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

Two bitumen samples “X” and “Y” have softening points 450C and 600C, respectively. Consider the following statements: I Viscosity of “X” will be higher than that of “Y” at the same temperature II Penetration value of “X” will be lasser than that of “Y” under standard conditions The correct option evaluating the above statements is

  1. Both I and II are True

  2. I is False and II is True

  3. Both I and II are False

  4. I is True and II is False

Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Bitumen with a higher softening point (Y) is harder and has higher viscosity at a given temperature compared to X. Therefore, X has lower viscosity and higher penetration value than Y.

Multiple choice

A 10 mm diameter electrical conductor is covered by an insulation of 2 mm thickness. The conductivity of the insulation is 0.08 W/m-K and the convection coefficient at the insulation surface is 10 W/m2-K. Addition of further insulation of the same material will

  1. increase heat loss continuously

  2. decrease heat loss continuously

  3. increase heat loss to a maximum and then decrease heat loss

  4. decrease heat loss to a minimum and then increase
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Multiple choice

The solidification time of a casting is proportional to $\Big(\frac{V}{A}\Big)^{2}$ , where V is the volume of the casting and A is the total casting surface area losing heat. Two cubes of the same material and size are cast using sand casting process. The top face of one of the cubes is completely insulated. The ratio of the solidification time for the cube with top face insulated to that of the other cube is
  1. $\frac{25}{36}$
  2. $\frac{36}{25}$

  3. 1
  4. $\frac{6}{5}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\text{It is given that solidification time of casting}(t_s)\\ t_s \alpha(\frac{V}{A})^2\\ \text {where $\hspace{0.5cm}$ v = Volume of casting} \\ \hspace{1.9cm}\text{A = surface area of casting losing heat}$

Multiple choice

Directions: Carry Two Marks Each.

A brick wall $\bigg(k=0.9\frac{w}{m.k}\bigg)$ of thickness 0.18 m separates the warm air in a room from the cold ambient air. On a particular winter day, the outside air temperature is -5oC and the room needs to be maintained at 27oC. The heat transfer coefficient associated with outside air is 20$\frac{W}{m^2k}$. Neglecting the convective resistance of the air inside the room, the heat loss (in $\frac{W}{m^2}$) is

  1. 88

  2. 110

  3. 128

  4. 160
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Multiple choice

Consider a long cylindrical tube of inner and outer radii, ri and ro, respectively, length, L and thermal conductivity, k. Its inner and outer surfaces are maintained at Ti and To, respectively (Ti > To). Assuming one-dimensional steady state heat conduction in the radial direction, the thermal resistance in the wall of the tube is
  1. $\frac{1}{2 \pi KL}1n \Big( \frac{r_i}{r_o} \Big)$
  2. $\frac{1}{2 \pi r_iL}$
  3. $\frac{1}{2 \pi KL}1n \Big( \frac{r_o}{r_i} \Big)$
  4. $\frac{1}{4 \pi KL}1n \Big( \frac{r_o}{r_i} \Big)$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Correct option is (3).

Multiple choice

A refrigerator operates between 120 kPa in an ideal vapour compression cycle withy R - 134a as the refrigerant enters the compressor as saturated vapour and leaves the condenser as saturated liquid. The mass flow rate of the refrigerant is 0..2 kg/s. Properties for R - 134a are as follows: || |---| | Saturated R - 134a| ||||||| |---|---|---|---|---|---| | P (kPa)| T (oC)| hf (kJ/kg)| Hg (kJ/kg)| Sf (kJ/kg. k)| Sg (kJ/kg. K)| | 120| - 22.32| 22.5| 237| 0.093| 0.95| | 800| 31.31| 95.5| 267.3| 0.354| 0.918| || |---| | Superheated R - 134a| ||||| |---|---|---|---| | P (kPa)| T (oC)| h (kJ/kg)| S (kJ/kg)| | 800| 40| 276.45| 0.95|

The rate at which heat is extracted (in kJ/s) from the refrigerated space is

  1. 28.3

  2. 42.9

  3. 34.3

  4. 14.6
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation