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[Thermodynamics] Heat Conduction Through Two Slabs

2021-11-17 14:49 作者:AoiSTZ23  | 我要投稿

By: Tao Steven Zheng (郑涛)

【Problem】

Consider two slabs of lengths %20L_1 and L_2, and cross-sectional area A. The respective thermal conductivity constants are k_1 and k_2.  

Part 1: Based on the below diagram, the temperature on the left is hotter than the temperature on the right (T_H%20%3E%20T_C) calculate the temperature between the two slabs T_m.

Part 2: Calculate T_m if %20k_1%20%3D%20k_2%20%3D%20k%20 and L_1%20%3D%20L_2%20%3D%20L.

【Solution】

Part 1

Heat naturally flows from higher temperature to lower temperature. Based on the diagram, the heat flows at a rate from left to right.

%5Cfrac%7BdQ%7D%7Bdt%7D%20%3D%20%5Cfrac%7BA%20k_2%7D%7BL_2%7D(T_h%20-%20T_m)

%20%5Cfrac%7BdQ%7D%7Bdt%7D%20%3D%20%5Cfrac%7BA%20k_1%7D%7BL_1%7D(T_m%20-%20T_c)

Thus,

%5Cfrac%7BA%20k_2%7D%7BL_2%7D(T_h%20-%20T_m)%20%3D%20%5Cfrac%7BA%20k_1%7D%7BL_1%7D(T_m%20-%20T_c)

%20%5Cfrac%7Bk_2%7D%7BL_2%7DT_h%20-%20%5Cfrac%7Bk_2%7D%7BL_2%7DT_m%20%3D%20%5Cfrac%7Bk_1%7D%7BL_1%7DT_m%20-%20%5Cfrac%7Bk_1%7D%7BL_1%7DT_c%20

%5Cfrac%7Bk_1%7D%7BL_1%7DT_m%20%2B%20%5Cfrac%7Bk_2%7D%7BL_2%7DT_m%20%3D%20%5Cfrac%7Bk_2%7D%7BL_2%7DT_h%20%2B%20%5Cfrac%7Bk_1%7D%7BL_1%7DT_c%20


%5Cleft(%5Cfrac%7Bk_1%7D%7BL_1%7D%20%2B%20%5Cfrac%7Bk_2%7D%7BL_2%7D%5Cright)T_m%20%3D%20%5Cfrac%7Bk_2%7D%7BL_2%7DT_h%20%2B%20%5Cfrac%7Bk_1%7D%7BL_1%7DT_c

Consequently,

%20T_m%20%3D%20%5Cfrac%7B%5Cfrac%7Bk_2%7D%7BL_2%7DT_h%20%2B%20%5Cfrac%7Bk_1%7D%7BL_1%7DT_c%7D%7B%5Cfrac%7Bk_1%7D%7BL_1%7D%20%2B%20%5Cfrac%7Bk_2%7D%7BL_2%7D%7D%20

Part 2

If k_1%20%3D%20k_2%20%3D%20k%20 and L_1%20%3D%20L_2%20%3D%20L,

T_m%20%3D%20%5Cfrac%7B%5Cfrac%7Bk_2%7D%7BL_2%7DT_h%20%2B%20%5Cfrac%7Bk_1%7D%7BL_1%7DT_c%7D%7B%5Cfrac%7Bk_1%7D%7BL_1%7D%20%2B%20%5Cfrac%7Bk_2%7D%7BL_2%7D%7D%20

becomes

T_m%20%3D%20%5Cfrac%7B%5Cfrac%7Bk%7D%7BL%7DT_h%20%2B%20%5Cfrac%7Bk%7D%7BL%7DT_c%7D%7B%5Cfrac%7Bk%7D%7BL%7D%20%2B%20%5Cfrac%7Bk%7D%7BL%7D%7D

Factoring out %5Cfrac%7Bk%7D%7BL%7D%20 yields

T_m%20%3D%20%5Cfrac%7B%5Cfrac%7Bk%7D%7BL%7D(T_h%20%2B%20T_c)%7D%7B%5Cfrac%7Bk%7D%7BL%7D(1%2B1)%7D

T_m%20%3D%20%5Cfrac%7BT_h%20%2B%20T_c%7D%7B2%7D




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