# SI System of Measurement

## What is SI System of Measurement?

SI system i.e., “International System of Units and measurement” is the most popular system of units and measurement. It is a modernized and extended form of the metric systems like CGS and MKS systems. It is used worldwide and covers all branches of science and technology.

Advantages of SI units over other system of units of measurements are –

1. SI system of measurement use coherent system of units. All derived units can be obtained by simple multiplication or division of fundamental units without introducing any numerical conversion factor.
2. It is a rational system of units and uses only one unit for a given attribute in different forms. For example – all forms of energy are measured in joule. On the other hand, in MKS system, the mechanical energy is measured in joule, heat energy in calorie and electrical energy in watt hour.
3. SI system is an absolute system of units. It does not use gravitational units. Hence, the use of gravitational acceleration 'g' is not required.
4. It is the internationally accepted system of units.

## Measuring Units

Units for the measuring of some most important physical quantities are given below –

### 1. Land Survey Measuring Units

Land area is measured in following units –

Square-foot ( ft^2 )  – It is a smaller unit used in FPS system of units.

Square-meter ( m^2 ) – It is a smaller unit used in MKS \ \& \ SI system of units.

Acre – It is a practical unit used for measuring very large land area.

1 \ \text {acre} = 4047 \ m^2

Hectare – It is a practical unit also used for measuring very large area.

1 \ \text {hectare} = 1 \times 10^{4} \ m^2

### 2. Mass Measuring Units

Mass of a body is defined as the amount of matter content in that body. It is measure in following units –

Kilogram – It is the unit used for measurement of mass in MKS \ \& \ SI system of units.

Tonne – It is a bigger unit used in MKS system. It is also called metric ton MT .

1 \ \text {tonne} = 1 \times 10^{3} \ kg

Chandra Shekhar limit – It is the largest practical unit for measurement of mass.

1 \ \text {CSL} = 1.4 \times \text {Mass of the sun}

Atomic mass unit – It is the unit for measuring very small masses like atomic mass. It is defined as the mass equivalent to \left ( \frac {1}{12} \right ) part of the mass of one carbon atom.

1 \ \text {amu} = 1.66 \times 10^{-27} \ kg

### 3. Force Measuring Units

As per Newton’s second law of motion, we got the definition of force as proportional to the product of mass and acceleration.

P \propto \text {Mass} \times \text {Acceleration}

Force is measured in following units –

dyne – It is the unit of force in CGS systems of measurement.

\text {1 dyne} = \text {1 gram mass} \times \text {1 cm per square second of acceleration}

poundal – It is the unit of force in FPS systems of measurement.

\text {1 poundal} = \text {1 pound mass} \times \text {1 foot per square second of acceleration}

newton – It is the unit of force in MKS \ \& \ SI systems of measurement.

\text {1 newton } = \text {1 kilogram mass} \times \text {1 meter per square second of acceleration}

### 4. Torque Measuring Units

The turning effect produced by a force is called a of a force or torque. Thus, a torque is measured by the product of the force and perpendicular distance of its line of action from the axis of rotation.

\tau = \text {Force} \times \text {Distance}

Therefore, units of torque measurement will be –

dyne-cm – It is the unit of torque in CGS system of measurement.

\text {1 dyne-cm} = \text {1 dyne force} \times \text {1 cm distance}

foot-poundal – It is the unit of torque in FPS system of measurement.

\text {1 foot-poundal} = \text {1 poundal force} \times \text {1 foot distance}

newton-meter – It is the unit of torque in MKS \ \& \ SI systems of measurement.

\text {1 newton-meter} = \text {1 newton force} \times \text {1 meter distance}

### 5. Air Pressure Measuring Units

The pressure at a point on a surface in contact with a fluid is defined as the thrust per unit area around that point acting normally.

p = \text {Total force on surface} \div \text {Normal area of surface}

Therefore, pressure is measured in following units –

kg per cm2 – It is the unit of pressure in CGS \ \& \ MKS system of measurement.

\text {1 kg per cm}^{2} = \text {1 kg force} \div \text {1 cm}^{2}

atmosphere – It is a unit commonly used for measurement of air pressure.

\text {1 atm} = \text {1 atmospheric pressure} = \text {1.0332 kg per cm}^{2}

### 6. Liquid Pressure Measuring Units

Liquid pressure is measured in following units –

bar – It is SI unit used for measurement of fluid pressure.

\text {1 atm} = \text {1.01325 bar}

\text {1 bar} = \text {0.98 atm} = \text {1.01 kg per cm}^{2}

pascal – It is SI unit used for measurement of liquid pressure.

\text {1 pascal} = \text {1 newton per m}^{2}

\text {1 bar} = \text {1 × 10}^{5} \text {newton per m}^{2} = \text {10}^{5} \text {Pascal}

kilo-pascal or mega-pascal – These are bigger SI units used for measurement of liquid pressures.

\text {1 kilo-pascal} = {10}^{3} \text {pascal}

\text {1 mega-pascal} = {10}^{6} \text {pascal}

### 7. Vacuum Measuring Units

Air pressures below atmospheric pressure are termed as vacuum.

Therefore : \text {Vacuum pressure} \le \text {1 atm}

Torr – It is the unit of measurement of negative air pressure or vacuum.

\text {1 torr} = \text {1 mm of mercury ( Hg ) column}

\text {1 atm} = \text {760 mm of Hg} = \text {760 torr}

pascal – Atmospheric pressure is equal to \text {101325 pascal} . Any pressure below this value in pascal is a vacuum. So, SI unit of measurement for vacuum is pascal.

\text {1 atm} \cong \text {101325 pascal}

\text {Vacuum} \le \text {101325 pascal}

### 8. Temperature Measuring Units

Temperature measuring units are based upon the range between upper and lower fixed temperatures of water. Different types of commonly used temperature scales are as follows –

Celsius or Centigrade scale – In this scale, lower fixed point is 0 \degree and upper fixed point is 100 \degree . Thus range of measurement is 100 unit of degree.

Fahrenheit scale – In this scale, lower fixed point is 32 \degree and upper fixed point is 212 \degree . Thus range of measurement in 180 units of degree.

Kelvin scale – In this scale, lower fixed point is 273 \degree and upper fixed point is 373 \degree . Thus range of measurement in 100 units of degree.

Conversion Relation : \left ( \frac { T_C - 0 }{ 100 } \right ) = \left ( \frac { T_F - 32 }{ 180 } \right ) = \left ( \frac { T_K - 273 }{ 100 } \right )

### 9. Light Intensity Measuring Units

Light intensity is defined as the rate of light spread in all directions through a surface of given area at certain distance from a light source. It is also called as luminous intensity.

The luminous intensity depends upon two factors –

1. The distance of surface from the light source.
2. Power of light source. Power is a property of the light source that describes the rate at which light energy is emitted by the source.

Units of luminous intensity measurement are as follows –

Candela (cd) – It is the power emitted by a light source in a particular direction. It is obtained by dividing the total luminous flux ( lm ) spread from surface by solid angle ( sr ) . A common candle emit light having luminous intensity of 1 candela.

Lumen – It is a modified form of candela.

\text {1 cd} = \text {1 lm sr}^{-1}