O International System of Units (SI) is an international measurement standard formed by a base of units for seven magnitudes of physics: mass, length, time, electric current, thermodynamic temperature, amount of substance and luminous intensity.
This metric system standard was created in France in the year 1960 during the General Conference on Weights and Measures (CGPM).
A quantity is defined as what can be quantified. The unit is the representation established to designate the measures of quantities. For example, kilogram (kg) is the unit assigned to the measurement of the mass quantity in the SI.
International System Units
From the creation of a pattern with a small group of quantities, called fundamental quantities, it was possible to organize the various known physical quantities. This base is especially important for scientific and technological development.
At 7 SI base units they are all defined in terms of fundamental constants. Are they:
- Subway (m): is the unit of the magnitude of length and corresponds to the distance traveled, in a vacuum, by light in 1/299 792 458 of a second.
- Kilogram (kg): is the unit of the mass quantity and its value is derived from Planck's constant, whose value is 6.62607015 x 10-34 J.s.
- Second (s): is the unit of the time quantity and corresponds to the duration of 9 192 631 770 periods of radiation in the transition between two hyperfine levels of the ground state cesium-133 atom.
- Ampere (A): is the unit of the electric current quantity established in terms of elementary charge, whose value is 1.602176634 x 10-19 Ç.
- Kelvin (K): is the unit of the thermodynamic temperature quantity fixed in terms of the Boltzmann constant, whose value is 1.380649 x 10-23 JK-1.
- Mol (mol): is the unit of the quantity of matter expressed in terms of Avogadro's constant, whose value is 6.02214076 x 1023 mol-1.
- Candela (cd): is the unit of the luminous intensity quantity defined in terms of luminous efficacy, whose value is 683 lm. W-1.
SI fundamental magnitude table
| fundamental magnitude | base unit | unit symbol |
|---|---|---|
| Pasta | kilogram | kg |
| Time | second | s |
| Length | subway | m |
| Electric current | ampere | THE |
| Light intensity | candela | CD |
| amount of substance | mol | mol |
| thermodynamic temperature | kelvin | K |
Derived quantities
Derived quantities are those that can be expressed using the base units and symbols of the multiplication and division operations.
For example, in the International System, energy is a quantity measured by the unit joule (J). Joule can be written in terms of fundamental units as follows:
1 J = 1 kg.m2/s2
It reads: One joule equals one kilogram square meter per second squared.
Examples of SI-derived quantities and units
| Derived quantity | Derived unit | unit symbol | Expression in SI base units |
|---|---|---|---|
| Area | square meter | m2 | — |
| Volume | cubic meter | m3 | — |
| Speed | meter per second | m/s | — |
| Acceleration | meter per second squared | m/s2 | — |
| Strength | newton | N | kg. m. s-2 |
| Pressure | paschal | Pan | kg. m-1. s-2 |
| Energy | joule | J | kg. m2. s-2 |
| power | watt | W | kg. m2. s-3 |
Learn more about Units of Measure.
Prefixes for units
To express quantities with very large or very small values, we use the cientific notation, which uses the x pattern. 10no, Where and the exponent n indicates the number of decimal places before or after the decimal point.
Examples:
2,430,000,000 watts = 2.43. 109 watts
0.0042 m = 4.2. 10-3 m
You prefixes used before a unit of measure are related to scientific notation, as they represent powers of 10 and are used as a multiplier factor to write multiples and submultiples of the units.
Examples:
2.43 x 109 watts = 2.43 gigawatt = 2.43 GW
4,2. 10-3 m = 4.2 mm = 4.2 mm
prefix table
| multiples | submultiples | ||||
|---|---|---|---|---|---|
| Prefix | Symbol | Factor |
Prefix | Symbol | Factor |
| deca | gives | 101 | I decided | d | 10-1 |
| hecto | H | 102 | centi | ç | 10-2 |
| kilo | k | 103 | milli | m | 10-3 |
| mega | M | 106 | micro | 10-6 | |
| gigantic | G | 109 | nano | no | 10-9 |
| will have | T | 1012 | peak | P | 10-12 |
| peta | P | 1015 | femtus | f | 10-15 |
| eh | AND | 1018 | act | The | 10-18 |
| zetta | Z | 1021 | zepto | z | 10-21 |
| yotta | Y | 1024 | yocto | y | 10-24 |
unit conversion
Many times to facilitate the calculations with the values we are working with, we need to convert the units. A very common process for converting is called chain conversion.
For example, if one meter and one hundred centimeters correspond to the same length, then dividing one by the other will result in 1.
These two reasons above can be used as conversion factor, because multiplying the quantity by a unit factor does not change it. This is useful for canceling unwanted drives.
For example, if a problem presents length data in centimeters, but asks for the result in meters, you can do it like this:
Example:
Also read about:
- unit conversion
- mass measurements
- Volume measurements
- Time measurements
- Capacity measures
- Length measurements
