Q40. Two conducting wires of the same material have lengths L and 1.2L. If the longer wire has half the volume of the shorter wire, how does its electrical resistance compare?
[UPSC NDA II, General Ability, 13 September 2026]
A. 288% larger
B. 200% larger
C. 188% larger
D. 108% larger
Answer: C. 188% larger
Explanation: Resistance is ρL/A. Since volume equals area times length, the longer wire has area A/2.4 if the shorter has area A. Its resistance is then 1.2 × 2.4 = 2.88 times the original, or 188% larger.
Q39. Three wires X, Y and Z have equal lengths and conductivity ratio 1 : 2 : 3. Z has twice the cross-sectional area of Y and X has the same resistance as Z. What is the ratio R(Y) : R(X)?
[UPSC NDA II, General Ability, 13 September 2026]
A. 1 : 1
B. 3 : 1
C. 2 : 1
D. 1 : 2
Answer: B. 3 : 1
Explanation: For equal lengths, resistance varies inversely with conductivity multiplied by cross-sectional area: R = L/(σA). Let Y have area A; Z has 2A. Equality of X and Z resistance gives X area 6A. Therefore R(Y)/R(X) = 6/2 = 3.
Q38. Why are cooking utensils often made with a copper bottom?
[SSC CGL 2025, 13 September, Shift 1]
A. Copper is a good insulator, preventing heat loss.
B. Copper has a low melting point, making it easy to manufacture.
C. Copper is a good conductor of heat, ensuring even heating.
D. Copper reacts readily with food, enhancing flavour.
Answer: C. Copper is a good conductor of heat, ensuring even heating.
Explanation: Copper has high thermal conductivity, so energy from the flame spreads rapidly across the utensil base. This reduces hot spots and helps food cook more uniformly. Copper bottoms are frequently combined with stainless steel to improve durability and avoid direct food contact.
Q37. The thermal conductivity of lightweight concrete is:
[SSC JE Civil Engineering, 6 December 2025, Shift 1]
A. Lower than dense concrete
B. Equal to dense concrete
C. Higher than dense concrete
D. Negligible
Answer: A. Lower than dense concrete
Explanation: Lightweight concrete contains more pores and air spaces than dense concrete. Trapped air restricts heat transfer, reducing the effective thermal conductivity. This makes lightweight concrete useful where thermal insulation and reduced structural load are desirable, although its conductivity is not zero.
Q36. What is the most common material used for the inner conductor of a coaxial cable, and what is the primary reason for its use?
[SSC JE Electrical Engineering, 4 December 2025, Shift 2]
A. Silver, for its superior conductivity
B. Aluminium, for its light weight and low cost
C. Copper, for its high conductivity and low resistance
D. Steel, for its low resistance
Answer: C. Copper, for its high conductivity and low resistance
Explanation: Copper offers high electrical conductivity with reasonable cost, ductility and mechanical reliability. A copper inner conductor allows high-frequency signals to travel with comparatively low resistive loss. Silver conducts still better, but the additional expense makes it less suitable for most coaxial cables.
Q35. A heating element of a heater is made of a material with high resistivity and high melting point. Which material property is most crucial for its function as a heating element?
[SSC Stenographer 2025, 8 August, Shift 2]
A. High modulus of elasticity
B. High thermal conductivity
C. Low specific heat capacity
D. High resistivity
Answer: D. High resistivity
Explanation: High resistivity allows a suitably designed element to develop substantial electrical resistance and produce heat through Joule heating. A high melting point also lets it tolerate operating temperatures. Nichrome is commonly used because it combines these properties with oxidation resistance.
Q34. The S.I. unit of thermal conductivity is:
[UPPSC Assistant Engineer Mains, Mechanical, 29 September 2025]
A. J/(m²·K)
B. J/(m·K)
C. W/(m·K)
D. None of the above
Answer: C. W/(m·K)
Explanation: Fourier’s law gives heat-transfer rate as kAΔT/L. Rearranging, k equals heat-transfer rate multiplied by length, divided by area and temperature difference. The corresponding SI unit is watt per metre per kelvin, written W/(m·K).
