87
As discussed earlier, work is the force timed distance, which, for a closed system, is comparable to the pressure timed the change in volume:
dw PdV
=
According to the law of conservation of energy, a heat of work that flows into or
out of the system should result in a change in the total energy stored in the system.
Mathematically:
∆E q w
= −
Where
∆E = change in internal energy of the system
q = energy flow into the system
w = work done by the system
If the volume of the system remains constant, then w = 0. Therefore,
∆E q
= v
Physical as well as chemical changes are usually encountered under constant
pressure during manufacturing to keep the system under control. Absence of such
control may result in the explosion of reactor leading to disaster.
Fig. 2.30 Water vapor from tank receiving hot water from a paper industry
If heat is absorbed by the system, q has a positive value.
If the heat is generated by the system, q has a negative value.
If the system does work on the surroundings, w has a positive value.
If surroundings do work on the system, w has a negative value.
2.9 Thermodynamics