Lect12 - PHYS 172: Modern Mechanics Fall 2009 Lecture 12...

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Fall 2009 PHYS 172: Modern Mechanics Lecture 12 – Energy Accounting Read 6.5 – 6.9
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CLICKER QUESTION #1 Reading Question (Sections 6.5 – 6.9) (This is a closed-book quiz, no consulting with neighbors, etc.) Consider a system containing only an object that weighs 50 N. It is in free fall, and after about 1 second has a speed of 10 m/s. Choose the correct statement: A. Earth does no work on this object. B. The energy of the system is conserved. C. At t=1 sec, gravity is adding energy to the system at a rate of 500 Watts (= F · v ). D. More than one of the above. E. None of the above.
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THIS WEEK: Same as usual NEXT WEEK: No labs No lectures There is still recitation Exam #2 is Wednesday, October 14 in Elliot Hall at 8 pm Same format as before (all about Energy!) Bring pencils, ID, calculator (graphing is ok) Alternative testing in room 8 of Physics building at same time
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Today’s Lecture • Review of thermal energy; Power • Open and Closed Systems • Energy Accounting: Choice of System • Example: Galloping Horse
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Review of thermal energy: Macroscopic Systems & Thermal Energy Even if a block of metal is at rest, the individual atoms have K (they’re jiggling) and the bonds have U spring . jiggling spring atoms bonds thermal energy K U ! " ! " + = # $ # $ % & % &
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Heat (Q): Thermal Energy Transfer Two objects in contact: thermal energy always flows from high-T to low-t. This is referred to as “ energy transfer by heating ”. Convention for Sign of Q system gets thermal energy from surroundings system gives thermal energy to surroundings
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sys E W Q ! = + “Q” is the energy transferred due to a temperature difference between the system and the surroundings . “Thermal energy” is the energy of a system associated with its temperature (atomic K & U). _____________________________________ An increase in temperature always means an increase in thermal energy. “Q” can flow into a system without changing its temperature. (Example?) Macroscopic work “W” can increase the temperature of a system. (Example?)
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Specific Heat Capacity thermal E T ! " !
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This note was uploaded on 10/26/2009 for the course CHM 111 taught by Professor Staff during the Spring '08 term at Purdue University-West Lafayette.

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Lect12 - PHYS 172: Modern Mechanics Fall 2009 Lecture 12...

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