The first law of thermodynamics is the law of energy transformation and conservation. In the process of thermal phenomena, internal energy and other forms of energy are transformed into each other.
The internal energy increment of the system is equal to the sum of the heat absorbed by the system from the outside and the work done by the outside to the system. Let the internal energy variation of the system be △U, the external work to the system be W, and the external heat absorbed by the system be Q, then △ U = W+Q..
When using this law, we should pay attention to the sign treatment of three quantities: for the external work of the system, W takes a positive value, and for the external work of the system takes a negative value. If the volume of the system remains unchanged, w = 0；; The system absorbs heat from the outside, q is positive, the system releases heat to the outside, and q is negative; The internal energy of the system increases, △U takes a positive value, and the internal energy of the system decreases, and △U takes a negative value.
Extended data
This law was passed by Meyer? J.R. Mayer Joule? Many physicists such as Joule confirmed this. The first law of thermodynamics is the law of energy conservation and transformation in the field of thermal phenomena.
All substances in nature have energy, and energy has different forms, which can be transformed from one form to another, and can also be transferred from one object to another. In the process of transformation and transfer, the sum of energy remains unchanged.
Suppose there is a closed system whose internal energy is U 1. The system absorbs heat Q from the environment, and the environment does work for the system. As a result, the system changes from the initial state with internal energy of U 1 to the final state with internal energy of U2.
According to the law of conservation of energy, U2=U 1+Q+W or △ U = Q+W (1-4) is the mathematical expression of the first law of thermodynamics, that is, the change of the internal energy of the system is equal to the heat absorbed by the system from the environment plus the work done by the environment on the system. ? When the pressure is constant and only volume work is done, the first law of thermodynamics can be expressed as △ U = Q-P △ V (1-5).
The general formula δ u = q+w
Adiabatic: Q=0, δ u = w
Constant volume (w = 0): w = 0, δ u = qv.
Constant pressure (w = 0): w =-p δ v =-δ (PV), δ u = q-δ (PV) δ h = qp.
Constant volume+adiabatic (w = 0): δ u = 0.
Constant pressure+adiabatic (w = 0): Δ h = 0
Definition of enthalpy: H = U+PV δ H = δ U+δ (PV)
References:
Baidu encyclopedia-the first law of thermodynamics