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How can we describe SI units?

If you’ve read any of our previous articles, you’ll see that we dive into various engineering topics, such as mechanical resonance or deflection of cantilever beams.  In this article, we’re going to learn all about SI units, standard engineering prefix notation, and how we can describe them.

Introduction to Engineering

Before we jump in, let’s figure out what engineering means, after all, it means different things to different people.  Sometimes it can be easier to clarify what it’s not:

  • It’s not the technician who fixes your car.
  • It’s not the person who repairs your computer when it won’t start.
  • It’s not the person who fixes a leaking pipe or installs the wiring in your house.
  • It’s not a welder that makes machine components.

Engineering is the manipulation of the forces of nature to benefit or advance humanity.  Engineers are interested in how we can do something, rather than why.  It results in products and services that are useful to humans in some way.

Engineering is also concerned with design, with a useful function in mind.  It could be design for a component, device, system, or machine.  For example, an air-conditioner, skyscraper, suspension bridge, electrical car, prosthetics or an aircraft gas-turbine.

SI Units

SI units are used in both engineering and science.  It stands for “Systeme Internationale d’Unites” (International System of Units) and is based on the metric system.  It’s now adopted by most countries as the official system of measurement.

The SI is made up of 7 ‘base units’ that are shown in the table below:

Physical QuantityPhysical QuantitySI UnitSI Unit
Electric CurrentIampereA
Amount of Substancenmolemol
Luminous Intensitylvcandelacd

Derived Units

We can define derived units from these base units.  For example, if we take the base unit of time, the inverse is commonly known as frequency.  The table below shows these derived units, and many are named after famous individuals, such as Sir Isaac Newton:

Physical QuantityQuantity
SI UnitUnit
Expression in SI base unitsAlternative expressions
frequencyv, fhertzHzs-1
forceFnewtonNkg m s-2J m-1
pressureppascalPakg m-1 s-2N m-2
energy (all forms)E, U, V, W,etc.jouleJkg m2 s-2N m = C V = V A s
powerPwattWkg m2 s-3J s-1 = VA
electric chargeQcoulombCA s
electric potential differenceE, φ, ζ, Φ, η, etc.voltVkg m2 s-3 A-1J A-1 s-1 = J C-1
electrical capacitanceCfaradFA2 s4 kg-1 m-2C V-1
electrical resistanceRohmΩkg m2 s-3 A-2V A-1
electrical conductanceGsiemensSA2 s3 kg-1 m-2A V-1 = Ω-1
magnetic fluxΦweberWbkg m2 s-2 A-1V s = T m2
magnetic inductionBteslaTkg s-2 A-1Wb m-2 = N A-1 m-1
inductanceL, MhenryHkg m2 s-2 A-2V A-1 s = Wb A-1
luminous fluxΦlumenlmcd sr
illuminationEluxlxcd sr m-2lm m-2
Celsius temperaturetdegree Celsius°CK
plane angleα , β , γ , θ , Φradianradm m-1dimensionless

Almost all equations used in engineering are based on these SI units.  This gives us a common standard framework for formulae and equations that is recognised globally.

In our next article, we’re going to look at how we can use engineering concepts to solve practical problems, so make sure you keep an eye out for it.

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