HARD

Earn 100

Define emissive power, coefficient of emission and absorptive power.

Important Questions on Thermal Properties of Matter

EASY

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

On observing light from three different stars

P

Q

and

R

, it was found that intensity of violet colour is maximum in the spectrum of

P

, the intensity of green colour is maximum in the spectrum of

R

and the intensity of red colour is maximum in the spectrum of

Q

. If

T_{p}, T_{Q}

and

T_{R}

are the respective absolute temperatures of

P

Q

and

R

, then it can be concluded from the above observations that :

EASY

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

A heat source at

T = 10^{3} K

is connected to another heat reservoir at

T = 10^{2} K

by a copper slab which is

1 m

thick. Given that the thermal conductivity of copper is

0 .1 W K^{- 1} m^{- 1}

, the energy flux through it in the steady-state is:

HARD

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

A thin piece of thermal conductor of constant thermal conductivity insulated on the lateral sides connects two reservoirs which are maintained at temperatures, $T_{1}$ and, $T_{2}$ as shown. Assuming that the system is in steady-state, which of the following plots best represents the dependence of the rate of change of entropy on the ratio of temperatures, $\frac{T_{1}}{T_{2}}$

Question Image

HARD

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Two rectangular blocks, having identical dimensions, can be arranged either in configuration

I

or in configuration

I I

as shown in the figure. One of the blocks has thermal conductivity

K

and the other

2 K

. The temperature difference between the ends along the x-axis is the same in both the configurations. It takes

9 s

to transport a certain amount of heat from the hot end to cold end in the configuration

I

. The time to transport the same amount of heat in the configuration

II

EASY

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Let there be four identical cubes having colours blue, red, black and white. When they are heated together and allowed to cool under identical conditions, which cube will cool at the earliest?

EASY

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Two spherical black bodies have radii

r_{1}

and

r_{2}

. Their surface temperatures are

T_{1}

and

T_{2}

. If they radiate same power then

\frac{r_{2}}{r_{1}}

MEDIUM

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Two conducting cylinders of equal length but different radii are connected in series between two heat baths kept at temperatures

T_{1} = 300 K a n d T_{2} = 100 K

as shown in the figure. The radius of the bigger cylinder is twice that of the smaller one and the thermal conductivities of the materials of the smaller and the larger cylinders are

K_{1} a n d K_{2}

, respectively. If the temperature at the junction of the two cylinders in the steady state is

200 K

, then

K_{1} / K_{2}

HARD

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Three rods of Copper, Brass and Steel are welded together to form a Y-shaped structure. Area of cross-section of each rod is

4 {cm}^{2}

. End of copper rod is maintained at

100 ° C

. Where as ends of brass and steel are kept at

0 ° C

. Lengths of the copper, brass and steel rods are

46, 13

and

12 cms

respectively. The rods are thermally insulated from surroundings except at ends. Thermal conductivities of copper, brass and steel are

0.92, 0.26

and

0.12

CGS units respectively. Rate of heat flow through copper rod is :

MEDIUM

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Heat energy is incident on the surface of a material at the rate of

1000 J \min^{- 1}

. If the coefficient of absorption is

0.8

and the coefficient of reflection is

0.1

, then the amount of heat energy transmitted through the material in

5 minutes

MEDIUM

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

A black body is at a temperature of

5760 K.

The energy of radiation emitted by the body at wavelength

250 nm

U_{1}

, at wavelength

500 nm

U_{2}

and that at

1000 nm

U_{3}

. Wien's constant,

b = 2.88 \times 10^{6} nm K.

Which of the following is correct?

MEDIUM

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

A metallic prong consists of $4$ rods made of the same material, cross-section, and same lengths as shown.

Question Image

The three forked ends are kept at $100^{o} C$ and the handle end is at $0^{o} C .$ The temperature of the junction is

HARD

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Three large identical plates are kept parallel to each other. The outer two plates are maintained at temperatures

T

and

2 T

, respectively. The temperature of the middle plate in steady state will be close to

MEDIUM

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

A spherical black body with a radius of

12 cm

radiates

450 W

power at

500 K

. If the radius were halved and the temperature doubled, the power radiated in watt would be

EASY

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

The two ends of a metal rod are maintained at temperatures

100 ° C

and

110 ° C

. The rate of heat flow in the rod is found to be

4.0 J s^{- 1}

. If the ends are maintained at temperatures

200 ° C

and

210 ° C

, the rate of heat flow will be:

EASY

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Two rods A and B of different materials are welded together as shown in figure. Their thermal conductivities are $K_{1}$ and $K_{2}$ . The thermal conductivity of the composite rod will be

Question Image

HARD

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

Parallel rays of light of intensity

I = 912 W m^{- 2}

are incident on a spherical black body kept in surroundings of temperature 300 K. Take Stefan-Boltzmann constant

σ = 5.7 \times 10^{-8} W m^{- 2} K^{- 4}

and assume that the energy exchange with the surroundings is only through radiation. The final steady state temperature of the black body is close to

HARD

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

A black coloured solid sphere of radius $R$ and mass $M$ is inside a cavity with a vacuum inside. The walls of the cavity are maintained at temperature $T_{0}$ . The initial temperature of the sphere is $3 T_{0}$ . If the specific heat of the material of the sphere varies as $α T^{3}$ per unit mass with the temperature $T$ of the sphere, where $α$ is a constant, then the time taken for the sphere to cool down to temperature $2 T_{0}$ will be

( $σ$ is Stefan Boltzmann constant)

MEDIUM

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

A cylinder of radius

R

is surrounded by a cylindrical shell of inner radius

R

and outer radius

2 R

. The thermal conductivity of the material of the inner cylinder is

K_{1}

and that of the outer cylinder is

K_{2}

. Assuming no loss of heat, the effective thermal conductivity of the system for heat flowing along the length of the cylinder is:

EASY

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

(A)	The energy emitted versus wavelength plot of a black body depends on the material of the black body.
(B)	The emissivity of a completely reflecting surface is zero.
(C)	Emissivity is equal to ${(absorptive power)}^{2}$ .
(D)	Black-body radiates more than normal body Which of the following is correct?

EASY

Physics>Heat and Thermodynamics>Thermal Properties of Matter>Radiation

For any material, if

R, T

and

A

represent the reflection coefficient, transparent coefficient and absorption coefficient respectively, then for a blackbody which one of the following is true

Define emissive power, coefficient of emission and absorptive power.

Important Questions on Thermal Properties of Matter

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