What is fundamental Physical Quantity?

In physics, there are seven fundamental physical quantities which are the basis in SI units system to derive any desired physical quantity. These seven fundamental physical quantities are listed below along with their SI units and symbols. Length. SI unit of length is called a metre having symbol as m. Time. SI unit of time is called the second having a symbol as s. Mass. SI unit of mass is kilogram having symbol as kg. Electric current. SI unit of electric current is called an ampere symbol as A. Luminous Intensity. SI unit of luminous intensity is called candela having the symbol as Cd. Amount of substance. SI unit of amount of substance is called mole having symbol as mol. Temperature. SI unit of temperature is called kelvin having the symbol as K.

What is SI unit of time?

The SI unit of time is known as seconds and the symbol is denoted as s. It is one of the fundamental physical quantities which is listed in SI units of measurement.

What is derived units?

There are seven fundamental physical quantities namely Length, Mass, Time, Luminous Intensity, Electric current, Amount of substance, Temperature. Other than these seven fundamental physical quantities listed by International system of units, all the physical quantities SI units can be derived from these seven units such as Area, Volume, Frequency and many more useful physical quantities, these are all derived from seven fundamental physical quantities hence known as derived units.

Which of the following is a derived unit? (A) Second (B) Kilogram (C) Newton (D) Ampere

According to International system of units, seven fundamental units are Length(metre), Time (seconds), Temperature (Kelvin), Luminous Intensity (Candela), Amount of substance (mole), Mass (Kilogram), Electric current (Ampere). Any SI unit other than these seven is called Derived quantities, so from given options Newton which is a derived SI unit of physical quantity Force is a derived unit. Hence, the correct option is (C) Newton, is a derived unit.

What is the unit meaning?

The term SI stands for International system of units, which is made to establish the common reference of measurements of all the physical quantities. The units defined by International system of units (SI units) are accepted by all the countries and use these standard units for measurement of all the physical quantities in scientific fields.

What is the significance of the Planck constant in the modern SI system?

The Planck constant (h) is a fundamental physical constant that plays a crucial role in quantum mechanics. In the 2019 redefinition of SI base units, the Planck constant was given an exact defined value, which in turn was used to define the kilogram. This shift from a physical artifact (the International Prototype Kilogram) to a definition based on an invariant constant of nature ensures long-term stability and universality of the mass unit. The Planck constant now links the macroscopic world of everyday measurements to the quantum world, underpinning the entire SI system.

How does the SI system address measurements in fluid dynamics, particularly viscosity?

In fluid dynamics, viscosity is a measure of a fluid's resistance to flow. The SI unit for dynamic viscosity is the pascal-second (Pa⋅s), which is equivalent to one newton-second per square meter (N⋅s/m²). For kinematic viscosity, which is dynamic viscosity divided by density, the SI unit is square meters per second (m²/s). These units allow for precise descriptions of fluid behavior across various scales, from the flow of blood in capillaries to the movement of tectonic plates. The consistency of SI units in this field enables accurate comparisons and calculations in diverse applications, from engineering to geophysics.

Why is the candela considered a base SI unit, and how does it relate to human perception of light?

The candela (cd) is the SI base unit for luminous intensity, which measures the power emitted by a light source in a particular direction, weighted to the sensitivity of the human eye. It's considered a base unit because light perception is a fundamental aspect of human interaction with the physical world, distinct from other physical quantities. One candela is defined as the luminous intensity of a source that emits monochromatic radiation of frequency 540 × 10¹² Hz (green light) with a radiant intensity of 1/683 watt per steradian. This definition links the physical measurement of light to human visual perception, making it crucial for fields like optics, photography, and lighting design.

How does the SI system address the measurement of very large masses, such as in astronomy?

While the kilogram is the SI base unit for mass, astronomical objects often have masses far beyond practical use of this unit. The SI system accommodates this through the use of prefixes and scientific notation. For example:

Si Units List - Definition, Advantages, FAQs

Team Careers360Updated on 02 Jul 2025, 04:35 PM IST

What is unit?

