S I Units: Base Units and Quantities

Quantity	Name	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric Current	ampere	A
Thermodynamic Temperature	kelvin	K
Amount of Substance	mole	mol
Luminous Intensity	candela	cd

S I is abbreviated from Système international d’unités, is the modern form of measurement and the most widely used in the world.

It’s just to make sure that scientists are all singing from the same hymn sheet.

When we talk about a base unit in physics we mean one which can be measured without reference to another unit.

For example, the metre is the unit of measurement of the base quantity length, or the Kelvin is the unit of the base quantity thermodynamic temperature. (A one Kelvin change is identical to a change of one degree Celsius; we just start the numbering from a different place, so 0K = -273 Celsius, approximately.)

Here they all are – we use six out of the seven at this level, the unit of luminous intensity, the candela, isn’t really mentioned.

Notice the ones named after people have capital letters – the others don’t.

We don’t need to know exactly how people defined a length of 1m or whatever – it has changed over the years, but more information can be found here.

When we do calculations – we obviously have to use the correct numbers – we can’t multiply 50cm by 0.5m and get 25m². It’s convenient to use powers of 10 and /or prefixes. For example, 100m = 0.1km, 3×10^-6g = 3μg or 3×10^-9kg and so on.

NB {Calculator: 3 EXP +/-9} Sometimes EE. Get to know your own machine.

Here’s a list of all the prefixes you might need: Example, one Gigabyte is 1×10⁹ bytes; 4TB hard drives are readily available.

Prefix	symbol	x times 10x	In full…
yotta	Y	24	1,000,000,000,000,000,000,000,000
zetta	Z	21	1,000,000,000,000,000,000,000
exa	E	18	1,000,000,000,000,000,000
peta	P	15	1,000,000,000,000,000
tera	T	12	1,000,000,000,000
giga	G	9	1,000,000,000
mega	M	6	1,000,000
kilo	k	3	1,000
hecto	h	2	100
deca	da	1	10
base		0	1
deci	d	-1	0.1
centi	c	-2	0.01
milli	m	-3	0.001
micro	μ	-6	0.000001
nano	n	-9	0.000000001
pico	p	-12	1E-12
femto	f	-15	1E-15
atto	a	-18	1E-18
zepto	z	-21	1E-21
yocto	y	-24	1E-24

You won’t come across all of these – the most useful ones I’ve highlighted for you.

A common error people make is to attempt to convert everything to SI before doing calculations. You don’t have to. As long as units match you can add/subtract/multiply or divide them, If they don’t, convert to the most convenient. It helps, however, to be quick at converting. In the next post, we’ll do some conversions.

Next, we’re going to think about derived units, combinations of base units. Simple example,: velocity (a vector): Did you know: –

m/s. is wrong

m/s is OK

ms^-1 is better

m.s^-1 is also wrong

Go figure…