SI Units — Set 1

Correct Answer: C. Candela

• **Candela** = Candela (cd) is the SI base unit of luminous intensity, measuring the power of light emitted in a specific direction. • **7 base units** — Candela is one of the seven SI base units; it quantifies perceived brightness rather than total radiant power. • 💡 Wrong-option analysis: Kelvin: SI unit of thermodynamic temperature, not light; Ampere: SI unit of electric current; Mole: SI unit of amount of substance.

Correct Answer: A. Radian

• **Radian** = The radian is the SI unit for measuring plane angles, defined as the angle subtended at the centre of a circle by an arc equal in length to the radius. • **2π radians** — A full circle equals exactly 2π radians (approximately 6.283 radians), making radian the natural unit in mathematics. • 💡 Wrong-option analysis: Minute: a subdivision of degree, not an SI unit; Steradian: SI unit for solid angle (3D), not plane angle; Degree: common but not the SI unit.

Correct Answer: C. Pascal

• **Pascal** = The pascal (Pa) is the SI unit of pressure, defined as one newton per square metre (N/m²). • **101,325 Pa** — Standard atmospheric pressure equals 101,325 Pa, named after French scientist Blaise Pascal. • 💡 Wrong-option analysis: Newton: SI unit of force, not pressure; Joule: SI unit of energy/work; Watt: SI unit of power.

Correct Answer: C. Mole

• **Mole** = The mole is the SI base unit for amount of substance, representing exactly 6.02214076 × 10²³ elementary entities (Avogadro's number). • **6.022 × 10²³** — This Avogadro constant was fixed in the 2019 SI redefinition, linking mole to a precise count of particles. • 💡 Wrong-option analysis: Gram: a unit of mass, not amount of substance; Liter: a unit of volume; Kilogram: SI base unit of mass, not substance amount.

Correct Answer: A. Ampere

• **Ampere** = The ampere (A) is the SI base unit of electric current, named after physicist André-Marie Ampère. • **1 coulomb per second** — One ampere represents the flow of one coulomb of electric charge per second through a conductor. • 💡 Wrong-option analysis: Coulomb: unit of electric charge (not current); Volt: unit of electric potential difference; Ohm: unit of electrical resistance.

Correct Answer: D. Joule

• **Joule** = The joule (J) is the SI unit for both work and energy, defined as the work done when a force of one newton acts through a distance of one metre. • **1 watt-second** — One joule equals one watt-second (W·s), and also equals one newton-metre (N·m). • 💡 Wrong-option analysis: Newton: unit of force, not energy; Pascal: unit of pressure; Watt: unit of power (rate of energy transfer, not energy itself).

Correct Answer: A. Hertz

• **Hertz** = Hertz (Hz) is the SI unit of frequency, named after German physicist Heinrich Hertz. • **1 cycle per second** — One hertz equals one complete cycle or oscillation per second, making it fundamental in wave physics and electronics. • 💡 Wrong-option analysis: Tesla: SI unit of magnetic flux density; Watt: SI unit of power; Decibel: a logarithmic unit for sound intensity, not frequency.

Correct Answer: D. Kelvin

• **Kelvin** = Kelvin (K) is the SI base unit of thermodynamic temperature, starting from absolute zero (the lowest possible temperature). • **273.15 K** — Water freezes at 273.15 K; absolute zero is 0 K (−273.15°C), where molecular motion theoretically stops. • 💡 Wrong-option analysis: Celsius: common temperature scale but not the SI base unit; Rankine: an absolute scale used in engineering but not SI; Fahrenheit: non-SI scale used mainly in the USA.

Correct Answer: C. Isaac Newton

• **Isaac Newton** = The newton (N) is the SI unit of force, named in honor of Sir Isaac Newton for his laws of motion. • **1 kg·m/s²** — One newton is the force required to accelerate a 1 kg mass at 1 m/s², derived directly from Newton's second law F = ma. • 💡 Wrong-option analysis: Michael Faraday: the farad (capacitance) is named after him; James Joule: the joule (energy) is named after him; James Watt: the watt (power) is named after him.

Correct Answer: B. Tesla

• **Tesla** = The tesla (T) is the SI unit of magnetic flux density (magnetic induction), named after inventor Nikola Tesla. • **1 Wb/m²** — One tesla equals one weber per square metre; Earth's magnetic field is about 25–65 microtesla. • 💡 Wrong-option analysis: Henry: SI unit of electrical inductance; Gauss: CGS unit of magnetic flux density (1 T = 10,000 gauss), not SI; Weber: SI unit of magnetic flux, not flux density.