Page 22 - Advanced Thermodynamics for Engineers, Second Edition
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6 CHAPTER 1 INTRODUCTION AND REVISION
1.5.3.2 Unit of heat (energy)
The standard unit of heat is the unit of energy, viz. the joule (J). In engineering practice the kilojoule
(kJ), is usually used, but the kilojoule is not a rational unit. The joule can be related to the calorie or
kilocalorie.
One kilocalorie is the quantity of energy required to raise the temperature of 1 kg of water by 1 C
if the water is at 15 C.
The joule will be defined later and is not based on the thermal capacity of a substance, but derives
from the First Law of Thermodynamics.
One kilocalorie (kcal) is equivalent to 4.1868 kJ.
1.5.3.3 Heat (transfer) processes on a state diagram
Heat and work transfer processes both have similar effects on the changes of state. It is more difficult to
see that the same change of state can be brought about by different heat transfer processes based on the
different thermodynamic constraints on the system, but this is the case.
The quantity of heat energy flowing into a system is dependent on the path followed during the
process: hence heat is not a property of the system.
1.5.4 RELATIONSHIP BETWEEN HEAT AND WORK
Both heat and work are transitory phenomena associated with interactions at the boundaries of
systems. They only exist when they are taking place and are not stored per se in the system.
1.5.4.1 The First Law of Thermodynamics
The First Law of Thermodynamics is a statement of the conservation of energy. It is based on the
equivalence of heat and work, which was first stated by Joule in the 1840s. The First Law enables the
concept of internal energy and enthalpy to be defined as thermodynamic properties.
1.5.4.1.1 Statement of the First Law of Thermodynamics
When a system executes a cyclic process, the algebraic sum of the work transfers is proportional to the
algebraic sum of the heat transfers.
This can be written mathematically as
I I I
dW ¼ J ~ dQ; where means integration around the cycle; (1.3)
which can be rearranged to give
I I I I
dW J ~ dQ ¼ J ~ dQ dW ¼ 0: (1.4)
~
In SI units the units of work and heat are both joules (J), and hence J ¼ 1. Thus Eqn (1.4) becomes
I I I I
dW dQ ¼ dQ dW ¼ 0;
which can be rewritten as
I
ðdQ dWÞ¼ 0: (1.5)