# Introduction to Units & Measurement

## Units of Measurement

### SI Units

The table below shows the Units of Measurement also called SI Units.
Units of Measurement: SI Units
Physical QuantityName of UnitAbbreviation
MassKilogramkg
LengthMeterm
TimeSecondks or sec
TemperatureKelvinK
Amount of substanceMolemol
Electric currentAmpereA or amp
Luminous intensityCandelacd
Derived SI units are constructed from the appropriate base units such the SI unit of speed is meters per second, m/s.

### Metric Prefixes

Base units are converted into units that are more common or convenient as illustrated in the table below.
Prefixes Used in the Metric System and with SI Units
PrefixAbreviationMeaningExample
PetaP10151 petabyte (PW) = 1 × 1015 bytes
TeraT10121 terawatt (TW) = 1 × 1012 watts
GigaG1091 gigajoule (GJ) = 1 × 109 joules
MegaM1061 megabyte (MB) = 1 × 106 bytes
Kilok1031 kilogram (kg) = 1 × 103 grams
Decid10− 11 decimeter(dm) = 1 × 10− 1 meter
Centic10− 21 centimeter(cm) = 1 × 10− 2 meter
Mellim10− 31 milliwatt (mW) = 1 × 10− 3 watt
Microμ10− 61 microjoule(μJ) = 1 × 10− 6 joule
Nanon10− 91 nanosecond(ns) = 1 × 10− 9 second
Picop10− 121 picowatt(pW) = 1 × 10− 12 watt
Femtof10− 151 femtosecond(fs) = 1 × 10− 15 second
Attoa10− 181 attowatt(aW) = 1 × 10− 18 watt
Zeptod10− 211 zeptojoule(zJ) = 1 × 10− 21 joule

### Temperature Units Used in Science

• Celsius scale: based on the properties of water.
• Kelvin scale: the “absolute” temperature, which is the SI unit of temperature.
• The Kelvin unit has the same size as the Celsius unit, but the two scales have different zero points.
• 0 K is the lowest possible temperature ("absolute zero").
• This the point at which all thermal energy would have been drained from an object; so the temperature cannot get any lower.
• 0 K = − 273.15 °C, so K = °C + 273.15