SI Units — Set 6

Correct Answer: A. Pico

• **Pico** = The prefix 'pico' (symbol p) represents 10⁻¹², meaning one-trillionth, used in picofarad and picosecond. • **picofarad** — Capacitors in electronics are often rated in picofarads (pF = 10⁻¹² F); 1 pF is one-trillionth of a farad. • 💡 Wrong-option analysis: Nano: represents 10⁻⁹ (one-billionth); Femto: represents 10⁻¹⁵ (one-quadrillionth); Micro: represents 10⁻⁶ (one-millionth).

Correct Answer: B. 10^9

• **10^9** = The prefix 'giga' (symbol G) represents one billion (10⁹), used in gigabyte, gigahertz, and gigawatt. • **uppercase G** — 'Giga' is represented by uppercase G; a modern computer processor runs at several gigahertz (GHz), meaning billions of cycles per second. • 💡 Wrong-option analysis: 10^15: this is 'peta' (P); 10^6: this is 'mega' (M); 10^12: this is 'tera' (T).

Correct Answer: C. Newton

• **Newton** = The newton (N) is a derived SI unit, not a base unit; it is derived from kilogram, metre, and second. • **F = ma → kg·m/s²** — One newton = 1 kg·m/s² (from Newton's second law); the seven base units do not include the newton. • 💡 Wrong-option analysis: Meter: SI base unit of length; Kelvin: SI base unit of thermodynamic temperature; Candela: SI base unit of luminous intensity.

Correct Answer: A. Reciprocal meter

• **Reciprocal meter** = The reciprocal metre (m⁻¹) is the SI unit of wave number, representing the number of wave cycles per unit length. • **spectroscopy** — Wave numbers are widely used in infrared spectroscopy; for visible light (~500 nm), the wave number is about 2 × 10⁶ m⁻¹. • 💡 Wrong-option analysis: Hertz: unit of frequency (cycles per second, not per metre); Meter: unit of length, not wave number; Radian: unit of plane angle.

Correct Answer: B. Tera

• **Tera** = The prefix 'tera' (symbol T) represents one trillion (10¹²), used in terabyte and terahertz. • **terabyte** — One terabyte = 10¹² bytes; modern hard drives are typically measured in terabytes, making 'tera' a familiar prefix. • 💡 Wrong-option analysis: Exa: represents 10¹⁸ (one quintillion); Giga: represents 10⁹ (one billion); Peta: represents 10¹⁵ (one quadrillion).

Correct Answer: A. Kilogram per mole

• **Kilogram per mole** = kg/mol is the SI unit of molar mass, representing the mass of one mole of a substance. • **g/mol in practice** — While kg/mol is the SI unit, chemists commonly express molar mass in g/mol; hydrogen's molar mass is 0.001 kg/mol (1 g/mol). • 💡 Wrong-option analysis: Kilogram per cubic meter: SI unit of density, not molar mass; Gram per mole: common practical unit but not the formal SI unit (uses gram, not kilogram); Mole per kilogram: this is molality, the inverse relationship.

Correct Answer: B. Ampere-square meter

• **Ampere-square meter** = A·m² is the SI unit of magnetic dipole moment, describing the strength and orientation of a magnetic dipole. • **equivalent to J/T** — One A·m² equals one joule per tesla (J/T), since energy U = −m·B where B is in tesla. • 💡 Wrong-option analysis: Ampere-meter: not the standard SI unit for magnetic dipole moment; Weber-meter: not a standard SI unit; Tesla-meter: related to magnetic flux calculations, not dipole moment.

Correct Answer: A. Candela

• **Candela** = The candela (cd) is the SI base unit of luminous intensity, measuring the power of light emitted in a specific direction weighted by human eye sensitivity. • **one of 7 base units** — Candela is unique among SI base units in being defined by a physiological response; it replaced the older unit 'candle' in 1948. • 💡 Wrong-option analysis: Watt: measures total radiant power regardless of visibility to the human eye; Lux: measures illuminance (lumens per m²); Lumen: measures total luminous flux from a source.

Correct Answer: C. Pascal

• **Pascal** = The pascal (Pa) is the SI unit of pressure, equal to one newton per square metre (N/m²), named after Blaise Pascal. • **101,325 Pa** — Standard atmospheric pressure is 101,325 Pa (1 atm); even modest pressures like a car tyre involve about 200,000 Pa. • 💡 Wrong-option analysis: Newton: SI unit of force, not pressure (pressure = force per area); Bar: practical unit of pressure (1 bar = 10⁵ Pa) but not the SI unit; Joule: SI unit of energy/work.

Correct Answer: B. Mole

• **Mole** = The mole (mol) is the SI base unit of amount of substance, one of the seven fundamental SI base units. • **6.022 × 10²³ entities** — One mole contains exactly 6.02214076 × 10²³ elementary entities (Avogadro's number), fixed in the 2019 SI redefinition. • 💡 Wrong-option analysis: Gram: a unit of mass, not amount of substance; Kilogram: SI base unit of mass; Dalton: atomic mass unit (1 Da ≈ 1.66 × 10⁻²⁷ kg), used for atomic/molecular masses, not amount of substance.