2. Work and heat are energy transfer from one
system to another and thus play a crucial role
in most thermodynamic system on devices. As
we want to analyze such systems, we need to
model heat and work as functions of properties
and parameters characteristic of the system or
how they function.
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4. In elementary mechanics, the work done by a
force F on a body displaced a distance in
the direction of the force is:
However, when treating thermodynamics
from a macroscopic point of view, it is
advantageous to tie in the definition of work
with the concepts of system, properties, and
processes.
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5. Work is done by a system if the sole effect
on the surrounding could be the raising of a
weight.
Work done by a system is positive; energy
leaves the system.
Work done on a system is negative; energy is
added to the system.
In general, work is a form of energy in transit,
that is , energy being transferred across a
system boundary.
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6. Figure 4.1 Example of work crossing the boundary of a system.
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7. Figure 4.2 Example of work crossing the boundary of a system
because of a flow of an electric current across the system boundary.
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10. Shaft work
Figure 4.3 Forceacting Specific work
at radius r gives a
torque T = Fr.
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11. 4.3
Work Done at the Moving Boundary of a
Simple Compressible System.
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12. Considering Fig. 4.4 as a
quasi-equilibrium process,
Figure 4.4 Example of work done at the moving boundary of a
system in a quasi-equilibrium process.
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13. Work done on the
system during the
process from state 1
to state 2
(∵ on ∴ is
negative .)
Figure 4.5 Use
of pressure-volume diagram to show work done at
the moving boundary of a system in a quasi-equilibrium process.
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14. Why the work done
during each process
is a path function
(inexact differentials)
not a point function
(exact differentials)?
Figure 4.6 Various quasi-equilibrium processes between two given
states, indicating that work is a path function.
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15. Remarks:
A work done during a process depends not
only on the end states of the process but also
on its path (e.g. the path from states “1” to “2”
in Fig. 4.6) too.
16. A: The area underneath each curve in Fig. 4.6
represents the work for each process.
The differentials of point functions are exact
differentials.
Thermodynamic properties are, of course,
point functions.
The differential of path functions are inexact
differentials
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17. Two classes of problems:
1. on graphical form (e.g.
experiments). Evaluate by graphical or
numerical integration.
2. Analytical relations between P and V, e.g.,
polytrophic process: .
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18. Remarks: Eqs. (4.5) or (4.6) are mathematical
results, because there are cases in which work is
not given by Eq. (4.4)! -- see Ex. 4.5.
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21. Consider as a system a stretched wire that is under a
given tension. The work done by the system when the
length of the wire is changed by an amount :
The minus sign above is due to work done by the
system when is negative
See Example 4.6
Electric power is volts time ampere.
(Electric) power See P=Vi H.W#4.63, P88
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23. Work, in general, is given by the integral of
the product of an intensive property (e.g. P)
and the change of an extensive property
(e.g. )
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24. The identification of work in a process is an
important aspect of many thermodynamic
problems.
25. Q1: What is the work of the system comprising
of both gas and vacuum shown below?
A: No work done because no work can be
identified at the system boundary.
Figure 4.17 Example of process involving a change of volume for
which the work is zero.
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26. Q2: Work of the system comprising of the gas
alone?
A: No work done in this processes of filling
the vacuum, because 1. there is no resistance at
the system boundary as V↑ 2. this is not a
quasi-eq’m process, and therefore the work
can not be calculated from .
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28. Heat is defined as the form of energy that is
transferred across the boundary of a system at
a given temperature to another system (or the
surroundings) at a lower temperature by virtue
of the temperature difference between these
two systems.
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29. Another aspect of this definition of heat is that a
body never contains heat. Rather, heat can be
identified only as it crosses the boundary.
Thus, heat is a transient phenomenon. Heat, like
work, is a form of energy transfer to (positive)
or from (negative) a system. The unit for it is
the same as that for work.
SI unit for heat is Joule.
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30. From a mathematical perspective, heat is a
path function, and is an inexact differential
: heat transferred during the process from
“1” to state “2”. Later, e.g. in Fig. 8.8 & 8.9
the rate of heat transfer
specific heat transfer
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32. Conduction: energy exchange between
molecules. Energy is given out by molecules
having more (energy) in the average (high
temperature) to those having less (energy) in
the average (low temperature).
This is Fourier law of conduction. The minus
sign gives the direction of the heat transfer
from a higher temperature to a low
temperature.
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33. Convection: the transfer of energy between a
solid surface and the adjacent flowing fluid.
This is the Newton’s law of cooling.
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34. Radiation: the transfer of energy due to the
emission of electromagnetic waves in space
(do not require any substances in space)
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