[[Thermodynamics MOC]]
# Second Law of Thermodynamics
The second law states that an [[Thermodynamic system|Isolated thermodynamic system]] will never undergo a decrease in entropy.
$$
\begin{align*}
\Delta S \geq 0
\end{align*}
$$
It is important to note that a non-isolated system can decrease in entropy,
but this will always result in a larger increase in entropy elsewhere.
Equality occurs only for a [[Reversible and irreversible processes|reversible process]].

## Kelvin-Planck version
It can be shown that the second law is equivalent to the statement

> It is impossible to construct a [[Heat engine]] operating in cycles
> that extracts heat from a reservoir 
> and delivers an equal amount of work

or, equivalently
$$
\begin{align*}
e \neq 1
\end{align*}
$$
See also [[Heat engine#Thermal efficiency of a heat engine]].

This is because a heat engine converts random energy of particle motion into ordered motion associated with mechanical work,
a process analogous to unscrambling an egg.[^2020]

[^2020]: 2020\. [[Sources/@wolfsonEssentialUniversityPhysics2020|Essential university physics]], p. 346

#
---
#state/tidy | #SemBr  | #lang/en