Page 38 - 4498

C M
c  (2.4.4)
M

2.5 The First Law of Thermodynamics

The first law of thermodynamics is a version of the law of

conservation of energy, adapted for thermodynamic systems. The law of
conservation of energy states that the total energy of an isolated system is
constant; energy can be transformed from one form to another, but cannot

be created or destroyed. The first law is often formulated by stating that
the change in the internal energy of a closed system is equal to the amount
of heat supplied to the system, minus the amount of work done by the
system on its surroundings. There are two equivalent definition of the first

law of thermodynamics.
The alteration of the internal energy of the system U 1 2 A U^ during
2
the process 1- 2 of the system transition from the state 1 to the state 2 is
equal to the sum of the mechanical work  A 1 2 done over the system by

external forces and the heat Q added to the system:
1 2

U 1 2  1 2  Q 1 2 (2.5.1)
 A

Causing  A 1 2  A 1 2 A' - —Ai-2 it is possible to obtain another form
U2
of the first law of thermodynamics

Q 1 2  A 1 2  U 1 2 (2.5.2)

The heat added to the system equals to the mechanical work done by
the system and the increase in internal energy of the system.

For infinitesimal:
dQ  dU  dU (2.5.3)
If dQ  0 - the heat is added to the system.

If dQ  0 - the heat is removed from the system.

If dA  0 - the mechanical work is done by the system
If dA  0 - the mechanical work is done over the system

For the cycle Q  A
1 2 1 1 2 1

38