produce the oxygen needed by animals. The sequence of chemical reactions is complex but can be summarized globally as follows:

6 CO2 + 6 H2O → C6H12O6 + 6 O2

Thus the plant produces both oxygen O2, and glucose C6H12O6 which is for it an energy reserve. For her… and also for men, hungry for energy. Plants do us a huge service: transforming intermittent solar energy into chemical energy, which can be stored and used whenever and wherever we want.

### 3.2. The conquest of energy by man

But what to do with this chemical energy? Absorbing it by eating plants is a good solution, within reach of any animal. Only one animal has been able to do better, it is man. And in many ways. First through the domestication of fire, hundreds of thousands of years ago, more recently applied to ceramics manufacturing and metallurgy (the oldest metallurgical remains found date back about 10,000 years).

Man has also been able to take advantage of the energy of water – water mills appeared shortly before the Christian era – and wind – the windmills dear to Cervantes (Figure 3) date from the Middle Ages. These industrial machines probably contributed to the decline of slavery in the Christian world before it reappeared in the colonies. They provided immense services until the 19th century, when thermal machines became widespread, already the subject of research in the 17th century and whose first prototypes appeared in the 18th century. A thermal machine heats material (e. g. water) and transfers heat from this “hot source” to a “cold source”. As we pass, we take some of this heat and transform it into mechanical energy or electricity. Unfortunately, only part of the heat can be transformed, all the more so as the temperature difference between hot and cold sources is greater (link to thermodynamic article).

It was in the 19th century that industry began to transform certain landscapes and factory chimneys began to pollute in a worrying way (Figure 4). This 19th century was also the century of major research on electricity, marked by the discoveries of Ampère, Volta, Faraday, Maxwell. The use of electricity in industry became widespread in the second half of the 20th century. Electricity is less polluting than 19th century coal-fired machines, but how is it produced in the 20th century? Most often, by coal, which only displaces the source of pollution. We also use the potential energy of the water coming down from the mountains (the amount mgh by which this article begins). This is called hydroelectricity. But this is not enough to feed the appetite of an ever-increasing number of people and an ever-increasing demand for energy.

### 3.3. The control of nuclear energy

It was also in the second half of the 20th century that nuclear energy began to be used, the same energy source that forced Japan to surrender in 1945. The principle of a nuclear reactor and the 1945 bomb is more or less the same (see “Harenessing Nuclear Energy“): heavy nuclei are used (e. g. isotopethe isotopes of an element are nuclei with the same number of protons, and therefore of electrons in the neutral state, but different in the number of neutrons. The isotopes of an element have similar chemical properties (these being determined by electrons), but differ in their nuclear properties. 235U of uranium) that are bombarded with neutrons. Neutrons cause the uranium nucleus to split into two lighter nuclei, with the emission of a few neutrons (about 3). If the piece of uranium is not too large, the neutrons escape into the atmosphere and nothing happens. If the uranium block exceeds a certain critical mass, the neutrons are likely to cause further fission, there is a chain reaction that releases more and more heat and finally an explosion that, on the one hand, ends the reaction and, on the other hand, causes more or less significant damage. In an atomic bomb, we make sure that this damage is as great as possible. In a nuclear reactor, on the other hand, efforts are made to control the chain reaction, so that the critical mass is reached but never exceeded. It is not easy, and there are sometimes accidents, some of which are serious.

Instead of using the fission of heavy nuclei, we can consider using the fusion of light nuclei. That’s what happens in an H-bomb. This is also what the Sun does to illuminate and heat us. We do not yet know how to control the nuclear fusion reaction and make electricity. Indeed, light nuclei are willing to combine if they are very close; they then release an enormous amount of energy; but it is very difficult, on Earth, to bring them close enough. The Sun is much larger than the Earth, and gravity imposes an enormous pressure that, at the centre of the star, exceeds 200 billion times that of the Earth’s atmosphere. On Earth, the march towards the industrial use of fusion is slow; the current stage is represented by the ITER reactor, which will be commissioned in the coming years. Thanks to ITER, we hope to demonstrate the “feasibility” of the project.

### 3.4. Innovative alternatives

However, there is a risk of a shortage of raw materials. Oil and gas reserves, which are very convenient fuels, are likely to run out before the end of the 21st century, at least the easy-to-exploit reserves from groundwater. We are therefore looking for more hidden reserves, those that impregnate certain rocks. The United States is firmly committed to this path. Should France follow their example and exploit its shale gas? The issue is controversial, as it involves inflicting treatments on our basement that can compromise its stability. On the other hand, the extent of the resources is not known.

