entropy: s = rev q/t (11.8)
DESCRIPTION
ENTROPY: S = rev Q/T (11.8). The novelist and physicist C. P. Snow once remarked that not knowing the 2 nd Law of Thermodynamics was analogous to never having read a work by Shakespeare. 1850 ~ 1 st & 2 nd Laws of Thermodynamics 1854 ~ ( Q 1 /T 1 ) rev = ( Q 2 /T 2 ) rev - PowerPoint PPT PresentationTRANSCRIPT
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The novelist and physicist C. P. Snow once remarked that not knowing the
2nd Law of Thermodynamics was analogous to never having read a
work by Shakespeare.
ENTROPY: S = rev Q/T (11.8)
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1850 ~ 1st & 2nd Laws of Thermodynamics
1854 ~ (Q1/T1)rev = (Q2/T2)rev
1865 ~ dS = dQ/Trev
dS > dQ/Tirrev
“I propose to name the magnitude S the entropy of the body, from The Greek word for transformation. I have intentionally formed the word entropy so as to
be as similar as possible to the word energy, sinceboth these quantities….are so nearly related…”
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Definition of Entropy:S = revQ/T or dS = [Q/T]rev (11.8)
Because of 2nd Law:dS [Q/T]irrev or TdS Q (11.9a)
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IRREVERSIBLE AND REVERSIBLE PROCESSES
A process is reversible if system remains in equilibrium, i.e. if the work and heat are added in such a way that there are no currents. A currents of heat arises from temperature gradients; a current of mass arises from concentration gradients; a current of momentumflows, if there are differences in velocity. Hence, reversible processwill have no density, velocity and temperature gradients.
All natural processes are irreversible, not in equilibrium, associatedwith currents.
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It can be shown for ideal gas and reversible cycle That QH/TH = QC/TC so over a cycle there is no gain or loss of Q/T. Call
Q/T the entropy, S.
isothermal
isothermal
adiabatic
adiabatic
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dS = [Q/T]rev (11.8)dS [Q/T]irrev or TdS Q (11.9a)Tds = Q/m = q (reversible/11.8) (11.9b)Tds Q/m = q (irrev./11.8) (11.9c)
ds = 0 (rev. & adiabatic/11.9b) (11.9d)ds > 0 (irrev. & adiabatic/11.9c) (11.9e)
Isentropic if reversible and adiabaticReversible if isentropic and adiabaticAdiabatic if reversible and isentropic
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Energy Equation:Q/m - pdv = duQ/m = Tds for reversible processTds = du + pdv (11.10a)
Note: Although 11.10a was derived for areversible processes, it is also valid for irrev. processes as it involves only exact differentials having integrated values that are independent of process.
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Tds = du + pdv (11.10a)Definition: h = u + pvdh = du +(dp)v + p(dv)Tds = du + dh - du - (dp)v Tds = dh - v(dp) (11.10b)
Note: Although 11.10b was derived for areversible processes, it is also valid for irrev. Processes as it involves only exact differentials having integrated values that are independent of process
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James Watt (1736-1819) is fancifully depicted here as a boy entranced by the nature of steam.
Timeline of steam:150 BC-159 AD Hero1600s Torricelli/Viviani – creation of vacuum1650 von Guericke – invents air pump1698 Savery – makes a steam pump1712 Newcomen – better pump using
condensing steam
1769 – Watt greatly improves Newcomen’s design
1804 – first stem driven rail road1812 – first stem driven ship
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isothermal
isothermal
adiabatic
adiabatic
For an ideal gas pv = RTH
For an ideal gas pvk = const
QH
QC
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pv = k
pv1.4 = k
Reversible Processes
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Work = pdV(Watt’s Secret)