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10-2.Transmission of Heat
medium
The graph. Shown in the adjacent diagram, represents the variation of temperature $(T)$ of two bodies, $x$ and $y$ having same surface area, with time $(t)$ due to the emission of radiation. Find the correct relation between the emissivity
A
${e_x} > {e_y}$ and ${a_x} < {a_y}$
B
${e_x} > {e_y}$ and ${a_x} < {a_y}$
C
${e_x} < {e_y}$ and ${a_x} > {a_y}$
D
${e_x} > {e_y}$ and ${a_x} > {a_y}$
(IIT-2003)
Solution
(c) Rate of cooling $\left( { – \frac{{dT}}{{dt}}} \right)\; \propto $ emissivity $(e)$
From graph, ${\left( { – \frac{{dT}}{{dt}}} \right)_x} > {\left( { – \frac{{dT}}{{dt}}} \right)_y}$
==> ${e_x} > {e_y}$
Further emissivity $(e)$ $\propto$ Absorptive power $(a)$
==> ${a_x} > {a_y}$
( good absorbers are good emitters).
Standard 11
Physics