• Biofuels are another temptation: instead of growing plants to eat them or have them eaten by animals, they are grown to make alcohol for fuel. It would obviously be dangerous to go too far in this direction. There is an optimum to be sought: part of the soil must be devoted to cultivation, but plants have other functions, including feeding us, as they have done for millennia. And, of course, to renew our oxygen.
• More innovative are fuel cells [3]. As a thermal machine can do, they use chemical energy, but they transform it directly into electrical energy without raising the fuel to a high temperature. The process can be more efficient.
• The heat pump can also be an interesting new feature. It is used for heating. The heat is pumped out and transported to the house to be heated. It is the same function as that of a refrigerator that takes heat from a chamber that you want to cool and transports it outside; the operation of a heat pump is therefore that of a refrigerator. This heat transport requires energy, which we have to pay to our electricity supplier. But you have to pay less for a given result if the energy we buy supplies a heat pump than if it is completely transformed into heat. At least if it’s not too cold outside.
• Wood heating is also a technique that has improved considerably recently. With the methods used for centuries, most of the energy was wasted. Better efficiencies can currently be achieved, and smoke pollution limited, by optimising the recovery of the heat produced.

### 3.5. Renewable energies

Most of the so-called renewable energy sources use the Sun’s energy, either directly or through the atmospheric or hydraulic movements it generates. The most common method is the use of river water (hydroelectric power) or wind (wind power). Solar energy is also directly exploited: we can use solar radiation to heat water, we can use the same heat to operate a thermal machine that will produce electricity, we can finally use photovoltaic cells that transform solar energy into electricity. Tides are also exploited and marine swells are being considered.

Most of these techniques have one disadvantage: intermittency. There are days without wind, and at night there is no sun. It is therefore necessary to be able to store energy. However, if you produce electricity, this form of energy has everything to please you, except that it is very difficult to store. It must be transformed into mechanical or chemical energy. For example, we can raise the water from the dams during the day and lower it at night. Or water is electrolyzed during the day to produce hydrogen: globally 2 H2O → 2 H2+O2. And at night, the reverse reaction restores energy, either through a thermal machine or fuel cells.

A simpler solution is to transform solar energy directly into chemical energy, easy to store. This is what nature does very well, through photosynthesis. Unfortunately, the process is a little slow for the busy animals that we are. The problem of energy storage has not yet been satisfactorily solved, which is why solar and wind energy can currently only make a small contribution compared to other sources: coal, nuclear energy, oil, and even hydroelectricity.

## 4. The environment pays the price

The most visible effect of energy production machines was first and foremost the transformation of the landscape. A transformation that is often harmonious. Windmills, like riverside mills, were often very beautiful (Figure 5), and our current wind turbines are not so bad (Figure 6). However, they are accused of scaring away some animals and even being a deadly danger to others, such as bats, thereby endangering biodiversity and consequently the ecological balance.

Since the end of the 19th century, the industry has been responsible for accidents and pollution, lasting or temporary, of the air, water and soil. The production and creation of energy is not always directly responsible, but it is often so. Accidents at nuclear power plants are of particular concern because of the high number of such plants in some countries (United States, Japan, France, etc.). The most serious was that of Chernobyl, in Ukraine, in 1986, responsible for thousands of deaths and massive radioactive pollution, which makes the surroundings uninhabitable even thirty years later. Dam failures are at least as dangerous. The Malpasset dam in southeastern France killed more than 400 people in 1959. This dam was not so much intended to produce energy as to regulate the region’s water supply, but the danger obviously threatens all dams, and defies any prevention if the failure is due to a major earthquake.

While such accidents are exceptional, pollution from industry is chronic. The most characteristic is that of coal-fired power plants. The most widespread is that due to motor traffic, particularly in large cities. Among other things, it produces sulphur dioxide (SO2), which causes lichens to disappear. The concentration of this gas in Paris in 2000 was almost twice as high as they can tolerate. In some English and American cities, smog is a combination of fog, dust and gases such as sulphur dioxide. Some anti-pollution measures have been taken, and smog is declining. Sulphur dioxide disappears fairly quickly after forming sulphuric acid that combines with the first organic or inorganic body it encounters. The same is not true for carbon dioxide (CO2). This gas is produced in very large quantities in thermal power plants by burning coal or oil and by transport (gasoline combustion). It is decomposed by plants, and they do so with considerable efficiency during their lifetime; but after their death they reconstitute, by fermentation, most of the carbon dioxide they had decomposed. CO2 is also partially absorbed by the seas, but this only limits the increase in atmospheric concentration (Figure 7). This results in effects on the climate and in particular a warming (greenhouse effect).

