SI units Why Do we need Units? • We need units because we want to measure the amount or quantity of some things. • To make this measurement globally acceptable we need to put some unique measurement value. This value is called a UNIT. The International System of Units (SI) UNIT NAME UNIT SYMBOL DERIVED FROM QUANTITY NAMED AFTER All lower case unit symbols Meter m Length kilogram kg Mass Second s Time Candela cd Luminous intensity Mole mol Amount of substance liter l, L 10-3 m3 volume Leading capital letter in unit symbol ohm WA-2 resistance Ampere A Electric current Kelvin K Thermodynamic temperature He