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.