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Ch15 Thermodynamics

Ch15 Thermodynamics. First Law of Thermodynamics The Internal Energy of a closed system will be equal to the energy added to the system by heating minus the work done by the system its surrounding. Zeroth Law of Thermodynamics

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Ch15 Thermodynamics

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  1. Ch15 Thermodynamics First Law of Thermodynamics The Internal Energy of a closed system will be equal to the energy added to the system by heating minus the work done by the system its surrounding Zeroth Law of Thermodynamics If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. Second Law of Thermodynamics Heat flows out from hot objects to cold; heat does NOT flow from cold to hot

  2. Internal Energy • The sum of all the energy of all the molecules in an object (thermal energy) • Internal Energy of an Ideal Gas

  3. Ch15 Thermodynamics • Heat – Transfer of energy due to ΔT • Work – Transfer of energy NOT due to ΔT • Q – Heat • W – Work • W done on the system is negative (Giancoli) • W done by the system is positive (Giancoli) • ΔU – Change in energy

  4. Ch15 Thermodynamics Law of Conservation of Energy First Law of Thermodynamics AP equation sheet Heat added is + Heat lost is - Work on system is – Work by system is +

  5. The distinction between work done on the gas and work done by the gas is one that is often made on the AP Exam Ch15 Thermodynamics The area under the P-V curve will always be the work done by the gas during the process

  6. First Law of Thermodynamics (Q) 2500J of heat will increase the Internal Energy (W) 1800J of work done ON the system will … Is the work positive or negative? Why? 2500J of heat is added to a system, and 1800J of work is done on the system. What is the changed in internal energy of the system? Did the temperature increase or decrease?

  7. Ch15 Thermodynamics • Isothermal process: Constant temperature • The system is in contact with a heat reservoir • Change of phase • The work done by the gas in an isothermal process equals the heat added to the gas

  8. Isothermal process: Constant temperature, i.e. PV is constant Which Isothermal process is at a higher Temperature? Which Isothermal process does more work?

  9. Adiabatic • Adiabatic Process: No heat in or out of the system • Well insulated (like a thermos) • The process happens very quickly (firing of a car cylinder

  10. Work Given the following two processes: Isothermal and Adiabatic. Both processes start at 10Pa and end with a volume of 10m3 During which process is more work done? Estimate the work done in each process.

  11. Ch15 Thermodynamics • Isovolumetric: (Isochoric) No change in volume • Inside a ridged container

  12. Ch15 Thermodynamics • Isobaric: No change in pressure • Movable piston

  13. c 400 b 300 Pressure (Pa) 200 d 100 a 40 60 20 10 30 50 Volume (m3) Internal Energy ΔU • 1 mole of an ideal gas is brought from point a to point c by 3 different process paths. Which path has the highest change in internal energy? • 1) • 2) • 3) • 4) All the same • 5) Unknown

  14. c 400 b 300 Pressure (Pa) 200 d 100 a 40 60 20 10 30 50 Volume (m3) Work (W) • 1 mole of an ideal gas is brought from point a to point c by 3 different process paths. During which path did the gas do the most work? • 1) • 2) • 3) • 4) All the same • 5) Unknown

  15. c 400 b 300 Pressure (Pa) 200 d 100 a 40 60 20 10 30 50 Volume (m3) Heat (Q) • 1 mole of an ideal gas is brought from point a to point c by 3 different process paths. During which path was the most heat added? • 1) • 2) • 3) • 4) All the same • 5) Unknown

  16. One mole of monatomic ideal gas is enclosed under a frictionless piston. A series of processes occur, and eventually the state of the gas returns to its initial state with a P-V diagram as shown below. Answer the following in terms of P0, V0, and R. • Find the temperature at each vertex. • Find the change in internal energy for each process. • Find the work by the gas done for each process.

  17. An ideal gas is slowly compress at constant pressure (2.0 ATM) from 10.0L to 2.0L • Heat is then added to the gas holding the volume constant and the pressure and temperate are allowed to rise until the temperature reaches its original value. • Calculate the total work done by the gas • Calculate the total heat flow into the gas Pressure Pa Isothermal line 2 10 Volume m3

  18. In an engine 0.25 moles of an ideal gas in the cylinder expands rapidly and adiabatically against the piston. In this process, the temperature of the gas drops from 1150K to 400K. • What type of process is this? b) How much work does the gas do? Is the work done by the gas positive or negative?

  19. Efficiency • Efficiency (e): the ratio of work W done by the system to the input heat QH

  20. An automobile engine has an efficiency of 20% and produces an average of 23,000J of mechanical work per second. • How much input heat is required? • How much heat is discharged as wasted per second? b) a)

  21. Is the car’s efficiency higher or lower as the car warms up?

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