SI Units — Set 2

Correct Answer: C. Volt

• **Volt** = The volt (V) is the SI unit of electric potential difference and electromotive force, named after Alessandro Volta. • **1 W/A** — One volt is defined as one watt per ampere; a conductor carrying 1 A with 1 W of power has a potential difference of 1 V. • 💡 Wrong-option analysis: Farad: SI unit of capacitance; Ohm: SI unit of electrical resistance; Ampere: SI base unit of electric current.

Correct Answer: B. Ohm

• **Ohm** = The ohm (Ω) is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm. • **1 V/A** — One ohm equals one volt per ampere; it quantifies how strongly a material opposes the flow of electric current. • 💡 Wrong-option analysis: Siemens: SI unit of electrical conductance (the inverse of ohm); Henry: SI unit of inductance; Farad: SI unit of capacitance.

Correct Answer: A. Farad

• **Farad** = The farad (F) is the SI unit of electrical capacitance, named after English scientist Michael Faraday. • **1 C/V** — One farad equals one coulomb per volt; a capacitor has a capacitance of 1 F when 1 C of charge causes a potential difference of 1 V. • 💡 Wrong-option analysis: Henry: SI unit of inductance; Weber: SI unit of magnetic flux; Coulomb: SI unit of electric charge, not capacitance.

Correct Answer: B. Henry

• **Henry** = The henry (H) is the SI unit of electrical inductance, named after American scientist Joseph Henry. • **1 V·s/A** — One henry is the inductance that produces an EMF of one volt when the current changes at one ampere per second. • 💡 Wrong-option analysis: Farad: SI unit of capacitance; Ohm: SI unit of resistance; Tesla: SI unit of magnetic flux density.

Correct Answer: C. Lumen

• **Lumen** = The lumen (lm) is the SI unit of luminous flux, measuring the total quantity of visible light emitted by a source. • **derived from candela** — One lumen equals one candela multiplied by one steradian (cd·sr); a higher lumen value means a brighter light source. • 💡 Wrong-option analysis: Watt: measures radiant power, not luminous flux perceived by the human eye; Candela: measures luminous intensity in a direction; Lux: measures illuminance (lumens per square metre).

Correct Answer: C. Lux

• **Lux** = The lux (lx) is the SI unit of illuminance, measuring the luminous flux incident on a surface per unit area. • **1 lm/m²** — One lux equals one lumen per square metre; direct sunlight provides about 100,000 lux on a clear day. • 💡 Wrong-option analysis: Lumen: total luminous flux emitted, not incident flux per area; Dioptre: unit of optical power of a lens; Candela: luminous intensity in a specific direction.

Correct Answer: B. Becquerel

• **Becquerel** = The becquerel (Bq) is the SI unit of radioactivity, named after Henri Becquerel who discovered radioactivity in 1896. • **1 decay/second** — One becquerel equals one radioactive decay per second; the older unit Curie (1 Ci = 3.7 × 10¹⁰ Bq) has been replaced by Becquerel in SI. • 💡 Wrong-option analysis: Curie: older CGS unit of radioactivity, not SI; Roentgen: unit of radiation exposure (ionisation in air), not SI; Sievert: SI unit of equivalent radiation dose (biological effect).

Correct Answer: D. Gray

• **Gray** = The gray (Gy) is the SI unit of absorbed dose of ionizing radiation, representing energy deposited in matter. • **1 J/kg** — One gray equals one joule of radiation energy absorbed per kilogram of matter; it replaced the older unit 'rad' (1 Gy = 100 rad). • 💡 Wrong-option analysis: Sievert: measures equivalent dose (biological effect), not absorbed dose; Curie: unit of radioactivity (decay rate), not absorbed dose; Becquerel: measures radioactivity (decays per second).

Correct Answer: B. Sievert

• **Sievert** = The sievert (Sv) is the SI unit of equivalent radiation dose, measuring the biological effect of ionizing radiation on human tissue. • **weighting factor** — Sievert = Gray × radiation weighting factor; 1 Sv of any radiation type causes the same biological harm, making it essential for radiation safety standards. • 💡 Wrong-option analysis: Becquerel: measures radioactivity (decay rate), not biological dose; Gray: measures physical absorbed dose without accounting for biological effect; Lux: measures illuminance (visible light), unrelated to radiation.

Correct Answer: A. Katal

• **Katal** = The katal (kat) is the SI unit for catalytic activity, measuring the rate at which a catalyst (especially enzymes) converts substrate. • **1999** — One katal equals one mole of substrate converted per second; it was officially adopted by the General Conference on Weights and Measures in 1999. • 💡 Wrong-option analysis: Mole: measures amount of substance, not catalytic rate; Pascal: SI unit of pressure; Joule: SI unit of energy.