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More Last updated date: 29th Nov 2023
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# Coefficient of emission of an opaque body is $0.6$. Radiant heat energy is incident on it at the rate of$20watts$. Determine the quantity of radiant heat reflected by it in $2$minutes​.A. $480J$B. $960J$C. $1920J$D. $1500J$ Verified
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Hint: In order to solve this question, we need to find the emissivity of the body given. Emissivity is the measure of degree to be able to radiate energy by a body. Use this concept to find the answer to this question.

Formula used:
$E = \varepsilon Pt$where $\varepsilon =$emissivity, $P =$Power, $t =$time taken

Emissivity, $\varepsilon = 1 - 0.6$ $\therefore$because it is an opaque body.
$\varepsilon = 0.4$
We have provided $P =$$20watts$, $t = 2$minutes and $\varepsilon = 0.4$
Substituting all these value in below formula we get energy:
$E = \varepsilon Pt \\ E = 0.4 \times 20 \times 2 \times 60 \\ E = 960J \\$(convert minutes into seconds by multiplying it by $60$)

The radiant heat energy is option(B) $960J$

Note: Points to be kept in mind while solving these questions:
Should have proper knowledge of emissivity because emissivity can be confusing sometimes.
Should learn all the important formulas as this topic contains lots of formulas which can be messy.