units and measurement Class 11 physics chapter 2 notes || WHAT IS THE INTERNATIONAL SYSTEM OF UNITS class 11 || WHAT IS THE MEASUREMENT OF LENGTH class 11 || What is the Measurement of Large Distances || explain SI Base Quantities and Units


👉Topics that mentioned here are as follows:-👇

  1. SI Base Quantities and Units
  2. Some units retained for general use


  1. Measurement of Large Distances

units and measurement Class 11 physics chapter 2 notes

units and measurement Class 11 physics chapter 2 notes


Estimation of any physical amount includes comparison with a certain fundamental, self-assertively chosen, globally acknowledged reference standard called unit. The result of a estimation of a physical amount is communicated by a number (or numerical degree) went with by a unit. In spite of the fact that the number of physical amounts shows up to be exceptionally huge, we require as it were a restricted number of units for communicating all the physical amounts, since they are inter-related with one another. The units for the basic or base amounts are called principal or base units. The units of all other physical amounts can be communicated as combinations of the base units. Such units gotten for the determined amounts are called derived units. A total set of these units, both the base units and inferred units, is known as the system of units.


In earlier time scientists of different countries were using different systems of units for measurement. Three such systems, the CGS, the FPS (or British) system and the MKS system were in use extensively till recently.

The base units for length, mass and time in these systems were as follows :- 
  • In CGS system they were centimetre, gram and second respectively.
  • In FPS system they were foot, pound and second respectively.
  • In MKS system they were metre, kilogram and second respectively.
The framework of units which is at show universally acknowledged for estimation is the Système Internationaled’ Joins together (French for International System of Units), truncated as SI. The SI, with standard scheme of symbols, units and shortened forms, created by the Bureau International des Poids et measures (The Worldwide Bureau of weights and measures, BIPM) in 1971 were as of late changed by the Common Conference on weights and measures in November 2018. The conspire is presently for worldwide utilization in logical, specialized, mechanical and commercial work. Since SI units utilized decimal framework, transformations inside the framework are very basic and helpful. We might take after the SI units in this pages.

In SI, there are seven base units as given in fig:- 1. Besides the seven base units, there are two 
more units that are defined for (a) plane angle dθ as 
the ratio of length of arc ds to the radius r and (b) 
solid angle d as the ratio of the intercepted area dA of the spherical surface, described about the apex O as the centre, to the square of its radius r, as shown 
in Fig.1(a) and (b) respectively. The unit for plane 
angle is radian with the symbol rad and the unit for the solid angle is steradian with the symbol sr. Both these are dimensionless quantities.

Fig. 1 Description of (a) plane angle dθ and (b) solid 
angle dΩ


Explanation  SI Base Quantities and Units

Figure 2

* The values mentioned here need not be remembered or asked in a test. They are given here only to indicate the extent of accuracy to which they are measured. With progress in technology, the measuring techniques get improved leading to measurements with greater precision. The definitions of base units are revised to keep up with this progress.

Some units retained for general use (Though outside SI)

Figure 3

Note that when mole is utilized, the rudimentary substances must be indicated. These substances may be iotas, atoms, particles, electrons, other particles or indicated bunches of such particles.

We utilize units for a few physical amounts that can be determined from the seven base units . A few inferred units in terms of the SI base units are given in . A few SI determined units are given uncommon names and a few inferred SI units make utilize of these units with uncommon names and the seven base units .


You're as of now recognizable with a few coordinate strategies for the estimation of length. For case, a meter scale is utilized for lengths from 10 (to the power minus)³ m to 10² m. A vernier callipers is utilized for lengths to an precision of 10(minus power4) m. A screw gage and a spherometer can be utilized to degree lengths as less as to 10(minus power 5) m. To degree lengths past these ranges, we make utilize of a few extraordinary backhanded strategies.

What is the Measurement of Large Distances

Huge separations such as the remove of a planet or a star from the soil cannot be measured specifically with a meter scale. An vital strategy in such cases is the parallax method.
After you hold a pencil before you against a few particular point on the foundation (a divider) and see at the pencil to begin with through your left eye A (closing the correct eye) and after that see at the pencil through your right eye B (closing the cleared out eye), you'd notice that the position of the pencil appears to alter with regard to the point on the divider. Usually called parallax. The remove between the two focuses of perception is called the basis. In this illustration, the premise is the separate between the eyes.
To degree the remove D of a distant absent planet S by the parallax strategy, we watch it from two diverse positions (observatories) A and B on the earth, isolated by remove AB = b at the same time as appeared in below figure. We degree the point between the two bearings along which the planet is seen at these two focuses. The ∠ASB in below figure spoken to by image θ is called the parallax angle or parallactic angle.

As the planet is very far away, b/D Smaller than 1, and therefore, b/D is very  small. 
Then we approximately take AB as an arc of length b of a circle with centre at S and the distance D as the radius AS = BS so that AB = b = D θ where θ is in radians.

D=b/θ (equation 1)

Having determined D, we can employ a similar method to determine the size or angular diameter of the planet. If d is the diameter of the planet and α the angular size of the planet (the angle subtended by d at the earth), we have

α = d/D (equation 2)

Parallax method

The angle α can be measured from the same location on the earth. It is the angle between the two directions when two diametrically opposite points of the planet are viewed through the telescope. Since D is known, the diameter d of the planet can be determined using Eq. (2.2).

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