SI Units
Quantity and Unit
SI unit
The International System of Units abbreviated has S.I. is a set of metric units now used in many countries. The value of a physical quantity is written as a number by a suitable unit. The International System of Units is adopted in Physics. The following table shows some of SI units.
Physical quantities
A physical quantity is any measurable feature or property of a body or an object, those properties that are not measurable e.g. love, hate etc are non-physical quantities.
There are many physical quantities in Physics. These Physical quantities can be divided into two types as shown below;
Prefixes
Sometime a physical quantity is too big or too small to be conveniently expressed in SI units. Then some symbols are used as the prefixes instead of Zeros or many places. Prefixes are multiples or decimals of ten. The following table shows some prefixes
Other SI Units
| Physical Quantity | SI Unit | Symbol for Unit | Example |
|---|---|---|---|
| Length | meter | m | 1 meter (1 m) = 100 centimeters (100 cm) |
| Mass | kilogram | kg | 1 kilogram (1 kg) = 1000 grams (1000 g) |
| Time | second | s | 1 second (1 s) = 60 seconds (1 minute) |
| Electric Current | ampere | A | 1 ampere (1 A) = flow of 1 coulomb of charge per second |
| Temperature | kelvin | K | 0 degrees Celsius (0 °C) = 273.15 kelvin (273.15 K) |
| Amount of Substance | mol | mol | 1 mole (1 mol) = 6.022 x 10^23 particles |
| Luminous Intensity | candela | cd | 1 candela (1 cd) = 1/683 watt of visible light power |
| Area | square meter | m^2 | 1 square meter (1 m^2) = 10000 square centimeters (10000 cm^2) |
| Volume | cubic meter | m^3 | 1 cubic meter (1 m^3) = 1000 liters (1000 L) |
| Speed | meter per second | m/s | Speed of light in a vacuum = 299,792,458 m/s |
| Acceleration | meter per second squared | m/s^2 | Acceleration due to gravity on Earth's surface ≈ 9.81 m/s^2 |
| Force | newton | N | 1 newton (1 N) = 1 kg * m/s^2 |
| Pressure | pascal | Pa | Standard atmospheric pressure at sea level ≈ 101,325 Pa |
| Energy | joule | J | 1 joule (1 J) = 1 N * m = 1 kg * m^2/s^2 |
| Power | watt | W | 1 watt (1 W) = 1 J/s |
| Electric Charge | coulomb | C | 1 coulomb (1 C) = 1 A * s |
| Electric Potential | volt | V | 1 volt (1 V) = 1 W/A |
| Resistance | ohm | Ω | 1 ohm (1 Ω) = 1 V/A |
| Frequency | hertz | Hz | 1 hertz (1 Hz) = 1 cycle per second |
| Electric Capacitance | farad | F | 1 farad (1 F) = 1 C/V |
| Inductance | henry | H | 1 henry (1 H) = 1 Wb/A |
| Magnetic Flux | weber | Wb | 1 weber (1 Wb) = 1 V * s |
| Magnetic Flux Density | tesla | T | 1 tesla (1 T) = 1 Wb/m^2 |
| Electric Conductance | siemens | S | 1 siemens (1 S) = 1 A/V |
| Temperature (Celcius) | degree Celsius | °C | Water boils at 100°C and freezes at 0°C at sea level |
| Illuminance | lux | lx | 1 lux (1 lx) = 1 lumen/m^2 |
| Radioactivity | becquerel | Bq | 1 becquerel (1 Bq) = 1 decay per second |
| Absorbed Dose | gray | Gy | 1 gray (1 Gy) = 1 J/kg |
| Dose Equivalent | sievert | Sv | 1 sievert (1 Sv) = 1 J/kg (for certain types of radiation) |
| Electric Potential Difference | volt | V | Standard AA battery voltage is around 1.5 V |
| Electric Charge Density | coulomb per cubic meter | C/m^3 | 1 C/m^3 = 1 C / (1 m^3) |
| Electric Current Density | ampere per square meter | A/m^2 | 1 A/m^2 = 1 A / (1 m^2) |
| Magnetic Field Strength | ampere per meter | A/m | Strength of Earth's magnetic field ≈ 25 to 65 µT (microtesla) |
| Luminous Flux | lumen | lm | 1 lumen (1 lm) = brightness of 1 candela over 1 steradian |
| Radioactivity (Old Unit) | curie | Ci | 1 curie (1 Ci) = 3.7 x 10^10 becquerels (Bq) |
| Angular Velocity | radian per second | rad/s | 1 radian/s (1 rad/s) = angle of 1 radian traversed in 1 second |
| Angular Acceleration | radian per second squared | rad/s^2 | Angular acceleration due to gravity on Earth's surface ≈ 9.81 rad/s^2 |
| Information Storage | bit | bit | 1 bit (binary digit) represents two states (0 or 1) |
| Electric Permittivity | farad per meter | F/m | Permittivity of free space (vacuum) ≈ 8.854 x 10^-12 F/m |
| Electric Permeability | henry per meter | H/m | Permeability of free space (vacuum) ≈ 4π x 10^-7 H/m |
| Electric Field Strength | volt per meter | V/m | 1 V/m = 1 V / 1 m |
| Specific Heat Capacity | joule per kilogram per kelvin | J/(kg·K) | Specific heat capacity of water ≈ 4186 J/(kg·K) |
| Thermal Conductivity | watt per meter per kelvin | W/(m·K) | Thermal conductivity of copper ≈ 401 W/(m·K) |
| Frequency (Angular) | hertz (radian per second) | Hz (rad/s) | 1 Hz = 2π rad/s |
| Electric Field Intensity | volt per meter | V/m | 1 V/m = 1 N/C (newton per coulomb) |
| Magnetic Field Intensity | ampere per meter | A/m | 1 A/m = 1 T (tesla) / μ₀ (permeability of free space) |
| Stress | pascal (newton per square meter) | Pa (N/m²) | 1 Pa = 1 N / m² |
| Electric Field Flux | coulomb per square meter | C/m² | 1 C/m² = 1 C / m² |
| Electric Field Flux Density | coulomb per square meter (farad per meter) | C/m² (F/m) | 1 C/m² = 1 F/m * V (volt) |
Other SI Prefixes
| Prefix | Symbol | Factor (Multiplication) |
|---|---|---|
| kilo- | k | 10^3 |
| mega- | M | 10^6 |
| giga- | G | 10^9 |
| tera- | T | 10^12 |
| peta- | P | 10^15 |
| exa- | E | 10^18 |
| zetta- | Z | 10^21 |
| yotta- | Y | 10^24 |
| deci- | d | 10^-1 |
| centi- | c | 10^-2 |
| milli- | m | 10^-3 |
| micro- | μ | 10^-6 |
| nano- | n | 10^-9 |
| pico- | p | 10^-12 |
| femto- | f | 10^-15 |
| atto- | a | 10^-18 |
| zepto- | z | 10^-21 |
| yocto- | y | 10^-24 |
Examples using SI Prefixes
With these prefixes, we can modify the base units accordingly to express various values conveniently. For example:
- 1 kilometer (1 km) = 1,000 meters (1,000 m)
- 1 megawatt (1 MW) = 1,000,000 watts (1,000,000 W)
- 1 milligram (1 mg) = 0.001 grams (0.001 g)
- 1 microsecond (1 μs) = 0.000001 seconds (0.000001 s)
These prefixes are widely used in scientific and engineering contexts to represent quantities across a wide range of scales. They play a crucial role in making measurements and calculations more manageable and understandable.