The term unit indicates the amount or magnitude of any quantity which is measurable. Different units are used to indicate different measurements of particular kind of quantities.

What is unit in Physics?

In physics, there are a lot of physical quantities which are measured to perform an experiment and a lot of physical quantities which form the basis of mathematical equations which govern laws of physics such as length, mass and many more. In order to measure such physical quantities, Appropriate units are made to measure them accurately.

Commonly Asked Questions

Q: How does the SI system handle units for quantities like pressure that have multiple common representations?

The SI system provides a standard unit for pressure - the Pascal (Pa), defined as one Newton per square meter (N/m²). However, it also recognizes that in different fields, other units might be more practical. For example, atmospheric pressure is often expressed in millibars (1 mbar = 100 Pa) or millimeters of mercury (1 mmHg ≈ 133.322 Pa). The SI system allows for the use of these alternative units but provides clear conversion factors to the standard Pascal, ensuring that all measurements can be easily converted and compared.

Q: What is dimensional analysis, and how does it relate to SI units?

Dimensional analysis is a method used in physics to check the consistency of equations and to derive relationships between physical quantities. It involves analyzing the dimensions (like length, mass, time) of the quantities in an equation. Each SI unit has a unique dimensional formula in terms of the seven base units. For example, force has the dimension [MLT⁻²] (mass × length × time⁻²). By ensuring that the dimensions on both sides of an equation match, scientists can verify the equation's validity and even derive new relationships between physical quantities.

Q: How does the SI system handle units for magnetic fields, and why is this sometimes confusing?

The SI unit for magnetic field strength is the tesla (T), named after Nikola Tesla. One tesla is defined as the magnetic flux density that, when acting on a one-ampere current flowing perpendicular to the field, produces a force of one newton per meter of conductor. This definition involves multiple concepts (force, current, length), reflecting the complexity of magnetism. The confusion often arises because there are two related but distinct concepts: magnetic field strength (measured in amperes per meter, A/m) and magnetic flux density (measured in tesla). In many contexts, both are referred to as "magnetic field," leading to potential misunderstandings.

Q: How does the SI system handle units for energy in different contexts, such as mechanics, thermodynamics, and electricity?

The SI system uses the joule (J) as the standard unit for energy across all contexts, promoting consistency and ease of conversion. In mechanics, joules measure work and kinetic energy. In thermodynamics, they quantify heat energy. In electricity, electrical energy is also measured in joules. However, other units are sometimes used for practical reasons:

Q: What is the relationship between the SI units for power (watt) and energy (joule)?

The watt (W) is the SI unit for power, while the joule (J) is the unit for energy. They are closely related: one watt is defined as one joule per second (1 W = 1 J/s). This means that power is the rate of energy transfer or conversion. For example, if a device uses 100 watts of power, it's consuming or producing 100 joules of energy every second. This relationship is crucial in many applications, from calculating electricity consumption to understanding the output of engines or solar panels. It demonstrates how derived units in the SI system are often interconnected, reflecting fundamental relationships in physics.

What is SI unit?

The term SI stands for International system of units. This is the standard system of measurements of units which is used by all the countries to measure any physical quantities. It was made to resolve the issue of different units used by different countries to measure a particular physical quantity.

The SI unit system consists of seven fundamental physical quantities units and these are fundamental because any other physical quantity in physics can be derived from these quantities relations. Any Physical quantity unit which is derived from fundamental physical quantities is known as Derived units.

Also read -

Fundamental Physical Quantities:

In physics, there are seven fundamental physical quantities which form the basis for deriving any other physical quantities. These seven fundamental physical quantities are listed as:

Length
Time
Mass
Electric current
Luminous intensity
Amount of substance
Temperature.

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Definition, Symbols and SI units List of Fundamental physical Quantities:

Below is the SI unit list chart indicating their definition, symbols along with their SI unit.

