JEE MAIN - Physics (2019 - 12th January Morning Slot - No. 19)
A cylinder of radius R is surrounded by a cylindrical shell of inner radius R and outer radius 2R. The thermal conductivity of the material of the inner cylinder is K1 and the of the outer cylinder is K2. Assuming no loss of heat, the effective thermal conductivity of the system for heat flowing along the length of the cylinder is :
K1 + K2
$${{{K_1} + 3{K_2}} \over 4}$$
$${{{K_1} + {K_2}} \over 2}$$
$${{2{K_1} + 3{K_2}} \over 5}$$
คำอธิบาย
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Keq = $${{{K_1}{A_1} + {K_2}{A_2}} \over {{A_1} + {A_2}}}$$
= $${{{K_1}\left( {\pi {R^2}} \right) + {K_2}\left( {3\pi {R^2}} \right)} \over {4\pi {R^2}}}$$
= $${{{K_1} + 3{K_2}} \over 4}$$