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:
- Metre (symbol m), which measures the length of an object.
- Kilogram (symbol kg), which measures the mass of an object.
- Mole (symbol mol), which measures the number of particles in a substance.
- Second (symbol s), which measures the time.
- Candela (symbol cd), which measures the luminosity of an object.
- Kelvin (symbol K), which measures the temperature of an object.
- Ampere (symbol A), which measures electrical current.
What are the advantages of using SI units?
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:
- Universal: the SI system is used in almost every country in the world.
- Versatile: the SI system provides several ways to measure large and small quantities.
- Extensive: the basic SI units can be combined to produce more complex units.
- Complete: we can describe any object using SI quantities.
- Repeatable: the SI system can be reproduced everywhere.
Figure 1. One of the main advantages of the SI system is that it is repeatable. Pouring 200 ml of the same liquid into all of these containers always results in the amount of volume and mass.
Reading SI units
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 \(33 \space kmh ^ {-1}\). So, 33 km/h is equal to \(33 \space kmh ^{-1}\).
Using SI units
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
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:
- Pressure: measured in Pascals (Pa) and used to describe the amount of force applied over an area. It is equal to Newtons over square metres (\(Nm ^ {-2})\).
- Force: measured in Newtons (N) and used to describe the acceleration of a mass at 1 metre per square second, using kilograms per metre per squared second (\(kg \cdot m \cdot s ^ {-2}\)).
- Area: measured in square metres (\(m ^ 2\)) and representing the surface inside a perimeter.
Any derived unit can be converted to its basic units.
Converting derived units to 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:
\[10\mathrm{Pa}=10\mathrm{\dfrac{kg}{m\cdot s^2}}\]
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:
\[39\mathrm{Pa}=39\mathrm{\dfrac{N}{m^2}}\]
So, 39 Pascals are 39 Newtons of force over a square metre. We can then convert Newtons to their basic units as below:
\[39\mathrm{Pa}=39\mathrm{\dfrac{N}{m^2}}=39\mathrm{\dfrac{kg\cdot m}{s^2\cdot m^2}}\]
Eliminating the metres, we get the same expression as in our first example.
\[39\mathrm{\dfrac{kg}{m\cdot s^2}}\]
Prefixes, form factors, and symbols
The SI system provides several ways to express quantities, including prefixes, form factors, and symbols.
- Prefixes: an example of using a prefix is to say that a million Newtons is the same as one Meganewton, with Mega being the prefix.
- Form factors: also known as standard form, form factors use exponents to shorten numbers. For example, 1 million Newtons can be expressed as \(1 \cdot 10 ^ 6\) Newtons.
- Symbols: these can replace both prefixes and form factors. For instance, 1 million Newtons equal 1 MN, with M meaning Mega and N being the symbol for Newtons.
What are the uses of SI units?
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.
Other unit systems and the history of the SI system
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 history of the SI system
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.
Conference on weights and measures (1860)
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.
Changes to the SI system
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.
Redefinition of the SI system
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.
Use of SI Units - Key takeaways
- The SI unit system is the main system used throughout the world.
- The main advantages of the SI system are that it provides a base system to measure any physical property and that it is a standard in most countries.
- To understand the derived SI units, we need to know how they are composed.
- Prefixes, form factors, and symbols allow us to express units more easily.
- There are other unit systems that are in use alongside the SI system in countries such as the United States and the United Kingdom.
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