And then the accumulation of carbon dioxide in the oceans could eventually disrupt marine life. This pollution is in addition to many others, which are not directly related to energy.

Nuclear power plants are currently low polluting. However, they accumulate radioactive waste that is beginning to cause problems (see “Harnessing Nuclear Energy“). On the other hand, nuclear power plants are often located on the banks of rivers, which serve as a cold source for the thermal machine, and are thus subject to local warming.

## 5. How to reduce pollution

• It is obviously desirable to develop renewable energies: mainly solar and wind energy. Tidal and wave energies also deserve to be developed, but their possibilities are more limited. As for dams on rivers, they are not far from reaching their limits. The main obstacle to the development of solar and wind energy is related to their intermittent nature, as the problem of energy storage does not currently have a satisfactory solution. However, some countries such as Denmark and Germany have already undertaken an “energy transition” to increase the share of renewable energy. The development of this company deserves to be monitored with interest (Figure 8).
• In addition, energy savings are possible. First, the efficiency of the installations can be improved.The pollution they impose can also be reduced. Thus, after the reunification of Germany, the old coal-fired power plants of the German Democratic Republic were replaced by modern plants that polluted significantly less. We have also seen that the efficiency of our ancestors’ wood heating can be considerably improved. The use of innovative devices, such as heat pumps and fuel cells, is also a way to save energy. Other paths require a transformation of our way of life. We have become accustomed to staying very far from work, which is a waste of time and energy. We have become accustomed to going to bed well after the Sun and rising after it. Changing this habit will be difficult, but it may be necessary to come to it. Already, in our cities, residents have massively resumed using public transit instead of their cars, reducing smog.
• Finally, the need for economic growth can be questioned. Our economists and politicians are currently unable to design an economy without growth in activity and energy consumption. This is an effect of technological progress, which makes it possible to increase yields and automate more and more activities. This reduces the burden on workers, but in the absence of growth, unemployment increases. And the growth in energy consumption has the deleterious effects mentioned above, in terms of pollution, impact on the climate and the environment. Moreover, can growth continue indefinitely in a finite world where resources are limited and starting to run out? Some economists are aware of the problem and are thinking about it.

## 6. What does the future hold for us?

Where are the resources now?

Oil and gas? It won’t be long now: a few decades, counting only the easily exploitable groundwater. A day will come when our cars will only be able to be electric, or use an artificial fuel (hydrogen, perhaps).

Coal? There are several centuries of it, even counting only proven resources, but its exploitation is highly polluting, hardly conceivable unless a way is found to “sequester” the carbon dioxide emitted.

Uranium? There are for two or three centuries, or a few millennia with breeder reactors. But the storage of waste, the dismantling of old power plants and security requirements will make energy more expensive.

How to do this? Nuclear fusion can be a solution. But probably not until the end of the twenty-first century. Renewable energies can be part of the solution. Energy sobriety, another one. In any case, the energy transition is necessary and will significantly change our way of life.

#### References and notes

Cover photo: Wind turbines and high voltage lines near Rye, England[source: DAVID ILIFF. CC BY-SA 3.0 license].

[1] The force between two distant charges of r is expressed as |qq’|/(4πε0 r2) (in vacuum) and the corresponding Coulomb potential energy is Wel=qq‘/(4πε0 r). The universal constant ε0 is called dielectric permeability of vacuum (the factor 4π in the denominator was arbitrarily introduced into this formula by 20th century physicists in order to make it disappear in other formulas).

[2] According to relativity, the energy of a particle of mass m and velocity v is W(v)=mc2 /(1-v2/c2)1/2. The kinetic energy is therefore Wcin=W(v)-mc2=mc2/(1-v2/c2)1/2-mc2. If v/c is much lower than 1, it is reduced to Wcin=mv2/2.

[3] Chatenet M. and Maillard F. “Les cellules à combustible“.

The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: VILLAIN Jacques (February 5, 2019), What is energy?, Encyclopedia of the Environment, Accessed August 12, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/physics/the-energy/.

