Adiabatic
Adiabatic refers to a process in which no heat is transferred into or out of a system, and the change in internal energy is only done by work. Often this is accomplished in an insulated container, where the process happens too quickly for heat to be transferred. Since heat moves because of a difference in temperature between a system and its surroundings, an adiabatic process can also happen when the system is in thermal equilibrium.[1] By the first law of thermodynamics, this then looks like
where:
is the change in the system's internal energy and
is the work done on the system (if the system does work to its surroundings, the work is negative and the systems internal energy thereby decreases).
For an adiabatic compression (decreasing the volume of the system, like a piston), the temperature must increase. Likewise, for an expansion (increasing volume), the temperature must decrease. This is because there is no heat able to flow in or out in order to balance the temperature, which is what occurs in an isothermal process.
Adiabatic processes are used in the Brayton cycle within gas turbines. Diesel engines also make use of a (somewhat) adiabatic compression in order to ignite its fuel. Adiabats are also important for the derivation of the Carnot efficiency (the maximum thermal efficiency of any thermodynamic system) which relies on two adiabatic processes.[2]
The video below (from Educational Innovations, Inc.) demonstrates an adiabatic compression: the temperature inside the chamber is increased enough to ignite cotton.
References
- ↑ H. Gould and J. Tobochnik, "Quasistatic Adiabatic Processes," in Statistical and Thermal Physics, 1st ed., Princeton, NJ: Princeton University Press, 2010, ch.2, sec.11, pp. 51-55
- ↑ R. D. Knight, "The Limits of Efficiency" in Physics for Scientists and Engineers: A Strategic Approach, 3nd ed. San Francisco, U.S.A.: Pearson Addison-Wesley, 2008, ch.19, sec.5, pp. 540-542