Latent Heats of Fusion and vaporization T(°C) 120 100 60 20 Water Steam Steam -20 Water ater Ice 62.7396 815 3080 Energy added() Physics 121: Lecture 25, Pg 6
Physics 121: Lecture 25, Pg 6 Latent Heats of Fusion and Vaporization Energy added (J) T (oC) 120 100 80 60 40 20 0 -20 -40 Water Water + Ice Water + Steam Steam 62.7 396 815 3080
Energy transfer mechanisms Thermal conduction(or conduction) Energy transferred by direct contact E.g. energy enters the water through ne bottom of the pan by thermal conduction Important: home insulation, etc Rate of energy transfer through a slab of area a and thickness AX, with opposite faces at different temperatures, Tc and Tr fo=Q/At=kA(Th-TC)/4X Energy k: thermal conductivity flo Physics 121: Lecture 25, Pg 7
Physics 121: Lecture 25, Pg 7 Energy transfer mechanisms Thermal conduction (or conduction): Energy transferred by direct contact. E.g.: energy enters the water through the bottom of the pan by thermal conduction. Important: home insulation, etc. Rate of energy transfer through a slab of area A and thickness x, with opposite faces at different temperatures, Tc and Th k : thermal conductivity x Th Tc A Energy flow =Q/t = k A (Th - Tc ) / x
Thermal Conductivities J/s m oC U/s m oc J/s m oc Aluminum 238 0.0234 Asbestos025 Copper 397 Helium 0.138 Concrete 1.3 Gold 314 Hydrogen.172 Glass 0.84 Iron 95 Nitrogen.0234 1.6 Lead 34.7 Oxygen.0238Water 0.60 Silver 427 Rubber. Wood 0.10 Physics 121: Lecture 25, Pg 8
Physics 121: Lecture 25, Pg 8 Thermal Conductivities Aluminum 238 Air 0.0234 Asbestos 0.25 Copper 397 Helium 0.138 Concrete 1.3 Gold 314 Hydrogen 0.172 Glass 0.84 Iron 79.5 Nitrogen 0.0234 Ice 1.6 Lead 34.7 Oxygen 0.0238 Water 0.60 Silver 427 Rubber 0.2 Wood 0.10 J/s m 0C J/s m 0C J/s m 0C
Energy transfer mechanisms Convection Energy is transferred by flow of substance E.g.: heating a room (air convection E.g. warming of North Altantic by warm waters from the equatorial regions Natural convection from differences in density Forced convection from pump of fan p Radiation: Energy is transferred by photons E.g.: infrared lamps Stephans law fo oAe T4: Power o=5.7x10-8 W/m2 K4 t is in Kelvin and a is the surface area e is a constant called the emissivity Physics 121: Lecture 25, Pg 9
Physics 121: Lecture 25, Pg 9 Energy transfer mechanisms Convection: Energy is transferred by flow of substance E.g. : heating a room (air convection) E.g. : warming of North Altantic by warm waters from the equatorial regions Natural convection: from differences in density Forced convection: from pump of fan Radiation: Energy is transferred by photons E.g. : infrared lamps Stephan’s law s = 5.710-8 W/m2 K4 , T is in Kelvin, and A is the surface area e is a constant called the emissivity = sAe T4 : Power
Resisting Energy Transfer The Thermos bottle also called a Dewar flask is designed to minimize Vacuum energy transfer by conduction convection and radiation the standard flask is a double-walled Pyrex glass with silvered walls and Silvered the space between the walls is surfaces evacuated Hot or cold liquid Physics 121: Lecture 25, Pg 10
Physics 121: Lecture 25, Pg 10 Resisting Energy Transfer The Thermos bottle, also called a Dewar flask is designed to minimize energy transfer by conduction, convection, and radiation. The standard flask is a double-walled Pyrex glass with silvered walls and the space between the walls is evacuated. Vacuum Silvered surfaces Hot or cold liquid