Quantity Name	Definition	SI Unit	Symbol
Length	Length is defined as the distance between two points of any object.	meter	m
Time	Time is the sequence of events that occurred in nature.	second	s
Mass	Mass is defined as the total amount of matter present in a body.	Kilogram	s
Electric current	Electric current is defined as the flow of electric charges inside a material.	Ampere	A
Luminous Intensity	Luminous Intensity is the total amount of visible light emitted in a unit of time per unit of solid angle.	Candela	Cd
Amount of Substance	The measurement of the number of atoms or molecules inside a material is called Amount of substance.	Mole	mol
Temperature	The measurement of intensity of heat inside a body is called temperature.	Kelvin	K

Related Topics,

Define Derived Units

In physics, other than these seven fundamental physical quantities, any physical quantity can be derived from the relationship between these fundamental physical quantities and hence, these quantities are called derived quantities, and units of such quantities are known as derived units.

Some of the most common and used derived units are listed below with their definition, SI units, and their symbols.

Quantity Name	Definition	SI unit	symbol
Area	It’s the amount of space covered by an object in two-dimensional plane	meter square	m²
Volume	It’s the amount of space covered by an object in three-dimensional space.	meter cube	m³
Velocity	Velocity is the ratio of distance covered by a body over time.	meter per second	ms^-1
Acceleration	Acceleration is the ratio of the velocity of a body over time.	meter per second square	ms^-2
Force	The product between mass of a body and the acceleration produced in it is called Force.	Newton	N
Frequency	The number of oscillations of a body in one second is called frequency.	Hertz	Hz
Work	The product of force applied on a body and the displacement covered by it is known as work done.	Joule	J
Pressure	Pressure is the ratio between force acting on a body to the area to which force is applied.	Pascal	Pa
Power	Power is defined as the ratio of work done by a body over a given period of time.	Watts	W
Density	Density is defined as the ratio between mass and volume of a body.	Kilogram per meter cube	Kgm^-3

These are some derived units and there are more, in brief other than those seven fundamental physical quantities every quantity that is measurable in physics will be derived from those seven fundamental physical quantities and their SI unit name and symbol is after the scientist who discovered a particular kind of physical quantity. For example, It was. Isaac Newton gave the idea of force so, the SI unit of force or derived unit of force is called Newton, named after S. Isaac Newton.

Different units for measurement of most used fundamental physical quantities other than SI base units:

In physics, a single physical quantity can be measured in many different units with respect to their SI units due to their very large numerical values sometimes, some of the most used Fundamental physical quantities can also be measured in different units and are listed as”

Length:

Since The SI unit of length is a meter and its symbol is m. Along with this unit, the following are the most commonly used units for the measurement of length are given in the unit table:

Name of unit	Symbol	Relation with SI unit of Length (m)
Centimetre	cm	1m=100cm
Nanometre	nm	1nm=10^-9m
millimetre	mm	1mm=10^-3m
decimetre	dm	1dm=0.1m
Angstrom	A°	1A°=10^-10m
Kilometre	Km	1km=1000m
Yard	yd	1yd=0.914m
Inch	in	1in=0.0254
foot	ft	1ft=0.305m

Time:

Since The SI unit of time is second and its symbol is s. Along with this unit, the following are the most commonly used units for the measurement of time are given in the unit table:

Name of Unit	Symbol	Relation with SI unit of Time (s)
Minute	min	1min=60s
Hour	hr	1hr=3600s
Day	Day	1day=24hr
Year	yr	1yr=365days

Mass:

Since The SI unit of mass is Kilogram and its symbol is Kg. Along with this unit, the following are the most commonly used units for the measurement of mass are given in the unit table:

Name of unit	Symbol	Relation with SI unit of Mass (Kg)
Grams	gm	1Kg=1000gm
Decigram	dgm	1dgm=0.1gm
Centigram	cgm	1cgm=0.01gm

Commonly Asked Questions

Q: How many base SI units are there, and what are they?

There are seven base SI units:

Q: What's the difference between base units and derived units in the SI system?

Base units are the fundamental units that cannot be broken down further, like meters for length or seconds for time. Derived units are formed by combining base units, such as meters per second (m/s) for velocity or kilogram per cubic meter (kg/m³) for density. Derived units allow us to express more complex physical quantities using combinations of the base units.

Q: What is the SI unit for energy, and why is it named after James Prescott Joule?

