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NEET UG PHYSICS MCQ: THERMAL PROPERTIES OF MATTER

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A perfect black body is one whose emissive power is

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Correct option: C

Unity

Option A (Maximum) is incorrect. Option C (Unity) is correct because a perfect black body absorbs all incident radiation, meaning its absorptivity is 1. Since emissivity equals absorptivity for a body in thermal equilibrium, its emissive power is also maximum, but it's more accurate to say its emissivity is unity.

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Four identical calorimeters painted in different colours, are heated to same temperature and then allowed to cool in vacuum. Which will cool fastest? An ice cube of dimensions

60 \mathrm{~cm} \times 50 \mathrm{~cm} \times 20 \mathrm{~cm}

is placed in an insulation box of wall thickness

1 \mathrm{~cm}

. The box keeping the ice cube at

0^{\circ} \mathrm{C}

of temperature is brought to a room of temperature

40^{\circ} \mathrm{C}

. The rate of melting of ice is approximately.

(Latent heat of fusion of ice is

3.4 \times 10^5 \mathrm{~J} \mathrm{~kg}^{-1}

and thermal conducting of insulation wall is

\left. {0.05{\text{W}}{{\text{m}}^{ - 1}}^\circ {{\text{C}}^{ - 1}}} \right)

A black body radiates heat energy at the rate of

2 \times 10^5 \mathrm{~J} / \mathrm{sm}^2

127^{\circ} \mathrm{C}

. The temp of black body at which rate becomes

32 \times 10^5 \mathrm{~J} / \mathrm{s}-\mathrm{m}^2

is: The amount of radiation emitted by a perfectly black body is proportional to Amount of heat required to raise the temperature of a body through 1K is called its If Poisson’s ratio

\sigma

+ \frac{1}{2}

for a material, then the material is