Q40. What is the SI unit of the resistivity of a material?
[RRB Group D – 25 August 2022]
A. Ohm-metre
B. Ohm
C. Reciprocal ohm-metre
D. Mho
Answer: A. Ohm-metre
Explanation: Resistivity depends on material characteristics and relates to a wire’s resistance through R = ρL/A. Solving for ρ gives R A/L, so the SI unit is Ω m. Ohm alone describes a particular conductor’s resistance.
Q39. Which unit is used to measure resistance in an electric circuit?
[RRB Group D – 29 September 2022]
A. Ampere
B. Ohm
C. Volt
D. Coulomb
Answer: B. Ohm
Explanation: The ohm is the electrical resistance unit. When one volt produces one ampere through an ohmic element, its resistance is one ohm. Ampere denotes current, volt denotes potential difference and coulomb denotes electric charge.
Q38. Which of these is NOT a unit of power?
[RRB Group D – 24 August 2022]
A. Kilowatt-hour
B. Watt
C. Joule per second
D. Horsepower
Answer: A. Kilowatt-hour
Explanation: Kilowatt-hour combines power and time, so it measures energy. Watt, joule per second and horsepower all express the rate of performing work. One mechanical horsepower is about 746 watts, while one kilowatt-hour is 3.6 million joules.
Q37. What is the standard unit for the power of a lens?
[RRB Group D – 11 October 2022]
A. Newton
B. Dioptre
C. Joule
D. Metre
Answer: B. Dioptre
Explanation: Optical power describes how strongly a lens converges or diverges light and equals the reciprocal of its focal length in metres. Therefore the unit is dioptre, symbol D. A focal length of 0.5 m corresponds to a magnitude of 2 D.
Q36. Which unit correctly represents electrical resistivity in SI?
[UPSC NDA-I GAT – 2022]
A. Ohm
B. Ohm per metre
C. Ohm-centimetre
D. Ohm-metre
Answer: D. Ohm-metre
Explanation: Electrical resistivity has dimensions of resistance multiplied by area divided by length. In SI, those combine to ohm-metres. Ohm-centimetre may be encountered in material specifications but is not the coherent SI expression; ohm measures resistance alone.
Q35. What are the dimensions of the gravitational constant G?
[UPSC NDA-I GAT – 2022]
A. [ML³T⁻²]
B. [M⁻¹L³T⁻²]
C. [M²L⁻²T⁻²]
D. [M²L⁻¹T⁻²]
Answer: B. [M⁻¹L³T⁻²]
Explanation: From F = Gm₁m₂/r², rearrange to G = Fr²/(m₁m₂). Force has dimensions [MLT⁻²], distance squared contributes L², and two masses contribute M². Thus the result is [M⁻¹L³T⁻²].
EXAM YEAR 2021
Q34. Why are immense astronomical distances conveniently expressed in light-years?
[UPSC Civil Services Prelims GS-I – 2021]
A. Distances between stars never change
B. The gravity of stars never changes
C. Light invariably follows a straight path
D. The speed of light in vacuum is constant
Answer: D. The speed of light in vacuum is constant
Explanation: A light-year is a distance defined using the constant speed of light in vacuum and the duration of a year. This provides a consistent, appropriately large unit for interstellar scales. It is not a time unit despite containing the word year.
Q33. What is the SI unit for amount of substance?
[SSC CGL (2020 cycle, held 18 August 2021)]
A. Kelvin
B. Candela
C. Mole
D. Metre
Answer: C. Mole
Explanation: The mole, symbol mol, is the SI base unit for amount of substance. One mole contains exactly 6.02214076 × 10²³ specified elementary entities. Kelvin, candela and metre instead correspond to thermodynamic temperature, luminous intensity and length.
Q32. Which of the following is the SI unit for pressure?
[SSC CHSL (2020 cycle, held 4 August 2021)]
A. Pascal
B. Candela
C. Kelvin
D. Ampere
Answer: A. Pascal
Explanation: A pascal equals one newton per square metre, as pressure is force per unit area. The other three options are SI base units, but they measure luminous intensity, temperature and electric current respectively rather than pressure.
Q31. In the SI system, what unit measures electrical resistance?
[RRB NTPC – 2021]
A. Volt
B. Ampere
C. Ohm
D. Coulomb
Answer: C. Ohm
Explanation: Resistance is the ratio of potential difference to electric current. Expressing this ratio in volt per ampere gives the derived unit ohm. Each distractor represents a separate electrical quantity, so identifying the defining relationship prevents confusion.
