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MHT CET · Physics · Thermal Properties of Matter

Which of the following statements is true/correct?

  1. A During clear nights, the temperature rises steadily upward near the ground level
  2. B Newton's law of cooling, an approximate form of Stefan's law, is valid only for natural convection
  3. C The total energy emitted by a black body per unit time per unit area is proportional to the square of its temperature in the Kelvin scale
  4. D Two spheres of the same material have radii \(1 \mathrm{~m}\) and \(4 \mathrm{~m}\) and temperatures \(4000 \mathrm{~K}\) and \(2000 \mathrm{~K}\) respectively. The energy radiated per second by the first sphere is greater than that radiated per second by the second sphere
Verified Solution

Answer & Solution

Correct Answer

(B) Newton's law of cooling, an approximate form of Stefan's law, is valid only for natural convection

Step-by-step Solution

Detailed explanation

During clear nights object on surface of earth radiate out heat and temperature falls. Hence option (a) is wrong. The total energy radiated by a body per unit time per unit area \(E \propto T^{4}\). Hence option (c) is
wrong. Energy radiated per second is given by
\(\frac{Q}{t} =P A \varepsilon \sigma T^{4} \)
\( \Rightarrow \frac{P_{1}}{P_{2}} =\frac{A_{1}}{A_{2}}\left(\frac{T_{1}}{T_{2}}\right)^{4}=\left(\frac{\pi}{r_{2}}\right)^{2} \cdot\left(\frac{T_{1}}{T_{2}}\right)^{4} \)
\( =\left(\frac{1}{4}\right)^{2}\left(\frac{4000}{200}\right)=\frac{1}{1}\)
\(\because P_{1}=P_{2}\), hence option (d) is wrong. Newton's law is an approximate from of Stefan's law of radiation and works well for natural convection. Hence option (b) is correct.