Page 109 - Mechanical Engineers' Handbook (Volume 4)

98 Thermodynamics Fundamentals

( C) 5 – [ ( F) 32]
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( F) 5 – ( C) 32
9
1 F 5 – C
9
The boundary that prevents the transfer of heat, regardless of the magnitude of the system–
environment temperature difference, is termed adiabatic. Conversely, the boundary that is
crossed by heat even in the limit of a vanishingly small system–environment temperature
difference is termed diathermal.
Measurements also show that a closed system undergoing a change of state 1 → 2in
the absence of work transfer experiences a heat interaction whose magnitude depends solely
on the end states:

2
Q
2
1 E E 1
W 0
In the special case of zero work transfer, the heat-transfer interaction is a thermodynamic
property of the system, which is by definition equal to the energy change experienced by
the system in going from state 1 to state 2. The last equation is the first law of thermody-
namics for closed systems incapable of experiencing work transfer. Note that, unlike work
transfer, the heat transfer is considered positive when it increases the energy of the system.
Most thermodynamic systems do not manifest purely mechanical ( Q 0) or purely
thermal ( W 0) behavior. Most systems manifest a coupled mechanical and thermal be-
havior. The preceding first-law statements can be used to show that the first law of ther-
modynamics for a process executed by a closed system experiencing both work transfer and
heat transfer is
2 2
Q W E E 1
2
1 1
energy
heat work change
transfer transfer
energy interactions (property)
(nonproperties)
The first law means that the net heat transfer into the system equals the work done by the
system on the environment, plus the increase in the energy of the system. The first law of
thermodynamics for a cycle or for an integral number of cycles executed by a closed system
is

Q W 0

Note that the net change in the thermodynamic property energy is zero during a cycle or an
integral number of cycles.
The energy change term E E appearing on the right-hand side of the first law can
1
2
be replaced by a more general notation that distinguishes between macroscopically identi-
fiable forms of energy storage (kinetic, gravitational) and energy stored internally,
mV 2 mV 2
E E U U 2 1 mgZ mgZ 1
2
2
2
1
1
2 2
energy internal kinetic gravitaional
change energy energy energy
change change
change