Conference 4 - Student Response 1
This response originally started as a search for a good description of enthalpy. I found the word all over the internet, yet somehow still don't have a good grasp of it. If anyone has a good way of explaining it, I'd love the answer and not the answer I found at NASA. Below I've included some great information on the Laws of Thermodynamics.
Thermodynamics is a branch of physics which deals with the energy and work of a system. As mentioned on the gas properties slide, thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. In aerodynamics, the thermodynamics of a gas obviously plays an important role in the analysis of propulsion systems but also in the understanding of high speed flows. In our observations of the work done on (or by) a gas, we have found that the amount of work depends not only on the initial and final states of the gas but also on the process (or path) which produces the final state. Similarly the amount of heat transferred into (or from) a gas also depends on the initial and final states and the process which produces the final state. Many observations of real gases have shown that the difference of the heat flow into the gas and the work done by the gas depends only on the initial and final states of the gas and does not depend on the process or path which produces the final state. This suggests the existence of an additional variable, called the internal energy of the gas, which depends only on the state of the gas and not on any process. The internal energy is a state variable, just like the temperature or the pressure. The first law of thermodynamics defines the internal energy (E) as equal to the difference of the heat transfer (Q) into a system and the work (W) done by the system. (E2 - E1 = Q - W) We have emphasized the words "into" and "by" in the definition. Heat removed from a system would be assigned a negative sign in the equation. Similarly work done on the system is assigned a negative sign. The internal energy is just a form of energy like the potential energy of an object at some height above the earth, or the kinetic energy of an object in motion. In the same way that potential energy can be converted to kinetic energy while conserving the total energy of the system, the internal energy of a thermodynamic system can be converted to either kinetic or potential energy. Like potential energy, the internal energy can be stored in the system. Notice, however, that heat and work can not be stored or conserved independently since they depend on the process. The first law of thermodynamics allows for many possible states of a system to exist, but only certain states are found to exist in nature. The second law of thermodynamics helps to explain this observation. If a system is fully insulated from the outside environment, it is possible to have a change of state in which no heat is transferred into the system. Scientists refer to a process which does not involve heat transfer as an adiabatic process. The implementation of the first law of thermodynamics for gases introduces another useful state variable called the enthalpy which is described on a separate page. Sources: http://www.grc.nasa.gov/WWW/K-12/airplane/thermo1.html
Thermodynamics is a branch of physics which deals with the energy and work of a system. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. In aerodynamics, the thermodynamics of a gas obviously plays an important role in the analysis of propulsion systems but also in the understanding of high speed flows. The first law of thermodynamics defines the relationship between the various forms of energy present in a system (kinetic and potential), the work which the system performs and the transfer of heat. The first law states that energy is conserved in all thermodynamic processes. We can imagine thermodynamic processes which conserve energy but which never occur in nature. For example, if we bring a hot object into contact with a cold object, we observe that the hot object cools down and the cold object heats up until an equilibrium is reached. The transfer of heat goes from the hot object to the cold object. We can imagine a system, however, in which the heat is instead transferred from the cold object to the hot object, and such a system does not violate the first law of thermodynamics. The cold object gets colder and the hot object gets hotter, but energy is conserved. Obviously we don't encounter such a system in nature and to explain this and similar observations, thermodynamicists proposed a second law of thermodynamics. Clasius, Kelvin, and Carnot proposed various forms of the second law to describe the particular physics problem that each was studying. The description of the second law stated on this slide was taken from Halliday and Resnick's textbook, "Physics". It begins with the definition of a new state variable called entropy. Entropy has a variety of physical interpretations, including the statistical disorder of the system, but for our purposes, let us consider entropy to be just another property of the system, like enthalpy or temperature. The second law states that there exists a useful state variable called entropy S. The change in entropy delta S is equal to the heat transfer delta Q divided by the temperature T. delta S = delta Q / T For a given physical process, the combined entropy of the system and the environment remains a constant if the process can be reversed. If we denote the initial and final states of the system by "i" and "f": Sf = Si (reversible process) An example of a reversible process is ideally forcing a flow through a constricted pipe. Ideal means no boundary layer losses. As the flow moves through the constriction, the pressure, temperature and velocity change, but these variables return to their original values downstream of the constriction. The state of the gas returns to its original conditions and the change of entropy of the system is zero. The second law states that if the physical process is irreversible, the combined entropy of the system and the environment must increase. The final entropy must be greater than the initial entropy for an irreversible process: Sf > Si (irreversible process) An example of an irreversible process is the problem discussed in the second paragraph. A hot object is put in contact with a cold object. Eventually, they both achieve the same equilibrium temperature. If we then separate the objects they remain at the equilibrium temperature and do not naturally return to their original temperatures. The process of bringing them to the same temperature is irreversible. The observation that heat is transferred from a hot object to a cool object is explained by this definition of entropy. Let us assume that the heat is transferred from the hot object (object 1) to the cold object (object 2). The amount of heat transferred is Q and the final equilibrium temperature for both objects we will call Tf. The temperature of the hot object changes as the heat is transferred away from the object. The average temperature of the hot object during the process we will call Th and it is the average of T1 and Tf. Th = (T1 + Tf) / 2 Similarly, for the cold object, the final temperature is Tf and the average temperature during the process is Tc which is the average of Tf and T2. Tc = (T2 + Tf) / 2 The entropy change for the hot object is -Q/Th, with the minus sign applied because the heat is transferred away from the object. For the cold object, the entropy change is Q/Tc, positive because the heat is transferred into the object. So the total entropy change for the whole system would be given by the equation: Sf = Si -Q/Th + Q/Tc, with Si and Sf being the final and initial values of the entropy. Th will always be greater than Tc, because T1 is greater than T2: Th > Tc So the absolute value of the term Q/Tc will always be greater than Q/Th |Q / Tc| > |Q / Th| Therefore, Sf will always be greater than Si, as the second law predicts. Sf > Si If, instead, we had assumed that the heat was being transferred from the cold object to the hot object, our final equation would be: Sf = Si +Q/Th -Q/Tc. The signs on the terms would be changed because of the direction of the heat transfer. Th would still be greater than Tc, and this would result in: Sf < Si The entropy of the system would decrease which violates the second law of thermodynamics Source: http://www.grc.nasa.gov/WWW/K-12/airplane/thermo2.html
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