Answer: when a process converts energy from one form to another, some energy converts into heat, a less usable form of energy.
Explanation:
The second law of thermdodynamics has several ways to be stated.
The original statement, made by Lord Kelvin, was that it is impossible to build a heat enginge that, operating in a cycle, converts all the heat (transfered from a hot source to a cold reservoir) into useful work: part of the heat is not a useful form of energy.
The second law of thermodynamics is also stated in terms of entropy: the spontaneous processes lead to an increase in entropy.
Entropy is a meausre of the disorder of the systems, and the disorder tends to increase. Entropy determines whether a a process will occurs spontaneously or not.
The second law of thermodynamics implies that the spotaneous flow of heat is from warmer subnstances to colder ones.
Energy can neither be created nor destroyed, but can change from one form to another is the first law of thermodynamics or energy conservation law.
