δU = Q+W
Or δU = Q-W (these two expressions are commonly used at present, and the former one is used more). In order to avoid confusion, the first one is widely used in physics, and it is generally said in chemistry that the system does external work, so the latter one will be used.
Energy is eternal, it will not be created by anyone, nor will it be destroyed by anyone. But heat energy can provide power for kinetic energy, and kinetic energy can be converted into heat energy. [ 1]
The universal law of energy transformation and conservation is the concrete expression of all macro-processes involving thermal phenomena. One of the basic laws of thermodynamics.
The first law of thermodynamics is an expression of the law of conservation and transformation of energy.
It is internal energy that characterizes the energy of thermodynamic system. By doing work and heat transfer, the system exchanges energy with the outside world, changing the internal energy. According to the general law of energy conservation, after the system reaches the final state II from the initial state I through an arbitrary process, the increment δ U of internal energy should be equal to the heat Q transferred to the system by the outside world during this process.
And the difference between the system and the work done by the system to the outside world, that is, u Ⅱ-u Ⅰ = δ u = q-w or Q =δU+W, which is the expression of the first law of thermodynamics. If, besides doing work and heat transfer, there is also energy Z brought by substances entering the system from the outside, it should be δ U = Q-W+Z. Of course, the above δ U, W, Q and Z can all be positive or negative (increasing the system energy to be positive and decreasing it to be negative). For infinitesimal processes, the differential expression of the first law of thermodynamics is
Δ q = dU+Δ w because u is a state function and du is fully differential; Q and W are process quantities, Δ q and Δ w only represent incomplete differential of tiny quantities, and the difference is represented by the symbol Δ. Because Δ u or dU only involves the initial state and the final state, only the initial state and the final state of the system are required to be balanced, regardless of whether the intermediate state is balanced. For quasi-static processes, there is δQ=dU+pdV.
One of the basic laws of thermodynamics is the expression of the law of energy conservation and transformation. The first law of thermodynamics points out that thermal energy can be transferred from one object to another, and can also be converted with mechanical energy or other energy. In the process of transfer and conversion, the total value of energy remains unchanged. Another expression is that it is impossible to realize the "perpetual motion machine of the first kind" that can do work without consuming energy.
Since18th century, the popular "heat theory" has been overthrown by Count von langford, J.R. von Meyer, J.P. Joule and others. They proved that heat is a manifestation of material movement and gradually summarized it as the expression of the first law. Among them, Joule's mechanical equivalent thermal experiment in 1840 ~ 1850 laid the foundation for the scientific expression of this law. Joule experiment shows that there is a strict quantitative relationship between the work W done by mechanical energy and the heat Q converted by it. Regardless of the conversion process, one unit of heat is always equivalent to e units of work, that is, W=EQ, where e is called the mechanical equivalent of heat. In the International System of Units (SI), the unit of heat and work is Joule (J), so E= 1.
For a closed system (see thermodynamic system), the first law of thermodynamics can be expressed as follows
q =δU+W
Or δQ=dU+δW
It shows that the heat q input into the system is equal to the sum of the increment δ u of the internal energy of the system and the work w of the system to the outside world.
In thermal equipment, we often encounter an open system in which the working medium stably enters and exits the equipment (see figure). At this time, the first law of thermodynamics can be expressed as
It shows that the heat q input into the system is equal to the increment of enthalpy h and kinetic energy before and after the fluid with mass m flows through the system.
And the sum of the mechanical work w output by the system to the outside world.
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