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22 Cards in this Set

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what is required for heat flow? what are the three modes of heat transfer?

radiation, convection, conduction

which mode of transfer is dominant in: solids,fluids

solids: conduction


liquids: convection

what factors determine the conductive heat flow?

area


difference in T


thermal conductance of material

what is thermal conductivity? does it change from one material to another? for a given material does it depend on the geometry?


its the measure of a materials ability to conduct heat, the larger K is, the more efficient a substance conducts heat

What is R-value? how does it depend on conductivity? is it a material property?

it gives a measure of that materials resistance to the flow of heat, its what helps rate the insulating quality of materials, larger R value the better at insulating

what is convection?

heat flow by the movement of a fluid

does heat transfer always require a medium?

no radiation doesnt

what are thermal radiations made of?

electromagnetic waves

what is prevost theory on heat exchange through thermal radiations?

-all objects radiate energy at all temp except at absolute zero


-rate of radiation depends on temp of the radiating object alone


-objects simultaneously emit and absorb radiation


-Temperature of the object changes with time due to this exchange and will become a constant only after the object reaches thermal equilibrium with the surrounding.



what determines the quality of energy? give ex of high quality energies

?

is thermal energy a high quality energy?

?

is it possible to convert all of the energy in random motions to energy in org motion? how about the other way?

its possible to convert ordered motions to disordered, they eventually do, but they can be converted the other way around

the process of converting directed kinetic energy to random kinetic energy is irreversible. which law of thermodynamics explains why?

order-disorder

what is the first law of thermodynamics? what is the second?

order-disorder


entropy

what is entropy? can the entropy of a closed system ever decrease?

it is the measure of a disorder of a system


?

what is a heat engine? is it possible to convert a heat engine that converts all of thermal energy to mechanical work?

a heat engine is a device that converts random thermal energy into organized mechanical energy



what is the definition for the efficiency of a heat engine? is it possible to construct a perfect heat engine that is 100% efficient?

-its the ratio of the mechanical energy delivered to total energy ex


-no

what is a carnot engine? what is carnot efficiency?

-an engine with maximum efficiently allowed by the laws of physics


-CE is related to the temp difference between the source and sink

what determines the carnot efficiency of a carnot engine?

e=1- (Tc/Th)

under what conditions will the carnot efficiency approach 100%?

?

what is cogeneration?

waste heat from the power generation plants are put to use for heating in industrial or residential sectors

heat pumps and refrigerators

-transfer heat from cooler to hotter systems


-heat pumps can provide efficient heating by using small amounts of high quality energy to transfer large amounts of heat