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The First Law of Thermodynamics
Definitions
•
system
•
isolated system
•
open system
•
closed system
•
properties
•
state of a system
•
change in state
•
path
•
process
•
cycle – cyclic process
•
state variable – state function
Each time you have a problem to solve in Thermodynamics you should pose the
following questions to yourself:
•
what is the system?
•
where is the boundary?
•
what is the initial state?
•
what is the final state?
•
what is the path of the transformation?
Work and Heat
work
–
any quantity that flows across the boundary of a
system during a change in its state and is completely
convertible into the lifting of a mass in the surroundings
Note that in this thermodynamic definition of work:
a.
work appears only at the boundary of the system;
b.
work appears only during a change in system;
c.
work is manifested by an effect in the surroundings;
d.
work is an algebraic quantity ( it can be calculated
from the change in PE of the mass,
E
potential
= mgh)
e.
W
< 0, the height of the mass in the surroundings is
lifted: work has been done by the system on the surroundings.
W
> 0, the
height of the mass in the surroundings is lowered.
Work has been
done
on the system by the surroundings.
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2
Heat
–
a quantity that flows across the boundary of a
system during a change in state by virtue of a difference in
temperature between the system and its surroundings and
flows from a point of higher to a point of lower temperature
Note that in this thermodynamic definition of heat:
a.
heat appears only at the boundary of the system;
b.
heat appears only during a change in state;
c.
the quantity of heat is manifested by an effect in the
surroundings;
d.
the quantity of heat is equal to
the number of grams of water in
the surroundings which are increased by one degree in
temperature starting at a specified temperature under a
specified pressure;
e.
heat is an algebraic quantity.
Heat is positive
if a mass
of water in the surroundings is cooled, in which case
heat has flowed from the surroundings to the system.
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 Spring '08
 STAFF
 Thermodynamics, Energy, Entropy, Heat, surroundings

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