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 kirchoff's laws black body radiation heat transfer thermal properties physics

A polished metal plate with a rough black spot on it is heated to about $1400 K$ and quickly taken into a dark room. Which one of the following statements will be true?

  1. The spot will appear brighter than the plate

  2. The spot will appear darker than the plate

  3. The spot and the plate will appear equally bright

  4. The spot and the plate will not be visible in the dark room
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

According to Kirchhoff law, good absorbers are good emitters. Since black spot is good absorbers so it is also a good emitter & will brighter than plate.

Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

A spherical body of area $A$ and emissivity $e = 0.6$ is kept inside a perfectly black body. Total heat radiated by the body at temperature T is :

  1. $0.4eAT^{4}$
  2. $0.8eAT^{4}$
  3. $0.6eAT^{4}$
  4. $1.0eAT^{4}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

When a non black body is placed inside a hollow enclosure, the total radiation from the body is the sum of what it would emit in the open ( with $\epsilon<1$ ) and the part of the incident radiation from the walls reflected by it. The two add up to a black body radiation.

Hence the total radiation emitted by the body is $1.0\epsilon AT^4$.

Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

The rates of heat radiation from two patches of skin each of area $S$, on a patient's chest differ by $2$%. If the patch of the lower temperature is at $300K$ and the emissivity of both the patches is assumed to be unity, the temperature of the other patch is closest to:

  1. $301.5K$
  2. $306K$
  3. $308.5K$
  4. $312K$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$P=\sigma e AT^4$

$\cfrac{P _1}{P _2}=(\cfrac{T _1}{T _2})^4\ \cfrac{P+P\times\cfrac{2}{100}}{P}=(\cfrac{T _1}{300})^4\1+\cfrac{2}{100}=(\cfrac{T _1}{300})^4\ \Rightarrow \cfrac{102}{100}=(\cfrac{T}{300})^4\ \Rightarrow T=301.488K\ \quad\simeq 301.5K$

Multiple choice components and importance of food science of kitchen evs

Solar constant at height at 83 km in

  1. 1 kcal/c$m^2$ /min
  2. 1 cal/c$m^2$/min
  3. 2 kcal/c$m^2$/min
  4. 2 cal/c$m^2$/min
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The solar constant is the amount of solar energy reaching the upper atmosphere. The standard value is approximately 2 calories per square centimeter per minute.

Multiple choice conduction in metals and non-metals thermal energy transfers physics

Two walls of thickness $d _1$ and $d _2$, thermal conductivities $K _1$ and $K _2$ are in contact. In the steady state if the temperature at the outer surfaces are $T _1$ and $T _2$, the temperature are the common wall will be 

  1. $\dfrac{K _1T _1+K _2T _2}{d _1d _2}$
  2. $\dfrac{K _1T _1d _2+K _2T _2d _1}{K _1d _2+K _2d _1}$
  3. $\dfrac{K _1d _1T _1+K _2d _2T _2}{K _1d _1+K _2d _2}$
  4. $\dfrac{(K _1d _1+K _2d _2)T _1T _2}{T _1+T _2}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\begin{array}{l} { k _{ 1 } }\dfrac { { { T _{ 1 } }-{ T _{ c } } } }{ { { d _{ 1 } } } } ={ k _{ 2 } }\dfrac { { { T _{ c } }-{ T _{ 2 } } } }{ { { d _{ 2 } } } }  \ where\, { T _{ c } }\, is\, \, common\, \, wall\, temperature,\, solving\, for\, \, { T _{ c } }\, we\, \, will\, get \ { T _{ C } }=\dfrac { { { T _{ 1 } }+\alpha { T _{ 2 } } } }{ { \alpha +1 } }  \ u\sin  g\, \, \alpha =\dfrac { { { d _{ 1 } } } }{ { { d _{ 2 } } } } \dfrac { { { k _{ 2 } } } }{ { { k _{ 1 } } } }  \ { T _{ c } }=\dfrac { { { T _{ 1 } }+\dfrac { { { d _{ 1 } } } }{ { { d _{ 2 } } } } \dfrac { { { k _{ 2 } } } }{ { { k _{ 1 } } } } { T _{ 2 } } } }{ { \dfrac { { { d _{ 1 } } } }{ { { d _{ 2 } } } } \dfrac { { { k _{ 2 } } } }{ { { k _{ 1 } } } } +1 } }  \ =\dfrac { { { T _{ 1 } }\left( { { d _{ 2 } }{ k _{ 1 } } } \right) +{ T _{ 2 } }{ d _{ 1 } }{ k _{ 2 } } } }{ { { d _{ 1 } }{ k _{ 2 } }+{ d _{ 2 } }{ k _{ 1 } } } }  \ Hence,\, the\, option\, B\, is\, the\, \, correct\, answer. \end{array}$

