Physics

Thermal Properties and Thermodynamics

431 Questions

Thermal properties and thermodynamics questions evaluate concepts of heat transfer, thermal efficiency, and temperature variations. Problems involve calculating heat content, conductivity, and the performance of heat engines. This subject is regularly tested in physics sections across multiple competitive platforms.

Heat transfer calculationsThermal efficiencyBlack body radiationTemperature variationsRefrigeration performance

Thermal Properties and Thermodynamics Questions

Multiple choice physics transfer of heat applications of heat conduction applications of insulation clothes - our necessity

Which of the following circular rods (given radius r and length l), each made of the same material and whose ends are maintained at the same temperature will conduct most heat?

  1. $r=r _0;l=l _0$
  2. $r=2r _0;l=l _0$
  3. $r=r _0;l=2l _0$
  4. $r=2r _0;l=2l _0$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Heat is given by $H=KA\Delta \theta _x=K\pi r^2 \dfrac{\Delta \theta} {l}$

For option B the value of $H$ will be more.

Multiple choice physics heat and energy zeroth law of thermodynamics

Two systems are in thermal equilibrium. The quantity which is common for them is

  1. heat

  2. momentum

  3. specific heat

  4. temperature
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Thermal equilibrium implies that there is no heat transfer taking place between the given two bodies i.e. there temperature is same.

Multiple choice physics heat and energy zeroth law of thermodynamics

Two metal spheres of different radii having same temperature are placed in thermal contact in a vaccum. Which of the following quantities is certainly correct?

  1. Each sphere has the same internal energy

  2. There is no net transfer of thermal energy between the spheres

  3. Both spheres radiate electromagnetic energy at the same rate

  4. The larger sphere a greater mean internal energy per atom than the smaller sphere
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

There is no net transfer of thermal energy between the spheres.

Option B is correct.

Multiple choice physics heat and energy zeroth law of thermodynamics

Three bodies $A, B$ and $C$ are in thermal equilibrium.
The temperature of $B$ is $45^{\circ}C$. Then the temperature of $'C'$ is _____

  1. $45^{\circ}C$
  2. $50^{\circ}C$
  3. $40^{\circ}C$

  4. Any temperature
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A, B and C are in thermal equilibrium and B is at temperature $45^0 C$, then according to zeroth law of thermodynamics both A and C will also be at temperature $45^0 C$.

Multiple choice physics nuclear physics hazards and safety measures of radiations harmful effects and safety precautions for radiations energy production

The effective area of a black body is 0.1 $m^2$ and its temperature is 1000 K. The amount of radiations emitted by it per min is -

  1. 1.34 k-cal

  2. 81 k-cal

  3. 5.63 k-cal

  4. 1.34 k-J.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\begin{array}{l} \dfrac { { d\emptyset  } }{ { dt } } =A6T{ Y^{ 4 } } \ =81\, \, kcal \end{array}$

Multiple choice modelling gases - the kinetic model ideal gases kinetic theory of gases physics

A body at a temperature of ${ 727 }^{ \circ  }C$ and having surface area ${ 5cm }^{ 2 }$ , radiated 300J of energy each minute. The emissivity (Given: boltzmann constant=$ 5.67\times { 10 }^{ -8 }{ Wm }^{ -2 }{ K }^{ -4 }$ is

  1. e=0.2

  2. e=0.02

  3. e=0.18

  4. e=0.15
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

According to the Stefan-Boltzmann law, radiated energy per unit time is E = e * sigma * A * T^4. Substituting the given values (T = 727 + 273 = 1000 K, A = 5 cm^2 = 5 * 10^-4 m^2, E = 300 J / 60 s = 5 W), solving for emissivity e yields 0.18.

Multiple choice energy efficiency energy transformations and energy transfers physics

A carnot engine is designed to operate between $480 \, K$ and $300 \, K$. If the engine actually produce $1.2 \, J$ of mechanical energy per cal. of heat absorbed, then the ratio of actual efficiency to theoretical efficiency is

  1. $3/4$
  2. $4/3$
  3. $1/3$
  4. $3/1$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Theoretical efficiency = 1 - (T_low / T_high) = 1 - (300 / 480) = 1 - 0.625 = 0.375. Actual efficiency = 1.2 J / 4.18 J (1 cal) approx 0.287. Ratio = 0.287 / 0.375 approx 0.765, which is 3/4.

