dr.salwa al saleh [email protected]. internal energy energy transfers conservation of energy and...
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Internal Energy Energy Transfers
Conservation of Energy
and Heat Work
Lecture 9
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Thermodynamic Systems
Remember….Thermodynamics: Fundamental laws that heat and work obey
System: Collection of objects on which the attention is being paid
Surrounding – Everything else around
System can be separated from surrounding by: Diathermal Walls – Allows heat to flow throughAdiabatic Walls - Perfectly insulating walls that do not allow
flow of heat
State of a system – the physical condition – can be defined using various parameters such as volume, pressure, temperature etc.
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Before We Start
• Why is T Ktrans?
• Why is C larger when there are more modes? • Why does energy partition between modes?
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Thermodynamic Paths
energy transfers
§ 19.3–19.4
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Energy Transfers
Between system and surroundings• Work• Heat
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Work
From a volume change of the system
W = - p dVV1
V2
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Heat and Work
dW = F.ds = (PA) ds = p (Ads = p dV
W= dw = p dV Work done represented by the area under the
curve on pV diagram.Area depends upon the path taken from i to f
state. Also PV= nRT
For b) from i to a process volume increase at constant pressure i.e
Ta=Ti (Va/Vi) then Ta>Ti . Heat Q must be absorbed by the
system and work W is done a to f process is at constant V (Pf>Pa) then
Tf=Ta(pf/pa) Since Tf<Ta , heat Q’ must be lost by the systemFor process iaf total work W is done and net
heat absorbed is Q-Q’
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Mechanical Work
Mechanical work:
Convention: work done on the system is taken as
positive.
Pi , Vi
m
Pf , Vf
piston held down by pins
removing pins h
extw P V
Expansion of a gas
Infinitesimal volume change:
extw P V
Work performed by the gas:
ext ( )f
i
V
Vw P V dV
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Reversible Processes
When the process is reversible the path can be reversed, so expansion and compression correspond to the same amount of work.
To be reversible, a process must be infinitely slow.
A process is called reversible if Psystem= Pext at all times. The work
expended to compress a gas along a reversible path can be completely recovered upon reversing the path.
.
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Reversible Isothermal Expansion/Compression of Ideal Gas
Reversible isothermal compression: minimum possible work
Reversible isothermal expansion: maximum possible work
PP
VVfVi
Pi
Pf
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Heat
From a temperature difference
dQ /dT Tsurr – Tsys
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Work and Heat
Depend on the path taken between initial and final states.
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Question
Is the work done by a thermodynamic system in a cyclic process (final state is also the initial state) zero.
Source: Y&F, Figure 19.12
A. True.
B. False.
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Cyclic Process
W 0
W
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Work between States
W is not uniquely determined by initial and final states
p
V
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What Are the Processes?
p
V
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Group Work
Qualitatively sketch a pV plot for each described process AB.
a) Volume is gradually doubled with no heat input, then heated at constant volume to the initial temperature.
b) System is heated at constant pressure until volume doubles, then cooled at constant volume to the initial temperature.
c) System is allowed to expand into a vacuum (free expansion) to twice its volume.
d) Volume is gradually doubled while maintaining a constant temperature.
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Group Work
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Conservation of Energy
E of a system = work done on the system
+ heat added to the system
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Internal Energy
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Question
All other things being equal, adding heat to a system increases its internal energy E.
A. True.
B. False.
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Question
All other things being equal, lifting a system to a greater height increases its internal energy U.
A. True.
B. False.
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Question
All other things being equal, accelerating a system to a greater speed increases its internal energy E.
A. True.
B. False.
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Question
All other things being equal, doing work to compress a system increases its internal energy E.
A. True.
B. False.
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Cyclic Processes
= E1 – E1 = 0
soQ – W = 0
soQ = W
• Work output = heat input
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Work out = Heat in
Does this mean cyclic processes convert heat to work with 100% efficiency?
(Of course not).
Waste heat is not recovered.
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Example Problem
A thermodynamic cycle consists of two closed loops, I and II.
d) In each of the loops, I and II, does heat flow into or out of the system?
c) Over one complete cycle, does heat flow into or out of the system?
p
V
I
II