In thermodynamics, entropy is the measure of particle disorder in a physical system. The letter is used **s** to represent this greatness.

Comparing this concept to everyday life, we may think that when a person starts an activity his objects are organized, and as he uses them and develops his activities, his objects tend to become increasingly disorganized.

Returning to the context of particles, as we know, as they undergo temperature changes, bodies change the agitation state of their molecules. So considering this agitation as the system disorder, we can conclude that:

- when a system receives heat
**Q> 0,**its entropy increases; - when a system gives in heat
**Q <0,**its entropy decreases; - if the system does not exchange heat
**Q = 0**, its entropy remains constant.

According to Rudolf Clausius, who first used the idea of entropy in 1865, for the study of entropy as physical greatness, it is more useful to know its variation than its absolute value. Thus, Clausis defined that the *entropy variation *(ΔS) in a system such as:

For processes where absolute temperatures (T) are constant.

For the case where the absolute temperature changes during this process, the calculation of entropy variation involves integral calculation, and its resolution is given by:

Looking at nature as a system, we can say that the universe is constantly receiving energy, but has no capacity to yield it, and then concludes that *entropy of the universe* it is increasing over time.