ENERGY, WORK, AND HEAT
Thermodynamics
As with work, the amount of heat transferred depends upon the path and not simply on the initial
and final conditions of the system. Also, as with work, it is important to distinguish between
heat added to a system from its surroundings and heat removed from a system to its
surroundings. A positive value for heat indicates that heat is added to the system by its
surroundings. This is in contrast to work that is positive when energy is transferred from the
system and negative when transferred to the system. The symbol q is sometimes used to indicate
the heat added to or removed from a system per unit mass. It equals the total heat (Q) added
or removed divided by the mass (m). The term "specific heat" is not used for q since specific
heat is used for another parameter. The quantity represented by q is referred to simply as the
heat transferred per unit mass.
(1-16)
q
Q
m
where:
q
=
heat transferred per unit mass (Btu/lbm)
Q
=
heat transferred (Btu)
m
=
mass (lbm)
Example:
Determine the heat transferred per unit mass if 1500 Btu’s are transferred to 40 lbm of
water.
Solution:
Using Equation 1-16
q
Q
m
q
1500 Btu
40 lbm
q
37.5 Btu/lbm
The best way to quantify the definition of heat is to consider the relationship between the amount
of heat added to or removed from a system and the change in the temperature of the system.
Everyone is familiar with the physical phenomena that when a substance is heated, its
temperature increases, and when it is cooled, its temperature decreases. The heat added to or
removed from a substance to produce a change in its temperature is called sensible heat. The
units of heat are often defined in terms of the changes in temperature it produces.
HT-01
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