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Thermodynamics
A thermodynamic system is one that is made up of
very many small interacting pieces
.
State of a thermodynamic system
 The state of a thermodynamic system is determined
by macroscopic variables like pressure (P), temperature (T), volume (V), internal energy
(U), number of particles (N), and other things like entropy (S) that we will talk about later.
These are all called “
state variables
”.
If we let our thermodynamic system interact with other systems then the state variables
may change.
The state our system arrives at when the
state variables
stop changing
is
called its “
equilibrium state
”.
It is an amazing experimental fact that we can only choose three of these state variables to
independently control.
Once we have chosen three of them and fixed their values (lets say
we choose a certain amount of stuff so we choose N, put it in a specific container so this
chooses V, and give it a certain amount of energy so this fixes U) then all the others are
fully determined and we can’t change them without changing one of our three fixed
variables.
Equations of state
let us calculate all other state variables from these three.
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 Summer '09
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