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Q.

The rectangular surface of  area  8cm × 4cm of a black body at a temperature of 1270C emits energy at rate of E per second. If the length and breadth are reduced to half of its initial value and the temperature is raised to 3270C, the rate of emission of energy will be

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a

large left ( frac {3E}{8} right )

b

large left ( frac {9E}{16} right )

c

large left ( frac {81E}{16} right )

d

large left ( frac {81E}{64} right )

answer is D.

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Detailed Solution

l1 = 8cm, b1 = 4cm; l2 = cm, b2 = 2cm
T1 = 1270C = 400k, T2 = 3270C = 600k
large frac {E_2}{E_1}=left ( frac {T_2}{T_1} right )^4left ( frac {l_2b_2}{l_1b_1} right )
large frac {E_2}{E}=left ( frac {600}{400} right )^4left ( frac {4times 2}{8times 4} right )
large E_2=frac {81}{64}E
 

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