System of Units

Subject: Physics

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Overview

The quantities which are measurable in physics is known as physical quantity.Kilogram ,meter ,second,mole candela ,ampere,Radian ,Steradian are fundamental units .This note provides us an information on the system of units.
System of Units

Physics is also known as the science of measurement. The quantities which are measurable in physics is known as physical quantity. Example: length, mass, time, force, etc. Physical quantities are divided into two groups:

  1. Fundamental physical quantity
    Certain physical quantities are selected independent of each other. These are called fundamental physical quantities. They are mass, length and time. The number of physical quantities depends upon the system of the unit.
  2. Derived physical quantity
    These are physical quantities which can be expressed by a proper combination of fundamental quantities.

Let us consider the velocity of a body. It is defined as the rate of change of displacement per unit time. It can be represented by the combination of length and time. Hence, it is a derived physical quantity. Other examples are Work, Energy, Power, etc.

Units

Unit is the standard quantity used in the measurement. First, a standard is selected and the physical quantity is compared with the standard which is the process of measurement.

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System of units

  1. FPS System
    Length – Foot (F)
    Mass – Pound (P)
    Time – Second (S)

  1. CGS system
    Length – centimeter (C)
    Mass – Gram (G)
    Time- Second (S)

  1. MKS system
    Length – meter (M)
    Mass – Kilogram (K)
    Time – Second (S)

  1. SI system
    Length – meter (M)
    Mass – Kilogram (K)
    Time – Second (S)
    Amount of system – Mole (mol)
    Temperature – Kelvin (K)
    Luminous intensity – Candela(C)
    Electric current – Ampere (A)

  1. Supplementary units

    Plain angle – radian (rad)

    Solid angle – steradian (sr)

Definitions of fundamental quantities

  1. Kilogram
    It is defined as the mass of the platinum-iridium cylinder kept at the International Bureau of Weights and Measurements in France.
  2. Meter
    It is defined as the distance traveled by light in \(\frac{1}{299792458}\) seconds in the vacuum.
  3. Second
    It is defined as the 9192631770 periods of electromagnetic radiations emitted during a transition between two lower levels of Cesium-133 atoms.
  4. Mole
    It is the amount of substance that contains as much atoms/molecules as there are in 0.012 kg of C12.
  5. Kelvin
    It is \(\frac{1}{273.16}^\text{th}\)of a thermodynamic temperature of the triple point of water.
  6. Candela
    It is defined as the luminous intensity in a certain direction emitted by a source of electromagnetic radiation of 540 x 1012Hz that has intensity of\(\frac{1}{683}\) watt per steradian in that direction.
  7. Ampere
    It is that constant current which when maintained in 2 parallel conductors of negligible cross-section and infinite length separated in the vacuum by a distance of 1m produces a force of 2 x 10-7N per meter on either way.
  8. Radian
    It is the angle between two radii of a circle that cuts an arc equal to the length of the radius on the circumference.
  9. Steradian
    It is the angle subtended at the center of a sphere by an area equal to the square of the radius.
Things to remember

The quantities which are measurable in physics is known as physical quantity.

Certain physical quantities are selected  independent of each other. These are called fundamental physical quantities. 

Kilogram is defined as the mass of the platinum-iridium cylinder kept at the International Bureau of Weights and Measurements in France.

Second  is defined as the 9192631770 periods of electromagnetic radiations emitted during a transition between two lower levels of Cesium-133.

Mole is the amount of substance that contains as much atoms/molecules as there are in 0.012 kg of C12.

Ampere is that constant current which when maintained in 2 parallel conductors of the negligible cross section and infinite length separated in vacuum by a distance of 1m produces a force of 2 x 10-7N per meter on either way.

Radian is the angle between two radii of a circle that cuts an arc equal to the length of the radius on the circumference.

Steradian is the angle subtended at the center of a sphere by an area equal to the square of the radius.

  • It includes every relationship which established among the people.
  • There can be more than one community in a society. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • common interests and common objectives are not necessary for society.
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