Tag: heat and thermodynamics

Questions Related to heat and thermodynamics

Two black metallic spheres of radius 4m, at 2000 K and 1m at 4000 K will have ratio of energy radiation as

laws of heat transfer heat and thermodynamics physics

  1. 1 : 1

  2. 4 : 1

  3. 1 : 4

  4. 2 : 1

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Correct Option: A

Two sphere of same material and of same emissivity have radii 1 m and 4 m and temperature 4000 K and 1000 K, respectively. The ratio of radiation emitted per sec is  

laws of heat transfer heat and thermodynamics physics

  1. 4:1

  2. 1:4

  3. 1:1

  4. 16:1

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Correct Option: C

We find that the temperature of air decreases as one goes up from the earth's surface because

laws of heat transfer heat and thermodynamics physics

  1. The atmospheric pressure drops with height

  2. The earth which radiates in the infrared region is the main heat source and temperature drops as we go away from it

  3. The density of air drops with height and the air therefore cannot hold stronger as we go up

  4. Winds are stronger as we go up

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Correct Option: B
Explanation:
The behaviour of atmosphere is different for visible and infrared radiations. Most of the infrared radiations are not allowed to pass through atmosphere, since it reflects them. The energy from the sun, heats the earth which in turn starts emitting radiations. Since, the earth gets heated to much lower temperature than the temperature of the sun, according to Planck's law, the radiations emitted by the earth are mostly in the infrared region. These radiations from the earth are not allowed to pass through the atmosphere which reflects them back. As a result of it the earth's atmosphere becomes richer in infrared radiations which are sometimes called heat radiations. So, earth radiates in the infrared region, this is the source of heat. As we go up, it decreases.

Choose the correct answer from the alternatives given.
Radiations of intensity $0.5\ W/m^2$ are striking a metal plate. The pressure on the plate is then

laws of heat transfer heat and thermodynamics physics

  1. $0.166 \, \times \, 10^{-8} \, N \, m^{-2}$

  2. $0.332 \, \times \, 10^{-8} \, N \, m^{-2}$

  3. $0.111 \, \times \, 10^{-8} \, N \, m^{-2}$

  4. $0.083 \, \times \, 10^{-8} \, N \, m^{-2}$

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Correct Option: A
Explanation:

Given: The intensity of the incident radiations is $0.5\ W/m^2$.


To find: The radiation pressure on the plate.

The radiation pressure experienced by the plate is given as;
$P = \dfrac{I}{c}\\Rightarrow \dfrac{0.5}{3 \times 10^8}\\Rightarrow  0.166 \times 10^{-8} N m^{-2}$

Option $(A)$ is correct.

A pan filled with hot food cools from $94^oC$ to $86^oC$ in$2$ minutes when the room temperature is at $20^oC$. The time taken to cool it from $71^oC$ to $69^oC$ is 

laws of heat transfer heat and thermodynamics physics

  1. $12\,s$

  2. $22\,s$

  3. $32\,s$

  4. $42\,s$

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Correct Option: D
Explanation:

For approximate calculation of the time taken,

$\cfrac { { T } _{ i }-{ T } _{ f } }{ \Delta t } =k\left[ \cfrac { { T } _{ i }+{ T } _{ f } }{ 2 } -{ T } _{ o } \right] $
where,
${ T } _{ o }\longrightarrow $room temperature
$T _{i} \longrightarrow$initial temperature
$T _{f} \longrightarrow$final temperature
$\Delta t \longrightarrow$time taken
$k \longrightarrow$constant
$\Longrightarrow \cfrac { 94-86 }{ 2 } =k\left[ \cfrac { 94+86 }{ 2 } -20 \right] \ \Longrightarrow 4=k[90-20]=k[70]\ \therefore k=\cfrac { 4 }{ 70 } \ \Longrightarrow \cfrac { 71-69 }{ \Delta t } =\cfrac { 4 }{ 70 } \left[ \cfrac { 71+69 }{ 2 } -20 \right] \ \Longrightarrow \cfrac { 2 }{ \Delta t } =\cfrac { 4 }{ 70 } \left[ 70-20 \right] \ \Longrightarrow \cfrac { 2 }{ \Delta t } =\cfrac { 4 }{ 70 } \times 50\ \therefore \Delta t=\cfrac { 70 }{ 2\times 50 } =0.7min=42sec$

Five kilomoles of oxygen is heated at constant pressure. The temperature of the oxygen gas is increased from 295 K to 305 K. If the molar heat capacity of oxygen at  constant pressure is 6.994 kcal/kmole K. The amount of heat absorbed is in kcal,

laws of heat transfer heat and thermodynamics physics

  1. 249.7

  2. 44

  3. 349.7

  4. 539.7

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Correct Option: C

Water is used to cool radiators of engines, because:

laws of heat transfer heat and thermodynamics physics

  1. Of its lower density

  2. It is easily available

  3. It is cheap

  4. It has high specific heat

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Correct Option: D

Star A emits radiation of maximum intensity at a wavelength of $5000 \mathring{A}$ and it has temperature $ 1227^oC $. If star B has temperature $ 2727^oC $ , then the maximum intensity would be observed at 

laws of heat transfer heat and thermodynamics physics

  1. $ 4000 \mathring{A} $

  2. $ 2250 \mathring{A} $

  3. $ 3000 \mathring{A} $

  4. $ 2500 \mathring{A} $

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Correct Option: D
Explanation:

$\begin{array}{l} \dfrac { { \lambda { m^{ 1 } } } }{ { \lambda m } } =\dfrac { T }{ { { T^{ 1 } } } }  \ \dfrac { { \lambda { m^{ 1 } } } }{ { 5000A } } =\dfrac { { 1227+273k } }{ { 2727+273k } }  \ \lambda { m^{ 1 } }=2500A \end{array}$

An ideal gas has initial volume V and pressure P. In doubling its volume the minimum work done will be in the following process(of given processes)

physics isothermal and adiabatic processes work done by an ideal gas in isothermal expansion thermodynamic processes heat and thermodynamics

  1. Isobaric process

  2. Isothermal process

  3. Adiabatic process

  4. None of the above.

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Correct Option: C

Two difference gases of molecular masses $M _1$ and $M _2$ are at the same temperature. What is the ratio of their mean square speeds?

physics isothermal and adiabatic processes work done by an ideal gas in isothermal expansion thermodynamic processes heat and thermodynamics

  1. $\dfrac{M _1}{M _2}$

  2. $\dfrac{M _2}{M _1}$

  3. $\sqrt {\dfrac{M _1}{M _2}}$

  4. $\sqrt {\dfrac{M _2}{M _1}}$

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Correct Option: A
Explanation:

Mean squared speed $=\cfrac{3RT}{M}$

$\cfrac{V _1}{V _2}=\cfrac{M _1}{M _2}$