Understanding Entropy: Examples of Unavailable Energy in Everyday Life

[trungdien]

From what I have heard, entropy is the amount of energy that is unavailable to do work. What exactly does it mean by "unavailable energy", and can someone give some examples of energy being unavailable to do work in real life?

 

[CWatters]

Water at room temperature (20C) contains energy (because it's not at absolute zero) but that energy can't be used to heat up a mug of coffee already at 30C (You could do it using a heat pump but that needs an external source of energy).

 

[anorlunda]

You can talk about the unavailable or the available fractions of total energy.

See the Wikipedia article https://en.wikipedia.org/wiki/Exergy
It explains both theory and practical examples.