**What are the most important topic of units and measurement of class 11 physics?**

Units and Measurements is a very important topic which builds a foundation for a meaningful learning and understanding of Physics. It must also be borne in mind that for appreciating Physics, one must also develop a love for Mathematics, because both of these two subjects go hand in hand. Needless to say, if Physics is a piece of bread, then Mathematics is the delicious filling, and together they make a wonderful sandwich. Therefore, the connection between Physics and Mathematics must be clearly understood when pursuing any course in Physics.

Many texts have covered this base topic of Units and Measurements in a variety of manner to help students gain the necessary understanding of the topic. The important topics, which form the core of the chapter on Units and Measurement, can be listed as follows,

- Units and Measurement – definitions
- Classification of Units
- Dimensions and its Uses
- Order of Magnitude
- Significant Figures and its rules
- Errors and causes of errors in measurement, its kinds

Measurement is defined as a method that is relative in nature. Every physical quantity needs its measurement. This measurement is done based on a reference and this reference is called as a standard. To measure any physical quantity, one needs to determine the number of times a standard amount (say a kilogram, meter, second) of that physical quantity is present in the quantity to be measured. The number that is obtained through this process is defined as its magnitude and the standard is the chosen unit of the physical quantity. A standard can be defined as the actual physical embodiment of the unit of a physical quantity.

Next, understanding of unit and its kinds is also very essential. There are a huge number of physical quantities that can be measured. Every of these quantities needs a defined unit. However, it is not necessary that these quantities are independent of one another. There needs to be a segregation 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 can be defined as a quantity completely independent of each other. As per the international CGPM body, there are only seven fundamental quantities. All the other physical quantities are derived quantities. Thus, as an example length and time are fundamental quantities while speed is a derived. There are several units and measurement systems prevalent in the world and every system has its own fundamental quantities. CGS system, FPS system, MKS system and SI are most widely used and accepted. Fundamental quantities are defined as Base quantities. Any fundamental or base unit should have the properties of Invariability and Availability.

As stated, there are seven fundamental quantities, which are,

- Length, meter (m)
- Mass, Kilogram (kg)
- Time, second (s)
- Electric current, Ampere (A)
- Thermodynamic temperature, Kelvin (K)
- Amount of substance, Mole, (mol)
- Luminous intensity, Candela (cd)

There exist two other units called supplementary units which are Plane angle with unit Radian (rad) and Solid angle with unit steradian (sr).

Every physical quantity can be derived from fundamental quantities. Expressing a physical quantity in its base units is done as a product of different powers of the base quantity. The exponent of a base quantity that enters this expression is called the Dimension of the Quantity in that Base. For example, Potential energy P is the product of mass, gravitational pull and height.

P=mgh

The base unit of mass is Kilogram, height is meters and gravitational pull is measured in meters per second square. For simplicity sake, For convenience the base quantities are represented by one letter symbols. Generally, mass is denoted by M, length by L, time by T and electric current by I. The thermodynamic temperature, the amount of substance and the luminous intensity are denoted by the symbols of their units K, mol and cd respectively. Thus, the dimension of potential energy can be obtained as M x LT-2 x L. This is simplified as ML2T-2 which means dimension of potential energy is 1 in mass, 2 in Length and -2 in Time. Dimensions are useful in establishing the homogeneity of dimensions in an equation. It also comes advantageous during the conversion of units and in deducing the relationships that exist amongst the physical quantities.

Physical quantities vary over a wide range in terms of its measurement. To simplify and to develop an idea of its size, one uses the power of ten method. It can be understood with examples such as the diameter of the sun is expressed as 1.39 x 10^9 m and the diameter of a hydrogen atom as 1.06 x 10^(- 10) m. We then get the order of magnitude of that number. Thus, the diameter of the sun is of the order of 10^9 m and that of a hydrogen atom is of the order of 10^(- 10) m.