Physics

Thermal Properties and Thermodynamics

380 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 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 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

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
C Correct answer
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
$\Longrightarrow \cfrac { 94-86 }{ 2 } =k\left[ \cfrac { 94+86 }{ 2 } -20 \right] \ \Longrightarrow 4=k[90-20]=k[70]\ \therefore k=\cfrac { 4 }{ 70 } \ \Longrightarrow \cfrac { 71-69 }{ \Delta t } =\cfrac { 4 }{ 70 } \left[ \cfrac { 71+69 }{ 2 } -20 \right] \ \Longrightarrow \cfrac { 2 }{ \Delta t } =\cfrac { 4 }{ 70 } \left[ 70-20 \right] \ \Longrightarrow \cfrac { 2 }{ \Delta t } =\cfrac { 4 }{ 70 } \times 50\ \therefore \Delta t=\cfrac { 70 }{ 2\times 50 } =0.7min=42sec$

Multiple choice physics measurement and effects of heat thermal expansion in gases thermal expansion of fluids volume elasticity constant of gases

A uniform steel rod has length $\ell$ at $0^oC$. Now one of its end is kept in ice $(0^oC)$ and the other end is kept in steam $(100^oC)$. If the coefficient of thermal expansion of the rod is $\alpha,$how much is the thermal expansion of the rod at steady state? 

  1. $50\ \alpha\ell$
  2. $100\ \alpha\ell$
  3. $200\ \alpha\ell$
  4. $150\ \alpha\ell$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Thermal expansion is defined as the change in length due to a change in temperature. Since the rod is at steady state with one end at 0C and the other at 100C, the temperature varies linearly along the rod, resulting in an average temperature of 50C. The expansion is given by delta L = L * alpha * delta T, where delta T is the difference between the average temperature and the initial temperature (50 - 0 = 50).

Multiple choice physics kinetic theory maxwell-boltzmann speed distribution function behavior of perfect gas and kinetic theory kinetic theory of matter

$T _1$ is the temperature of oxygen enclosed in a cylinder. The temperature is increased to $T _2$ and Maxwellan distribution curves for $O _2$ at temperature $T _1$ and $T _2$ are plotted. If $A _1$ and $A _2$ are the areas under the curves and the speed axis, in both cases , then 

  1. $A _1 > A _2$
  2. $A _1 < A _2$
  3. A_1 = A_2$
  4. $A _1=\sqrt {A _2}$
Reveal answer Fill a bubble to check yourself
A Correct answer