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

A small copper ball of 5 mm diameter at 500 K is dropped into an oil bath whose temperature is 300 K. The thermal conductivity of copper is 400 W/m.K, its density 9000 kg/m3 and its specific heat 385 J/kg.K. If the heat transfer coefficient is 250 W/m2.K and lumped analysis is assumed to be valid, the rate of fall of the temperature of the ball at the beginning of cooling will be

  1. 8.7 K/s

  2. 13.9 K/s

  3. 17.3 K/s

  4. 27.7 K/s
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Multiple choice

In two air standard cycles - one operating on the Otto and the other on the Brayton cycle - air is entropically compressed from 300 K to 450 K, heat is added to raise the temperature to 600 K in the Otto cycle and to 550 K in the Brayton cycle. If $\eta_0$ and $\eta_B$are the efficiencies of the Otto and Brayton cycles, then
  1. $\eta_0=0.25,\eta_B=0.18$
  2. $\eta_0=\eta_B=0.33$
  3. $\eta_0=0.25,\eta_B=0.45$

  4. it is not possible to calculate the efficiencies unless the temperature after the expansion is given.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Multiple choice

In two air standard cycles - one operating on the Otto and the other on the Brayton cycle - air is entropically compressed from 300 to 450 K, heat is added to raise the temperature to 600 K in the Otto cycle and to 550 K in the Brayton cycle. If Wo and WB are work outputs per unit mass, then

  1. Wo >WB

  2. Wo <WB

  3. Wo =WB

  4. It is not possible to calculate the work outputs unless the temperature after the expansion is given.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Multiple choice

An un-insulated air conditioning duct of rectangular cross section 1 m × 0.5 m, carrying air at 20°C with a velocity of 10 m/s, is exposed to an ambient of 30°C. Neglect the effect of duct construction material. For air in the range of 20°C-30°C, data are as follows: thermal conductivity =0.025 W/m.K; viscosity = 18$\mu$ Pa.s; Prandtl number = 0.73; density = 1.2 kg/m3. The laminar flow Nusselt number is 3.4 for constant wall temperature conditions and, for turbulent flow, Nu=0.023 Re0.8 Pr0.33. The Reynolds number for the flow is

  1. 444

  2. 890

  3. 4.44 × 105

  4. 5.33 × 105
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Multiple choice

An un-insulated air conditioning duct of rectangular cross section 1 m × 0.5 m, carrying air at 20°C with a velocity of 10 m/s, is exposed to an ambient of 30°C. Neglect the effect of duct construction material. For air in the range of 20°C-30°C, data are as follows: thermal conductivity =0.025 W/m.K; viscosity = 18$\mu$ Pa.s; Prandtl number = 0.73; density = 1.2 kg/m3. The laminar flow Nusselt number is 3.4 for constant wall temperature conditions and, for turbulent flow, Nu = 0.023 Re0.8 Pr0.33. The heat transfer per metre length of the duct is

  1. 3.8 <span style="font-family:" arial;="" font-size:="" small;="">watt

  2. 5.3 <span style="font-family:" arial;="" font-size:="" small;="">watt

  3. 89 <span style="font-family:" arial;="" font-size:="" small;="">watt

  4. 769 <span style="font-family:" arial;="" font-size:="" small;="">watt
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$\frac{O}{L}=25.64\times3\times10=769.2W\simeq769W$

Multiple choice

Spot welding of two 1 mm thick sheets of steel (density = 8000 kg/m3) is carried out successfully by passing a certain amount of current for 0.1 second through the electrodes. The resultant weld nugget formed is 5 mm in diameter and 1.5 mm thick. If the latent heat of fusion of steel is 1400 kJ/kg and the effective resistance in the welding operation is 200$\Omega$, the current passing through the electrodes is approximately

  1. 1480 A

  2. 3300 A

  3. 4060 A

  4. 9400 A
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Multiple choice physics electric current and its effects coil and fuse heating effect of electric current- uses applications of heating effect of electric current

Thermal energy produced by the resistance R in time duration 1 $ \mu s $. using the source at resonance condition is 

  1. 0 J
  2. $ 1 \mu J $
  3. $ 100 \mu J $

  4. not possible to calculate from the given information.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

To calculate thermal energy produced by resistance, we need the current through the resistance, which requires knowledge of the circuit components (voltage source, impedance at resonance, circuit configuration). The question states 'using the source at resonance condition' but provides no numerical values for voltage, current, resistance, frequency, or circuit parameters. Without this information, the energy cannot be calculated.

Multiple choice physics electric current and its effects coil and fuse heating effect of electric current- uses applications of heating effect of electric current

If p.d. across a conductor is constant and resistivity of its material is $\rho $, the joule heat produced in 1 s is proportional to .. 

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

Joule heat in 1 second is H = V²/R (since P.d. is constant). Resistance of a conductor is R = ρL/A, where L is length and A is cross-sectional area. For a fixed geometry (L and A constant), R ∝ ρ. Therefore H = V²/R ∝ 1/R ∝ 1/ρ. The heat produced is inversely proportional to resistivity.

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.