Thermodynamics
From SkepticWiki
Thermodynamics is the branch of physics that studies heat and work. It originated to study heat engines but has expanded to be of central importance in physics.
There are two cattegories of thermodynamics. Classical Thermodynamics describes the overall macroscopic properties of bulk matter. Statistical Thermodynamics is the study of the microscopic properties of matter and their relationship to macroscopic properties.
[edit] The Laws of Thermodynamics
Some of the relationships discovered in the study of thermodynamics are so fundamental and well-supported by evidence that they have been stated as laws:
[edit] Important Concepts
- Variable of State: A macroscopic thermodynamic property, often abbreviated as state variable. Pressure, temperature and volume are variables of state.
- Temperature: A variable of state measuring the average kinetic energy of the particles in a body. The Zeroth Law of Thermodynamics allows thermometers to be constructed to measure temperature.
- Heat: A form of energy that can flow by thermal conduction from one body to another.
- Work: In thermodynamics, a transfer of energy other than the flow of heat.
- Entropy: A variable of state measuring what is often described metaphorically as the disorder of a system. See Second Law of Thermodynamics. Changes of entropy are associated with the flow of heat in and out of a system, which is what distinguishes heat from other forms of energy and necessitates a separate science of thermodynamics. Entropy can be defined microscopically using probability and statistics.
- Thermal Equilibrium: A condition where there is no net heat flow between two bodies in contact. From the Zeroth Law of Thermodynamics, they will have the same temperature.
- Heat Engine: A device that uses the flow of heat between reservoirs of different temperature to do work, or an idealized description of such a device. A heat engine can be run backward as a refrigerator.
- System: A portion of the universe under study.
- Environment: The remainder of the universe outside a system. Typically, a system and its environment are defined such that the system draws heat from and expends heat to the environment and also does work on or recieves work from the environment.
- Internal Energy: A variable of state measuring the total kinetic and potential energy within a body or system due to vibration and interaction of its component parts. This omits any kinetic energy given by net motion of the entire body or system and all energy given by its mass due to Relativity.
- Enthalpy: A variable of state defined as the internal energy plus the quantity of the pressure multiplied by the volume. This is most useful when studying the thermodynamics of compressible gases.
- Thermal reservoir: A body sufficently large that we can think of heat flow to or from the reservoir as not affecting its temperature.
- Sink: A body to which heat can flow.
- Cycle: A process that a system can run though such that all variables of state of the system are the same before and after the cycle. A good example of a cycle is two rotations of an automobile engine, after having been warmed up.
- Reversible process. A process is said to be reversible if it can be reversed : if you can follow the process backwards to return to the intial state. A process is said to be internally reversible if it is possible to reverse the system under consideration to its initial state, but not necessarily possible to reverse the effect of the process on the surroundings of the system. In nature, there is no such thing as a reversible thermodynamic process, as this would require parts without friction, moving parts that move infinitessimally slowly, infinitely large areas for heat to be conducted through, and so forth. Nonetheless, although such a machine cannot actually exist, the concept has considerable theoretical and practical value.
[edit] Misconceptions
The language employed in discussing thermodynamics consists of one big misconception: that heat is a sort of fluid. Thus we speak of the flow of heat, thermal reservoirs, heat capacity, heat pumps, et cetera. This supposed fluid once even had a name - it was called caloric. It is still tempting to apply the metaphor - to think of a heat engine as being like a waterwheel and a refrigerator as being like a pump rasing water. But the metaphor is not exact. In a waterwheel, it is the potential energy of the water which is converted into mechanical work, while the quantity of water stays the same. But in a heat engine, some of the heat itself is converted into work. The conversion of work into heat can be demonstrated simply by rubbing one's hands together: this was one of the earliest arguments against the "caloric" theory.
This slightly misleading metaphor of fluid flow remains attached to thermodynamics, just as the dead hypothesis that all acids contained oxygen is still enshrined in the name of that element (oxygen = "producer of acidity").
