Lect10 - Physics 213 Lecture 10 Pg 1 Misc Notes Misc Notes...

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Unformatted text preview: Physics 213: Lecture 10, Pg 1 Misc. Notes Misc. Notes • The end is near – don’t get behind. • All Excuses must be taken to 233 Loomis before noon, Friday, Dec. 4. • The PHYS 213 final exam times are * 8:00 – 10:00 am Wed., Dec. 16 and * 1:30 - 3:30 pm Wed., Dec. 16. The deadline for changing your final exam time is 10pm, Tuesday, Dec. 1. • Homework 6 is due Saturday , Dec. 5 at 8 am. • Course Survey = 2 bonus points (accessible at the top of HW6) • BEWARE!!! The midterm average (85%) was significantly higher than usual. Don’t become complacent!! Physics 213: Lecture 10, Pg 2 Lecture 10 Lecture 10 Heat Pumps, Heat Pumps, Refrigerators, Refrigerators, Available Work and Free Energy Available Work and Free Energy Agenda for today Agenda for today • Pumping Heat – Refrigerators • Free Energy and Available Work • Work from Hot and Cold Bricks Reference for this Lecture: Elements Ch 10 Reference for Lecture 11: Elements Ch 11 Physics 213: Lecture 10, Pg 3 Ideal Refrigerators and Heat Pumps Ideal Refrigerators and Heat Pumps • The Carnot cycle is reversible (each step is reversible) : T c T h p V Q h Q c Hot reservoir at T h Cold reservoir at T c Q h Q c W o n Fridge: • Heat is being transferred from cold to hot by action of work on the gas. Note that heat never flows directly from cold to hot; the cold stuff is being heated by the adiabatic compression. • Q c / Q h = T c / T h is still true. Coefficient of performance (efficiency): • Refrigerator: K = ------------------------------ = --------- = -------- • Heat Pump: K = ------------------------------ = --------- = -------- Heat transferred from T c Q c T c Work input Q h- Q c T h- T c Work input Q h- Q c T h- T c Heat delivered to T h Q h T h Physics 213: Lecture 10, Pg 4 Helpful Hints in Dealing with Engines and Fridges Helpful Hints in Dealing with Engines and Fridges ● Quickly sketch the process. Define Q h and Q c and W by (or W on ) as positive and show directions of flow. ● Determine which Q is given. ● Write the First Law of Thermodynamics (FLT). There are only 3 configurations of Carnot ‘engines’: Engine (Q h given) Refrigerator (Q C given) Heat Pump (Q h or Q C given) FLT: W by = Q h- Q C W on = Q h- Q C W on = Q h- Q C W by = Q h (1- T C /T h ) W on = Q C (T h /T C- 1) W on = Q h (1- T C /T h ) or Fridge eqn. Q C Q h W by T h T C Q C Q h W on T h T C Q C Q h W on T h T C Q leak =Q C Q leak =Q h Physics 213: Lecture 10, Pg 5 Example: Refrigerator There is a 70 W heat leak from a room at temperature 22 °C into an ideal refrigerator. How much electrical power is needed to keep the refrigerator at -10 °C? Hint: For the fridge, Q c must exactly compensate the heat leak. So Q c = 70 J for each second of operation. (Watt = J/s) Assume Q c /Q h = T c /T h (Carnot) Heat Leak Hot reservoir at T h Cold reservoir at T C Q h Q C W on Fridge: Food, 263K Kitchen, 295K Physics 213: Lecture 10, Pg 6 Example: Refrigerator There is a 70 W heat leak from a room at temperature 22 °C into...
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Lect10 - Physics 213 Lecture 10 Pg 1 Misc Notes Misc Notes...

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