MadSci Network: Physics Query:

### Re: what is the difference between adiabatic and isothermal change?

Date: Mon Mar 12 19:25:25 2001
Posted By: Sidney Chivers, , Nuclear Engineering, retired
Area of science: Physics
ID: 984076416.Ph
Message:
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First, a few definitions:

- "A process that takes place in such a way that no heat flows into or out
of the system is called an adiabatic process." (P. 564, Halliday and
Resnick, Physics, 1966)

- "A reversible process of special importance is the reversible adiabatic
process ... the entropy of a control mass undergoing a reversible
adiabatic process will not change; such a process of constant entropy is
called an isentropic process."  (P. 185, Reynolds and Perkins, Engineering
Thermodynamics, 1970)

- "The Carnot cycle is an ideal power cycle that is impractical to
implement.  However, its work output sets the maximum attainable from any
heat engine ..." (P. 25-8, Michael R. Lindeburg, Engineer-in-Training
Reference Manual, 8th Edition, 1992)

- Processes that occur at constant temperature are isothermal.

The Carnot cycle is frequently illustrated by a pressure vs. volume,
temperature vs. entropy, or enthalpy vs. entropy curve showing the path
followed by a Carnot cycle in transitioning between four states.  Two of
the transitions are isothermal (isothermal expansion of saturated liquid
to saturated vapor and isothermal compression of vapor), and two of the
transitions are isentropic (reversible adiabatic processes) (an isentropic
expansion of vapor and an isentropic compression).

To put the four processes in context, and assuming a, b, c, and d are
different states, the processes are

- a to b: isothermal expansion of saturated liquid to saturated vapor

- b to c: isentropic expansion of vapor

- c to d: isothermal compression of vapor

- d to a: isentropic compression

Of course, the above would make a lot more sense if accompanied by an
illustration.  On the web I found the following site with an applet that is at
least intriquing

http://www.phy.ntnu.edu.tw/java/carnot/carnot.html

and the following website with illustrations I thought were better matched to
the above

http://www.taftan.com/thermodynamics/CARNOT.HTM

Hope this helps.

Thanks for your question.

sid

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