Work%20and%20Heat

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Chapter 4

• Work and Heat

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4.1 WORK

• From a macroscopic point of view,Work is done by
  a system if the sole effect on the surroundings
  (everything external to the system) could be the
  raising of a weight.
• Work is a form of energy in transit.

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Work Crossing Boundary of a System

• Work done by a system is considered positive
• Work done on a system is considered negative.

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4.3 WORK DONE AT THE MOVING BOUNDARY of a SIMPLE
COMPRESSIBLE SYSTEM
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Pressure versus Volume
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Work Is a Path Function.
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Point Functions

• Thermodynamic properties are point functions that
  for a given point on a diagram (such as Fig. 4.5)
  or surface (such as Fig. 3.12),
• the state is fixed, and thus there is a definite
  value of each property corresponding to this
  point.

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• Point functions are exact differentials.

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• Path functions are inexact differentials.

, never write W2 -W1
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Polytropic Process

• Path-functional relationship is a process called
  a polytropic process.
• PV n constant

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Work done in polytropic process
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Example 4.1
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Work Done in Various Processes
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Example 4.2
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Example 4.3
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Solution
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4.4 OTHER SYSTEMS THAT INVOLVE WORK

• Other types of systems in which work is done at a
  moving boundary.
• In this section we briefly consider three such
  systems, a stretched wire, a surface film, and
  electrical work.

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A Stretched Wire
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Example 4.4
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Work Done on a Liquid Film System
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Electrical Work
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4.5 CONCLUDING REMARKS REGARDING WORK
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Example of process involving a change of volume
for which the work is zero ( Fig. 4.13 )
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• However, this is not a quasi-equilibrium process,
  and therefore the work cannot be calculated from
  this relation.
• Because there is no resistance at the system
  boundary as the volume increases, we conclude
  that for this system no work is done in this
  process of filling the vacuum.

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4.6 DEFINITION OF HEAT

• Definition of heat is out of a transfer of energy
  .
• 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 the
  two systems.
• Heat can be identified only as it crosses the
  boundary. Thus, heat is a transient phenomenon.

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• A body never contains heat.
• If we consider the hot block of copper as one
  system and the cold water in the beaker as
  another system, we recognize that originally
  neither system contains any heat (they do contain
  energy, of course).

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• The units for heat are the same as the units for
  work.
• In the International System the unit for heat
  (energy) is the joule.
• Heat transferred to a system is considered
  positive, and heat transferred from a system is
  negative.
• A process in which there is no heat transfer (Q
  0) is called an adiabatic process.

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• From a mathematical perspective, heat, like work,
  is a path function and is recognized as an
  inexact differential.

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4.7 HEAT TRANSFER MODES

• Conduction - The energy exchange between
  molecules is heat transfer by conduction, and
• it increases with the temperature difference and
  the ability of the substance to make the transfer.

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Convection

• Convection takes place when a media is flowing.
• In this mode, the bulk motion of a substance
  moves matter with a certain temperature over or
  near a surface with a different temperature.

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Radiation

• Radiation -The final mode of heat transfer
  transmits energy as electromagnetic waves in
  space.
• The transfer can happen in empty space and does
  not require any matter, but the emission
  (generation) of the radiation and the absorption
  does require a substance to be present.

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4.8 COMPARISON OF HEAT AND WORK

• Similarities between heat and work
• Both are transient phenomena.
• Both are boundary phenomena.
• Both are path functions and inexact differentials.

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• Difference between heat and workWhich crosses
  the boundary of the system, heat or work?

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Which crosses the boundary of the system, heat
or work?
Fig. 4.16
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