VCE Physics/Appendices/Appendix A : SI Units

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Introduction[edit]

If you make a measurement, and then communicate this measurement to others, it's essential you include the correct unit.

So, imagine you're buying a bookshelf from Linda. You tell Linda the space on your wall where you want to put the bookshelf is 2.7 wide. She says, "No problem, the shelves will easily fit there." You get the bookshelf home and it does not fit!... you meant 2.7 ft and she thought you meant 2.7 m.

You might think this would rarely happen. But there are many examples of such errors in real life, including some infamous ones with significant consequences, like the case of the [wikipedia:Mars_Climate_Orbiter|Mars Climate Orbiter] which was a much more expensive mistake than buying the wrong bookshelf – and you'd probably be able to take the bookshelf back!

SI units[edit]

To communicate a measurement we need to agree on a unit. So, for example, we might agree on a unit to measure length based on the length of your forearm. We'll call the unit simply the "forearm" and use the symbol "F". So a length of 2F is a length that is twice the length of your forearam.

There are several issues with this definition of length for use by the wider public (further discussion to come).

The system of SI units is endorsed by overwhelming international agreement (see the introduction to the appendices) and is even endorsed by the US which still also extensively uses non-SI units. For example, the National Institute of Standards (NIST) – the leading organisation in the US for measurement standards – maintains a list of the most up to date values for many physical constants, and they list the values in SI units, for example, they express the value for the speed of light in m s-1 (NOT ft s-1 or mph, both of which are commonly used in the US).

Naming convention[edit]

it is interesting to note that SI follows the convention that units named after a scientist (e.g. the kelvin, named after Lord Kelvin) are written in lower case, but the symbol is in upper case. So you should write "kelvin" and NOT "Kelvin" but the symbol is "K" not "k". This does not change the physics or the maths, but it can help in remembering correct use for the name and symbol of many units.

SI units in VCE Physics[edit]

The SI system includes many units. So while you can, and should, look at the section of the BIPM website on SI units, for your convenience, we have also listed here all the SI units that are used in VCE physics:

Note: this list does not include units that are only relevant to one of the many particular options in Unit 2, Area of Study 2.

Quantity Unit name Unit Symbol Named After
Length / Distance / Displacement metre m
Time second s from the Latin "secunda pars minuta"
Mass kilogram kg
Weight newton N Isaac Newton
Temperature kelvin K Lord Kelvin
Temperature degree Celsius °C Anders Celsius
Current ampere A André-Marie Ampère
Potential difference volt V Alessandro Volta

Usually the SI prefix "k" (short for kilo) would only be used in front of a base unit to indicate 1000 times that base unit. For example, 1 km = 1000 m, or 1 kN = 1000 N. It is correct that 1 kg = 1000 grams, and this would seem to imply that the gram is the base unit of mass, however, this is NOT correct. That the kilogram is the base unit is an historical quirk.

degree Celsius is not strictly an SI unit. The SI unit for temperature is the kelvin. However, degrees Celsius and kelvin have been defined so that in terms of a difference in temperature 1°C = 1K, so the translation between the two is a simple addition or subtraction. Also, because it has been common practice to use °C, and people are usually familiar with temperatures in °C, often °C is used in science to provide a more immediate sense of the temperatures involved. This common use is acknowledged by the BIPM and is why they were defined to have a simple relationship between them.