30. Why is the logarithmic mean temperature difference used in heat-exchanger calculations?
[MPPSC Assistant Engineer Mechanical, July 2022]
(A) Heat-flow rate varies along the exchanger
(B) Fluid temperatures change from place to place
(C) Both of the above
(D) Neither of the above
Answer: (C) Both of the above
Explanation: As two fluids travel through a heat exchanger, their temperatures and local temperature difference usually vary with position. The logarithmic mean gives a suitable effective temperature difference for integrated heat-transfer calculations. It is used with Q = UAΔT_lm.
29. Why is the freezer compartment traditionally positioned near the top of a refrigerator?
[WBCS Preliminary Examination, August 2021]
(A) It is easiest to access
(B) It lowers manufacturing cost
(C) Cooling occurs only through conduction
(D) It promotes convection currents
Answer: (D) It promotes convection currents
Explanation: Cold air near the freezer is denser and descends; warmer air elsewhere rises to replace it. Natural convection distributes cooling through the interior. This is particularly important in older direct-cool refrigerators without fans circulating the air mechanically.
28. What is the approximate boiling point of liquid nitrogen at atmospheric pressure?
[RRB NTPC CBT-I, February 2021]
(A) −27°C
(B) −132°C
(C) −273°C
(D) −196°C
Answer: (D) −196°C
Explanation: Nitrogen boils at about 77 K, or −196°C, at standard atmospheric pressure. Its low boiling point makes it useful for cryogenic cooling and preserving biological material. The value depends on pressure, as with the boiling point of any liquid.
27. What is the usual temperature range of a clinical thermometer?
[SSC MTS 2020 paper, October 2021]
(A) 35°C to 42°C
(B) 32°C to 38°C
(C) 38°C to 48°C
(D) 25°C to 32°C
Answer: (A) 35°C to 42°C
Explanation: Traditional clinical thermometers are calibrated over approximately 35°C to 42°C, the relevant range for human body temperature. Laboratory thermometers cover much wider ranges. Digital clinical instruments may have different technical limits, but standard school-textbook coverage uses 35–42°C.
26. Which device is NOT primarily based on Joule heating?
[RRB NTPC CBT-I, April 2021]
(A) Electric iron
(B) Electric plug and switch
(C) Electric kettle
(D) Electric fuse
Answer: (B) Electric plug and switch
Explanation: An iron and kettle intentionally heat a resistive element, while a fuse melts when excessive current generates heat. Plugs and switches mainly establish, interrupt or route electrical contact; Joule heating in them is unwanted energy loss rather than their intended operating principle.
25. Consider furnace oil:
1. It is a refinery product;
2. Industries can use it to generate power;
3. Burning it can emit sulphur compounds.
Which statements are correct?
[UPSC Civil Services Prelims, October 2021]
(A) 1 and 2 only
(B) 2 and 3 only
(C) 1 and 3 only
(D) 1, 2 and 3
Answer: (D) 1, 2 and 3
Explanation: Furnace oil is obtained during crude-oil refining and burned in industrial boilers and power-generating equipment. Its chemical energy becomes heat through combustion. Sulphur compounds in the fuel may produce sulphur dioxide, contributing to air pollution if emissions are uncontrolled.
24. What does the Kelvin–Planck statement of the second law forbid?
[MPPSC Assistant Engineer Mechanical, 2021]
(A) Heat flowing from cold to hot without work only
(B) A cyclic engine producing net work while taking heat from only one reservoir
(C) Conversion of work to heat
(D) Conversion of some heat into work
Answer: (B) A cyclic engine producing net work while taking heat from only one reservoir
Explanation: A cyclic heat engine cannot turn all heat from a single thermal reservoir into net useful work without another effect. A heat sink is necessary. This formulation of the second law explains why ideal thermal efficiency cannot equal 100 percent.
23. For a closed system undergoing a complete thermodynamic cycle, what is the cyclic integral of (δQ − δW)?
[SSC JE Mechanical, October 2020]
(A) Positive
(B) Negative
(C) Zero
(D) Impossible to determine
Answer: (C) Zero
Explanation: The first law gives dU = δQ − δW for a closed system. Internal energy U is a state function, so its total change over a complete cycle equals zero. Therefore the cyclic integral of heat input minus work output vanishes.
22. Why is the thermal efficiency of a real Otto cycle lower than that of an ideal constant-specific-heat Otto cycle?
[RPSC Lecturer Technical Education Mechanical, 2020]
(A) The specific-heat ratio increases sharply with temperature
(B) The specific-heat ratio decreases as temperature rises
(C) The specific heat always falls with rising temperature
(D) Specific heats never vary with temperature
Answer: (B) The specific-heat ratio decreases as temperature rises
Explanation: In an actual high-temperature working gas, heat capacities change with temperature and the ratio γ = Cp/Cv often decreases. This reduces efficiency relative to an air-standard Otto-cycle model with constant γ, along with other real losses.
21. Why does liquid kept in a vacuum thermos remain hot or cold for a long period?
[UPPSC State Services Prelims, 2019]
(A) Conduction is reduced only
(B) Convection and radiation are reduced only
(C) Conduction, convection and radiation are all reduced
(D) None of these heat transfers matter
Answer: (C) Conduction, convection and radiation are all reduced
Explanation: A vacuum between double walls greatly suppresses conduction and convection. Reflective silvered surfaces reduce radiative exchange, and an insulating stopper limits heat leakage at the mouth. Because all three modes are restricted, the contents change temperature slowly.
