Unformatted text preview: carry enough heat away—at the same
time reducing T across wood
– h may tend higher due to selfinduced airflow with large T
– also, a breeze/wind would help cool it off
Winter 2008 Lecture 4 15 – assume you can put out P = 100 W metabolic power
• 2000 kcal/day = 8,368,000 J in 86400 sec –
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– 100 W P = h·(Tskin Tout) ·A + A (Tskin4 Tout4) (hA + 4 A T3) T
with emissivity = 0.8, T = 293 K
100 = ((5)(1) + 4.56) T
T = 10.5°
so the room is about 310 10.5 = 299.5 K = 26.5° C = 80° F
iterating (using T = 299.5); 4.56
4.87; T
10.1°
assumes skin is full internal body temperature
• some conduction in skin reduces skin temperature
• so could tolerate slightly cooler Winter 2008 16 4 Thermal Considerations 01/17/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 The whole enchilada The enchilada calculation • Let’s take a cubic box with a heat source inside and
Let’
consider all heat transfers • power generated = power conducted = power
convected plus power radiated away –
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– P = 1 W internal source
inside length = 10 cm
thickness = 2.5 cm
Rvalue = 5
so 5.67 t/ = 5
= 0.028 W/m/K
effective conductive area is 12.5 cm cube
A c = 0.09375 m2
external (radiative, convective) area is 15 cm cube
A ext =
0.135 m2
– assume h = 5 W/K/m2, = 0.8, Text = 293 K
– assume the air inside is thoroughly mixed (perhaps 1 W
source is a fan!) Winter 2008 17 P = ·(Tin Tskin) ·Ac/t = ( hAext + 4 A ext T3)·( Tskin Text)
– first get Tskin from convective/radiative piece
– Tskin = Text + P/ (hAext + 4 A ext T3) = 20° + 1.0/(0.675+0.617)
– Tskin = 20.8° (...
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This note was uploaded on 01/30/2014 for the course PHYS 121 taught by Professor Staff during the Winter '08 term at UCSD.
 Winter '08
 staff
 Energy, Thermal Energy

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