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22-The First Law of Thermodynamics

22-The First Law of Thermodynamics - The First Law of...

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The First Law of Thermodynamics B. A. Rowland 53750/53760 Types of Systems We can classify thermodynamic systems as one of three types. The first is the open system. In an open system, the system may exchange both matter and energy with the surroundings. An example of an open system is the human body—we release excess thermal energy (from metabolism) to the surroundings (sweat is actually a fairly effective heat dispersal system) and we exchange matter (we take in oxygen in respiration and expel carbon dioxide). A system which cannot trade matter with its surroundings but can trade energy is know as a closed system. An example of a close system would be a sealed bottle of Coca-Cola. As long as the seal is not broken, no matter can enter or leave the bottle (of course, assuming a perfect seal). The plastic (or glass) used to make Coca-Cola bottles isn't a very good thermal insulator, so energy can be transferred between the Coke bottle and the surroundings (this is why even a sealed Coke bottle will warm to room temperature if left out of the fridge). Finally, we have the isolated system. An isolated system cannot exchange matter or energy with its surroundings. An example: coffee in a thermos. Sealing the thermos will ensure that no matter can get in or out, and a thermos is a fairly good insulator (let's assume here it is a perfect insulator) and no energy will get in our out of the thermos (if the insulator was perfect, the coffee would still be a toasty 100 o C in 100 years!). Reversibility in Thermodynamics Reversibility in thermodynamics implies that you could reverse some arbitrary system change without dissipating heat or energy in the process. This means that the system must be in equilibrium with the surroundings at all times, and would have to perform the process infinitely slowly; because of this, there are no perfectly reversible processes in nature. For a reversible process involving an ideal gas, you may use the Ideal Gas Law as an equation of state: pV = nRT. Reversibility will be more important conceptually when we discuss the Second Law of Thermodynamics.
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