Which is a fundamental unit?

Step 1: Analyze the question We are asked to identify which option is a fundamental (base) unit, not a derived one.

Step 2: Explain the concept The SI system has seven base units:

• Metre (m): length
• Kilogram (kg): mass
• Second (s): time
• Ampere (A): electric current
• Kelvin (K): temperature
• Mole (mol): amount of substance
• Candela (cd): luminous intensity

All other units are derived, meaning they are built from combinations of these base units.

Step 3: Check each option

• A. Ampere is the base unit of electric current, so it is fundamental.
• B. Joule is the unit of energy/work: J = N·m = kg·m²/s². It is derived.
• C. Newton is the unit of force: N = kg·m/s². It is derived.
• D. Ohm is the unit of resistance: Ω = V/A = kg·m²·s⁻³·A⁻². It is derived.
• E. Watt is the unit of power: W = J/s = kg·m²/s³. It is derived.

Step 4: Answer Only the ampere is a fundamental unit.

Tip: Remember the seven base units with the phrase "My Kind Sister Always Keeps Many Cats" (Metre, Kilogram, Second, Ampere, Kelvin, Mole, Candela). If a unit is named after a scientist (Newton, Joule, Watt, Ohm), it is almost always derived. Ampere is the exception, since it is named after a scientist but is still a base unit.