THE FIRST LAW OF THERMODYNAMICS

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CHAPTER 4 THE FIRST LAW OF THERMODYNAMICS
Sub-chapter covered 4.1 The First Law of
Thermodynamics 4.2 Energy Balances for Closed
Systems 4.3 Energy Balances for Steady Flow
Systems
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The First Law of Thermodynamics

• The first law of thermodynamics is an expression
  of the conservation of energy principle.
• Energy can cross the boundaries of a closed
  system in the form of heat or work.
• Energy transfer across a system boundary due
  solely to the temperature difference between a
  system and its surroundings is called heat.
• Work energy can be thought of as the energy
  expended to lift a weight

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• Energy Balances
• The net change in the total energy of the system
  during a process is equal
• to the difference between the total energy
  entering and the total energy
• leaving the system during that process

Change in the total energy of the system
Total energy entering the system
Total energy leaving the system
-

Ein - Eout ?Esystem
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Energy Change of a System
Energy Change Energy at final state -
Energy at initial state
? Esystem Efinal - Einitial E2
- E1
? E ? U ? KE ? PE
where ? U m (u2 u1 ) ? KE ½ (m
)(V22 - V12) ? PE mg (z2 z1 )
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• Mechanisms of Energy Transfer
• Heat Transfer, Q - Heat transfer to a system
  (heat gain) increase the
• energy of the molecules and the heat transfer
  from a system (heat loss)
• decrease the energy of the molecules.
• Work, W Energy interaction that is not caused
  by a temperature difference
• between system and surroundings.
• - Work transfer to a system
  (work done on a system) increase the
• energy of the system and work transfer from a
  system (work done by the
• system) decrease the energy of the system.
• Mass Flow, m Mass flow in and out of the
  system as an additional
• mechanism of energy transfer.
• - Mass enters a
  system, increase the energy of the system
• and mass leaves the system,
  decrease the energy of the system

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Ein - Eout ?Esystem
kJ
.
.
.
Ein - Eout ?Esystem
rate form
kW
ein - eout ?esystem
per unit mass
kJ/kg
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Energy Balances for Closed Systems

• Closed system (control mass) no mass can
  enter or leave a system

Overall energy balance Ein Eout (Qin Qout)
(Win Wout) (Emass,in Emass,out)
?Esystem
0
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Various forms of the first law relation for
closed system
General Q - W ?E
Stationary system Q - W ?U
Per unit mass q - w ?e
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• Example 1
• A piston cylinder device content 25g of
  saturated water vapor that
• is maintained at constant pressure of 300 kPa.
  A résistance heater
• within the cylinders turned on and passes
  current of 0.2 A for 5 min
• from a 12V source. At the same time, a heat
  loss of 3.7 kJ occurs.
• a) Show the process in P-v diagram
• b) Final specific enthalpy
• c) Final temperature of the process

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Energy Balances for Steady Flow Systems

• steady flow process a process during which a
  fluid flows through a
• control volume steadily
• no intensive or extensive properties within the
  control volume ( the mass, m
• the volume, V and the total energy content, E
  remain constant)

Under steady flow conditions
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.
.
Mass balance for steady flow process min
mout Multiple inlets and exits ? mi ?
me One inlet and one exit m1 m2 or
?1V1A1 ?2V2A2
.
.
.
.
Energy Balance for Steady Flow Process
0 (steady)
Energy balance
(kW)
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Energy can be transferred by heat, work and mass
only the general steady flow system can also be
written as
where
for inlet and exit
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.
where Q heat transferred into the system
(heat input) W work produced by the system
(work output)
.
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Steady flow for single stream
per unit mass
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• Exercise
• Water is being heated in a closed pan on top of a
  range while being
• stirred by paddle wheel. During the process,
  30 kJ of heat is
• transferred to the water and 8 kJ of heat is
  lost to the surrounding
• air. The paddle wheel work amounts to 500
  Nm. Determine the
• final energy of the system if its initial
  energy is 10 kJ

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• Exercise
• A well insulated rigid tank contains 5 kg of
  saturated liquid-vapor
• mixture of water at 100 kPa. Initially
  three quarters of the mass is in
• the liquid phase. An electric resistor
  placed in the tank is connected
• to a 110V source and a current of 8 A flows
  through the resistor when
• the switch turned on. Determine how long it
  will take to vaporize
• all the liquid in the tank. Also show the
  process on a T-v diagram with
• respect to the saturation lines