S.I. Units
From DT Online
The modern metric system is the International System of Units (French: Système International d'Unités, SI). The system was published in 1960 and was based on the metre-kilogram-second system of units (MKS). There are now seven internationally agreed units plus units derived from them to cover all aspects of technology together with a set of prefixes which serve as multipliers or fractions of them.
The preferred prefixes multiply or divide the unit names by 1000 (or 10^{-3}) thus, 10mm is preferred to a centimetre, especially in construction and engineering.
Note: Units named after people use capital letters for the letter symbol - but not for the unit name itself.
Measurement of Length
Metre | ||||
---|---|---|---|---|
Prefix Name | Symbol | 10^{n} | Decimal | Description |
kilometre | km | 10^{3} | 1000 | A thousand metres |
metre | m | 10^{0} | 1 | A metre |
centimetre | cm | 10^{-2} | 0.01 | A hundredth of a metre |
millimetre | mm | 10^{-3} | 0.001 | A thousandth of a metre |
micrometre | μm | 10^{-6} | 0.000001 | A millionth of a metre |
Units used in Mechanics
Prefix Name | Symbol | 10^{n} | Decimal | Description |
---|---|---|---|---|
gram | g | 10^{-3} | 1/1000 | Originally the weight of a cubic centimetre of pure water but now defined as one-thousandth of a kilogram |
kilogram | kg | 10^{0} | 1 | S.I. Base Unit of Mass - equal to the mass of the International Prototype of the Kilogram |
tonne | t | 10^{3} | 1000 | Known as Metric Ton in USA (i.e. equivalent to approximately 2,204.6lbs whereas Imperial Ton = 2240lbs) |
megatonne | Mt | 10^{9} | 1,000,000,000 | Used for the energy of nuclear explosions and other similar events. |
Newton | N | 10^{0} | 1 | S.I. Base Unit of Force named after Sir Isaac Newton - the amount needed to accelerate a 1kg mass at 1 metre per second each second (1N = 1kg X ms^{2}) |
Newton Metre | Nm | 10^{0} | 1 | S.I. Base Unit of Torque or Moment - resulting from applying a force of one newton perpendicularly to a moment arm which is one metre long (1Nm). It is calculated as for a joule but is preferred for Torque to avoid confusion with Work and Energy. |
Joule | J | 10^{0} | 1 | S.I. Base Unit of Work or Energy named after James Prescott Joule - the amount needed to apply a newton over the distance of a metre (1J = 1Nm) |
Pascal | Pa | 10^{0} | 1 | S.I. Base Unit of Pressure or Stress named after Blaise Pascal - the pressure exerted by applying a newton over a square metre (1Pa = 1Nm^{2}) |
Units used in Electronics
Ohms (resistance) - named after Georg Simon Ohm | ||||
---|---|---|---|---|
Prefix Name | Symbol | 10^{n} | Decimal | Description |
mega-ohm | MΩ | 10^{6} | 1000000 | A million ohms |
kilo-ohm | kΩ (or K) | 10^{3} | 1000 | A thousand ohms |
ohm | Ω (or R) | 10^{0} | 1 | An ohm |
Note:
The unit or its prefix may be used to replace a decimal point - e.g. 2.2mega-ohm = 2M2; 2.2kilo-ohm = 2K2; and 2.2ohm = 2R2. | ||||
Ampere (electric current)' - named after André-Marie Ampère | ||||
Prefix Name | Symbol | 10^{n} | Decimal | Description |
ampere | A | 10^{0} | 1 | Amp |
milli-amp | mA | 10^{-3} | 0.001 | A thousandth |
Farads (electric capacitance) - named after Michael Faraday | ||||
Prefix Name | Symbol | 10^{n} | Decimal | Description |
farad | F | 10^{0} | 1 | A Farad |
micro-Farad | μF | 10^{-6} | 0.000001 | A millionth |
nano-Farad | nF | 10^{-9} | 0.000000001 | A thousand millionth |
pico-Farad | pF | 10^{-12} | 0.000000000001 | A trillionth |