The 1st Law of Thermodynamics
The 1st Law of Thermodynamics states “Matter is not being created or destroyed.” It is because God has finished His creation that nothing now is being created. But, as a corollary to that, why is it that nothing is being destroyed? This is the second half of the statement of the law. Matter may change form, but in the end the total amount of matter in the Universe remains the same.
Understanding heat
Understanding heat means understanding energy. Conservation of Energy, or the first law of thermodynamics, tells us that energy cannot be created or destroyed. Energy is simply converted from one form to another. In most cases, some type of energy is converted into heat or heat is converted into something else.
Heat
In any incremental process, the change in the internal energy is considered due to a combination of Heat added to the system and Work done by the system. Taking dU as an infinitesimal (differential) change in internal energy, one writes