JEE MAIN - Physics (2021 - 17th March Morning Shift - No. 5)
Two identical metal wires of thermal conductivities K1 and K2 respectively are connected in series. The effective thermal conductivity of the combination is :
$${{2{K_1}{K_2}} \over {{K_1} + {K_2}}}$$
$${{{K_1} + {K_2}} \over {{K_1}{K_2}}}$$
$${{{K_1} + {K_2}} \over {2{K_1}{K_2}}}$$
$${{{K_1}{K_2}} \over {{K_1} + {K_2}}}$$
Explanation
_17th_March_Morning_Shift_en_5_1.png)
$${R_{eq}} = {R_1} + {R_2}$$
$$ \Rightarrow $$ $${1 \over {{K_{eq}}}}{{2l} \over A} = {l \over {{K_1}A}} + {l \over {{K_2}A}}$$
$$ \Rightarrow $$ $${2 \over {{K_{eq}}}} = {l \over {{K_1}}} + {l \over {{K_2}}}$$
$$ \Rightarrow $$ $${2 \over {{K_{eq}}}} = {{{K_1} + {K_2}} \over {{K_1}{K_2}}}$$
$$ \Rightarrow $$ $${K_{eq}} = {{2{K_1}{K_2}} \over {{K_1} + {K_2}}}$$
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