The SI unit for energy is the Joule (J). It's named after James Prescott Joule, a 19th-century physicist who studied the nature of heat and its relationship to mechanical work. One Joule is defined as the work done when a force of one Newton is applied over a distance of one meter (J = N⋅m). The unit honors Joule's contributions to understanding the relationship between different forms of energy, a fundamental concept in physics.

Q: What is the SI unit for force, and how is it derived?

The SI unit for force is the Newton (N). It's a derived unit, defined as the force needed to accelerate a mass of one kilogram at a rate of one meter per second squared. In terms of base units, one Newton is equal to one kilogram meter per second squared (kg⋅m/s²). This derivation shows how force relates to mass and acceleration, key concepts in Newton's laws of motion.

Q: What is the difference between mass and weight in the SI system?

In the SI system, mass and weight are distinct concepts with different units. Mass is a measure of an object's amount of matter, measured in kilograms (kg), and is constant regardless of location. Weight is the force exerted on an object due to gravity, measured in Newtons (N), and can vary depending on the strength of the gravitational field. On Earth, an object's weight in Newtons is approximately its mass in kilograms multiplied by 9.8 (the acceleration due to gravity).

Advantages of SI units:

From country to country, we don’t need to convert one unit of any physical quantity measured to another system of units, SI units provide standard units used by all countries in the scientific field. This is the most important advantage of using SI units.
Some physical quantities have very long expressions for their measurements of units, SI units provide special memorable symbols for each fundamental physical quantity SI units which are easy to represent the unit of measurement of any physical quantity.
The fundamental physical quantities are used to determine units of measurements of other physical quantities known as derived units so, the SI unit of fundamental physical quantities gives the advantage to derive any physical quantity SI unit.

Also, check-

NCERT Physics Notes:

Commonly Asked Questions

Q: What are SI units and why are they important?

SI units, or the International System of Units, are a standardized set of measurements used globally in science and everyday life. They're important because they provide a universal language for measurements, ensuring consistency and accuracy in scientific research, international trade, and communication across different fields and countries.

Q: How does the SI unit system help in solving complex physics problems?

The SI unit system helps in solving complex physics problems by providing a consistent framework for measurements and calculations. It allows for easy conversion between units, simplifies dimensional analysis, and helps in checking the validity of equations. By using SI units, physicists can more easily identify errors, compare results across different experiments, and communicate their findings clearly to the global scientific community.

Q: Why was the kilogram redefined in 2019, and how does it affect measurements?

The kilogram was redefined in 2019 to base it on fundamental physical constants rather than a physical artifact. It's now defined in terms of the Planck constant, which is invariable. This change ensures long-term stability and accuracy of the unit, as the previous physical prototype could change slightly over time. For everyday measurements, this change is imperceptible, but it's crucial for high-precision scientific and technological applications.

Q: Why do some countries still use non-SI units, and how does this affect scientific collaboration?

Some countries, like the United States, still use non-SI units (e.g., inches, pounds) due to historical and cultural reasons. This can complicate scientific collaboration, as it requires careful unit conversion and can lead to errors. However, in scientific contexts, SI units are generally used worldwide to ensure consistency and ease of communication. Scientists must be proficient in both systems and careful about conversions to avoid misunderstandings or calculation errors.

Q: How does the concept of significant figures relate to SI units and measurement precision?

Significant figures in measurements using SI units indicate the precision of the measurement. They show how many digits are reliably known, plus one estimated digit. For example, 5.00 m has three significant figures, indicating a more precise measurement than 5 m. Understanding significant figures is crucial when working with SI units to properly represent the accuracy of measurements and calculations, especially when combining measurements of different precisions.

Frequently Asked Questions (FAQs)

Q: What is the significance of the fine-structure constant in physics, and how does it relate to SI units?

The fine-structure constant (α) is a fundamental physical constant that characterizes the strength of the electromagnetic

Q: How does the SI system handle very large or very small measurements?

