Physical quantities are those quantities that can be measured. Basically, there are two types of physical quantities (Base quantities or fundamental quantities) and (Derived quantities).
These are quantities that are used to describe the laws of physics. Physical quantities may be divided into six categories.
- Constant or ratio
Such quantities have the only magnitude, for example, refractive index, dielectric constant and specific gravity. Such quantities have no units.
These quantities have magnitude and unit. Some of them may have direction also but vector laws are not applied. Examples are the charge, mass, distance, speed, and current.
These quantities possess magnitude, unit, and direction. They also follow the triangle law of addition. For example velocity, force, momentum, torque, and displacement.
These possess magnitude (amplitude) and phase. Try follows triangle law. Simple harmonic motion (SHM), waves. AC voltage and AC current are phasors.
Such quantities do not have any specified direction but have different values in different directions. For example moment of inertia. In anisotropic media even density, refractive index, dielectric constant, electric conductivity, stress, strain and so on become tensor. A physical quantity that has only one component is called a scalar or a tensor of zero ranks. If it has more than one component but less than or equal to four, it is called a vector or a tensor of rank 1. If the components are greater than four, it is termed as a tensor of a higher rank.
- Conversion Factors
Some physical quantities convert into another when multiple by a factor. For example, in a wave y = y0 sin (ωt – kx), k is a conversion factor. When k is multiplied by displacement or path difference it generates phases or phase difference. In frequency modulation kEmfc = δ, k converts the voltage into the angle and is termed as a conversion factor. Many other conversion factors can be thought of.
In general, a physical quantity = magnitude x unit. If the unit changes, the magnitude will also change. We apply n1u1 = n2u2.
Physical quantities may be divided into fundamental and derived quantities.
The quantities that do not depend upon any other quantity are called “fundamental or absolute or basic quantities”. Initially, only three fundamental quantities –length, mass and time – were considered. With the development of science, four more physical quantities were added. These are temperature, electric current, luminous intensity and amount of a substance.
Physical quantities examples
- Electric current
- Electric flux
- Electric potential
The physical quantities derived from the fundamental quantities are called derived quantities like velocity, acceleration, force, and momentum.