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# 什么是能量？

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能量是以各种形式存在的：机械能、势能或者动能、电能、化学能、核能，亦或是热能。能量对所有生物来说都是至关重要的。首先，对于植物来说，它们将太阳能转化为氧气和营养物质，为动物提供养分。其中，人类及其所使用的机器消耗的能量远比其他动物多得多。而这也带来了很多问题，尤其是碳污染、事故风险，从长远来看，也会最终造成资源枯竭。一些人希望能从可再生能源和节约能源中找到解决办法。另一些人则认为，我们必定要大规模使用裂变或聚变的核能才能解决眼前的能源问题。

## 1. 各种形式的能量

### 1.1. 能量不会凭空消失，只能相互转化

物理学中的一个重要定理就是能量是守恒的。能量可以多种形式存在，但只能从一种形式转化成另一种形式，既不会凭空产生，也不会消失，这就是能量守恒定律。我们将依次讨论势能、动能热能等形式的能量。

我们先从与力相关的势能开始说起（有关介绍，请参见“动力学定律”）。一个物体在离地高度为 h 的重力势能（还有一个常数）是该物体从高度 h 掉落时重量 P 所做的，记为 Wpot = Ph，重量与质量 m 成正比，记为 P = mg，式中的 g 重力加速度。假如我们来考虑一部电梯，它需要至少 mgh 电能才能把它提升到更高的楼层。事实上也许还需要更多的能量，这是因为电梯本身运转也会消耗一部分能量（如一部分电能转换成了热能）。如果现在物体换成一个苹果，它长在距离地面高度为 h0 的树枝上。当它掉落时，随着高度的降低，它也获得了一个取决于时间 t 的速度 v(t) = –dh/dt。最终它获得的动能为 Wcin = mv²(t)/2。能量守恒定律告诉我们总的能量是恒定的，即为 mgh(t)+mv²(t)/2。因此，它对时间的导数为零。所以有 mgv(t) = mv(t)dv(t)/dt 为常数。经化简可得：dv(t)/dt = g，常数 g 就是加速度。

据说在17世纪，英国人艾萨克·牛顿（Isaac Newton）在冥想德国人约翰-开普勒几年前解决的行星运行问题时，观察到一个苹果掉落（图1）。突然，他灵光一闪：苹果的掉落和行星围绕太阳旋转所描述的椭圆不正是一个普遍现象的两个方面吗?牛顿的直觉经计算结果得以证实：两个质量为 m m’ 的物体在距离 r 时，相互吸引的力 F 等于 Gmm’/，其中 G 是引力常量。同样地，如果这两个物体是太阳和地球，或者是地球和月球，又或者是苹果和地球，它们之间的引力都会满足以上计算。这种现象叫做万有引力。与这个力 F 对应的势能为 Wpot = –Gmm’/r。它是在最初两个物体相距无限远的情况下，把它们拉近在一起时力 F 做的机械功。换句话说 F = –dWpot/dr。引力是相互吸引的。通常规律是力会使势能减小。

苹果的坠落说明了总能量 Wtot = Wpot + Wcin 的守恒，这称为机械能。相比之下运动持续时间长一点，因此我们可以观察到动能的增加伴随着势能的减小，然后是相反的交替转变。类似的振荡可以通过在螺旋弹簧上附加一定质量（无需太重）的物体来实现。让我们轻轻拉一下弹簧，然后释放：它会收缩，然后拉伸，再收缩，如此往复。这里我们再回顾一下，势能和动能的总和是恒定的，或者说至少在弹簧不吸收能量的情况下是恒定的（有可能部分能量会转变成热量）。这里的势能 Wpot 本质是弹性势能，与弹簧长度到平衡点之差 δz 的平方成正比：Wpot = γδz²

因此，在不同的情况下，能量可以用不同的公式表示。通常情况下，我们更倾向于用提供或吸收的功率来表示，即单位时间内的能量。因此，如果把一个电器（例如电梯）连接到电压为 V = 220V的电源上，流经的电流为 I，其消耗的功率则为 W = VI。如果电梯爬升需要的时间为 t = 20s，那么我们需要向电力供应商支付电量为 VIt 的费用。

### 1.2. 热能

然而，供应商提供的能量不只用于运行发动机，还能加热。因为我们可以将其转化为热量。空气储存的热量本质上是分子的动能之和（对于速度 v 和质量 m 的 ñ 个分子，总动能为 ñmv²/2 ）。由于我们很难知道房间里包含的几百万亿个分子的速度，所以我们更倾向于说一个物体储存的热量是其温度和压强的函数，这个函数至少可以大致估算出来（有关介绍，请参见“压强、温度和热量”）。

因此，我们可以回到上面提到的例子，并问问我们自己，如果我们不设法维持弹簧和秋千的运动，为什么它们会很快停止运动呢？这是因为存在摩擦，而即使我们并没有注意到这一现象，摩擦也会产生热量。所以，能量仍是守恒的，不过有部分能量会损耗成热。