Multiple choice conduction in metals and non-metals thermal energy transfers physics

Consider a compound slab consisting of two different materials having equal thickness and thermals conductivities K and $2K$ in series. The equilvalent conductivity of  the slab is

  1. $\frac{2}{3}K$
  2. $\sqrt {2K} $
  3. $3K$
  4. $\left( {\frac{4}{3}} \right)K$
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice conduction in metals and non-metals thermal energy transfers physics

A solid sphere and a hollow sphere of same material and size are heated to some temperature and allowed to cool in the same surroundings.If the temperature difference between each sphere and its surrounding is T, then:-

  1. The hollow sphere will cool at a faster rate for all values of T

  2. The solid sphere will cool at a faster rate for all values of T

  3. Both spheres will coll at the same rate for all values of T

  4. Both spheres will coll at the same rate only for small values of T
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

According to Newton's law of cooling, the rate of cooling depends on the surface area and emissivity. Since the spheres have the same material, size, and surroundings, they have the same surface area and emissivity, resulting in the same rate of cooling.

Multiple choice conduction in metals and non-metals thermal energy transfers physics

Two identical bodies are made of a material for which the heat capacity increases with temperature. One of these is at ${ 100 }^{ 0 }$, while the other one is at ${ 0 }^{ 0 }C$. If the two bodies are brought into contact, then assuming no heat loss, the final common temperature is

  1. ${ 50 }^{ 0 }C$
  2. more than ${ 50 }^{ 0 }C$
  3. less than ${ 50 }^{ 0 }C$ but greater than ${ 0 }^{ 0 }C$
  4. ${ 0 }^{ 0 }C$
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice conduction in metals and non-metals thermal energy transfers physics

A semicircular rod is joined at its end to a straight rod of the same material and the same cross-sectional area. The straight rod forms a diameter of the other rod. The junctions are maintained at different temperatures. Find the ratio of the heat transferred through a cross-section of the semicircular rod of the heat transferred through a cross-section the straight rod in the given time.

  1. $\dfrac 2\pi$
  2. $\dfrac 1\pi$
  3. $\dfrac 1{2\pi}$
  4. $\dfrac \pi2$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Heat flow rate is proportional to cross-sectional area and inversely proportional to length. For a semicircular rod of radius R, the length is pi*R and area is A. For the straight diameter rod, length is 2R and area is A. The ratio of heat flow (H_semi / H_straight) = (K*A*dT / (pi*R)) / (K*A*dT / (2*R)) = 2/pi.

Multiple choice conduction in metals and non-metals thermal energy transfers physics

Bimetallic thermostat consists two metals of different thermal co-efficient thermostat always bends towards material having
(i)large thermal expansion on heating 
(ii)large thermal expansion on cooling
(iii)small thermal expansion on heating
(iv)small thermal expansion on cooling

  1. I only

  2. II only

  3. III and II only

  4. I and IV only
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice conduction in metals and non-metals thermal energy transfers physics

Two angular blocks A and B different metal have same length and same area of cross-section . they are kept in such a way that their cross-sectional area touch each other. the temperature at one end of A is $ 100^0 $ C and that of B at the other end is $0^0 C $ C . if the ratio of their conductivity is 1:3 then under steady state, the temperature of the junction in contact will be:-

  1. $ 25^0 C $
  2. $ 50^0 C $
  3. $ 75^0 C $
  4. $ 100^0 C $
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice conduction in metals and non-metals thermal energy transfers physics

On heating one end of a rod, the temperature of the whole rod will be uniform when the :

  1. $k $ $ = 1$
  2. $k $ $ = 0$
  3. $k $ $ = 100$
  4. $k $ $ = \infty $
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

If temperature of whole rod is same, it means that heat is being conducted from one end to other infinitely fast. This is possible only when thermal conductivity is infinite.