Heat Chapter 12: Thermodynamics

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Transcript Heat Chapter 12: Thermodynamics

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
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
H is the thermal conductivity of the material
A 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 conductor
d is the thickness of the conductor
** On the gold sheet L is used to represent thickness.
• The ideal gas law is a thermodynamic equation of state.
pV = nRT
pV = NkBT
On Gold Sheet
p is pressure in pascals (Pa)
V is volume in cubic meters (m3)
n is the number of moles
R is the universal gas constant, 8.31 J/(molK)
T is the temperature in kelvin
N is the number of molecules
kB 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 energy
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 = - pV
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 nRT
U = 3/2 NkBT
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
On Gold Sheet
thermal pump – the reverse of a heat engine
• example: a refrigerator
• Coefficient of performance is a measure of
efficiency for a thermal pump
Carnot cycle – the ideal heat engine
• give the upper limit of efficiency
e C = TH – TC
On Gold Sheet
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
**Remember to use Kelvin for all thermodynamic formulas.