The Second Law of Thermodynamics

The second law of thermodynamics is the one that relates to entropy if you're familiar with that concept. Entropy is basically the measure of disorder and I'll talk a bit more about it later on. The no 100% heat engine means that if we are using heat to generate work, such as in a power station which burns cold to heat up water then uses this energy to create electricity. This also tells us that mechanical work which is measured in joules is directly convertible into thermal energy  measured in calories.