lecture_12_4_20_2011

lecture_12_4_20_2011 - Transfer of thermal energy by heat...

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Unformatted text preview: Transfer of thermal energy by heat conduction and convection Lecture 12 Three ways to transfer Heat Occurs in both solids and fluids Energy Energy transfer thru matter Occurs only in fluids Can take place in a vacuum transfer due to flow of matter Energy transfer by EM radiation The Heat Flow Problem Fouriers Law (1822) uniform temperature gradient k = thermal conductivity W/(mK) ( 29 J cal W or s s cold end hot end Q T kA t x = - Why negative sign? Good thermal insulators How much energy is lost through a glass picture window (1.5 m x 1.0 m x 0.005 m) on a cold winter day? The thermal conductivity of glass is 0.8 W/(mK). The temperature difference between the inner and outer surface of the glass is 0.5 o C. Q T kA t x = = I T in = 20 o C T out = -10 o C | I | = 0.8 W/(mK) x 1.5 m x 1.0 m x 0.5 K /0.005 m = 120 W In one day, 120 J/s x 3600s/hr x 24 h/day = 1.04 x 10 7 J T in = 20 o C T out = -10 o C 19.5 o C xx T T...
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lecture_12_4_20_2011 - Transfer of thermal energy by heat...

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