### 1.3. 物质——电能和核能的储存器

在牛顿发现万有引力定律一个世纪以后，法国物理学家查尔斯·库仑（Charles Coulomb）对两个电荷 q q’ 之间的相互作用建立了类似的定律。力与乘积 qq’ 成正比，与距离的平方成反比[1]。但是，不同于质量，电荷可以是正的，也可以是负的。因此，如果两个电荷符号相反，则是吸引力，但如果符号相同，则是排斥力。

在电池或其他发电机中，正电荷分布在正极，负电荷分布在负极。在电场中移动的电子或任何其他电荷 q’ 会具有势能，它可以转化为热能（在电阻中）或机械能（在电动机中）。电势可以通过势能除以电荷 q’ 得到。它相当于重力场的 gh。如果电势差为 V，对应可用的能量为 Vq’，消耗的功率是 VI，其中电流 I 是单位时间内通过两极间电路的电荷。

除了在电学上的表现形式以外，这种“库仑”相互作用也是材料稳定性的原因。带正电荷的原子核会吸引带负电荷的电子，从而形成原子，而原子本身也会相互吸引。此外，当发生化学反应时，它会导致原子核和电子的重组以及库仑能的改变，这被称为化学能。煤、汽油或氢气等燃料是化学能的储存容器，而这种化学能就是库仑能。上面提到的弹簧的弹性势能也是库仑相互作用的结果。

这里有一个有趣的练习。比较电子和质子在0.1nm距离的引力 Fg = Gmm’/ 和库仑力 Fel = qq’/(4πε0r²) 大小。其中，引力常数为 = 6.67×10-11 Nm²/kg²，质量分别为 m = 1.67×10-27Kg m = 0.91×10-30 Kg。由此我们推导出 Fg = 10-47N。此外，1/(4πε0) = 9 x10Nm²/，载荷用库仑 (C) 表示。质子的电荷与电子的电荷相反，即 q = -q’ = 1.6×10-19 C，所以 Fel = 2.3×10-8N，我们看到引力完全可以忽略不计，因为库仑力是引力的2.3×1039倍！

在原子核内，也存在相互作用，这些相互作用距离非常短，仅在这些原子核内才变得明显。它们将核子，即质子中子结合在一起。因此，我们可以通过结合轻核来释放巨大的能量（这就是氢弹中核聚变的原理）。我们也可以通过分裂重核(如铀)来获得巨大的能量，这会在原子弹或核反应堆中核裂变时发生。然后质子间作用力以排斥力为主导并释放库仑能（见“放射性与核反应”）。在这两种情况下，核电势能或库仑势能都转化为原子核的动能，继而转化为热能。

### 1.4. 光能

还有另一种形式的能量：由光和更普遍的电磁辐射携带的能量。它受到量子力学相对论这些奇特定律的制约。量子力学要求光能只能以有限的数量或光子被吸收，每个光子具有的能量为 hv，其中 v 是频率（与颜色有关），h 是普朗克常数。相对论使我们尝试能够理解光子可以同时具有能量，且质量 m 为零。如果动能为 mv²/2， 而 m = 0，即使光子的速度 c = 300,000 km/s，它的能量也应该为零。爱因斯坦发现了解决这个问题的方法：与光速相比，公式 Wcin = mv²/2 只是对较小速度 v 有效的近似。正确的（相对论）公式导致接近光速时能量无限增加，所以像光子这样质量为零的粒子在光速下能量不为零。

爱因斯坦的另一个发现是一个质量为 m 的粒子即使在静止状态下（如果 v = 0）也拥有一个能量，它等于 W = mc²。这就是著名的爱因斯坦方程[2]，它得到了核物理学的证实：如果两个轻核结合成一个较重的核，并伴有能量辐射。那么大核比两个小核的质量之和轻一点，其差值就等于辐射的能量。这就是氢弹的奥秘。

## 2. 单位和量级

能量的单位是什么？在国际单位制中，它的单位是焦耳。它也等于 kgm²/s²。国际单位制中另一个重要的单位是瓦特（符号 W），它是功率的单位。提供或消耗的功率是指单位时间内提供或消耗的能量。因此，一瓦特等于一焦耳/秒。

例如，如果将质量为 m = 200kg 的电梯升至高度为 h = 10m 的三楼，需要消耗（假设电梯完全不浪费任何能量）能量 W = mgh，其中 g = 9.81 m/s²。这大约是20000J。

全球每年消耗的能量约为0.6×1021J，这相当于太阳在地球上辐射的总能量的1/10000，但却占光合作用总能量——所有陆地生命的能源——的近1/5。至于地球自转的动能，则在1029J左右。