Q33. Calorimeter is a small container made of a thin sheet of _______ with good thermal conductivity.
[SSC CGL 2024 Tier I, 17 September, Shift 1]
A. Silver
B. Platinum
C. Copper
D. Caesium
Answer: C. Copper
Explanation: A calorimeter commonly uses a thin copper vessel because copper transfers heat efficiently and quickly reaches a nearly uniform temperature. The thin wall also minimizes heat capacity of the container, helping measurements of heat exchange between substances remain more precise.
Q32. Which property of the material of a conductor determines its resistivity?
[RRB Technician Grade I, 19 December, Shift 2]
A. Melting point
B. Electrical conductivity
C. Temperature coefficient
D. Density
Answer: B. Electrical conductivity
Explanation: Resistivity and conductivity are reciprocal intrinsic electrical properties: resistivity ρ equals 1/σ, where σ represents electrical conductivity. A highly conducting material therefore has low resistivity. The value also depends on the material and conditions such as temperature.
Q31. Graphite is a good conductor of electricity due to:
[WBCS Mains (Science), 2023]
A. Strong electrostatic bonding
B. Free localized bonds
C. Free delocalized electrons
D. Tetrahedral arrangement of particles
Answer: C. Free delocalized electrons
Explanation: In graphite, each carbon atom forms three covalent bonds within a hexagonal layer. Its fourth valence electron contributes to a delocalized electronic system. These mobile electrons carry electric charge along the layers, unlike diamond where valence electrons are localized in bonds.
Q30. Which of the following has the highest value of resistivity?
[SSC CGL 2023 Tier I, 17 July, Shift 1]
A. Silver
B. Nichrome
C. Chromium
D. Iron
Answer: B. Nichrome
Explanation: Nichrome is a nickel–chromium alloy with far greater resistivity than the listed pure metals. Its high resistance and oxidation tolerance make it suitable for heating elements. Silver, by contrast, has extremely low resistivity and is one of the best electrical conductors.
Q29. Which of the following material is a conductor of electricity?
[SSC MTS 2022 cycle, held 8 May 2023, Shift 2]
A. Glass
B. Mica
C. Silver
D. Ebonite
Answer: C. Silver
Explanation: Silver is a metal with abundant mobile charge carriers and exceptionally high electrical conductivity. Glass, mica and ebonite are normally electrical insulators. Although silver conducts better than copper, its cost limits extensive use in ordinary household wiring.
Q28. Which of the following is a good conductor of heat but a bad conductor of electricity?
[RSMSSB Forest Guard, 13 November 2022, Shift 2]
A. Copper
B. Mica
C. Gold
D. Ebonite
Answer: B. Mica
Explanation: Mica is an effective electrical insulator and can transmit heat while withstanding elevated temperatures. Its combination of heat tolerance and dielectric strength is valuable in electrical equipment. Relative to metals, however, its thermal conductivity is modest; the comparison is among these options.
Q27. Which of the following elements has the highest conductivity at 298.15 K?
[Delhi Police Head Constable AWO/TPO, 28 October 2022, Shift 1]
A. Gold
B. Iron
C. Copper
D. Sodium
Answer: C. Copper
Explanation: At approximately room temperature, copper conducts electricity better than gold, iron and sodium. Its low electrical resistivity and practical availability make it highly useful in cables and electrical apparatus. Silver would rank above copper, but silver is not one of the choices.
Q26. Arrange the following materials in ascending order of electrical conductivity at 298.15 K: (a) Copper, (b) Sodium, (c) Iron, (d) Gold.
[Delhi Police Head Constable AWO/TPO, 28 October 2022, Shift 3]
A. (c), (b), (d), (a)
B. (a), (b), (d), (c)
C. (d), (a), (c), (b)
D. (b), (c), (a), (d)
Answer: A. (c), (b), (d), (a)
Explanation: Electrical conductivity generally varies with electron mobility and scattering in the material. Among these four elements near room temperature, iron has the lowest conductivity, followed by sodium, then gold, while copper conducts best. Therefore the increasing sequence is c–b–d–a.
