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**SI Units|Definition & Meaning**

**Definition**

**SI** **stands** for **International** **System** of **Units**, which we call the **Metric** **system**. The system **defines** **seven** **base** **quantities** and **units**: time **(second),** mass **(kilogram),** length **(meter),** electrical current **(ampere),** luminous intensity **(candela),** temperature **(kelvin),** and amount of substance **(mole).** All other quantities in SI Units are derived using these base quantities and units.

Figure 1 – Quantities in International system of units

**Quantities** can be **divided** into **two groups** in the International System of Units (SI): **base quantities** and **derived quantities.**

**Base Quantity**

The term “base quantity” refers to a **quantity** that **cannot** be **defined** in **terms** of **another quantity.** The SI system has seven fundamental units: **length, mass, time, electricity, thermodynamic temperature, substance amount,** and **luminous intensity.**

**Derived Quantity**

**Quantities** that can be **defined** in **terms** of **one** or **more base quantities** are referred to as derived quantities. For instance, **velocity** is a derived quantity representing the displacement of an object over time, and acceleration is another derived quantity defined as the rate of change of velocity over time.

The **derived quantities force, energy, power,** and **electric charge** are additional examples.

The **flowchart** of SI units is shown below.

Figure 2 – Flow of Base Units and Derived Units

**Base Units**

Figure 3 – SI Base Units

For a specific system of measurement, such as the metric or SI system, the base units are the accepted units of measurement (SI). The seven basic SI units are the **meter** (length), the **kilogram** (mass), the **second** (time), the **mole** (amount of substance), the **kelvin** (temperature), the **ampere** (electric current), and the **candela** (luminous intensity).

Here is a quick description of each.

**Meter**

The distance that light covers in a vacuum in **1/299,792,458** of a **second** is measured in **meters,** or m.

**Kilogram**

The kilogram (kg), the fundamental unit of mass, is defined as the weight of the **platinum-iridium cylinder** known as the **International Prototype** of the **Kilogram,** which is kept at the **International Bureau** of **Weights** and **Measures.**

**Second**

The duration of **9,192,631,770 radiation periods,** or the base unit of time, the second, is equal to the change in the **cesium-133 atom’s** ground state between its **two hyperfine levels.**

**Mole**

The mole, or mol, is the basic unit of substance quantity and is the **amount** of a **substance containing** the **same number** of entities is described **as** there are **in 12 grams** of **carbon-12.**

**Kelvin**

The base unit of temperature is the **kelvin (K),** which is equal to **1/273.16** of the triple point of water’s thermodynamic temperature.

**Ampere**

The ampere (A), the fundamental unit of electrical current, is the **constant current** that, **if maintained** in two **parallel, straight conductors** of **infinite length** and negligible **circular cross-section,** spaced one meter apart in vacuum, would result in a force between these conductors equal to 2 x 10^{-7} **newtons per meter** of length.

**Candela**

The **candela (cd),** the basic unit of luminous intensity, is defined as the amount of light emitted in a given direction by a source with a radiative intensity of **1/683 watt per steradian** and a **frequency** of **monochromatic radiation** of 540 x 10^{12} Hz**.**

**Derived Units**

Figure 4 – SI Derived Units

**Defined** in **terms** of the **base units** of a specific system of measurement, such as the International System of Units, derived units are units of measurement (SI). Contrary to base units, derived units are **defined** in **terms** of **combinations** of the **base units** rather than having their own independent definition.

The following are some instances of derived SI units:

**Velocity** v = m/s

**Acceleration** a = m/s^{2}

**Force** N = kgm/s^{2}

**Energy** J = Nm = kgm^{2}/s^{2}

**Power** W = J/s

**Pressure** Pa = N/m^{2}

**Electric charge** C = As

**Derived units** offer a practical and **standardized method** of **expressing various** physical **quantities,** including speed, force, energy, power, and pressure, which are used to measure a wide range of physical quantities.

**Significance of Learning SI Units**

The most popular system of measurement in use today, the International System of Units (SI), offers a consistent and standardized method for measuring physical quantities. Learning SI units is crucial for the following reasons.

**Consistency:**The use of SI units guarantees that measurements of physical quantities are uniform across nations and cultures, facilitating comparison and information exchange.**Precision:**The SI units have a precise definition, making it possible to make accurate measurements and lowering the chance of errors.**Clarity:**Using standardized units of measurement facilitates the comprehension and exchange of data and information in both academic and real-world settings.**Applications in science:**Since SI units offer a standard language for expressing and comparing measurements of physical quantities, they are crucial for scientific research.

International communication and collaboration in fields like science, technology, and business are made possible in large part by the use of SI units.

**Applications of SI Units**

Numerous scientific and technical applications employ the International System of Units (SI units), including.

**Physics:**Physical quantities like length, mass, time, and energy are expressed using SI units.**Chemistry:**In chemical experiments and reactions, SI units are used to express quantities like concentration, volume, and temperature.**Biology:**In biological studies, SI units are used to express quantities like length, mass, volume, and time.**Engineering:**To design and measure structures, machines, and systems, SI units are used in engineering applications.**Medicine:**In medical diagnoses, treatments, and research, SI units are used to express quantities like length, mass, volume, time, and dose.**Meteorology:**In order to express atmospheric parameters like temperature, pressure, and wind speed, SI units are used.**Astronomy:**To express astronomical quantities like distance, mass, and time, SI units are used.

There are many other uses as SI units can be used to depict any quantity.

**SI vs. MKS vs. CGS**

**SI**

Despite being a **contemporary** and **widely used system,** the International System of Units isn’t the only one that has been applied to scientific and technical fields. The **MKS** (meter-kilogram-second) and **CGS** (centimeter-gram-second) units are two **older systems.**

**MKS**

The **three base units** of the SI system—the **meter** (length), **kilogram** (mass), and **second** (time)—are the **foundation of** the **MKS system.** The SI system has largely taken the place of the MKS system, which was widely used in the late 19th and early 20th centuries.

**CGS**

The **centimeter, gram,** and **second** units form the **basis of** the **CGS system** (time). In the 19th century, the CGS system was widely used to express electrical and magnetic quantities in physics and engineering.

## An Example of SI Units Used To Derive Other Quantities

**Consider** the **SI units** of mass, length, and time: kilogram, meter, and second. **Derive** the **units** of **velocity, acceleration,** and **force** using them.

**Solution**

**Velocity**

**Velocity** is given by **displacement over time.** Displacement is essentially length. Therefore, we can say the units will be that of length over time, so **meters/second** is the unit of velocity, or **m/s (meters per second).**

**Acceleration**

Acceleration defines the rate at which velocity changes. This is given by dividing the velocity of an object by time elapsed. The combination of the units of velocity and time in the form of a fraction gets us: **(meters/second)** **/ second** = **meters / second ^{2},** which is the unit of acceleration. We usually write it as

**m/s**(meters per second squared).

^{2}**Force**

Given that force is the product of mass and acceleration (F = ma), combining the units of mass and acceleration gets us the following result:

**F = kg * m/s ^{2}**

The unit **kgms ^{2}** or

**kgm/s**pronounced as kilogram meter per second squared, is

^{2},**also called**and

**written**as the

**newton**or N (after the physicist Isaac Newton).

*All mathematical drawings and images were created with GeoGebra.*