PHYSICS:
MEASUREMENTS:
-Measurement is the determination of the size or magnitude of something.
-The International System of Units, (symbol: SI) (for the French phrase Système International d Unités), is the most widely used system of units.
-SI was selected from the existing Metre-Kilogram-Second system of units (MKS), with the addition of extra units, rather than the older Centimetre-Gram-Second system of units (CGS).
SI system
The SI system of units defines seven SI base units: All other physical units can be derived from these base units: these are known as SI derived units.
The SI system of units defines seven SI base units: All other physical units can be derived from these base units: these are known as SI derived units.
| Name | Unit | Definition/ Remarks |
| Length (l) | metre (m) | defined as the distance light travels in a vacuum in 1/299,792,458 second |
| Mass (m) | kilogram (kg) | defined to be the mass of a specific cylinder of platinum-iridium alloy, kept at the International Bureau of Weights and Measures near Paris. |
| Time (t) | second (s) | defined as the time required for 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of caesium 133 |
| Electric current (I) | ampere (A) | defined as the constant current that, flowing in two parallel conductors one meter apart in a vacuum, will produce a force between the conductors of 2 × 10-7 newtons per meter of length |
| Thermodynamic temperature (T) | kelvin (K) | is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water |
| Amount of substance (n) | mole (mol) | contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; |
| Luminous Intensity (I) | candela (cd) | The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. |
Derived units
All other physical units can be derived from these base units: these are known as SI derived units.
All other physical units can be derived from these base units: these are known as SI derived units.
| Quantity | Special name | Special symbol | Expressed in SI Derived Units | Expressed in SI Base Units |
| frequency | hertz | Hz | - | s-1 |
| force | newton | N | - | m• kg • s-2 |
| pressure, stress | pascal | Pa | N/m2 | m-1•kg • s-2 |
| energy, work, quantity of heat | joule | J | N • m | m2 • kg • s-2 |
| Power, radiant flux | watt | W | J/s | m2 • kg • s-3 |
| Electric charge, quantity of electricity | coulomb | C | - | s.A |
| electric resistance | ohm | W | V/A | m2 • kg • s-3 • A-2 |
| Quantity | Name | Symbol |
| Area | square Meter | m2 |
| Volume | cubic | m3 |
| Velocity | meter per second | m/s |
| Accelera- tion | meter per second squared | m/s2 |
| Density | kilogram per cubic meter | kg/m3 |
| Current density | ampere per square Meter | A/m2 |
| Magnetic field strength | ampere per meter | A/m |
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