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The International System of Units or SI system is a set of values used to measure things all over the world. The SI system consists of seven base units that further define 22 derived units, each with its special name and symbol. The system works as a standard to communicate agreed values. The kilogram, the metre, and the second are part of the SI system, which is also known as the metric system.
The SI system is used in multiple disciplines, from natural sciences to arts and social sciences. Knowing how to read and use the SI system allows us to exchange information. The seven basic units are:
The SI system is a set of values officially used all over the world to make measurements. It is employed in every area of development to measure and share information. Global trade depends on the SI to exchange products. There are many advantages of using the SI system, which is:
Figure 1. One of the main advantages of the SI system is that it is repeatable. Pouring 200ml of the same liquid into these vases always results in the same amount of volume and mass. Source: Knowing Roger, Flickr (Public Domain).
To understand how the SI units work, it is essential to be familiar with their meaning. In some cases, the meaning of an SI unit is easily grasped. If, for instance, you wish to know the velocity of your car, the speedometer might tell you that this is 33 km/h or 33 kilometres per hour. In this case, your car would be travelling 33 kilometres every hour.
Sometimes, units are expressed with negative exponents, such as ms ^ -1. A negative exponent is used instead of the slash to mean ‘per’. The example above could also be expressed as 33kmh ^ -1. So, 33 km/h is equal to 33 kmh ^ -1.
One way of using SI units is to convert derived units to basic units to understand what they mean. Other tools of the SI system are prefixes, standard form, and symbols that allow us to express units more easily.
Derived units in the SI system, which result from combining basic ones, are used to explain more complex processes, such as pressure, velocity, area, capacitance, work, etc. Examples of derived units are the following:
Any derived unit can be converted to its basic units.
To convert derived units to basic units, we need to know the equivalence of every derived unit. Replacing all derived units with their basic units gives us expressions that use only the seven elemental units. See the following two examples.
If you have 10 Pascals, you can convert these as follows:
Here, 10 Pa is 10 kilograms per metre per second squared.
Let’s say we have 39 Pascals. Pascal is pressure over the surface, with pressure being a force. Pascal, therefore, is force over an area, and as force is measured in Newtons, we can change 39 Pascals to 39 Newtons over a square metre:
So, 39 Pascals are 39 Newtons of force over a square metre. We can then convert Newtons to their basic units as below:
Eliminating the metres, we get the same expression as in our first example.
The SI system provides several ways to express quantities, including prefixes, form factors, and symbols.
Figure 2. On a ruler, cm is the short name of the ‘centimetre’ unit. It is also the symbol of the centimetre unit in the SI system. Source: Tim, Flickr (CC BY 2.0).
The SI system is not only used to measure but also to manufacture. Measuring once, we can use those measurements to produce the same part multiple times. The SI system is used to exchange and sell goods all over the world. Science and research also use SI units to measure scientific quantities.
The SI system, which is the world standard for commerce, technology, and economy, is the de facto system used all over the world. However, there are two other systems in use in some countries, including the United States, United Kingdom, Thailand, and Liberia, in combination with the SI. These are the Imperial System and the United States Customary System (USCS), which use different units.
The SI system was initially created in nineteenth-century France to unify measurements. Before this, France’s measurement system consisted of many units with no easy way to convert one to the other. This made things very difficult in everyday life and also in science and commerce.
During the French revolution, the National Assembly was organised by a group of revolutionaries, thinkers, and free labour, known as the third state. One of the many aims of the group was to create a modern system of units for the New Republic of France. Our modern SI system is based on the system developed by that group.
In 1875, a diplomatic treaty was signed by seventeen countries. This established the SI system as a standard system of measurements. The signed treaty introduced the metre and kilogram prototypes but lacked quantities such as the ampere, mole, candela, and kelvin.
The temperature unit used in the eighteenth century was Celsius, which was in use until the 10th General Conference of Weights and Measures, where the kelvin was adopted. In 1948, the candela was introduced as a unit to measure the luminosity of an object. In 1960, the second was adopted as a small portion of the time it takes for the earth to rotate.
The metric system, which is the base of the SI system, used a set of objects with a weight of one kilogram and a length of one metre. As copies of these objects have slight variations in their length/weight, they were changed to new definitions that use universal constants.
The new definitions introduced later use universal constants as the mass of the platinum-iridium cylinder stored in Paris defines the kilogram and the length travelled by the speed of light to define the metre.
To use SI units, we need to know each measured value. For example, to use SI units to express velocity, we need the travelled distance in metres and the time it took to travel that distance in seconds.
SI units allow us to exchange measurements that can be reproduced. For example, to manufacture something all over the world, we need the same measurements for every piece of equipment.
Yes, all properties of an object can be expressed using SI units, except for non-dimensional quantities that measure ratios such as the radian.
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