# 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
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 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)
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.