chapter 11: heat chapter 12: thermodynamics
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Chapter 11: Heat
Chapter 12: ThermodynamicsReview
Chapter 11Heat
heat – a form of energy in transit• SI unit is the joule (J)• common nonstandard units are the kilocalorie
(kcal) and the British thermal unit (BTU)
mechanical equivalent of heat – relates joules to kilocalories
1 kcal = 1000 calories = 1 Calorie = 4,186 J Memorize this or write it on your blue sheet.
heat energy work• all have the same units; they are different forms
of the same thing
thermal conductivity – the heat-conducting ability of a material
H = Q = KA T On Gold Sheet t d
where H is the thermal conductivity of the materialA is the cross-sectional area T is the temperature of the hot side of the conductor minus
the temperature of the cold side of the conductord is the thickness of the conductor
** On the gold sheet L is used to represent thickness.
Chapter 12Thermodynamics
• The ideal gas law is a thermodynamic equation of state.
pV = nRT or pV = NkBT On Gold Sheet
where p is pressure in pascals (Pa)V is volume in cubic meters (m3)n is the number of molesR is the universal gas constant, 8.31 J/(molK)T is the temperature in kelvinN is the number of moleculeskB is Boltzmann’s constant, 1.38 x 10-23 J/K
• Know the vocabulary of thermodynamics.
• Know the 1st Law and sign conventions.
The First Law of Thermodynamics is a statement of energy conservation for thermodynamic systems.
∆U = Q + W On Gold Sheet Q : heat∆U : change in internal energyW: work
Sign Conventions
The system is the gas, fluid, etc. you are analyzing.
+Q means heat added to the system
-Q means heat removed from the system
+W means work done on the system (compression)
-W means work done by the system (expansion)
isothermal – constant temperature• U = 0 ; Q = -W• As U goes, so goes T.
isobaric – constant pressure• W = - pV On Gold Sheet
isometric – constant volume• W = 0; Q = U
adiabatic – no heat is exchanged• Q = 0; U = W
• The area under the curve on a P-V graph is equal to work.
• Internal energy is linked to temperature. Recall from Chapter 10, for ideal monatomic gases:
U = 3/2 nRT or U = 3/2 NkBT
The Second Law of Thermodynamics specifies the direction in which a process can naturally or spontaneously take place.
• Heat does not flow spontaneously from a colder to a warmer body.
• In a thermal cycle, heat energy cannot be completely transformed into mechanical work.
• The total entropy of the universe increases in every natural process.
The Third Law of Thermodynamics – It is not possible to lower temperature to absolute zero, since it would violate the Second Law: no heat engine can be 100% efficient.
heat engines – convert heat to work• For one cycle, the system returns to the same temperature and
heat is converted to work done by the system.• U = 0; Q = -W
thermal efficiency – work out divided by work in
e = W On Gold Sheet Q
thermal pump – the reverse of a heat engine • example: a refrigerator• Coefficient of performance is a measure of
efficiency for a thermal pump
QH
W
QC
QH
W
QC
Carnot cycle – the ideal heat engine• give the upper limit of efficiency
eC = TH – TC On Gold Sheet TH
entropy – a measure of the disorder of a system
• The entropy of an isolated system increases for every natural process as well as all irreversible processes, such as free expansion.
• The entropy of an isolated system stays the same for all reversible processes and reversible cycles.
• Entropy of an isolated system never decreases. Entropy can only decrease in a non-isolated system by doing work or expending energy.
**Remember to use Kelvin for all thermodynamic formulas.
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