Units of measurement

The study of science, including physics, is quantitative in nature. We study natural phenomena and events in terms of quantities, which can be measured. A measurement is basically observations to estimate a physical quantity. Its basic objective is to reduce uncertainty and to give definitive stature to the quantities being described.

The measurement of quantity is done by comparing it with some standard called “unit”. A unit, therefore, is any division of quantity, which is accepted as one unit of that quantity. A quantity (Q) is expressed as the product of a number (number of times in comparison to the standard) and the name given to the unit or standard.

$$Q=nX\text{name of unit}$$

$$Q=nu$$

We can have a look at some of the physical quantities that we use in our everyday life: 5 kg of sugar, 110 Volt of electric potential, 35 Horse Power of an engine and so on. The pattern of all these quantitative expression follows the same construct as defined above.

System of units

Our earlier units have been human based (in the context of what we use in our daily life) and, therefore, varied from country to country and even from society to society. We had measure of length (foot) in terms of the length of a foot step as unit.

Relating units to immediate physical world is not wrong; rather it is desirable. What is wrong is that there are many units for the same quantity with no relative merit over each other. We are led to a situation, where we have different units for the same quantity, based on experiences in different parts of the world. These different units of the same quantity do not bear any logical relation amongst themselves. We, therefore, need to have uniform unit system across the world.

Further, it is seen that there are scores of physical quantities. If we assemble all quantities, which are referred in the study of physics, then the list will have more than 100 entries. Fortunately, however, most physical quantities are “dependent” quantities, which can be expressed as combination of other quantities. This fact leads us to classify quantities in two groups :

1. Basic or fundamental quantities
2. Derived quantities

Basic or fundamental units are a set of units for physical quantities from which other units can be derived. This classification i.e. existence of basic quantities has a great simplifying effect. We are limited to study few of basic units; others (derived) are derived from them.

Here, we should also strike difference between “basic” quantity of a system of measurement to that of quantity of basic nature. The presence of length, mass and time in the basic category may give impression that all members are basic in nature. Neither it is required nor it is so. We can have a system of measurement with quantities, which are not basic in nature.

In modern SI unit system, for example, the electric current is included in the list of “basic” quantities. We, though, know that it is equal to time rate of charge. A quantity of basic nature in the universe is not derivable from other quantities, but current is. As such, current is not basic in nature. We could have included “charge” as the basic quantity instead. But, then there are other requirements of a basic unit like reproducibility and ease of measurement etc, which need to be taken into consideration.