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Laws of Thermodynamics

The laws of thermodynamics are empirical rules, that is, determined by experimentation, that indicate the natural mechanisms that govern energy exchanges. Being empirical rules, the laws of thermodynamics should be called thermodynamic principles. However, the use of both terms is common.

The first law of thermodynamics states

The total amount of energy in any isolated system (without interaction with any other system) remains constant over time, although that energy can take another form of energy.

This is the well-known law of conservation of energy, which has become popular as the energy is neither created nor destroyed, it only transforms. However, despite being an expression that has become colloquial, it is worth stopping to think for a moment about the implications of this principle. The most obvious is that we cannot generate energy out of nothing and, therefore, we must find “stores” of energy, which we call energy sources. On the other hand, any system that receives energy has to, to be in balance, give out the same amount. For example, when heating a body, its temperature increases, and its response is to emit a greater amount of heat until the body emits the same amount of energy it receives, and reaches equilibrium.

The first principle of thermodynamics says:

The entropy of the universe tends to increase over time.

This principle indicates that in every energy exchange, part of the energy becomes unusable and cannot be used again. This increase in entropy is what we know as process losses. There are multiple formulations of this principle, which ultimately boil down to the fact that every real process will have losses.

Despite their apparent simplicity, these two principles are fundamental and necessary when analyzing the correct use of energy in engineering, and the viability of energy sources, such as hydrogen, or solar energy.