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.