Q25. Which of the following properties is required for a material to be used in the filament of an electric bulb?
[RRB Group D, 11 October 2022, Shift 2]
A. Low temperature
B. Low melting point
C. High resistivity
D. High conductivity
Answer: C. High resistivity
Explanation: An incandescent bulb filament needs sufficient electrical resistance to become very hot when current passes through it. It also needs an exceptionally high melting point to survive incandescent temperatures. Tungsten satisfies these requirements; a highly conducting, low-resistance filament would be less suitable.
Q24. Which of the following represents materials in increasing order of their electrical resistance?
[RRB Group D, 6 October 2022, Shift 3]
A. Insulators, conductors, alloys
B. Insulators, alloys, conductors
C. Conductors, alloys, insulators
D. Alloys, conductors, insulators
Answer: C. Conductors, alloys, insulators
Explanation: For comparable dimensions, good conductors offer the least resistance to charge flow. Many alloys resist electron movement more than pure conducting metals. Insulators offer the greatest resistance because they have very few freely moving charges. Hence the listed progression is conductors, alloys, insulators.
Q23. Copper is used in _______ due to its low resistivity.
[RRB Group D, 27 September 2022, Shift 2]
A. Toasters
B. Heaters
C. Transmission lines
D. Electric bulbs
Answer: C. Transmission lines
Explanation: Copper has low electrical resistivity, which reduces energy loss as current flows through a conductor. Therefore it is suited to power-carrying cables and transmission conductors. Toasters and heaters deliberately employ comparatively high-resistance alloys to generate useful heat.
Q22. Which of the following statements about electrical conductivity in ionic compounds is correct?
[RRB Group D, 19 September 2022, Shift 1]
A. Ionic compounds conduct poorly in both solid and molten states.
B. Ionic compounds conduct in the solid state but not when molten.
C. Ionic compounds conduct when molten but not in the solid state.
D. Ionic compounds conduct in both solid and molten states.
Answer: C. Ionic compounds conduct when molten but not in the solid state.
Explanation: In a solid ionic crystal, positive and negative ions occupy fixed lattice positions and cannot move freely. Melting breaks the rigid arrangement and allows mobile ions to carry electric charge. Thus ionic compounds ordinarily conduct electricity when molten, but not as solids.
Q21. Consider the statements:
I. Graphite is a bad conductor of electricity.
II. Each carbon atom in graphite bonds to three other carbon atoms in a hexagonal plane.
Which is correct?
[RRB Group D, 25 August 2022, Shift 3]
A. Both statements are correct.
B. Both statements are incorrect.
C. I is correct and II is incorrect.
D. II is correct and I is incorrect.
Answer: D. II is correct and I is incorrect.
Explanation: Statement I is false because delocalized electrons make graphite conductive. Statement II is true: graphite has layered hexagonal sheets in which carbon atoms form three covalent bonds each. This bonding arrangement leaves electrons available for movement through the structure.
Q20. The SI unit of resistivity is:
[RRB Group D, 25 August 2022, Shift 2]
A. Ohm-metre
B. Ohm
C. Ohm per metre
D. Mho
Answer: A. Ohm-metre
Explanation: The resistance of a uniform wire is R = ρL/A, giving resistivity ρ = RA/L. Multiplying ohms by area in square metres and dividing by length in metres leaves ohm-metres, written Ω·m. Conductivity instead has SI unit siemens per metre.
Q19. Which of the following materials is preferably used for electrical transmission lines?
[RRB Group D, 24 August 2022, Shift 2]
A. Nichrome
B. Tungsten
C. Tin–lead alloy
D. Copper
Answer: D. Copper
Explanation: Copper has high conductivity and therefore low resistive losses for a given cable size. Nichrome is designed for resistance heating, tungsten withstands very high temperatures, and tin–lead alloys are used as solders. Of these options copper best serves as a power conductor.
