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

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