The rate of emission of heat energy of an iron ball of radius 5 cm is 10 J/s, then rate of emission of heat energy by a copperr ball of radius 1 cm at same temperature will be (If emissivity of both the balls is same)

Options

A. 2 J/s
B. 0.4 J/s
C. 2 cal/s
D. 250 J

Answer

B

Explanation

`(R_(2))/(R_(1))=(A_(2))/(A_(1))((T_(2))/(T_(1)))^(4)` as `T_(1)=T_(2)`
`(R_(2))/(R_(1))=(1)/(25)`
`therefore R_(2)=(10)/(25)=0.4J//s`