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lecture_12_4_20_2011

lecture_12_4_20_2011 - Transfer of thermal energy by heat...

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Transfer of thermal energy by heat conduction and convection Lecture 12
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Three ways to transfer Heat Occurs in both solids and fluids Occurs only Energy Energy transfer thru matter in fluids Can take place in a vacuum transfer due to flow of matter Energy transfer by EM radiation
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The Heat Flow Problem
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Fourier’s Law (1822) uniform temperature gradient k = thermal conductivity W/(m•K) hot ( 29 J cal W or s s cold end end Q T kA t x = - Why negative sign?
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Good thermal insulators
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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/(m•K). The temperature difference between the inner and outer surface of the glass is 0.5 o C. o Q T kA t x = = I T in = 20 C T out = -10 o C | I | = 0.8 W/(m•K) 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
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T T I T h T c I =constant= I o L Steady State Heat Flow in 1-dimension means I o
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