Which Of The Following Is Not An Si Base Unit

So, the International System of Units, universally known as the SI (from the French Système International d'Unités), forms the bedrock of modern science, engineering, and global commerce. It provides a coherent framework where every measurable quantity can be expressed through a combination of just seven defining constants and seven base units. In real terms, understanding the distinction between these SI base units and the vast array of derived units is a fundamental milestone for any student of physics or chemistry. Because of that, a common examination question asks: *which of the following is not an SI base unit? * To answer this confidently, one must first memorize the exclusive list of the seven base units and then recognize the imposters—units that are derived, non-SI, or obsolete That's the part that actually makes a difference..

The Magnificent Seven: The SI Base Units

Since the 2019 redefinition, the SI system rests on seven defining constants, but the structural architecture remains built upon seven base units. Plus, these are dimensionally independent; none can be expressed as a combination of the others. They are the alphabet from which the language of measurement is written.

1. Second (s) – The unit of time. It is defined by taking the fixed numerical value of the caesium frequency $\Delta\nu_{Cs}$, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s$^{-1}$.
2. Metre (m) – The unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum $c$ to be 299,792,458 when expressed in the unit m s$^{-1}$.
3. Kilogram (kg) – The unit of mass. It is defined by taking the fixed numerical value of the Planck constant $h$ to be $6.62607015 \times 10^{-34}$ when expressed in the unit J s, which is equal to kg m$^2$ s$^{-1}$.
4. Ampere (A) – The unit of electric current. It is defined by taking the fixed numerical value of the elementary charge $e$ to be $1.602176634 \times 10^{-19}$ when expressed in the unit C, which is equal to A s.
5. Kelvin (K) – The unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant $k$ to be $1.380649 \times 10^{-23}$ when expressed in the unit J K$^{-1}$, which is equal to kg m$^2$ s$^{-2}$ K$^{-1}$.
6. Mole (mol) – The unit of amount of substance. One mole contains exactly $6.02214076 \times 10^{23}$ elementary entities (Avogadro's number).
7. Candela (cd) – The unit of luminous intensity. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency $540 \times 10^{12}$ Hz, $K_{cd}$, to be 683 when expressed in the unit lm W$^{-1}$, which is equal to cd sr kg$^{-1}$ m$^{-2}$ s$^3$.

These seven—and only these seven—are the SI base units. If a multiple-choice question lists any unit not on this list, that unit is the correct answer to "which is not an SI base unit?"

Common Imposters: Derived Units Masquerading as Base Units

The most frequent distractors in physics exams are SI derived units. These are formed by multiplying or dividing base units. Because they are used daily, they often feel fundamental, but they are mathematically dependent on the seven base units.

The Newton (N) – Force

Perhaps the most common trap. Force feels like a primal concept. On the flip side, Newton’s Second Law ($F = ma$) dictates its dimensions: mass $\times$ acceleration. $1 \text{ N} = 1 \text{ kg} \cdot \text{m} \cdot \text{s}^{-2}$ Because it combines kilogram, metre, and second, the Newton is a derived unit, not a base unit.

The Joule (J) – Energy, Work, Heat

Energy is the capacity to do work. Work is force $\times$ distance. $1 \text{ J} = 1 \text{ N} \cdot \text{m} = 1 \text{ kg} \cdot \text{m}^2 \cdot \text{s}^{-2}$ The Joule is derived Worth keeping that in mind. And it works..

The Watt (W) – Power

Power is the rate of energy transfer. $1 \text{ W} = 1 \text{ J} \cdot \text{s}^{-1} = 1 \text{ kg} \cdot \text{m}^2 \cdot \text{s}^{-3}$ The Watt is derived And that's really what it comes down to. But it adds up..

The Pascal (Pa) – Pressure

Pressure is force per unit area. $1 \text{ Pa} = 1 \text{ N} \cdot \text{m}^{-2} = 1 \text{ kg} \cdot \text{m}^{-1} \cdot \text{s}^{-2}$ The Pascal is derived Less friction, more output..

