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What is SI Units in Physics?

Table of SI Units

a. Table of Base SI Units

These seven base units form the foundation of the SI system and we use them as building blocks for expressing other physical quantities. Here is a brief table of SI (International System of Units) base units:

Electric CurrentAmpere(A)
Amount of SubstanceMole(mol)
Luminous IntensityCandela(cd)

b. Table of Derived SI Units

These derived units are due to the combinations of the base SI units in various mathematical expressions to represent different physical quantities. Here is a brief table of 20 derived SI units:

Derived QuantityUnitSymbol
SpeedMeter per Second (m/s)(m/s)
AccelerationMeter per Second Squared (m/s²)(m/s2)
ForceNewton (N)(N)
EnergyJoule (J)(J)
PowerWatt (W)(W)
PressurePascal (Pa)(Pa)
FrequencyHertz (Hz)(Hz)
VoltageVolt (V)(V)
Electric CurrentAmpere (A)(A)
ResistanceOhm (Ω)(Ω)
Electric ChargeCoulomb (C)(C)
Electric PotentialVolt (V)(V)
CapacitanceFarad (F)(F)
Magnetic FluxWeber (Wb)(Wb)
Magnetic Flux DensityTesla (T)(T)
InductanceHenry (H)(H)
Luminous FluxLumen (lm)(lm)
IlluminanceLux (lx)(lx)
Electric ConductanceSiemens (S)(S)
Catalytic ActivityKatal (kat)(kat)

What is SI Units?

SI units in Physics (International system of units), also known as the metric system, were established to promote international collaboration and facilitate clear and efficient communication of measurements. The system is based on decimal multiples and submultiples of units, making it easy to convert between different magnitudes.

Definition of SI Units

SI units in Physics (International system of units) are defined by the International Bureau of Weights and Measures (BIPM). It is the standard of measurement all across the globe in the form of a metric system. The definitions are based on fundamental constants of nature that are considered to be universal and unchanging. These constants provide a reliable and consistent basis for measurement.

What is SI Units in Physics
What is SI units in Physics?

Most Commonly Used SI Units in Physics

There are three commonly used systems of units in science and engineering. These systems are:

  1. Meter – Kilograms Second: system or MKS system
  2. Gaussian system: The Gaussian system consists of (a) Centimetre – grams – second system (b) British engineering system i.e foot-pound system or fps

The meter, centimetre, and the foot are the units of length. Kilogram and gram are the units of mass. Finally, the pound is the unit of force.

The MKS system of units is, however, the metric system of units universally used in scientific work. It provides the common basic units used in most countries of the world. Today, the new international unit system of measurement is systeme internationale d’unite (or SI unit). This unit system is derived and extended from the MKS system of units along with agreed symbols and abbreviations).

Types of Units of Measurement

There are two main units of measurement:

  1. Fundamental units: These are units that can help us to obtain other units.
  2. Derived Units: To obtain these, we simply multiply or divide two or more fundamental units.

The History of SI Units in Physics

We can trace the origins of SI units back to the French Revolution in the late 18th century when there was a need for a standardized system of measurement. The metric system was initially developed, and over time, it evolved into the SI system that we use today. The International System of Units was officially adopted in 1960 by the General Conference on Weights and Measures.

Table of Units

QuantityUnit of MeasurementSymbol
Fundamental unitMassKilogramkg
Electric CurrentAmpereA
Amount of SubstanceMolemol
Luminous IntensityCandelacd
Derived UnitsForceNewtonN
DensityKilogram per meter cubekg/m3
Electric chargeCoulombc
MomentumKilogram metre per second or kgm/s or
Newton SecondNs
Table of units

The Seven Base SI Units

The SI system is built upon seven base units, they are the foundation for measuring different physical quantities. These base units are as follows:

1. Meter

The meter (m) is the unit of length. It is the distance travelled by light in a vacuum during a specific time interval.

2. Kilogram

The kilogram (kg) is the unit of mass. Previously, it was defined by a physical prototype known as the International Prototype of the Kilogram, but since 2019, it is defined in terms of the Planck constant.

3. Second

The second (s) is the unit of time. It is defined based on the radiation emitted by a caesium-133 atom.

4. Ampere

The ampere (A) is the unit of electric current. It is defined in terms of the elementary charge.

5. Kelvin

The kelvin (K) is the unit of temperature. It is defined based on the Boltzmann constant.

6. Mole

The mole (mol) is the unit of the amount of substance. We can define it as the amount of a substance that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12.

7. Candela

The candela (cd) is the unit of luminous intensity. We can define it based on the power of a light source.

