Chapter 415

Energy is one of the basic concepts of physics. From classical mechanics to relativity, quantum mechanics and cosmology, energy is always a core concept.

In common terms or popular science books, energy refers to what a system can release or obtain from it, which can be equivalent to doing a certain amount of work. For example, 1 kilogram of gasoline contains 12 kilowatt-hours of energy, which means that if all the chemical energy in 1 kilogram of gasoline is released, it can do 12kh of work.

Energy is a physical quantity that describes a system or a process in physics. The energy of a system can be defined as the sum of the work required to transition from a state of zero energy to the current state of the system. How much energy a system has is not a definite value in physics, it changes with the description of the system. In the process of life activities of the human body, all life activities need energy, such as the synthesis reaction of material metabolism, muscle contraction, gland secretion and so on. And this energy mainly comes from food. Nutrients contained in animal and plant foods can be divided into five categories: carbohydrates, lipids, proteins, minerals and vitamins, plus water into six categories. Among them, carbohydrates, fats and proteins can be oxidized in the body to release energy. The three are collectively referred to as "capacity nutrients" or "heat source substances".

The law of conservation of energy states that energy cannot be created or destroyed, but can only be transformed from one form to another, while the total amount of energy remains the same. Energy is a scalar, not a vector, and has no direction. As for positive matter and antimatter, it does not mean that the mass is positive or negative, but that the electric properties of the atomic nucleus are opposite, and the mass is converted into energy after meeting. Any movement requires energy. There are many forms of energy, such as light energy, sound energy, thermal energy, electrical energy, mechanical energy, chemical energy, nuclear energy, etc. As an example, look at the energy of a solid with mass 1kg:

In classical mechanics, its energy is the sum of the work done to accelerate from rest to the current speed.

In classical heat, its energy is the sum of the work done to heat the existing temperature from absolute zero.

In physical chemistry, its energy is the sum of the work added to the raw materials when the solid is synthesized.

In atomic physics, its energy is the sum of the work done on the atom from its state of zero energy to its present state.

The energy contained in this solid can also be defined in the opposite way. Give two examples:

The internal energy of the solid is the sum of the work released in cooling it to absolute zero.

The atomic energy of the solid is the kinetic energy of releasing its binding energy in a nuclear fission or fusion reaction into reaction products.

Although energy is a commonly used and basic physical concept, it is also an abstract physical concept.

In fact, physicists did not really understand the concept of energy until the middle of the 19th century. Before that, it was often confused with concepts such as force and momentum.

The energy requirement of the human body refers to the energy intake necessary for the body to maintain good health for a long time, with good body shape, body composition and activity level, to achieve energy balance and maintain the necessary energy intake for productive labor and social activities.

In Einstein's special theory of relativity, energy is a component of four-dimensional momentum. In any closed system, when observed in any inertial system, each component of this vector will be conserved and will not change with time, and the length of this vector will also be conserved. The length of the vector is the rest mass of a single particle, which is also a system composed of multi-mass particles Constant quality.

Therefore, as long as the observer’s reference frame does not change, the conservation of energy to time in the special theory of relativity still holds true, and the energy of the entire system remains unchanged. Observers in different reference frames will measure different amounts of energy, but each observation None of the measured energy values ​​will change over time. The invariant mass is defined by the energy-momentum relationship, and is the minimum value of the mass and energy of the system that can be observed by all observers. The invariant mass is also conserved, and the values ​​measured by all observers are the same.

In quantum mechanics, the energy of a quantum system is described by a self-adjoint operator called the Hamiltonian, which acts on the system's Hilbert space. If the Hamiltonian operator is a time-invariant operator, as the system changes, the measurement of its occurrence probability does not change with time, so the expected value of energy does not change with time. The local energy conservation in quantum field theory can be obtained by using the energy-momentum tensor operator with Noether's theorem. Since there is no global time operator in quantum theory, the uncertain relationship between time and energy can only be established under some specific conditions, which is different from the nature of the uncertain relationship between position and momentum as the basis of quantum mechanics. different. The energy at each fixed time can be accurately measured without being affected by the uncertain relationship between time and energy, so even in quantum mechanics, energy conservation is a well-defined concept.

Energy must obey the law of conservation of energy. According to this law, energy can only change from one form to another and cannot be created or destroyed. Energy conservation is a mathematical conclusion derived from the translational symmetry of time.

According to the law of conservation of energy, the incoming energy is equal to the outgoing energy plus the change in internal energy. This law is a fairly fundamental criterion in physics. According to the translational symmetry of time, the laws of physics hold true at any time.

The law of conservation of energy is a feature of many laws of physics. From a mathematical point of view, energy conservation is a consequence of Noether's theorem. If a physical system satisfies continuous symmetry as it translates in time, its energy is conserved. On the contrary, if the physical system has no symmetry in time translation, its energy is not conserved, but if it is considered that this system exchanges energy with another system, and the synthesized larger system does not change with time, the energy of this larger system will be Conservation. Since any time-varying system can be placed within a larger time-invariant system, conservation of energy can be achieved by appropriately redefining energy. For the physical theory under flat space-time, since quantum mechanics allows non-conservation in a short period of time, energy conservation is not observed in quantum mechanics, and in special relativity, the law of energy conservation will be transformed into the law of mass-energy conservation.

The law of conservation of mass and energy means that in an isolated system, the sum of the relativistic kinetic energy and static energy of all particles remains unchanged during the interaction process. The law of conservation of mass and energy is a special form of the law of conservation of energy.

Energy, something that Lin Feng longed for very much, if he doesn't have enough energy, he can't do anything, what he can do now is to continuously enhance his energy. To be continued.