SI (Systéme International) units:
- Length: the meter (m) is defined as the distance light travels in a vacuum in 1/299,792,458 sec (note that this is truly universal: in principle it can be determined by anyone, anytime, anywhere in the Universe).
- Mass: the kilogram (kg) however is still based on the International Prototype Kilogram in Paris and the derived standard kilogram standards held by governments around the world.
- Time: the second (s) is defined today as the duration of 9,192,631,770 periods of the radiation of two hyperfine levels of the ground state of the cesium 133 atom.
- Amount of substance: the mole (mol) is defined as the number of atoms in 0.012 kg (defined, so sig figs not restricted) of carbon 12 atoms.
- Temperature: the kelvin (K) is defined as 1/273.15 of the thermodynamic temperature of the triple point of water.
- Electric current: the ampere is defined as the the current which carries one coulomb (6.24146 x 1018 times the charge on an electron or proton) of charge through a conductor in one second.
You should know (memorize) and be able to interconvert the prefixes in the table below:
|| (or mc)
© R A Paselk
Last modified 26 January 2009