Derived SI Units

In addition to the base units, the SI system also includes derived units. They are due to the combinations of the base units and we use them to measure various physical quantities. Some commonly used derived SI units include:


The newton (N) is the unit of force, defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second squared.


The joule (J) is the unit of energy and work. We can define it as the work done or energy transferred when a force of one newton is applied over a distance of one meter.


The watt (W) is the unit of power. We can define it as one joule per second and we use it to measure the rate of doing work or the transfer of energy.


The coulomb (C) is the unit of electric charge. We can define it as the quantity of electricity transported in one second by a current of one ampere.


The hertz (Hz) is the unit of frequency. It represents the number of cycles or oscillations per second and we commonly use it to measure the frequency of waves and vibrations.


The volt (V) is the unit of electric potential difference or voltage. We can define it as one joule of energy per coulomb of charge.


The ohm (Ω) is the unit of electrical resistance. It represents the resistance when a voltage of one volt is applied to a conductor and a current of one ampere flows through it.


The lumen (lm) is the unit of luminous flux. It measures the total amount of light emitted by a light source.

These derived units provide a comprehensive and standardized way to measure a wide range of physical quantities, enabling scientists, engineers, and researchers to communicate and collaborate effectively.

Advantages of SI Units

The SI system offers several advantages over other systems of measurement. These advantages include:

Universal Application

SI units are recognized and used internationally, providing a common language for scientists, engineers, and researchers worldwide. This universal application allows for seamless communication and collaboration across borders and disciplines.

Consistency and Accuracy

SI units are based on precise and well-defined definitions, using fundamental constants of nature. This ensures consistency and accuracy in measurements, reducing errors and discrepancies in scientific experiments and industrial processes.

Ease of Conversion

SI units are based on decimal multiples and submultiples, making conversions between different magnitudes straightforward. This ease of conversion simplifies calculations and enables easy comparison of measurements.

Simplicity and Clarity

The designation of SI system is very simple and intuitive. The base and their derived units cover a wide range of physical quantities, eliminating the need for numerous measurement systems and reducing complexity.

Applications of SI Units in Physics

SI units find extensive applications in various fields, including:

Science and Research

SI units are fundamental to scientific research and experimentation. We use them to measure and quantify physical quantities in fields such as physics, chemistry, biology, and astronomy. Scientists rely on SI units to ensure the accuracy, consistency, and reproducibility of their findings.

Engineering and Technology

SI units play a crucial role in engineering and technology. From designing structures to developing electronic circuits, engineers use SI units to make precise measurements and calculations. The consistency of SI units enables interoperability and compatibility in engineering designs and specifications.

Commerce and Trade

SI units are essential in commerce and trade, facilitating fair and accurate transactions. Industries rely on SI units to determine product specifications, ensure quality control,

and facilitate international trade. SI units provide a standardized system that allows for easy comparison and exchange of goods and services across different countries and markets.

Education and Academia

We use SI units extensively in educational institutions and academic research. Students learn to use SI units to perform experiments, analyze data, and communicate their findings. The use of SI units fosters a common understanding and promotes consistency in scientific literature.

Healthcare and Medicine

SI units are vital in the field of healthcare and medicine. We also them to measure various parameters such as weight, temperature, blood pressure, and medication dosages. Consistent and accurate measurements using SI units are crucial for diagnosing illnesses, monitoring patient health, and ensuring effective treatment.


The International System of Units (SI) provides a universal, consistent, and standardized approach to measurement. It encompasses base units and derived units that cover a wide range of physical quantities. SI units offer advantages such as universality, accuracy, ease of conversion, simplicity, and clarity. They find applications in diverse fields including science, engineering, commerce, education, healthcare, and more. Embracing SI units ensures clear communication, precise measurements, and facilitates collaboration in a globalized world.

Frequently Asked Questions

  1. Can we use SI units to measure non-physical quantities? SI units are primarily designed for measuring physical quantities, such as length, mass, time, etc. They may not be directly applicable to non-physical quantities like emotions or abstract concepts.
  2. Are SI units universally adopted? While the majority of countries have using the SI system, some regions may still use a mix of traditional and metric systems. However, SI units are widely recognized and used in scientific and international contexts.
  3. Can I use SI units in everyday life? Absolutely! SI units are not limited to scientific or technical use. They can be used every day to measure distances, weights, temperatures, and other common quantities.
  4. What are the advantages of using SI units in research publications? Using SI units in research publications ensures clarity, consistency, and compatibility. It allows for easy replication of experiments and facilitates collaboration among researchers from different countries.
  5. How can I convert between SI units? Converting between SI units is straightforward as it involves multiplying or dividing by powers of ten. You can consult conversion tables or use online conversion tools for precise conversions.

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Difference between Fundamental Quantities and Derived Quantities