出于各种各样的原因，物理学家、化学家和工程师经常使用除焦耳以外的其他单位。例如，电力供应商以千瓦时 (kWh) 作为收费单位。它的换算很简单：1小时=3600秒，所以1千瓦时=360000瓦特·秒=3600000焦耳，因此我们上面提到的20000焦耳就是 (2/360) 千瓦时，或者说是略高于0.005千瓦时。经济学家经常使用吨油当量 (toe) ：它是指燃烧一吨油平均产生的热量，约为42×109焦耳。

另一方面，物理学家经常使用电子伏特 (eV) 及其倍数 (KeV，MeV，GeV) 或约数 (milli-ev或meV) 。电子伏特是指电子通过一伏特的电势差的能量变化。伏特 (V) 是电动势的单位，因此它等同于焦耳/库仑。电子的电荷量为1.6×10-19C，则可得1eV = 1.6×10-19C·V = 1.6×10-19J。

但是，有些科学家也使用其他单位，诸如：

• 卡路里，常被用来作为热量的单位。它的值等于4.18焦。
• 开尔文，它必须乘以玻尔兹曼常数才能得到焦耳（见“压强、温度和热量”）。

## 3. 没有能量就没有生命

### 3.1. 植物的贡献

在地球上，生命的出现是一个奇迹。这要归功于各种有利条件的特殊组合（见“生物圈，主要的地质行为者”），其中太阳的能量发挥了重要作用。这种能量是核能，是轻核聚变的结果，但它以光辐射的形式来到我们身边。它的作用首先是维持适当的温度，但远不止于此。阳光的一个特殊用途是光合作用。植物因光合作用才产生了动物所需的氧气。这个化学反应的顺序很复杂，但可以概括如下：

6CO2 + 6H2O → C6H12O6 + 6O2

因此，植物产生氧气 O2 和葡萄糖 C6H12O6 。葡萄糖对植物来说是一种能量储备，对于需要能量的人类也是如此。植物将间隙性的太阳能转化成了可以储存起来并能随时随地使用的化学能，为人类做出了重大贡献。

### 3.2. 人类所掌握了的能源

那么，该如何利用这些化学能呢？通过吃掉植物来获取其中能量是一个很好的解决办法，任何动物都能做到这一点。但只有一种动物做得最好，那便是人类。我们的方法很多，首先在几十万年前，我们就能熟练使用火，时间再近一些时，我们又掌握了陶瓷制造和冶金技术（已发现的最古老的冶金遗迹可追溯到大约1万年前）。

人类也可以利用水的能量，在公元前不久就出现了水车。人类还会利用风的能量，中世纪作家塞万提斯（Cervantes）喜欢写风车（图3）。在奴隶制出现在殖民地之前，这些工业机器可能促成了基督教世界奴隶制的衰落。直到19世纪热机普及之前，这些工业机械一直发挥着巨大作用，实际上，17世纪热机已经成为研究对象，18世纪出现了第一个原型。热机对物质（如水）进行加热，并将热能从这个“热源”转移到“冷源”。我们可将其中的部分热能转化为机械能或电能。遗憾的是，只有部分热能可以转化。当冷热源之间的温差较大时更是如此（链接到热力学文章）。

### 3.3. 核能的控制

20世纪下半叶核能开始得到利用。也正是这种能源让日本于1945年被迫投降。核反应堆和1945年出现的原子弹原理是差不多的（见“利用核能”）：重核（例如，铀的同位素 235U ）受到中子的轰击分裂成两个较轻的原子核，辐射出若干中子（约3个）。如果铀块不大，中子就会逃逸到大气中，无事发生。如果铀块超过一定的临界质量，中子就有可能引起进一步的裂变，发生链式反应，释放出越来越多的热能，最终发生爆炸。爆炸导致核反应结束，也会造成不同程度的重大破坏。对于原子弹，我们要确保其破坏力愈大愈好。但是，对核反应堆，我们要努力控制链式反应，使其达到临界质量，但决不能超过临界质量。做到这一点并非易事，有时可能会造成事故，甚至会酿成惨剧。

除了使用重核裂变，我们还可以考虑如何使用轻核聚变。这就是氢弹的原理，也是太阳为我们提供照明和热量的原因。目前，我们还不知道如何控制核聚变反应生产电力。事实上，如果轻核离得很近，它们倾向于结合，然后释放出巨大的能量。但在地球上，要让它们足够接近是极其困难的。太阳比地球大得多，引力产生了巨大的压强。在太阳的中心，压强将超过地球大气压的2000亿倍。在地球上，核聚变的工业化应用进展缓慢。当前阶段以ITER反应堆为代表，它将在未来几年内投入使用。我们希望利用ITER验证核聚变工业化项目的“可行性”。

