Heat loss Energy is never lost, but it can be burned up. Indeed, every time energy is used, some is turned into heat. This is why you feel hot after running and why light bulbs get hot. This matters because heat spreads out thinly in all directions. When wood burns, for instance, its energy is scattered through the air and is hard to use again.
Scientists use the term entropy to describe how much energy has become unusable.
The less free energy there is, the greater the entropy. Entropy is at a maximum when there is no free energy left at all. The word entropy was coined by German physicist Rudolf Clausius in 1868, but the idea dates back to the 1820s and a young Frenchman called Sadi Carnot. Carnot was trying to find out why a steam engine works. He realized it is because it is hot in one place and cold in another.