Answer
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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
Complete answer:
Emissivity, $\varepsilon = 1 - 0.6$ $\therefore $because it is an opaque body.
$\varepsilon = 0.4$
We have provided $P = $$20watts$, $t = 2$minutes and $\varepsilon = 0.4$
Substituting all these value in below formula we get energy:
$ E = \varepsilon Pt \\
E = 0.4 \times 20 \times 2 \times 60 \\
E = 960J \\
$(convert minutes into seconds by multiplying it by $60$)
The radiant heat energy is option(B) $960J$
Note: Points to be kept in mind while solving these questions:
Should have proper knowledge of emissivity because emissivity can be confusing sometimes.
Should learn all the important formulas as this topic contains lots of formulas which can be messy.
Formula used:
$E = \varepsilon Pt$where $\varepsilon = $emissivity, $P = $Power, $t = $time taken
Complete answer:
Emissivity, $\varepsilon = 1 - 0.6$ $\therefore $because it is an opaque body.
$\varepsilon = 0.4$
We have provided $P = $$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.
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