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Heat and Work
The forms of energy, which constitute the total energy of a system,
can be contained or stored in a system, and thus can be viewedas
the static forms of energy.
The forms of energy not stored in a system can be viewed as the
dynamic forms of energy or as energy interactions. The only two forms of energy interactions associated with a closed
system are heat transfer and work.
Energy transfer across a system boundary due solely to the
temperature difference between a system and its surroundings is called
heat.
Energy transferred across a system boundary that can be thought of as
the energy expended to lift a weight is called work. work is the energy
transfer associated with a force acting through a distance. Or any
energy interaction that is not due to temperature difference.
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Heat and Work
Heat and work are energy transport mechanisms between a system
and its surroundings. The characteristic properties of heat and work
are as follows:
1. Both are recognized at the boundaries of a system as they cross
the boundaries. They are both boundary phenomena.
2. Systems possess energy, but not heat or work.
3. Both are associated with a process, not a state. Unlike
properties, heat or work has no meaning at a state.
4. Both are path functions (i.e., their magnitudes depends on the
path followed during a process as well as the end states.
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Heat and Work
Since heat and work are path dependent functions, they have inexact
differentials designated by the symbol . The differentials of heat and
work are expressed as Q and W. The integral of the differentials of
heat and work over the process path gives the amount of heat or work
transfer that occurred at the system boundary during a process.
3
2
12
1,
2
12
1,
(not Q)
(not )
along path
along path
Q Q
W W W
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Sign Convention
A sign convention is required for heat and work energy transfers,
and the classical thermodynamic sign convention is used.
According to the classical sign convention,
heat transfertoa system and work done bya system are positive;
heat transferf roma system and work done on system are negative.
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Energy Transfer by Heat
Energy transfer across a system boundary due solely to the
temperature difference between a system and its surroundings is called
heat. The net heat transferred to a system is defined as
Heat is a boundary phenomena.
Thus in thermodynamics, the term heat simply means heat transfer.5
Q Q Qnet in out
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Energy Transfer by Heat
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m
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Energy Transfer by Heat
A process during which there is no heat transfer is called an
adiabatic process.
The system is well insulated so that only a negligible amount of heat can pass
through the boundary.
Both the system and the surroundings are at the same temperature.
An adiabatic process should not be
confused with an isothermal process
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Forms of Heat Transfer
Heat is transferred by three mechanisms
Conduction
Convection
Radiation
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Energy Transfer by Work
The energy crossing the boundary of a closed system other than
heat, is work.
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Work Heat
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Energy Transfer by Work
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Forms of Work
Mechanical work
Electrical work
The rate of electrical work done by electrons crossing a system boundary is
called electrical power and is given by the product of the voltage drop in voltsand the current in amps.
The amount of electrical work done in a time period is found by integrating the rate of
electrical work over the time period.
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(W)eW V I
2
1(kJ)eW V I dt
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Mechanical Forms of Work
Definition of work
There are several different ways of doing work, each in some way
related to a force acting through a distance.
In elementary mechanics, the work done by a constant force Fon a
body displaced a distance s in the direction of the force is given by
In many thermodynamic problems, mechanical work is the only form
of work involved. It is associated with the movement of the boundary
of a system or with the movement of the entire system as a whole. 12
2
1
2
112 FdsWW
For constant force
For variable force
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Mechanical Forms of Work
Some common forms of mechanical work are :
Shaft Work
Spring Work
Work done on Elastic Solid Bars
Work done to Raise a Body
Work done to Accelerate a Body
Work Associated with the Stretching of a Liquid Film
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Shaft Work
Energy transmission with a rotating shaft is very common in
engineering practice.
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Spring Work
Combination of springs are used in various common engineering
machines.
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Work done on Elastic Solid Bar
Most solids, contract or elongate under the action of a force.
They can be modeled as linear springs
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Work Done to Raise or Accelerate a
body
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