Misconceptions About SI

SI stands for ‘Système Internationale d’Unités’ which translates into ‘International System of Units’.

SI is the latest of several historical variations of the metric system. Previous versions include the ‘cgsA‘ (centimetre-gram-second-ampere) system and the ‘mksA‘ (metre-kilogram-second-ampere) system. SI is based on the mksA system.

Base Units

SI defines seven fundamental or ‘base units‘, from which all other units are derived. Not surprisingly, these other units are termed, ‘derived units‘.

The base units for SI are: metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), candela (luminous intensity), and mole (amount of substance).

Note, in particular, that the base unit for mass is the kilogram (kg), and not the gram (g). The gram is considered to be a sub-multiple of the kilogram base unit. Note, too, that the kilogram is a measure of mass, and not weight. Mass and weight are related, but different, quantities; with weight being a force due to the effect of gravity upon a mass. In SI, force and, therefore, weight, are measured in newtons (symbol: N), a derived unit.

Since these base units were established in the late 1940s, they have been defined in terms of both physical constants and artifacts. For example, the second has been defined in terms of the speed of light, c, while the kilogram has been defined in terms of the mass of a metal cylinder kept in a laboratory in the suberbs of Paris.

However, since May of 2019, all this has changed and, now, all SI base units are defined in terms of physical constants. These constants include the speed of light, the charge on an elementary particle, Planck’s Constant, and others. This fully-explained in my Page on ‘Understanding the New SI‘, elsewhere in this blog.

Derived Units

SI derived units are each defined in terms of base units. For example, the SI unit for electric potential difference, the volt (symbol: V) is defined as the potential-difference between two points such that the energy used in conveying a charge of one coulomb from one point to the other is one joule.

So the volt is defined in terms of the coulomb and the joule. The coulomb, in turn, is defined in term of the ampere and the second (both base units). The joule is defined in terms of the newton and the metre (a base unit). Finally, the newton is defined in terms of the kilogram, the metre, and the second (all base units).

So, by ‘deconstructing’ the volt, we find that it is ultimately derived from a combination of the base units: ampere, second, kilogram, and metre.

Most derived units have been given special names in honour of famous physicists whose research has contributed to our knowledge of the quantity concerned —for example, as we have learnt, the derived unit for potential difference is the ‘volt’, which is simply a special name given to a ‘joule per coulomb’, and is named after the Italian nobleman and professor of physics, Count Alessandro Volta (1745 – 1827).

Non-SI Metric Units

Not all metric units are SI units, although many may be ‘used alongside’ SI units.

Commonly-used, but non-SI metric, units include:

  • watt second (symbol: W·s) or, more-commonly, the kilowatt hour (symbol kW·h), is used to measure energy. The SI unit is the joule.
  • tonne (symbol: t), spoken as ‘metric ton’, is equivalent to 1000 kg and is used to measure mass. In the United States, the tonne is not generally used; instead, they call it a ‘metric ton‘. The SI unit is the kilogram.
  • litre (symbol: L or l), used to measure volume. The SI unit is the cubic metre.
  • Celsius (symbol: ºC), used to measure temperature. The SI unit is the kelvin. (NOT ‘degree kelvin’).

Multiples and Sub-Multiples

Frequently, we have to deal with very large, or very small, quantities. For example, the resistance of insulation is measured in millions of ohms, while the resistance of a conductor is measured in thousandths of an ohm.

To avoid having to express very large or very small values in this way, we use, instead, multiples and submultiples. These are indicated by assigning a prefix to the SI unit. The more common are listed below:

  • terra (symbol: T), meaning

     , e.g. terrawatt (TW).

  • giga (symbol: G), meaning

    , e.g. gigawatt (GW).

  • mega (symbol: M) meaning

    , e.g. megavolt (MV).

  • kilo (symbol: k), meaning

    , e.g. kilovolt (kV).

  • milli (symbol: m), meaning

    , e.g. milliampere (mA).

  • micro (symbol: μ), meaning

    , e.g. microvolt (μV).

  • pico (symbol; p), meaning

    , e.g. picofarad (pF).

Note that SI recommends using the ‘engineering system‘ of multiples —i.e. ten, raised to the power of multiples of 3. This means that multiples, such as centi, deca, etc., should not normally be used with SI units. For example, we should avoid using centimetres, and use millimetres instead.