Today’s Lecture Specific Heats Cyclic Processes Second Law of Thermodynamics Carnot Cycle Four Processes Between Two Isotherms How do you order them in terms of heat transferred to the gas? Things to remember: U depends on T only! W = ∫ PdV Q = ΔU + W The largest work done by the gas will require the most heat transfer! What is the heat transferred to the gas in process 3? Q = ΔU + W W = ∫ PdV ΔU = nCv ΔT Summary of Temperature Changes Process 1 – Adiabatic Q = 0 => ΔU = -W => T1 < Ta Process 2 – Isochoric W = 0 => Q = nCvΔT => T2 < Ta Process 3 – Isobaric P3 = Pa => Q = nCpΔT => T3 > Ta Process 4 – Isothermal T4 = Ta => ΔU = 0 => T4 = Ta Adiabatic Processes Q=0 ΔU = −W We can also find the work done by an ideal gas from: For an adiabatic expansion PVγ = const. Hence the work integral is: Ti Tf Vi Vf This result is consistent with W = -ΔU as it had to be! Example: Isothermal and Adiabatic Expansion Consider expanding .05 mol of He from 2.5atm to 1atm. Starting at T=295K. Find the final temperature and volume for both an adiabatic and isothermal process. From the ideal gas law the initial volume is: For He γ = 5/3. For isothermal expansion the volume increases by 2.5 or Vf = 1.21L. For adiabatic expansion: Specific Heats of an Ideal Gas From kinetic theory we showed the average (translational) kinetic energy per molecule is: For n moles the internal energy due to KE is: This means we can find Cv : For inert gases we have: Specific Heats of an Ideal Gas However for diatomic gases, nitrogen, oxygen, etc What’s up?? Internal energy may be more than just translational kinetic energy! Equipartition Theorem: In thermodynamic equilibrium, the average energy per molecule is 1/2kT for each degree of freedom This means Cv=R/2 for each degree of freedom For a diatomic molecule there are 5 degrees of freedom Summary of the First Law of Thermodynamics First Law of Thermodynamics Thermodynamic processes, Quasi-static - work is given by Isothermal – constant temperature Isochoric – constant volume Isobaric – constant pressure Adiabatic – no heat transfer Equipartition theorem – 1/2kT average molecular energy for each degree of freedom Cyclic Processes What is so special about them? The system returns to the same point (state) P What can we say about change ΔU, Q and W? Since the system returns to the same state and V U is a function of state only, ΔU = 0! By the 1st law ΔU = Q – W and we have Q = W. A B That is the system gets heat Q from the outside and does an equal amount of work W. Or the other way around! Both Q and W may be negative! What simple thermodynamic process are cyclic processes similar to? In what are they different? Cyclic Processes Since ΔU = 0, all we have to do is calculate W. How do we do that? In the simplest case we can divide the process into two parts: expansion A → B, with positive work done; contraction B → A with negative work done. WAB > 0 WBA < 0 Work Done in a Cycle Determine the work done by an Ideal gas in the cycle ACB. Examine this cycle one process at a time and them sum them up. Example: Work by a Gas with Isothermal Expansion At point B an ideal gas under a pressure of 250kPa has a volume of 1L. The gas expands along an Isotherm to a volume of 5L. Assume γ=1.4. Find the work done by the gas in the cycle BACB. First the work done along BA, The work done by the gas along AC, Since the work along CB is zero the total work during the cycle is Example: Work by a Gas with Adiabatic Expansion At point B an ideal gas under a pressure of 250kPa has a volume of 1L. The gas expands along an adiabat to a volume of 5L. Assume γ=1.4. First we need the pressure at A. The work along BA is The net work is An Adiabatic, Isothermal Cyclic Process A gas occupies 4L at 300K and 100kPa. It is compressed adiabatically to 1L (red) and then cooled at constant volume to 300K (black). Then it is allowed to expand isothermally to 4L (green). How much work is done by the gas in the cyclic process ABCA? Adiabatic compression: An Adiabatic, Isothermal Cyclic Process A gas occupies 4L at 300K and 100kPa. It is compressed adiabatically to 1L (red) and then cooled at constant volume to 300K (black). Then it is allowed to expand isothermally to 4L (green). How much work is done by the gas in the cyclic process ABCA? Isothermal expansion: The total work done by the gas in the cycle is: Why is it negative?? A Circular Cyclic Process An ideal gas is taken through a circular cyclic process, shown. (a) How much work does the gas do? (b) If there are 1.3 moles of gas, what are the max and min temperatures? The expressions for the pressure and volume are: The maximum and minimum temperatures occur the largest and smallest distance from the origin. This occurs when θ = π/4 and 5π/4 respectively or: Question: The gas goes from the state i to the state f. What is the minimum of work that the gas can do? What if the pressure is not allowed to drop below Pf ? Any other suggestions? 