Multiple choice laws of heat transfer heat and thermodynamics physics

The temperature of the black body that radiates heat at a rate of $459.27 \times 10^4 W m^{-2}$ is:

  1. 1000K

  2. 2000K

  3. 3000K

  4. 4000K
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

According to Stefan-Boltzmann law, the total energy radiated per unit surface area per unit time is E = sigma * T^4. Given E = 459.27 * 10^4 W/m^2 and taking Stefan's constant sigma approx 5.67 * 10^-8 W/m^2/K^4, solving for T gives T = (459.27 * 10^4 / 5.67 * 10^-8)^(1/4) = (8.1 * 10^12)^(1/4) = 3000 K.

Multiple choice laws of heat transfer heat and thermodynamics physics

Pervost's theory of heat exchange is not applicable at temperature

  1. $0^oR$
  2. $0^oC$
  3. $0 K$
  4. $0^oF$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Prevost postulated that  radiative equilibrium is the condition where a steady state system is in dynamic equilibrium, with equal incoming and outgoing radiative flux and negligible heat transfer by conduction and convection.
So by extended logic, at $0K$ equilibrium cannot be reached.

Multiple choice laws of heat transfer heat and thermodynamics physics

Two spheres made of same material have radii in the ratio 2 : 1. If both the spheres are at same temperature, then what is the ratio of heat radiation energy emitted per second by them?

  1. 1 : 4

  2. 4 : 1

  3. 3 : 4

  4. 4 : 3
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Radiation emitted per second depends on the temperature of the body.
Stefan's law states that the rate of emission of radiant energy by unit area of perfectly black body is directly proportional to the fourth power of its absolute temperature.
      $E \propto AT^4$
or   $E \propto r^2$
($\because A= \pi r^2$ and T is same for both the spheres)
where r is radius of sphere.
$\frac{E _1}{E _2} = \frac{r^2 _1}{r^2 _2}$
$=\left(\frac{2}{1}\right)^2=\frac{4}{1}$
$=4:1$
Note : A black body at absolute temperature T surrounded by another black body at absolute temperature $T _0$ not only loses an amount of energy $\sigma T^4$, thus the amount of heat lost by the former per unit time is given by 
$E=\sigma (T^4-T _0^4)$
This law is stefan Boltzmann's law.

Multiple choice laws of heat transfer heat and thermodynamics physics

A temperature of a body is ${400^ \circ }$ C. Assuming the surrounding temperature to be negligible. At what temperature will body emit double energy radiation?

  1. ${200^ \circ }$ c

  2. 200 K
  3. ${800^ \circ }$ c

  4. 800 K
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Energy radiated E is proportional to T^4. If E' = 2E, then (T'/T)^4 = 2, so T' = T * 2^(1/4). T = 400 + 273 = 673 K. T' = 673 * 1.189 = 800 K. Converting back to Celsius is not required as the options provide Kelvin.

Multiple choice laws of heat transfer heat and thermodynamics physics

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  

  1. 4:1

  2. 1:4

  3. 1:1

  4. 16:1
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The rate of radiation emitted by a body is given by Stefan-Boltzmann law: P = e * sigma * A * T^4 = e * sigma * (4 * pi * r^2) * T^4. Thus, P is proportional to r^2 * T^4. For sphere 1: r1 = 1 m, T1 = 4000 K. For sphere 2: r2 = 4 m, T2 = 1000 K. The ratio P1 / P2 = (r1/r2)^2 * (T1/T2)^4 = (1/4)^2 * (4000/1000)^4 = (1/16) * (4^4) = (1/16) * 256 = 16:1.

Multiple choice laws of heat transfer heat and thermodynamics physics

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 

  1. $12\,s$
  2. $22\,s$
  3. $32\,s$
  4. $42\,s$
Reveal answer Fill a bubble to check yourself
D Correct answer
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
$\Longrightarrow \cfrac { 94-86 }{ 2 } =k\left[ \cfrac { 94+86 }{ 2 } -20 \right] \ \Longrightarrow 4=k[90-20]=k[70]\ \therefore k=\cfrac { 4 }{ 70 } \ \Longrightarrow \cfrac { 71-69 }{ \Delta t } =\cfrac { 4 }{ 70 } \left[ \cfrac { 71+69 }{ 2 } -20 \right] \ \Longrightarrow \cfrac { 2 }{ \Delta t } =\cfrac { 4 }{ 70 } \left[ 70-20 \right] \ \Longrightarrow \cfrac { 2 }{ \Delta t } =\cfrac { 4 }{ 70 } \times 50\ \therefore \Delta t=\cfrac { 70 }{ 2\times 50 } =0.7min=42sec$