International System of units (SI units)
‘The International System of Units (SI Units)’ was developed to meet the energy conversion requirements. The abbreviation ‘SI’ is due to French influence. It is called ‘Le Système International d’Unités’ in French. The SI units were established in 1960 by the 11th General Conference on Weights and Measures. This committee was established mainly to promote the Metric system developed in France and hence the French influence.
The following list provides the ‘Base’ units in the SI system:
Quantity Unit Symbol
Length metre m
Mass kilogram kg
Time second s
Electric current ampere A
Temperature kelvin K (ºC for Temp diff)
Luminous intensity candela cd
Amount of substance mole mol (used for atoms, ions, etc)
The base units are predefined standard values. They must be treated as axioms! In other words, they must be accepted as they are – no questions asked!
All other units are derived from the above base units. For example, the unit of force is a derived unit. It is a product of mass (kg) and acceleration (m/s2). Hence, the unit is kg-m/s2, which has been given the name ‘newton (N)’ in SI units, to honour Isaac Newton – the famous English physicist.
Similarly, the unit for pressure is ‘force per unit area’, which is kg-m/s2 / m2. It is given the name ‘pascal (Pa)’, to honour Blaise Pascal – the French mathematician.
Finally, SI units use the same units for all forms of energy. It makes sense since the energy can be converted from one form to another. For example, the potential energy of a tank full of water above ground is expressed in joules. Similarly, the amount of heat stored in a bucket of hot water is also expressed in joules.
It is important to have a good understanding of SI units, as it makes the energy conversion calculations much simpler. The practical examples, which are presented in a later section, are designed to provide an appreciation of SI units for energy conversion calculations.
Obsolescence of units
This section illustrates the need to adapt the units with advancements in technology.
The mechanical ‘energy’ or the ‘work done’ is defined as a product of force and the distance (W = F x d). The ‘force’ in turn is defined as a product of mass and acceleration (F = m x a). The ‘weight’ is defined as the force exerted by the mass due to gravity. Hence, the weight is obtained as a product of mass and the acceleration due to gravity (W = m x g).
Before the discovery of gravitational force, there was no distinction between ‘mass’ and ‘weight’. Mass is now defined as the ‘amount’ or ‘quantity’ of material and is related to the volume and density of the material. Weight is defined as the ‘force’ acting on the ‘mass’ due to gravity.
The ‘pound’ has been used a measure of ‘weight’ since the Roman times. The acceleration due to gravity was unknown at that time. The discovery of gravitational force resulted in the introduction of the terms ‘mass’ and ‘weight’. This obviously created confusion. It was not clear whether ‘pound’ was to be used as a unit of ‘mass’ or ‘weight’. Some effort was made to reconcile this by introducing the term ‘slug’ as the unit of ‘mass’, and the unit of weight as ‘pound’ or ‘pound-force (lbf)’. This only resulted in more confusion. A ‘slug’ is defined as the mass that is accelerated by 1 ft/s2 when a force of one pound (lbf) is exerted on it. Hence, one slug has a mass of 32.174 pounds.
Confused? I am not surprised! I am in the same boat. But there is good news—SI units are much simpler and versatile. So, it is time to get off the Roman chariot and get into the Electric vehicle!
To avoid the confusion created by the obsolete system of units, it is best not to use them. The good news is that the whole world has accepted the SI units as the international standard and have been using it. However, the Americans still use the Imperial units, even though they are a signatory to the SI units. In fact, the engineering textbooks and standards in the United States use the SI units. However, the American society and the industry still prefer to use the Imperial units!