1 2 We can start at i , go to 1, make, say 10 circles, go to 2 and only then continue to f. 10 loops 1 W1→2→1 < 0 2 Wi→1→2→ f > 0 Wnet = 10 ⋅ W1→2→1 + Wi→1→2→ f < 0 With 10 loops, where the work is negative, the net work is certain to be negative! To convert internal energy or heat into work? We build a heat engine. ΔU = Q − W PV = nRT W = ∫ PdV Isothermal engine ΔU = 0 W =Q 100% of the heat transferred to the system is converted to work…. ⎛ V2 ⎞ W = nRT ln⎜⎜ ⎟⎟ ⎝ V1 ⎠ In principle one can get an unlimited amount of work… BUT it will require an infinitely large expansion! What are we going to do after the gas expands? Run it back? Isothermal Engine W = ∫ PdV ⎛ V2 ⎞ W = nRT ln⎜⎜ ⎟⎟ ⎝ V1 ⎠ As the system expands all the heat transferred to the system is converted to work…. W<0 W>0 As the system contracts back, though, the same amount of work is done by the surroundings and all the energy is returned to the reservoir. Adiabatic Engine W = − ΔU Q = 0 W = ∫ PdV The positive work is now limited by the internal energy of the insulated system. But again, no net work is done if you go back and forth along the same adiabat. W<0 Ti Tf W>0 Vi Vf We need an engine working in cycles and converting heat supplied from the outside into mechanical work with a possibly high efficiency… How efficient can it be? The isothermal engine could convert 100% heat into work, but did not work cyclically. Can we match this performance with an engine operating in cycles? Any fundamental law prohibiting it? The Second Law of Thermodynamics (Kelvin-Plank statement): It is impossible to construct a heat engine operating in a cycle that extracts heat from a reservoir and delivers and equal amount of work. It is impossible to construct a heat engine operating in a cycle that extracts heat from a reservoir and delivers and equal amount of work That would be an ideal heat engine… What is a real heat engine doing? • Works between two temperatures, a hot reservoir and a cold reservoir. (Hot side and cold side.) • Gets some heat Qh (obtained from, say, burning a fuel) from the hot side • Rejects some heat Qc to the cold side. • Works in a cycle, so that the internal energy does not change, ΔU=0. • Does work W = Qh - Qc • Has an efficiency e = W/Qh W Qh − Qc = e= Qh Qh Carnot Cycle http://perso.orange.fr/olivier.granier/thermo/simul/carnot/simul.htm Carnot Cycle What is the net work and efficiency of a Carnot cycle? Wtotal = WAB + WCD A B D C Since ThVBγ−1=TcVCγ-1 and ThVAγ-1=TcVDγ-1 Qc Tc = Qh Th Efficiency We have VB / VA = VC / VD and W Qh − Qc Th − Tc = = e= Qh Qh Th Carnot Cycle Efficiency W Qh − Qc Th − Tc e= = = Qh Qh Th Irreversible engines are necessarily less efficient, but so are many reversible engines! In a Carnot cycle all of the heat exchange takes place only between the highest and lowest temperatures, hence the Carnot cycle is the most efficient cycle. Carnot Cycle Efficiency W Qh − Qc Th − Tc = = e= Qh Qh Th • For highest efficiency we would want to run our engine between a very hot and a very cold reservoirs. • Large temperature difference, Th-Tc, and low temperature of the cold reservoir, Tc, are very helpful. • Efficiency can in principle reach 100% for Tc = 0, but we normally do not have such reservoirs available… Example: Carnot Cycle During one cycle a Carnot engine extracts 890J from a 550K reservoir and rejects 470J to a cooler reservoir. (a) How much work does the engine do during each cycle? (b) What is the efficiency of the engine? (c) What temperature is the cool reservoir? Example: Carnot Cycle A Carnot engine operates between Helium’s melting point and its boiling point at 4.25K. It has an efficiency of 77.7%. At what temperature does Helium melt?