### 3.4. 创新的替代办法

但是，我们依然面临着能源短缺的风险。石油和天然气等非常方便获取的燃料储量很可能在21世纪末之前耗尽，至少是对地下水层容易开采的这些储量而言。因此，我们正在寻找更多蕴藏在某些岩层中不易勘探的能源储量。美国坚定地致力于这一方面的研究。法国又是否应该效仿去开采页岩气？这个问题存在争议。因为它可能会影响地基稳定性。另一方面，资源的规模也尚不清楚。

• 生物燃料是另一种可替代方案：种植植物不用来食用或喂养动物，而是将其用来制造酒精作为燃料。如果种植太多植物去生产酒精，也会对我们造成不利影响。我们要寻求一个最佳方案：我们虽可以使用部分耕地，但也要考虑植物的其他功能：植物养育着我们，为我们补充氧气，几千年来都是如此。
• 更具创新性的是燃料电池[3]。类似于热机一样，燃料电池使用化学能，但它们直接将化学能转化成电能，而不需要将燃料提升至高温。因此这个过程更高效。
• 热泵也是一种有吸引力的的供暖新方式。它可将产出的热能输送到需要取暖的屋内。它与冰箱的功能相同，冰箱要从冷却的腔室中吸收热量并将其传输到外部。因此，热泵的操作原理类似于冰箱。这种热能的输送是消耗能量的，我们必须向电力供应商支付电力供给费用。如果我们购买的能源是只用于供给热泵，而非全部转化为热能，我们所支付的费用则相对低廉，特别是在外面不是太冷的情况下。
• 木材加热也是一种近期有很大改进的技术。几世纪以来，人们都使用这一方法，但大部分燃烧产生的热量都被浪费了。目前，通过优化对所产生的热量的回收，可以达到更高的效率，同时也减轻了烟雾污染。

### 3.5. 可再生能源

大多数所谓的可再生能源直接利用太阳能或是通过其它方式如大气或水力运动而产生的能源。最常见的方法是利用河水（水力发电）或风（风力发电）。也可直接利用太阳能：我们可以利用太阳辐射来加热水；也可以利用同样的热量来运转热能装置，产生电力；我们还可以利用光伏电池，将太阳能转化为电力；同时还可以利用潮汐或者海浪发电。

这些技术大多有这么一个缺点：间歇性。有时一整天都没有风，而晚上也没有太阳，那么我们必须想办法储存能量。我们通过发电得到的电力可以满足我们所有的需求，但却难以储存。因此，必须将它转化为机械能或化学能才能够储存起来。例如，我们可以在白天把水坝的水抬升起来，晚上再把水排放下去。或者在白天对水进行电解生成氢气：2H2O → 2H2 + O2。到了晚上，通过热机或燃料电池，利用逆向反应释放出能量。

一个更简单的解决办法是将太阳能直接转化为便于储存的化学能。自然界的光合作用就是一个很好的例子。可惜的是，对于非常重视效率的我们来说，这个过程有点漫长。储能的问题依然没有得到满意的解决方案，这也是为什么与其他能源：煤炭、核能、石油，甚至与水电相比，目前太阳能和风能对于发电帮助不大的原因。

## 4. 环境付出了代价

能源生产设备最明显的影响首先是改变了景观。这种变化通常让景观变得更加温馨和谐。风车和河边的磨坊都令人赏心悦目（图5）。我们现在的风力发电机也同样美观（图6）。然而，有人认为它们吓跑了一些动物，并严重威胁到了蝙蝠之类动物的生存，从而了危及生物多样性，乃至破坏生态平衡。

自19世纪末以来，无论从长远还是眼前来看，空气、水和土壤的事故或污染一直被归咎于工业的发展，能源的生产有时也被视为直接原因。由于一些国家（美国、日本、法国等）的核电站数量很多，核电站事故特别令人关注。最严重的是1986年乌克兰切尔诺贝利事件，它造成数千人死亡和大量人员伤亡。即使三十年过去了，周围被放射性污染的环境依然无法居住。溃坝也同样危险，1959年法国东南部的马尔帕塞大坝溃坝造成400多人死亡。建造这座大坝的目的并不是为了生产能源，而是为了调节该地区的供水。但如果大地震来临，所有大坝显然都面临风险，这样的事故亦无法防范。

