Statements of the third law The third law of thermodynamics is concerned with the behavior of...
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Statements of the third law Statements of the third law •The third law of thermodynamics is concerned with the behavior of systems in equilibrium as temperature approaches absolute zero. 0 T đQ S S T •As long as we deal in differences in entropy, knowledge of S 0 is unnecessary. •However, absolute entropy IS important, e.g. F = U - TS F = U - TS G = H - TS G = H - TS
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Transcript of Statements of the third law The third law of thermodynamics is concerned with the behavior of...
Statements of the third lawStatements of the third law•The third law of thermodynamics is concerned with the behavior of systems in equilibrium as temperature approaches absolute zero.
0
T đQS S
T
•As long as we deal in differences in entropy, knowledge of S0 is unnecessary.
•However, absolute entropy IS important, e.g.
F = U - TSF = U - TSG = H - TSG = H - TS
Statements of the third lawStatements of the third law•The Gibbs-Helmholtz equation:
P
GG H T
T
•For an isothermal process:
P
GG H T
T
•These equations imply:
0 0
0 0
lim lim
lim limT T
T T
G H
G H
Statements of the third lawStatements of the third law
0 0
0 0
lim lim
lim limT T
T T
G H
G H
•Nernst postulated the following:
0 0
lim 0, lim 0T T
P P
G H
T T
Statements of the third lawStatements of the third law
0 0
0 0
lim lim
lim limT T
T T
G H
G H
•Nernst postulate implies that:
2 1 2 11 2
0 0 0lim lim lim 0T T T
P
G G G GS S
T T T
Statements of the third lawStatements of the third law
0 0
0 0
lim lim
lim limT T
T T
G H
G H
•The Nernst formulation of the Third Law:
‘‘All reactions in a liquid or solid in thermal equilibrium All reactions in a liquid or solid in thermal equilibrium take place with no change of entropy in the neighborhood take place with no change of entropy in the neighborhood of absolute zero.’of absolute zero.’
Statements of the third lawStatements of the third law
0 0
0 0
lim lim
lim limT T
T T
G H
G H
•Planck later postulated that:
0 0 0lim 0, lim 0 lim 0T T T
P P
G HS
T T
Statements of the third lawStatements of the third law
0 0
0 0
lim lim
lim limT T
T T
G H
G H
•Planck’s statement of the Third Law:
‘‘The entropy of a true equilibrium state of a system at The entropy of a true equilibrium state of a system at absolute zero is zero.’absolute zero is zero.’
Statements of the third lawStatements of the third law
0 0
0 0
lim lim
lim limT T
T T
G H
G H
•Another statement of the Third Law is:
‘‘It is impossible to reduce the temperature of a system to It is impossible to reduce the temperature of a system to absolute zero using a finite number of processes.’absolute zero using a finite number of processes.’
Step 1: Isothermal increase of HStep 1: Isothermal increase of H
T1T2T3
H1
H2 > H1
SS
TT
HHMM
( , ) /H H
dU TdS HdM
TdS T H C dT T M T dH
Step 2: Adiabatic demagnetizationStep 2: Adiabatic demagnetization
T1T2T3
B1SS
TT
MM
B2 > B1
( , ) /H H
dU TdS HdM
TdS T H C dT T M T dH
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