Energy is one of the basic concepts in physics. From classical mechanics to relativity, quantum mechanics and cosmology, energy is always a core concept.
In common language or popular science readings, 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 12 kilowatt hours of work.
Energy is a physical quantity in physics that describes a system or a process. The energy of a system can be defined as the sum of the work required to convert the system 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 human life activities, all life activities require energy, such as synthetic reactions of material metabolism, muscle contraction, gland secretion, etc. And these energy mainly come from food. Nutrients contained in animal and plant foods can be divided into five major categories: carbohydrates, lipids, proteins, minerals and vitamins, plus water, which is the sixth category. Among them, carbohydrates, fats and proteins can release energy when oxidized in the body. The three are collectively called "energy-producing nutrients" or "pyrogen substances".
The law of conservation of energy states that energy cannot be created or destroyed out of thin air, but can only be converted from one form to another, while the total amount of energy remains unchanged. Energy is a scalar quantity, not a vector quantity, and has no direction. As for positive matter and antimatter, it does not mean that the mass is positive or negative, but that the electrical properties of the atomic nucleus are opposite, and the mass is converted into energy when they meet. 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, consider the energy of a solid with a mass of 1Kg:
In classical mechanics, its energy is the sum of the work done in accelerating from rest to its current velocity.
In classical heat, energy is the sum of the work done in heating an existing temperature from absolute zero.
In physical chemistry, its energy is the sum of the work added to the raw materials when synthesizing the solid.
In atomic physics, its energy is the sum of the work done on an atom from a state where its energy is zero to its current state.
The energy contained in this solid can also be defined in the opposite way. Give two examples:
The internal energy of a solid is the sum of the work released by cooling it to absolute zero.
The atomic energy of the solid is the kinetic energy that releases 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, for individuals with good body shape, body composition and activity level to achieve energy balance and maintain 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 frame, each component of this vector (one of which is energy and the other three are momentum) will be conserved and does not change with time, and the length of this vector will also be conserved (Minkowski module length) ), the vector length is the rest mass of a single particle, and is also the constant mass (i.e. constant energy) of a system composed of multi-mass particles.
Therefore, as long as the observer's reference frame does not change, the conservation of energy with respect to time in special relativity still holds, and the energy of the entire system remains unchanged. Observers in different reference frames will measure different amounts of energy, but each observation The measured energy values will not change over time. Invariant mass is defined by the energy-momentum relationship. It is the minimum value of the system mass and energy that can be observed by all observers. Invariant mass is also conserved, and the values measured by each observer are the same.
In quantum mechanics, the energy of a quantum system is described by a self-adjoint operator called the Hamiltonian operator, which operates in the Hilbert space (or wave function space) of the system. 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 under quantum field theory can be obtained by using the energy-momentum tensor operator in conjunction with Noether's theorem. Since there is no global time operator in quantum theory, the uncertain relationship between time and energy will only hold true 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 (see uncertainty principle). The energy at each fixed time can be measured accurately and is not affected by the uncertain relationship between time and energy. Therefore, even in quantum mechanics, energy conservation is a clearly defined concept.
Energy must obey the law of conservation of energy. According to this law, energy can only change from one form to another but cannot be created or destroyed out of thin air. Conservation of energy is a mathematical consequence of the translational symmetry of time (translational invariance).
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 basic criterion in physics. According to the translational symmetry of time (translational invariance), physical laws (theorems) hold true at any time.
The law of conservation of energy characterizes many physical laws. From a mathematical point of view, conservation of energy is the result of Noether's theorem. If a physical system satisfies continuous symmetry when translated in time, its energy (the conjugate physical quantity of time) is conserved. On the contrary, if a physical system has no symmetry during time translation, its energy is not conserved. However, if we consider that this system exchanges energy with another system, and the resulting larger system does not change with time, the energy of this larger system will Conservation. Since any time-varying system can be placed in a larger time-invariant system, energy conservation can be achieved by appropriately redefining energy. For physical theories under flat space and time, since quantum mechanics allows non-conservation in a short period of time (such as forward-antiparticle pairs), energy conservation is not observed in quantum mechanics. In special relativity, the law of energy conservation will be converted into matter. Law of conservation of energy.
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 is something that Lin Feng longs for. If he doesn't have enough energy, he can't do anything. All he can do now is to continuously enhance his energy.
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