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# The Physical Quantities and Units

In the measurement of any physical quantity, we require some reference standard. This reference standard of measurement is called a unit. The unit of a physical quantity is defined as the reference standard used to measure it. The magnitude of a physical quantity is the product of unit (u) in which the quantity is measured and the number (n) of times that unit is contained in the given quantity.
The unit selected for measuring a physical quantity must have the following characteristics:
• The unit should be well defined.
• The unit should be neither too small nor too large in comparison with the physical quantity to be measured. In simple words, the unit should be of some suitable size.
• The unit should be imperishable.
• The unit should be neither change with time nor with physical conditions like pressure, temperature etc.The unit should be easily reproducible.

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## Physical Quantities

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Physical quantities are organized in systems of units. To describe objects and phenomena of the physical world, many quantities are used, both additive and non additive, connected by analytical relations. In principle, one could choose an arbitrary unit standard for every additive quantity. This choice would, however, lead to the introduction of inconvenient proportionality factors and would require us to define and to maintain a large number of measurement standards. There are as many units as there are independent quantities. Let us consider three physical quantities mass, length and time. These quantities are independent of each other. So, three separate units are required for the measurement of these quantities. Thus, it becomes important to establish a system of units.

Fundamental physical quantities are those which cannot be defined in terms of other quantities. Derived physical quantities are those which can be defined in terms of fundamental physical quantities.

## SI Units

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According to the latest information from International Organization for Standardization (ISO) there are seven basic units from which a wide range of quantities can be derived in the form of products and quotient of these basic units. The seven basic units are metre, kilogram, second, ampere, kelvin, candela and mole. The SI system is a decimal system in which calculations using the numeral 10, multiplied or divided by itself is often used.

## The Avogadro Constant

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The Avogadro constant NA may be taken as the number of carbon atoms in 12 g of carbon-12 and has been determined experimentally to have a value of about 6.02×1023 mol-1.

One mole of any substance is the amount of substance containing a number of particles equal to the Avogadro constant.

## Scalar and Vectors

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A scalar is a physical quantity which has magnitude only. Examples of scalars include mass, temperature and pressure. A vector is a physical quantity which has both magnitude and direction. There are many examples of such quantities, including velocity, force and electric field. Vectors can be represented diagrammatically by a line with an arrow at the end. The length of the line shows the magnitude of the vector and the arrow indicates its direction. If the vector has magnitude one, it is said to be a unit vector. Two vectors are said to be equal if they have the same magnitude and the same direction.
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