ELI5-What is entropy?
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Suppose you have a container of fluid. Half the container is filled with hot fluid while the other is filled with cold and there is a divider in between. Now, suppose you remove the barrier and allow the hot and cold fluid to mix. Once the two parts of the fluid have fully mixed, it will all be the same temperature. The entropy has increased: even though the total amount of heat in the system has remained the same, there is no more *free energy*.
That is, if there is a *cold load* and a *hot load*, then you can use the difference in temperature to do work: to lift a weight or move a vehicle forward. To do this, you would use a *heat engine*. The second law of thermodynamics says that you cannot do work by moving heat from a cold spot to a warm spot. Conversely, you cannot move heat from a cold spot to a warm spot without expending energy. This is the same principle that governs the flow of entropy.
The process of allowing the hot and cold fluids to mix is an *irreversible* one. That is, you cannot easily separate out the hot (high energy) and cold particles (low energy) to return the system to its original configuration. You can expend energy to return it to a similar configuration, but the hot and cold particles will be different than in the original configuration. An irreversible process always increases the entropy of a system.
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