Question

The graph, shown in the diagram, represents the variation of temperature (T) of the bodies, x and y having same surface area, with time (t) due to the emission of radiation. Find the correct relation between the emissivity and absorptivity power of the two bodies

• 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$

Answer 1

Answer: (C)

Here from the graph we see that both $x , y$ are at same temperature but they are varying w.r.t time, so $\left(\frac{ dT }{ dt }\right)_{ x }>\left(\frac{ dT }{ dt }\right)_{ y }$
Now from the Kirchoff's law, we know that emissivity is equal to absorptivity, so the body which emits more will also absorb more.
Therefore, $E _{ x }> E _{ y }$ and $a _{ x }> a _{ y }$