Physical Quantities, Standards and their SI Units

What are physical quantities called?

Physical Quantities: All those quantities which we can express and directly measure by a number. We call them physical quantities. For example, mass, length, force, speed, distance, electric current, density of the object etc.

How many types of physical quantities are there?

Physical quantities can be divided into two categories:

1. Scalar signs.
2. Vector quantities.

1. Scalar Quantities: Those physical quantities which have only magnitude and no direction are called scalar quantities. For example, mass, density, temperature, electric current, time, speed, volume, work etc.

2. Vector Quantities: Those physical quantities which have magnitude as well as direction and which are added according to the definite laws of sum, are called vector quantities. For example, velocity, displacement, force, momentum, acceleration, torque, electric intensity, etc.

Who is Measurement?

Measurement: The process in which we find out how many times a given quantity is of a standard quantity is called measurement. If you look at the above picture carefully, you will find that if 1 meter is the standard, then if the tree is compared with this standard, then you will find that the length of the tree is 4 times this standard. Now we can say that to find the value of a physical quantity, measurement is to compare it with a standard.

Who are you calling a unit?

Unit: A fixed quantity of a physical quantity is taken as the standard and the numerical value of the result is taken as 1. The name of this standard is called the unit of that quantity. Units of Measurement: To measure any quantity, a quantity of the same quantity is taken as the standard and is given a name. This is called the unit of that amount.

Unit Type:

There are two types of units:- (i) basic units (ii) derived units.

Fundamental Unit / Unit: To express a physical quantity, some such standards are used, which are independent of other standards, these are called fundamental units. For example, the units of length, time and mass are the basic units, meter, second and kilogram respectively.
Derived Unit: When a physical quantity is expressed in two or more basic units, then it is called derived unit. For example, Newton, Pascal, Joule and Volt are derived units for force, pressure, work and potential respectively.

System of Units: The following four methods are prevalent for the measurement of physical quantities:-

• CGS System (Centimetre Gram Second System): In this system the units of length, mass and time are centimeter, gram and second respectively. That's why it is called Centimeter Gram Second or CGS method. It is also called the French or Metric system.
• FPS System (Foot, Pound, Second System): In this system the units of length, mass and time are foot, pound and second respectively. It is also called British method.
• MKS Method (Metre Kilogram Second System): In this system the units of length, mass and time are meter, kilogram and second respectively.
• International System of Units or S.I. Units: In 1960 AD, SI was accepted in the International Convention of Measures, whose full name is de Systeme International d' Units. There are seven basic units and two supplementary units in this system.

The seven basic units of SI are:

1. Basic unit of length 'Meter': The basic unit of length in the SI unit is the meter. 1 meter is the distance that light travels in a vacuum in 1/299792458 second.
2. Basic unit of mass 'Kilogram': The standard kilogram is the mass of a cylinder made of platinum-iridium alloy, which is kept at the International Bureau of Measurement and Weights in Severis, France. It is indicated in kg. (kg) writes.
3. The basic unit of time is 'second': 9192631770 The period of radiation produced by the transition between two fixed energy levels of the basic state of the cesium-133 atom is called 1 second.
4. The basic unit of electric current is 'ampere': If two long and thin wires are placed parallel to each other at a distance of 1 meter in vacuum and the same current is passed through them of such magnitude that the distance between the wires per meter If a force of 2 x 10-7 Newton is applied, then that magnitude of electric current is said to be 1 ampere. Its symbol is A.
5. Basic unit of temperature 'Kelvin': 1/273.16th of the thermodynamic temperature of the triple point of water is called Kelvin. Its symbol
6. Basic unit of light intensity 'candela': The light intensity of a light source in a certain direction is said to be 1 candela, when the source emits monochromatic light of 540 x 1012 Hz and 1/683 Watt/steradian intensity in that direction.

Note: If 1 joule of light energy is emitted per second inside the cubic angle, then it is called 1 watt/steradion.

7. The basic unit of the amount of a substance 'mole': A mole is the amount of a substance in which the number of its constituent elements (atoms, molecules, ....... etc.) is 6.023 x 1023. This number is called Avogadro's constant.

The following are the two complementary units of SI:

1. Radian: The angle subtended by an arc of length equal to the radius of a circle at its center is one radian. This unit is used to measure the angles made on the plane.
2. Steradian: The cubic angle subtended at the center of the sphere by a square area of side equal to its radius on the surface of a sphere is called 1 steradian. It is the unit for measuring solid angles.

Basic unit

Some other important units:

Units used to measure very long distances:- 1. Astronomical Unit (A.U.): The mean distance between the Sun and the Earth is called 'Astronomical Unit'. 1 A.U. = 1.495 x 1011 m 2. Light Year: A light year is the distance traveled by light in a vacuum in one year. 1 ly = 9.46 x 1015 m 3. Parsec: This is the largest unit of measure for distance. 1 parsec = 3.08 x 1016 meters

Units of length/distance

Units of mass

Units of time

Units of area

Units of volume

Important questions asked from measurement in major competitive exams:

1. There are 10 quintals in a metric ton.
2. There are 1 pico second =10^(-12) seconds.
3. There are 0.4536 kilograms in 1 pound.
4. The Chandrasekhar limit is used to show the mass of stars.
5. 1 degree is equal to 60 minutes.
6. Relative Density =(Density of Matter)/█(Density of Water @ 4°C)
7. There is no unit of relative density.
8. Ammeter is used to measure the intensity of electric current.
9. The altimeter is used to measure the altitude of a flying aircraft.
10. An actinometer is used to measure the intensity of the sun's rays.
11. The tape recorder tape is coated with a ferromagnetic powder.
12. The working pressure of rhinestone crystals in a clock is based on the electric effect.
13. Cinematograph works on the principle of vision.
14. Television was invented by JL Ward.
15. The laser was invented by Theodor Meimann.
16. The revolver was invented by Samuel Colt.
17. Radar was discovered by Robert Watson Watts.
18. The Fadometer instrument is an instrument used to measure the depth of the ocean.
19. Hydrometer is used to measure the relative density of liquids.
20. The National Physical Laboratory is located in New Delhi.
21. Manometer is an instrument to measure the pressure of gases.
22. The law of scattering of light was propounded by CV Raman.
23. National Science Day is celebrated on 28 February.
24. The unit of viscosity is poise.
25. There are 159 liters in 1 barrel.
26. The distance of the Earth from the Sun is about 14.95 crore km.
27. There are one million millimeters in 1 kilometer.
    

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☞ Major Standards and Units GK Questions and Answers 🔗

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