#### Transcript Heat Chapter 12: Thermodynamics

Review Heat 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 t d where 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. Thermodynamics • The ideal gas law is a thermodynamic equation of state. pV = nRT where or 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/(molK) 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 W: 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. QH • U = 0; Q = -W W QC thermal efficiency – work out divided by work in e= W Q 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 QH W QC Carnot cycle – the ideal heat engine • give the upper limit of efficiency e C = TH – TC TH 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 energy. **Remember to use Kelvin for all thermodynamic formulas.