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Lecture6%20Thermal%20Energy%20and%20Heat%20Conduction-slides

Lecture6%20Thermal%20Energy%20and%20Heat%20Conduction-slides...

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Today First law of thermodynamics Heat transfers Home heating
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Deep Questions ... “If energy can never be destroyed or created where did it all come from?” maybe consult: ' A Brief History of Time ' and ' The Grand Design ' by Stephen Hawking (Cosmologist)
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Energy and First Law of Thermodynamics E th = W + Q Change in thermal energy of a closed system (energy but no mass transfer) is due to either A. Work done on the system (pos. work) B. Work done by the system (neg. work) C. Heat transferred to the system (pos. heat) D. Heat removed from the system (neg. heat) Example: If the food in a food processor is our system under consideration, which of the four processes above occurs? Will the temperature of the food A) increase B) decrease or C) stay the same?
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Energy Balance We have learned that the body is an open system: Chemical energy in; thermal energy and mechanical energy out. [Aside: How do we measure the energy content of food?] If you think about energy transfers, what are the similarities between rooms or houses and your body?
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Heat Transfer Humans, animals and houses have essentially the same energy balance: Internal source for generating thermal energy Extra heat from sun/environment Insulation for slowing down heat conduction: Fur, fat, sweaters, and fibreglass. Heat transfer mechanisms (Conduction, Convection, Radiation) are the same. We neglect evaporation (sweating) and discuss convection only qualitatively.
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Question If you want to keep your room at a constant temperature, what can you say about the amount of heat into the room and out of the room? A. Heat in = Heat out B. Heat in > Heat out C. Heat in < Heat out
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House Heating Constant temperature inside: Energy balance: E in = E out .
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