The first law of thermodynamics: the concrete expression of the universal law of energy conservation and transformation in all processes involving macroscopic thermal phenomena. The first law of thermodynamics proves that the heat absorbed by the system from the surrounding medium, the work done to the medium and the internal energy increment of the system are all conserved in any process.
The first law of thermodynamics is the law of conservation of energy, which is a summary of human experience, and no other principle can prove it. The energy of a thermodynamic system is represented by internal energy, heat and work, and the first law of thermodynamics is an expression of energy conservation. The conclusions drawn from it are not found to contradict the facts. According to the first law of thermodynamics, it can be imagined that a machine should be made, which does not depend on external energy supply, and does not reduce energy itself, but constantly does work outside without consuming energy. People call this imaginary machine the first perpetual motion machine. Because you have to consume energy to do external work, you can't do external work without consuming energy, so the first law can also be expressed as "the first perpetual motion machine is impossible to cause." On the contrary, the perpetual motion machine of the first kind will never be built, which proves that the first law is correct.
After the thermodynamic system reaches state 2 from state 1, the internal energy of the system will generally change. According to the law of conservation of energy:
δU = Q-W( 1)
Where δ u = U2-U 1 is the internal energy increment of the system; Q is the heat absorbed by the system from the environment during this process; W is the work done by the system to the environment in this process. Equation (1) is the mathematical expression of the first law of thermodynamics.
In the equation (1), u is a state function, that is, the value of δ u depends only on the initial state and the final state of the system, and has nothing to do with the specific process of the system changing from the initial state to the final state, while Q and W are related to this process. When applying the equation (1), it should be noted that the symbols of q and w are: the system absorbs heat Q>0, the system emits heat q: 0, and the environment does work on the system w < 0.
If the system state changes slightly, the first law of thermodynamics is written as:
dU=δQ-δW (2)
Where Δ q and Δ w are the micro-heat and micro-work of the process respectively, and they are not fully differential, so Δ is used instead of Δ d to distinguish them from fully differential.
The first law of thermodynamics can also be expressed as the first perpetual motion machine (a machine that can automatically do work without consuming any fuel and energy). When the system is open, there is not only thermal and mechanical interaction between it and the medium, but also material exchange, so the expression of the first law of thermodynamics should also add an increase or decrease of energy caused by material exchange.
Mechanical energy is the energy form of objects in mechanical phenomena, including kinetic energy and potential energy (potential energy), that is, mechanical energy = kinetic energy+potential energy.
In a closed mechanical system (conservative mechanical system), only conservative forces do work. When there is no mutual conversion between mechanical energy and other forms of energy, mechanical energy is conserved and system energy is expressed as mechanical energy. Conservation of energy is embodied in the law of conservation of mechanical energy. The law of conservation of mechanical energy is a special case of the law of conservation of energy.
The law of conservation of energy shows that energy can only be transformed from one form to another, and cannot be generated or destroyed out of thin air. Conservation of energy is a mathematical conclusion drawn from translation symmetry (translation invariance) of time (see Nott theorem).
According to the law of conservation of energy, inflow energy is equal to outflow energy plus internal energy change.
This law is a fairly basic principle in physics. According to the translation symmetry (translation invariance) of time, the laws (theorems) of physics hold at any time.
In the special theory of relativity, the law of conservation of energy is the law of conservation of mass and energy. The law of conservation of mass and energy is a special form of law of conservation of energy. The mass-energy formula E=mc2 describes the corresponding relationship between mass and energy. In classical mechanics, mass and energy are independent of each other, but in relativistic mechanics, energy and mass are the same expression of mechanical properties of objects. In the theory of relativity, mass is extended to a mass energy value. It turns out that in classical mechanics, independent conservation of mass and energy are combined into a unified law of conservation of mass and energy, which fully embodies the unity of matter and motion.
The relativistic energy of a single mass particle includes its rest mass and kinetic energy. If the kinetic energy of a mass particle is zero (or in a relatively static reference frame), or a system with kinetic energy is in a momentum center system, its total energy (including the kinetic energy inside the system) is related to its static mass or invariant mass, and the famous relationship is E=mc2.
Therefore, as long as the observer's reference frame has not changed, the conservation of energy to time in special relativity still holds, the energy of the whole system remains unchanged, and the energy measured by observers in different reference frames is different, but the energy measured by each observer will not change with time. Invariant mass is defined by the relationship between energy and momentum, which is the minimum value of system mass and energy that all observers can observe. Invariant mass will be conserved, and the values measured by all observers are the same.
According to a large number of experiments, people have confirmed the law of conservation of energy, that is, when different forms of energy are transformed into each other, their sizes are conserved. Joule-mechanical equivalent thermal experiment is a famous experiment that confirmed the law of conservation of energy in the early stage and then established the first thermodynamic law of energy conversion and conservation in the macro field. Compton effect proves that the law of conservation of energy is still correct in the microscopic world, and then gradually realizes that the law of conservation of energy is determined by the invariance of time translation, thus making it a universal law in physics (see symmetry and conservation law).
It should be pointed out that the concept of energy has its scope of application. According to the general theory of relativity, energy can no longer be used as a measure under certain conditions.