The SI system uses prefixes to handle very large or very small measurements without changing the base unit. For example, instead of writing 1,000,000 meters, we can write 1 megameter (1 Mm). Similarly, 0.000001 seconds can be written as 1 microsecond (1 μs). These prefixes range from yotta- (10²⁴) to yocto- (10⁻²⁴), allowing for convenient expression of quantities across a vast range of scales while maintaining the benefits of the standardized SI system.

Q: How does the SI unit for temperature (Kelvin) relate to Celsius and Fahrenheit?

The Kelvin scale is the SI unit for temperature, with 0 K being absolute zero. It uses the same increment as Celsius, so a change of 1 K equals a change of 1°C. To convert from Celsius to Kelvin, add 273.15 (0°C = 273.15 K). Fahrenheit uses a different scale, where 32°F = 0°C and 212°F = 100°C. The relationship between Kelvin and Fahrenheit is more complex: K = (°F + 459.67) × 5/9. Scientists prefer Kelvin because it has an absolute zero and simplifies many physical laws and equations.

Q: Why is the speed of light important in the SI system, and how is it used to define the meter?

The speed of light (c) is a fundamental constant in physics and plays a crucial role in the SI system. Since 1983, the meter has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second. This definition links the unit of length (meter) to the unit of time (second) using a universal constant. It provides a more stable and precise definition than previous physical artifacts and allows for extremely accurate measurements in fields like astronomy, particle physics, and precision engineering.

Q: How do SI units help in understanding and comparing astronomical distances?

SI units, particularly the meter, provide a standardized way to express and compare astronomical distances. However, given the vast scales involved in astronomy, scientists often use larger derived units. For example, the astronomical unit (AU), approximately 149,597,870,700 meters, represents the average distance from Earth to the Sun. Light-years (the distance light travels in one year, about 9.46 trillion kilometers) are used for even greater distances. These units, while large, can always be expressed in terms of the base SI unit of meters, allowing for precise calculations and comparisons across different scales in the universe.

Q: What is the significance of the Avogadro constant in the SI system, and how does it relate to the mole?

The Avogadro constant (NA) is a fundamental constant in chemistry and physics, representing the number of entities (usually atoms or molecules) in one mole of a substance. Its value is approximately 6.022 × 10²³. In the SI system, the mole is defined as the amount of substance containing exactly 6.02214076 × 10²³ elementary entities. This definition links the microscopic world of atoms and molecules to the macroscopic quantities we can measure, making it crucial for calculations in chemistry, thermodynamics, and other fields of science.

Q: How does the SI system address the measurement of very small time intervals in particle physics?

While the second is the SI base unit for time, particle physics often deals with extremely short time intervals. The SI system accommodates this through the use of prefixes. For instance, nanoseconds (10⁻⁹ s), picoseconds (10⁻¹² s), or even attoseconds (10⁻¹⁸ s) are commonly used in particle physics. These small units allow scientists to precisely measure and describe events happening at incredibly short time scales, such as the lifetime of certain subatomic particles or the duration of laser pulses, while still maintaining consistency with the SI system.

Q: What is the relationship between the SI units for electric current (ampere) and charge (coulomb)?

The ampere (A) is the SI base unit for electric current, while the coulomb (C) is the SI unit for electric charge. They are closely related: one coulomb is defined as the amount of charge transferred by a current of one ampere in one second. Mathematically, this can be expressed as 1 C = 1 A × 1 s. This relationship demonstrates how derived units (coulomb) can be expressed in terms of base units (ampere and second), showcasing the interconnected nature of the SI system.

Q: Why is water's triple point important in the SI system, and how was it used to define the Kelvin scale?

The triple point of water is the temperature and pressure at which water can coexist in solid, liquid, and gas phases in thermodynamic equilibrium. It occurs at 273.16 K and a partial vapor pressure of 611.66 Pa. Until 2019, the Kelvin scale was defined by setting the triple point of water at exactly 273.16 K. This provided a fixed reference point for temperature measurements. While the definition has changed to be based on the Boltzmann constant, the triple point of water remains an important calibration point in thermometry due to its stability and reproducibility.

Q: How does the SI system handle units for radiation and radioactivity?

The SI system includes several units related to radiation and radioactivity:

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