Work, Energy

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Work, Energy Heat
The First Law Some Terminology
System Well defined part of the
universe Surrounding Universe outside the
boundary of the system Heat (q) may flow between
system and surroundings
Closed system No exchange of matter with
surroundings Isolated System No exchange of q,
or matter with surroundings
Isothermal process Temperature of the system
stays the same Adiabatic No heat (q) exchanged
between system and surroundings
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THE CONCEPT OF REVERSABILTY
Irreversible processes
Hot
Warm
Temperature equilibration
Cold
Warm
Mixing of two gases
P 0
Expansion into a vacuum
P 0
Evaporation into a vacuum
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THE CONCEPT OF REVERSABILTY
Reversible processes
Tiny weight
Po
Po
Po ?P
Condensation (pressure minimally increases by
adding tiny weight)
Evaporation (pressure minimally decreases by
removing tiny weight)
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IRREVERSIBLE EXPANSION
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THE CONCEPT OF REVERSABILTY
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REVERSIBLE EXPANSION
Pext Pressure of gas. If the gas is ideal, then
Pext nRT/V
How does the pressure of an ideal gas vary with
volume?
This is the reversible path. The pressure at
each point along curve is equal to the external
pressure.
P
V
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Reversible expansion gives the maximum work
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Reversible compression gives the minimum work
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A system from a state 1 (or 2) to a new state 2
(or 1). Regions A, B, C, D, and E correspond to
the areas of the 5 segments in the diagram.
1. If the process is isothermal reversible
expansion from state 1 to state 2, the total work
done by the system is equal to A. Area C Area
E B. Area C only C. Area E only D. Area A Area
C Area E E. Area A only
3. If the process in question 1 was carried out
irreversibly against a constant pressure of 2
atm, the total work done by the system is equal
to A. Area C Area E B. Area C only C. Area A
Area C Area E D. Area E Area D E. Area E only
2. If state 2 undergoes irreversible compression
to state 1 against an external pressure of 5
atm, the work done by the surroundings on the
system is equal to A. Area A Area C Area E B.
Area C only C. Area A Area C E. Area E only F.
Area A only
4. For a reversible adiabatic expansion of a gas,
which one of the following is correct? A. Heat
flows to maintain constant temperature B. The gas
suffers a maximum drop in temperature C. The gas
suffers a minimum drop in temperature D. The work
done is a positive quantity E. There is zero
change in internal energy
5. The heat capacity (Cp) for a solid at low
temperatures is approximately represented by Cp
AT3, where A is a constant. Using the equation
for Cp, the change in entropy (?S) for heating a
solid from 0K to 1K is A. A/4 C. A/2 B. A/3 D. A
6. Which one of the following is not true? A.
There is no heat flow between system and
surrounding for a reversible adiabatic process B.
Work done by the gas in a reversible expansion is
a maximum C. Work done by the gas in a reversible
expansion is a minimum D. Work done by the gas in
a reversible compression is a minimum E. Work
done by the gas in a reversible expansion is not
the same as the work done against a constant
external pressure
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Statistical Entropy
Consider four molecules in two compartments
If N ? ? the even split becomes overwhelmingly
probable
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Boltzmann
S kB lnW
Consider spin (or dipole restricted to two
orientations)
or
W 2, and S kB ln2
1 particle
2 particles
W 4, and S kB ln4
,
,
,
3 particles
W 8, and S kB ln8
1 mole
W 2NA, and S kB ln(2NA) NA kB ln2 Rln?
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