As we have seen, heat (or, rather thermal energy) from fuels can be trans- formed to mechanical energy. And, let us say it one more time, there is a range of thermal engine types that can be “employed” to perform many useful tasks.
It is essentially what The First Law of Thermodynamics states. It’s the good guy, everybody is happy with it, But it’s not the only law that governs the transformation of heat to work. The other one is the Second Law of Thermodynamics, and this is the bad guy.
This Section is mainly about the Second Law and its consequences – but before we start talking about it, we should briefly recapitulate what the First Law says in a more exact manner. We need some definitions:
- A system: A system: a single body, or more bodies that are in contact with one another.
- Internal Energy: There is a physical quantity called the internal ther- mal energy of a system or “internal energy” in short. Conventionally, it is denoted as U .
- Energy may be added to the system, thus increasing its U . (we call such a process “heating”) – either through a contact with another system, or by performing work on the system (for instance: if the system is a portion of gas, one way of performing work on it is compressing it ).
- Energy may be taken away from a system, thus lowering its U (we call such a process “cooling”) – either through a contact with another system, or by causing the system to perform work (for instance: if the system were a gas, it would perform work by expanding ). However, the process of taking energy away can not continue forever, when all energy is taken away, the system reaches the absolute zero point, at which its temperature is zero Kelvins (0 K).
Now, we can write the First Law in mathematical terms: essentially, it’s the Energy Conservation Law, but expressed in a way specifically applying to thermal phenomena:
\[ \Delta U=\Delta Q+\Delta W \]
\(\Delta U\) : The total change in the system internal energy.
\(\Delta Q\) : The change of \(U\) due to transfer of heat (heat flowing in or out from another system, in contact with the system considered).
\(\Delta W\) : The change of \(U\) due to mechanical work done on the system, or the work delivered by the system (then, negative).
Important: A common misconception is to confuse heat with the internal energy. Internal energy is the amount of energy contained by the system. Heat is the energy that flows in or out from/to a warmer/cooler body which is in contact with the system.