虽然这些事故是偶发的，但工业污染却是长期存在的。最典型的是燃煤电厂的污染，分布最广的是汽车污染源，特别是在大城市中。其中，污染产生的二氧化硫 (SO2) 会导致地衣消失，2000年巴黎二氧化硫的浓度几乎是地衣所能忍受的两倍。在一些英美城市，雾霾是雾、尘和二氧化硫等气体的复合体。目前人们已经采取了一些防治措施，雾霾污染正在减轻。二氧化硫在形成硫酸后会与所接触的第一个有机体或无机体结合，消失得非常快。但二氧化碳 (CO2) 不会这么快消失。在火力发电厂中，燃煤或燃油以及运输（汽油燃烧）产生了大量的二氧化碳。植物一生都在高效分解它们，但在植物死亡后，它们会通过发酵重新释放其分解的大部分二氧化碳。CO2 也可被海水吸收一部分，但这只能限制大气中 CO2 浓度的增加（图7）。这将最终导致气候变化，特别是全球变暖（温室效应）。

海洋中积累的二氧化碳，最终可能会扰乱海洋生物的生存环境。除了这种污染以外，能源还会造成大量间接污染。

目前核电站是低污染的。但是，它们积累的放射性核废料已经开始造成新的问题（参见“利用核能”）。另一方面，核电站往往建在河流两岸。河流是热机的冷源，因此也会造成局部地区变暖。

## 5. 如何减轻污染

• 开发可再生能源显然是可取的：主要是太阳能和风能。潮汐能和波浪能虽也值得开发，但可能有一定局限性。至于河流上的水坝也将很快达到利用的极限。太阳能和风能发展的主要障碍与其间歇性有关，因为储能问题目前没有令人满意的解决方案。不过，丹麦、德国等一些国家已经开展了“能源转型”项目以提高可再生能源的比重。它们的发展值得关注（图8）。
• 人类造成的污染是可以减轻的。因此，在德国统一后，东德的旧式燃煤电厂被清洁环保的现代电厂所取代。此外，我们也看到祖先曾使用的木材加热的效率显著提升。使用创新型设备，如热泵和燃料电池，也成为节约能源的一种方式。此外，我们需要转变生活方式。我们已经习惯于居住在远离工作地点的地方，通勤时既浪费时间又消耗能源。我们也已经习惯于晚起晚睡。改变习惯很困难，但却很有必要。在我们的城市中，大量居民已经搭乘公交替代汽车出行以减轻雾霾。
• 最后，人们对经济增长的必要性提出质疑。我们的经济学家和政治家目前还没有设计出一种无需增加工作岗位和能源消耗也能够得到增长的经济模式。经济增长是技术进步带来的，技术进步提高了产量，并且使越来越多的生产活动自动化。这样一来减轻了工人的劳动强度，但如果经济不增长，失业率也会上升。能源消耗的增长还会产生上述提及的如污染、对气候和环境的影响等危害。此外，地球资源本就有限，若我们进一步开采资源直至枯竭，这种经济增长又能否无限期地持续下去呢？一些经济学家已经意识到并开始思考这个问题。

## 6. 未来什么样的能源适合我们?

那么资源在哪里？

石油和天然气够吗？现在不会用太久了：只算容易开采的地下水层的资源也只够几十年了。也许会有一天，我们的汽车只能使用电力驱动，或者通过人工制备燃料（氢气）驱动了。

煤的储量还够吗？仅算已探明的煤矿资源可以使用好几百年。但煤的开采带来的严重污染不堪设想，除非找到一种方法来“封存”所排放的二氧化碳。

铀可以用吗？我们可以用两、三百年，对增殖堆甚至可用几千年。但核废料的储存、旧电厂的拆除和安全要求将使能源变得更加昂贵。

我们该如何做？核聚变可以是一个解决方案。但可能要到21世纪末才能解决。可再生能源可以成为解决方案的一部分。节能也是一种方案。无论如何，能源转型是必要的，这也将极大地改变我们的生活。

#### 参考资料及说明

[1] 两个距离为 r 的电荷受力表示为 |qq’ |/4πε0（在真空中）并且对应的库伦势能等于 Wel = qq’/(4πε0r)。宇宙常数 ε0 被称为真空中的介电磁导率（分母中的因子是由20世纪物理学家引入的，目的是带入其他公式便于相消）。

[2] 根据相对论，一个质量为m，速度为v的粒子的能量为 W(v) = mc² /(1-/c²)¹/²。因此其动能为 Wcin = W(v)mc² = mc²/(1-/c²)¹/² – mc² 。如果 v/c 远小于1，此式可以化简为 Wcin = mv²/2。

[3] 夏特内（Chatenet M.）和美拉德（Maillard F.）。燃料电池

The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: VILLAIN Jacques (April 12, 2024), 什么是能量？, Encyclopedia of the Environment, Accessed August 12, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/zh/physique-zh/the-energy/.

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