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Question

# A cylindrical rod has a hot end at 70∘C and cold end at 30∘C. The rate of heat flow is 9 W. If the dimensions of the cylindrical rod are tripled keeping the temperature difference constant, then find the value of new rate of heat flow.

A
9 W
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B
81 W
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C
18 W
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D
27 W
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Solution

## The correct option is D 27 WLet the length of the cylindrical rod be l1, Area of cross-section of cylindrical rod be A1, Temperature difference between the ends of rod: (T1−T2)=40∘C Given, Rate of heat flow: (Q1)=9 W If the dimensions tripled, then: l2=3l1;A2=πr22=π(3r1)2=9(πr21)=9A1 ∴ Applying the equation of thermal conductivity: Q1Q2=(A1A2)×(l2l1)=19×3=13 ⇒Q2=3Q1=3×9=27 W

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