Q70. Which is the SI unit of surface tension?
[SSC JE Mechanical – 7 June 2024]
A. N/m²
B. No unit
C. N/m
D. N/m³
Answer: C. N/m
Explanation: Surface tension is force acting tangentially per unit length along a liquid surface, so its SI unit is newton per metre. It can also be expressed as joule per square metre when interpreted as surface energy per unit area.
Q69. What is the SI base unit of electric current?
[SSC CGL Tier I – 13 September 2024]
A. Ohm
B. Ampere
C. Metre
D. Volt
Answer: B. Ampere
Explanation: The ampere, symbol A, is the SI base unit of electric current. Current measures the rate of flow of electric charge. Ohm and volt are derived electrical units for resistance and potential difference, while metre measures distance.
Q68. The unit pascal is used for measurement of which quantity?
[SSC JE Electrical – 5 June 2024]
A. Pressure
B. Density
C. Power
D. Energy
Answer: A. Pressure
Explanation: Pascal is the SI derived unit of pressure and equals one newton per square metre. Pressure characterises how a perpendicular force is distributed over an area. Density uses kg/m³, power uses watts, and energy uses joules.
Q67. Which unit expresses a couple or moment of force?
[SSC JE Mechanical – 6 November 2024]
A. Kilogram-second
B. Newton-second
C. Pascal-metre
D. Newton-metre
Answer: D. Newton-metre
Explanation: A couple produces rotation and is quantified by force times the perpendicular distance between forces, yielding newton-metres. Torque shares this SI unit with energy, but torque and energy are distinct physical quantities and their directions or definitions differ.
Q66. What is the SI unit of electrical resistivity?
[SSC CGL Tier I – 26 September 2024]
A. Farad
B. Ampere
C. Coulomb
D. Ohm-metre
Answer: D. Ohm-metre
Explanation: Resistivity is a material property related to resistance by R = ρL/A. Therefore ρ = RA/L, giving ohm-metres. A farad measures capacitance, an ampere measures current, and a coulomb measures electric charge.
Q65. The watt is the SI unit of which physical quantity?
[SSC GD Constable – 6 March 2024]
A. Electrical resistance
B. Power
C. Energy
D. Momentum
Answer: B. Power
Explanation: Power is the rate at which work is performed or energy is transferred. Its SI unit is the watt, equivalent to one joule per second. Energy itself is in joules, resistance in ohms, and linear momentum in kg m/s.
Q64. Which expression is the SI unit of mass density?
[SSC Stenographer – 11 December 2024]
A. kg·m/s
B. m³
C. kg/m³
D. m²
Answer: C. kg/m³
Explanation: Mass density is defined as mass divided by volume. SI mass uses kilograms, and SI volume uses cubic metres. Consequently density is expressed in kilograms per cubic metre. A quantity expressed only as m³ is volume, not density.
Q63. What temperature in degree Celsius corresponds to absolute zero?
[SSC MTS – 15 October 2024]
A. −273.15°C
B. 0°C
C. −100°C
D. 273.15°C
Answer: A. −273.15°C
Explanation: Absolute zero corresponds to zero kelvin, the origin of the thermodynamic temperature scale. As Celsius temperature is kelvin temperature minus 273.15, the value is −273.15°C. This is a temperature-scale conversion, not an energy unit.
Q62. Does refractive index have a physical unit?
[RRB Technician – 23 December 2024]
A. Cubic metre
B. No unit
C. Metre per second squared
D. Metre per second
Answer: B. No unit
Explanation: Refractive index is defined as the ratio of light speed in vacuum to its speed in a medium. Both speeds have the same dimensions, so they cancel. Refractive index is therefore dimensionless and has no SI unit.
Q61. What is the SI unit of angular momentum?
[RRB Technician Grade I – 19 December 2024]
A. kg·m²/s
B. kg·m⁻¹·s
C. kg·m²/s²
D. kg·m/s
Answer: A. kg·m²/s
Explanation: Angular momentum equals moment of inertia multiplied by angular velocity, or position crossed with linear momentum. Its SI dimensions are mass times length squared divided by time: kg m²/s. The kg m/s expression instead belongs to linear momentum.
