PROFESSOR: Another way
to do a process would be to seal it in a thermal
container so heat can't flow. When heat can't flow, we call
those processes adiabatic. For adiabatic compression
and expansion, we can also talk
about energy changes. So let's do that. Here we have
adiabatic expansion. Now if I expand, or let the
gas expand adiabatically, in order for the gas to
expand it has to do work. Now it can't absorb
a joule of heat to replace the joule of work
it did to re-energize itself. So I do a joule of work,
I use a joule of energy. Do a joule of work,
use a joule of energy. My energy drops. And for an ideal gas,
if the energy drops, the temperature drops. So ideal gas is
expanding adiabatically, work is negative, and the
change in energy, negative. It can also be zero because I
could expand against a vacuum and not have to do work. What about compression? If I compress the gas, now
work is being done on the gas. And the gas is saying I'm
getting this energy from work, but I can't release it as
heat, q has to be zero, so my energy just goes up. So in this case work is
positive and the energy change is positive for
adiabatic compression. Practically, it usually means
you just do the process quickly because you can quickly do
work, but heat flow always takes time. So a rapid process is
often an adiabatic process. And we can expand
either adiabatically, or we can be compressed
adiabatically. For ideal gases these
are the summary.
