Q10. The condenser and evaporator tubes in a Freon refrigeration plant are made of _______.
[SSC JE Mechanical, 1 March 2017, Evening]
A. Steel
B. Copper
C. Brass
D. Aluminium
Answer: B. Copper
Explanation: Refrigeration heat exchangers need materials that readily conduct thermal energy between refrigerant and surroundings. Copper tubing combines good heat transfer, corrosion resistance and convenient fabrication. These characteristics make it a common choice for condenser and evaporator tubing in suitable refrigeration systems.
Q9. Consider these statements about thermal conductivity:
1. It decreases with increasing molecular weight.
2. For non-metallic liquids it generally decreases as temperature rises.
3. It is generally lower in gases and liquids than in solids.
Which statements are correct?
[UPSC Engineering Services Examination, Mechanical Paper I, 2016]
A. 1 and 2 only
B. 1 and 3 only
C. 2 and 3 only
D. 1, 2 and 3
Answer: D. 1, 2 and 3
Explanation: For the conventional materials and comparisons assumed in this examination, all three trends are accepted. Thermal conductivity is influenced by molecular transport and structure; non-metallic liquid values frequently fall with temperature, while solids usually transport heat more efficiently than fluids. Individual exceptions exist.
Q8. A furnace wall has a 150 mm-thick refractory lining. The heat flux is 50 W/m² and its thermal conductivity is 0.05 W/(m·K). What is the temperature drop across the lining?
[UPSC Engineering Services Examination, Mechanical Paper I, 2016]
A. 140°C
B. 150°C
C. 160°C
D. 170°C
Answer: B. 150°C
Explanation: Using steady conduction through a plane wall, heat flux q = kΔT/L. Therefore ΔT = qL/k = 50 × 0.15 / 0.05 = 150 kelvin. A temperature difference of 150 kelvin is numerically equal to 150°C.
Q7. Thermal conductivity of aluminium, copper and stainless steel increases in the order:
[UPSC NDA I, General Ability, 19 April 2015]
A. Copper < Aluminium < Stainless steel
B. Stainless steel < Aluminium < Copper
C. Aluminium < Copper < Stainless steel
D. Copper < Stainless steel < Aluminium
Answer: B. Stainless steel < Aluminium < Copper
Explanation: Stainless steel is a comparatively poor metallic heat conductor, aluminium transfers heat more effectively, and copper conducts heat better still. Their usual room-temperature thermal conductivities justify the ascending order stainless steel, aluminium and copper. Composition and temperature can modify precise values.
Q6. Which of the following is the SI unit of the thermal conductivity of a material?
[UPSC NDA II, General Ability, 16 December 2015]
A. W·m⁻¹·K⁻¹
B. W·m/K
C. W·m·K
D. J·s⁻¹·m⁻¹·K
Answer: A. W·m⁻¹·K⁻¹
Explanation: Thermal conductivity connects heat-flow rate with a temperature gradient. From the conduction equation, the unit is watts divided by metre and kelvin. It can be expressed as W/(m·K), equivalent to W·m⁻¹·K⁻¹, and is not the same as thermal diffusivity.
Q5. Which of the following is not a desirable feature of a moderator in a nuclear reactor?
[RPSC Mechanical Engineering Lecturer, 2014 official paper]
A. It has low thermal conductivity
B. It slows down neutrons
C. It is non-corrosive
D. It has good chemical stability
Answer: A. It has low thermal conductivity
Explanation: A moderator primarily slows fast neutrons so they can sustain fission efficiently. Stable, corrosion-resistant materials are desirable under reactor conditions. Low thermal conductivity is generally undesirable because efficient heat removal reduces temperature gradients and the risk of localized overheating.
Q4. In ionic crystals, electrical conductivity is:
[UPSC Engineering Services Examination, Electronics Paper I, 2010]
A. Very high
B. Depends on material
C. Depends on temperature
D. Practically zero
Answer: D. Practically zero
Explanation: In an ordinary solid ionic crystal, the ions are locked in a lattice and charge carriers cannot move freely. Consequently its direct-current electrical conductivity is extremely small. When molten, or sometimes dissolved, ionic substances can conduct because ions become mobile.
Q3. Assertion: At low temperature, semiconductor conductivity increases as temperature rises.
Reason: Higher temperature can break covalent bonds and generate more electrons and holes.
[UPSC Engineering Services Examination, Electronics Paper I, 2010]
A. Both are true and the Reason correctly explains the Assertion.
B. Both are true but the Reason does not explain the Assertion.
C. Assertion true, Reason false.
D. Assertion false, Reason true.
Answer: A. Both are true and the Reason correctly explains the Assertion.
Explanation: Rising temperature excites additional electrons into the conduction band, creating electron–hole pairs. Although carrier mobility may fall through increased scattering, the growth in carrier concentration usually dominates for intrinsic semiconductors, so electrical conductivity increases over the stated regime.
Q2. Four identical-material rods of differing radii r and lengths L connect the same two temperature reservoirs. Which rod will conduct the most heat?
[ISRO Mechanical Engineering Paper I, 2009]
A. r = 2 cm, L = 2 m
B. r = 1 cm, L = 1 m
C. r = 2 cm, L = 0.5 m
D. r = 0.5 cm, L = 0.5 m
Answer: C. r = 2 cm, L = 0.5 m
Explanation: For rods of equal material and temperature difference, heat-flow rate is proportional to cross-sectional area divided by length, or r²/L. The third rod maximizes this ratio: 2²/0.5 = 8. It therefore transfers heat at the greatest rate.
Q1. Which of the following is arranged in decreasing order of electrical conductivity?
[SSC Combined Matric Level, 2002 (reported PYQ)]
A. Copper, Aluminium, Steel, Silver
B. Aluminium, Silver, Copper, Steel
C. Copper, Silver, Aluminium, Steel
D. Silver, Copper, Aluminium, Steel
Answer: D. Silver, Copper, Aluminium, Steel
Explanation: Silver has the highest electrical conductivity among these metals at ordinary temperature. Copper follows, then aluminium, while ordinary steel is substantially less conductive. Electrical conductivity reflects the ease with which electrons move through a material, and generally decreases when scattering is stronger.
