Q30. Which of these cannot serve as a unit of frequency?
[UPSC NDA-I GAT – 2021]
A. Decibel
B. Second⁻¹
C. Hertz
D. Minute⁻¹
Answer: A. Decibel
Explanation: Frequency is cycles per unit time, so reciprocal seconds, reciprocal minutes and hertz can all represent it. The decibel instead expresses a logarithmic ratio, usually of power or intensity. It does not measure cycles per time.
EXAM YEAR 2020
Q29. What is the SI base unit of temperature?
[SSC CHSL Tier I – 20 October 2020]
A. Kelvin
B. Joule
C. Watt
D. Candela
Answer: A. Kelvin
Explanation: Thermodynamic temperature uses kelvin as its SI base unit. Joule measures energy, watt measures power, and candela measures luminous intensity. Kelvin is written simply as K, without a degree symbol, unlike the Celsius scale.
Q28. Which is the SI unit of luminous intensity?
[SSC CHSL Tier I – 14 October 2020]
A. Pascal
B. Candela
C. Mole
D. Ampere
Answer: B. Candela
Explanation: Candela is one of the seven SI base units and measures luminous intensity in a given direction. Pascal is derived for pressure, mole measures amount of substance and ampere measures electric current. Candela has the symbol cd.
Q27. Which is NOT a unit of energy?
[UPSC NDA (I & II combined) GAT – 2020]
A. Joule
B. Watt-hour
C. Newton-metre
D. kg·m·s⁻²
Answer: D. kg·m·s⁻²
Explanation: Energy can be measured in joules, watt-hours or newton-metres in suitable contexts. The expression kg m s⁻² is one newton, the derived SI unit of force. It lacks an additional metre factor required for mechanical energy.
EXAM YEAR 2019
Q26. One dyne of force is equivalent to which value in SI units?
[UPSC NDA-I GAT – 2019]
A. 10³ kg·m/s²
B. 10⁻³ kg·m/s²
C. 10⁵ kg·m/s²
D. 10⁻⁵ kg·m/s²
Answer: D. 10⁻⁵ kg·m/s²
Explanation: Dyne is the CGS force unit, equal to one gram-centimetre per second squared. Because one gram is 10⁻³ kilogram and one centimetre is 10⁻² metre, one dyne equals 10⁻⁵ newton.
Q25. What is the SI unit of the amplitude of a displacement wave?
[RPF Constable – 25 January 2019]
A. Volt
B. Hertz
C. Metre
D. Farad
Answer: C. Metre
Explanation: Amplitude is the maximum displacement from the equilibrium position of a mechanical displacement wave. Displacement has dimensions of length, giving the SI unit metre. Hertz denotes frequency, volt electric potential difference and farad electrical capacitance.
EXAM YEAR 2018
Q24. Which unit is used for magnetic flux density?
[SSC JE Electrical – 25 January 2018]
A. Henry
B. Tesla
C. Coulomb
D. Weber
Answer: B. Tesla
Explanation: Tesla measures magnetic flux density and equals one weber per square metre. Henry is an inductance unit, coulomb an electric charge unit, and weber magnetic flux rather than flux density. Their different dimensions help distinguish these electromagnetic quantities.
Q23. What is the SI expression for a unit of pressure?
[64th BPSC Prelims – 2018]
A. Newton per square metre
B. Newton-metre
C. Newton
D. Newton per metre
Answer: A. Newton per square metre
Explanation: Pressure is force divided by the surface area over which it acts normally. Hence its SI unit is N/m², or pascal. Newton-metre represents torque or energy, newton represents force, and newton per metre can represent surface tension.
EXAM YEAR 2017
Q22. Which of the following gives the SI unit of electrical conductivity?
[SSC Scientific Assistant (Electronics) – 23 November 2017]
A. Ω·m
B. m
C. Ω/m
D. 1/(Ω·m)
Answer: D. 1/(Ω·m)
Explanation: Electrical conductivity σ is the reciprocal of electrical resistivity ρ. Resistivity has unit ohm-metre, so conductivity has unit reciprocal ohm-metre, also written siemens per metre. This inverse relationship connects material conduction to electrical resistance.
Q21. Pressure is measured in which SI unit?
[SSC MTS – 21 October 2017]
A. Newton
B. Pascal
C. Watt
D. Joule
Answer: B. Pascal
Explanation: Pressure expresses the force spread over each unit area, so it is measured in pascals. Force itself is measured in newtons. Watt refers to power and joule to energy, demonstrating the importance of distinguishing a quantity from related quantities.