The Coulomb (C) – Electric Charge

Charge is current $\times$ time. $1 \text{ C} = 1 \text{ A} \cdot \text{s}$ The Coulomb is derived. (Note: Since the 2019 redefinition, the Ampere is defined via the elementary charge, making the Coulomb technically a specific number of elementary charges, but structurally it remains a derived unit in the SI brochure) Most people skip this — try not to..

The Volt (V) – Electric Potential

Potential difference is energy per unit charge. $1 \text{ V} = 1 \text{ J} \cdot \text{C}^{-1} = 1 \text{ kg} \cdot \text{m}^2 \cdot \text{s}^{-3} \cdot \text{A}^{-1}$ The Volt is derived That's the part that actually makes a difference..

The Ohm ($\Omega$) – Electrical Resistance

$1 \Omega = 1 \text{ V} \cdot \text{A}^{-1} = 1 \text{ kg} \cdot \text{m}^2 \cdot \text{s}^{-3} \cdot \text{A}^{-2}$ The Ohm is derived.

The Hertz (Hz) – Frequency

Frequency is the reciprocal of time. $1 \text{ Hz} = 1 \text{ s}^{-1}$ The Hertz is derived.

Critical Distinction: The Kilogram (kg) is the only SI base unit that contains a prefix (kilo-) in its name and symbol. This is a historical artifact. The base unit of mass is the kilogram, not the gram. The gram (g) is a submultiple ($10^{-3}$ kg), making the gram a derived unit (or more precisely, a decimal submultiple of the base unit), not a base unit itself.

Non-SI Units Accepted for Use with SI

Another category of "not an SI base unit" includes units that are not part of the SI system at all but are accepted for use alongside it because of their widespread historical, cultural, or practical importance. If these appear in a multiple-choice list, they are definitively not SI base units.

We're talking about where a lot of people lose the thread.

• Minute (min), Hour (h), Day (d) – Units of time. (Base unit is the second).
• Litre (L or l) – Unit of volume. ($1 \text{ L} = 1 \text{ dm}^3 = 10^{-3} \text{ m}^3$).
• Tonne (t) – Unit of mass. ($

… ($1 \text{ t} = 10^{3} \text{ kg}$). The tonne is therefore a decimal multiple of the kilogram and, although accepted for use with the SI, it is not a base unit It's one of those things that adds up. But it adds up..

• Electronvolt (eV) – Commonly used in particle physics to express energies. ($1 \text{ eV} = 1.602 176 634 × 10^{-19} \text{ J}$). It is derived from the joule and the elementary charge, thus not an SI base.
• Astronomical unit (au) – Employed for distances within the Solar System. ($1 \text{ au} = 149 597 870.7 \text{ m}$). Like the tonne, it is a accepted non‑SI unit.
• Neper (Np) and Bel (B) – Logarithmic units for ratios of field‑quantities and power‑quantities respectively. The neper is dimensionless (defined as $\ln(x)$) and the bel is $\log_{10}(x)$; both are accepted for use with the SI but are not base units.
• Bar (bar) – Frequently used in meteorology and engineering for pressure. ($1 \text{ bar} = 10^{5} \text{ Pa}$). Again, an accepted non‑SI unit.

These examples illustrate that the SI system deliberately limits its foundation to seven invariant base units—second, metre, kilogram, ampere, kelvin, mole, and candela—while allowing a broad catalogue of derived units and a select set of external units for practical convenience. Any unit that cannot be traced directly to one of these seven through multiplication or division (including those with prefixes, historical names, or distinct definitions) falls outside the category of SI base units.

Conclusion: Understanding the distinction between SI base units, derived units, and accepted non‑SI units is essential for clear scientific communication. While the SI provides a coherent, universally accepted framework built on seven base units, the derived units (such as the joule, watt, pascal, etc.) and the accepted external units (like the litre, tonne, electronvolt, etc.) extend its applicability without compromising the underlying dimensional consistency. Recognizing which units belong to each class helps avoid confusion, ensures correct conversion factors, and reinforces the robustness of the International System of Units.