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Question

A solid copper sphere of density ρ, specific heat c and radius r is at temperature T1. It is suspended inside a chamber whose walls are at temperature 0 K. The time required for the temperature of sphere to drop to T2 is rρcxeσ(1T321T31). Find the value of x ?
Take the emmissivity of the sphere to be equal to e.

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Solution

Rate of heat loss dQdt=eAσT4
mc(dTdt)=eAσT4
Temperature decreases with time
m=43πr3ρ,A=4πr2

dTdt=3eσρcrT4t0 dt=rρc3eσT2T1dTT4t=+rρc9eσ[1T321T31] x=9

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