Q18. Electric conduction in a semiconductor occurs due to:
[WBCS Mains 2021, Paper IV, held May 2022]
A. Electrons only
B. Holes only
C. Neither electrons nor holes
D. Both electrons and holes
Answer: D. Both electrons and holes
Explanation: Semiconductor current can be carried by negatively charged conduction-band electrons and by positive holes representing missing valence electrons. Temperature and doping affect the concentrations of these carriers. The combined movement of electrons and holes accounts for semiconductor electrical conduction.
Q17. When do electrons move for the flow of charges in a conducting metallic wire?
[SSC CHSL 2021 cycle, held 30 May 2022, Shift 3]
A. When there is no difference of electric pressure
B. When the charges stop flowing
C. Only if there is a difference of electric pressure
D. Due to gravity
Answer: C. Only if there is a difference of electric pressure
Explanation: A potential difference establishes an electric field within a metallic conductor. Free electrons then acquire a small average drift velocity opposite to the field, producing net electric current. Thermal random motion exists even without voltage, but it produces no net current.
Q16. Which of the following carbon allotropes is/are good conductors of electricity: 1. Diamond, 2. Graphite, 3. Fullerene?
[UPSC CDS II, General Knowledge, 14 November 2021]
A. 1 only
B. 1 and 2 only
C. 2 only
D. 1 and 3 only
Answer: C. 2 only
Explanation: Graphite possesses extended layers of delocalized electrons, allowing electric charge to move. Diamond is ordinarily an electrical insulator because all four carbon valence electrons form localized covalent bonds. In this examination comparison, graphite alone is identified as a good conductor.
Q15. What is the SI unit of thermal diffusivity?
[MPPSC Assistant Engineer Mechanical 2021 official paper]
A. m³/s
B. m²/s
C. m/s
D. W/(m·K)
Answer: B. m²/s
Explanation: Thermal diffusivity measures how rapidly a temperature disturbance spreads through matter. It is calculated as α = k/(ρc), with thermal conductivity k, density ρ and specific heat c. Dimensional simplification produces square metres per second, m²/s.
Q14. Which of the following is NOT generally a property of non-metals?
[RRB NTPC CBT I, 18 January 2021, Shift 2]
A. Dull and lacklustre
B. Light substances
C. Conductors
D. Brittle
Answer: C. Conductors
Explanation: Most non-metals have electrons localized in bonds, so they do not conduct electricity well. They are commonly dull and, when solid, brittle. Graphite is a notable non-metallic exception because its delocalized electrons can carry current through carbon layers.
Q13. A steel rod has thermal conductivity 50.2 W/(m·K), cross-sectional area 0.02 m² and length 15 cm. With a 300°C temperature difference across it, what is the rate of heat flow?
[RRB ALP Electronics Mechanic, 21 January 2019, Shift 1]
A. 2.0 kJ/s
B. 1.0 kJ/s
C. 4.0 kJ/s
D. 3.0 kJ/s
Answer: A. 2.0 kJ/s
Explanation: For steady one-dimensional conduction, heat-flow rate is kAΔT/L. Substituting 50.2 × 0.02 × 300 / 0.15 gives 2008 watts, approximately 2.0 kilojoules per second. The calculation assumes a uniform temperature gradient and constant conductivity.
Q12. The good conductor of heat and electricity is _______.
[ISRO Technician B (Fitter), 24 March 2019]
A. Stainless steel wire
B. Iron wire
C. Copper wire
D. Brass wire
Answer: C. Copper wire
Explanation: Copper is more conductive than stainless steel, iron or brass under ordinary conditions. Its mobile electrons allow electrical current to pass readily and also contribute to thermal energy transfer. This combination makes copper useful in electrical wiring and numerous heat-transfer applications.
Q11. Which of the following is not a property of a metal?
[SSC CPO Tier I, 7 July 2017, Shift 2]
A. Hardness
B. Lustrous appearance
C. Malleability
D. Poor conductivity of heat
Answer: D. Poor conductivity of heat
Explanation: Most metals conduct heat well because mobile electrons and lattice vibrations transfer energy efficiently. Their typical properties also include lustre and malleability. Thus poor heat conductivity is not a general property of metals, although the exact conductivity varies considerably among metallic elements.
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.