What are physical quantities and their types?
The study of nature and its laws is the simplest and yet the most meaningful way to define physics. It is estimated that every event that occurs in nature, has some straightforward law according to which it occurs. Physics is the study of observation and admission of such natural laws and phenomenon. Physics concerns itself with the guidelines that are applicable to all life, thus, critical knowledge and understanding of physics is vital.
These laws give rise to physical parameters that are used for measurement. Therefore, a physical quantity is a material or system characteristic that can be quantified through measurement. A physical quantity is expressed as a value that is the algebraic multiplication of a number and a designated unit. For example, the physical quantity time can be enumerated as n sec, where n is the number and sec are the unit. Thus, the point to note is that a physical quantity takes at least two characteristics in common – numerical magnitude and the unit.
If one talks about gravity, then one needs to understand the quantity and quality of this gravitational force and how it can be expressed, making it distinct from other parameters. This is where the concept of fundamental quantities and derived quantities comes into play. This led to the development of standard units. The units are defined by a body named Conférence Générale des Poids et Mesures or CGPM also known as General Conference on Weight and Measures in English. This body has been given the authority to decide the units by international agreement.
As one is aware, a large number of physical quantities is measured. Every of these quantities requires a defined unit. However, it is not necessary that these quantities are independent of one another. Consider take a simple example of speed. This unit is dependent on the unit of length (meters) and the unit of time (second), to yield a unit for speed that is meter per second.
Thus, there needs to be a separation of units that are independent and those units, which are dependent. This is where the concept of Fundamental units /quantity and Derived units or quantity comes into play.
A fundamental quantity is defined as follows:
- Every identified fundamental quantity must be independent of each other
- All the other quantities are defined with respect to these fundamental quantities.
As per the international body, there are only seven fundamental quantities. All the other physical quantities that are known to us and expressed by us, are derived quantities, which means that these quantities are derived from fundamental quantities. Thus, taking the example above, it can be concluded that length and time are two distinct fundamental quantities, having distinct fundamental units, however speed is a derived quantity which depends on these two fundamental quantities.
There are several systems prevalent in the world and every system has its own fundamental quantities, such as SI units, FPS units, CGS units etc. Most prevalent are SI Units as they have international acceptance. In many Physics’ texts, fundamental quantities are defined as Base quantities having their own Base unit. Any fundamental or base unit should have the properties of Invariability and Availability.
As stated, there are seven fundamental quantities, which are described as below.
- Length, fundamental unit – meter (m)
- Mass, fundamental unit – Kilogram (kg)
- Time, fundamental unit – second (s)
- Electric current, fundamental unit – Ampere (A)
- Thermodynamic temperature, fundamental unit – Kelvin (K)
- Amount of substance, fundamental unit – Mole, (mol)
- Luminous intensity, fundamental unit – Candela (cd)
Besides these, there exists two other units called supplementary units.
- Plane angle with unit Radian (rad)
- Solid angle with unit steradian (sr)
Each of these fundamental units can be defined as per the CGPM body.
- Second: It is the time that makes the unperturbed ground state hyperfine transition frequency to be 9192631770 when expressed in the unit.
- Meter: It is the length that makes the speed of lighting vacuum to be 299792458 when expressed in the unit ms‑1
- Kilogram: It is the mass that makes the Planck’s constant h to be 6.62607015 x 10^- 34 when expressed in the unit J-s
- Ampere: Ampere is the current which makes the elementary charge e to be 1.602176634 x 10^-19 when expressed in the unit C
- Kelvin: Kelvin is the temperature that makes the Boltzmann constant to be 380649 x 10^-23 when expressed in the unit J/K.
- Mole: 1 mole of a substance is defined to contain 6.02214076 x 1-^23 elementary entities.
- Candela: It is the SI unit of luminous intensity, which makes the luminous efficacy of a monochromatic radiation of frequency 540 x 10^12 Hz to 683 when expressed in lm/W.
All the other quantities that exist are called derives quantities such as speed, volume, density, efficacy, voltage, resistance etc.