Temperature > Temperature > 05 - First Law of Thermodynamics

The first law of thermodynamics is nothing more than the principle of conservation of energy and, despite being studied for gases, it can be applied to any processes in which the energy of a system is exchanged with the external environment in the form of heat and work.

When we supply a system with a certain amount of energy, this energy can be used in two ways:

1. Part of the energy can be used for the system to perform work (t), expanding or contracting, or it may also happen that the system does not change its volume (t = 0);

2. The other part can be absorbed by the system, becoming internal energy, that is, this other part of energy is equal to the energy variation (ΔU) of the system. If the energy variation is zero (ΔU = 0) the system has used all the energy in the form of work.

ΔU= Q – t

Thus we have stated the first law of thermodynamics: the variation in internal energy ΔU of a system is equal to the difference between the heat Q exchanged with the external environment and the work t performed by it during a transformation.

Applying the law of conservation of energy, we have:

ΔU= Q - t à Q = ΔU + t

Q à Amount of heat exchanged with the environment:
Q > 0 à the system receives heat;
Q < 0 à the system loses heat;

ΔU à Variation of the internal energy of the gas:
ΔU > 0 à the internal energy increases, therefore, its temperature increases;
ΔU < 0 à the internal energy decreases, therefore, its temperature decreases;

t à Energy that the gas exchanges with the environment in the form of work:
t > 0 à the gas provides energy to the environment, therefore, the volume increases;
t < 0 à the gas receives energy from the environment, therefore, the volume decreases;