Thermodynamics Thermodynamics AP Physics Notes 2010-2011 Thermodynamics • Heat • Internal Energy • Work • Heat Reservoir / Heat Sink Thermodynamics Thermodynamics • Zeroth Law • First Law • Second Law Thermodynamics • Adiabatic • Isobaric • Isochoric (Isovolumetric) • Isothermic Thermodynamics • Carnot Theorem • Thermal Efficiency • Carnot Efficiency Thermodynamics • Entropy • Reversible • Irreversible Thermodynamics Thermodynamics • Thermodynamics - study of properties and movement of thermal energy (Q). Q is measured in Joules like all other forms of energy Thermodynamics Thermodynamics • Laws of Thermodynamics - each are associated with a variable. Zeroth law First law Second law Temperature, T Internal energy, U Entropy, S Thermodynamics • Adiabatic A thermodynamic process that occurs without gain or loss of heat and without a change in entropy Look for gases that are insulated from the environment From Greek a “not“, dia “through”, and batos “passable” Thermodynamics • Isobaric A thermodynamic process that occurs while the pressure remains constant From Greek isos "equal“ and baros "weight" Thermodynamics • Isochoric (Isolvolumetric) A thermodynamic process that occurs while the volume remains constant Look for gases that are contained in a closed or fixed container From Greek isos "equal“ and choro "place" Thermodynamics • Isothermal A thermodynamic process that occurs while the temperature remains constant Usually a relatively slow process to allow the gas to maintain its temperature From Greek isos "equal“ and therme "heat" Thermodynamics • Combustion Engine Thermodynamics • What is work as defined by the Collegeboard? Work is the work done by a gas Therefore… Positive work is the compression of a gas Negative work is expansion of a gas • What is work as defined by the textbook? Work is the work done on a gas • To avoid confusion, we use the Collegeboard’s definition only Thermodynamics W = PDV In order for work to be accomplished by the gas, it must expand or contract (change volume) A change in pressure, only, will not result in any work being accomplished In most examples a piston or object atop the gas must be moved for work to be accomplished Since we are considering the work done by the gas, as the piston moves, the gas loses energy Expansion is negative work; contraction is positive work Thermodynamics PV Diagram basics P(Pa) V(m3) Thermodynamics PV Diagram basics Given a point on the diagram we can use P(Pa) PV = nRT to find the gas’s temperature (K) V(m3) Thermodynamics PV Diagram basics P(Pa) If we see a line or curve connecting points then we know the gas has changed its properties in some way V(m3) Thermodynamics PV Diagram basics P(Pa) When we see an arrow on that line, then we know the original and final states of the gas V(m3) Thermodynamics PV Diagram basics P(Pa) Movement to the right shows expansion (remember this is negative work) V(m3) Thermodynamics PV Diagram basics Movement to the left shows compression (remember this is positive work) P(Pa) V(m3) Thermodynamics PV Diagram basics Movement directly up or down shows no change in volume (remember this is zero work) P(Pa) V(m3) Thermodynamics PV Diagram basics P(Pa) When the path closes then we have one complete cycle The gas has returned to its original pressure, temperature, and volume V(m3) Thermodynamics Carnot Cycle and PV Diagram (Animation) Ideal Diesel Engine (Animation) Rubber Bands as Spokes Engine ( Explanation of Rubber Bands as Spokes Engine ) Thermodynamics A. Be able to perform simple Carnot efficiency calculations. B. Be able to describe an ideal gas. C. Be able to interpret PV diagrams. 1. Which part of the cycle has DU greater than zero? 2. Which part of the cycle has Q greater than zero? 3. Which part of the cycle W greater than zero? 4. In which part of the cycle is no work done? 5. At which part of the cycle is the gas at is highest temperature? 6. What is the net work done? 7. Which part of the cycle does the most work? 8. Compare the temperatures at different points in the cycle. 9. Which part of the cycle represents an adiabatic, isothermal, isochoric (isovolumetric), or isobaric process? D. Be able to perform simple calculations using the ideal gas law or the combined gas law. For example, pressure is doubled and volume is quadrupled. What would be the relationship between the initial and final temperatures? E. Know the characteristics of the processes. F. Predict what would happen to temperature if kinetic energy is changed. G. Be able to perform simple calculations using the first law of thermodynamics. Thermodynamics A. Be able to interpret PV diagrams. 1. Use ideal gas law (or combined gas law) to calculate the temperature at different points. 2. Sketch the PV diagram from information given. 3. Predict whether positive, negative, or no work is done for the cycle. 4. Calculate the work done for the cycle (or part of a cycle). 5. Calculate the heat absorbed or given off for a cycle (or part of a cycle). 6. Predict which segments absorb the most heat. 7. Predict if the cycle represents a heat engine or refrigerator. B. Remember the equation for power (P=W/t or P=Fv) and that P=F/A. C. Sometimes they ask about the rate at which heat is discarded by an engine. This can be found knowing that W=QH-QL D. Be able to perform simple Carnot efficiency calculations.