# Measurement in Physics & SI units of Measurement

Kindly go through this article which will give you a simpler description about the need for measurement, fundamental quantities and derived quantities, the system of units, SI units of measurement, advantages and definitions of SI units etc. in a detailed manner.

## Importance of measurement in physics:

When faced with questions like what is the distance of the sun from the earth, what is the speed of light, what is the mass of an electron, one realises the importance of measurement. From the very ancient time, the man started measuring different physical quantities by using conventional methods. His footsteps were used for measuring length while the shadow produced by the sun for measuring the time. But as science progressed, those types of measurements became inadequate. He introduced precise and well-defined methods for measuring different physical quantities, through which he reached to the perfection in measurements.

The measurement means the action of measuring something or measurement is defined as the process of determining the value of an unknown quantity by comparing it with some pre-defined standard.

## What is unit in physics?

Any quantity that can be measured is called a physical quantity. The measurement of a physical quantity always involved the comparison of the quantity to be measured with a reference standard of the same kind. This reference standard used for the comparison is called the *unit* of the physical quantity.

The *standard unit* *of measurement* is defined as a unit of measurement which has a fixed value that does not change from person to person or place to place. For example, ‘second’ is a standard unit of measuring time. Whether a second is used by one person or another person, whether the second is used in one country or another country, it always represents exactly the ‘same time duration’. The duration of a second does not change from person to person or place to place. In fact, wherever we go in the world, a second has a fixed time duration, which never changes. Thus, a ‘second’ means the ‘same duration’ to everyone. So, second is a standard unit of measuring time. It is necessary to have standard units of measurements for the sake of uniformity in measurements.

### Characteristics of a standard unit

The desirable characteristics of a standard unit are

- A unit must be well defined.
- It must be highly precise.
- It must be easily reproducible.
- A unit should remain unaltered irrespective of place, time and physical conditions.
- It should be easily comparable with other similar units.

## Difference between fundamental and derived quantities

There are certain physical quantities that cannot be explained in terms of other physical quantities. They are called *fundamental quantities*. They are the length, mass, time, electric current, temperature, luminous intensity and the amount of substance. The units used to measure the fundamental quantities are called *fundamental units *or *basic units*; that is the fundamental units are the units of length, mass, time, electric current, temperature, luminous intensity and amount of substance.* *

The quantities which are derived from fundamental quantities are called *derived quantities*. *e.g. *volume, speed etc. The units of derived quantities are called *derived units* and are deducted from fundamental units. e.g. units of density, velocity, force, work etc.

For example, , and the unit is

Since the physical quantity velocity is derived from the fundamental quantities length and time, it is a derived quantity and its unit meter/second is a derived unit.

So, in short, we can write the difference between fundamental quantities and derived quantities as:

- Fundamental quantities are the base quantities of a unit system which are independent of other physical quantities.
- Derived quantities are the quantities which are derived from the fundamental quantities.

## Different system of units

A system of units is a set of related units, including both the fundamental and derived units, which are used for calculations. Some units exist in more than one system of units.

The different system of units used for measurement of physical quantities are:

### C.G.S Unit

The C.G.S. system of units (Centimetre, Gram, Second system) is a French system. This system deals with only three fundamental units – the Centimetre, Gram and the Second for length, mass and time respectively.

### F.P.S Unit

The F.P.S. system of units (Foot, Pound, Second system) is a British system. This system deals with only three fundamental units – the Foot, Pound and the Second for length, mass and time respectively.

### M.K.S Units

The M.K.S. system of units (Metre, Kilogram, Second system) was set up by France. This system also deals with three fundamental units – the Metre, kilogram and the Second for length, mass and time respectively. This system is also called the metric system of units and is closely related to C.G.S system of units.

### SI units of Measurement

The measurement system which is internationally accepted now is the one suggested by the Eleventh general conference of weights and Measures held in 1960 in France, and is known as Systeme Internationale d’ Unites or International System of Units abbreviated as SI units of measurement.

According to this system, there are seven basic or fundamental units and three supplementary units. The basic units are- the metre (m) for length,
- the kilogram (kg) for mass,
- the second (s) for time,
- the Kelvin (K) for temperature,
- the ampere (A) for electric current,
- the candela (cd) for luminous intensity and
- the mole (mol) for the amount of substance.

The supplementary units are

- the radian (rad) for angle,
- the steradian (sr) for solid angle,
- the becquerel (Bq) for radioactivity.

## SI Units List

The SI units list of the fundamental and the supplementary quantities and the symbols used to represent them are mentioned in the below table.

Sl. No | Physical Quantity | Unit | Symbol for the unit |

1. | Length | Metre | m |

2. | Mass | Kilogram | kg |

3. | Time | Second | s |

4. | Temperature | Kelvin | K |

5. | Electric Current | Ampere | A |

6. | Luminous Intensity | Candela | cd |

7. | Amount of substance | Mole | mol |

SI Units List 1: Fundamental/Basic quantities and their SI units

Sl. No | Physical Quantity | Unit | Symbol for the unit |

1. | Angle | Radian | rad |

2. | Solid Angle | Steradian | sr |

3. | Radioactivity | Becquerel | Bq |

SI Units List 2: Supplementary quantities and their SI units

The SI units of measurement list of some derived quantities and the symbols used to represent them are given below.

Sl. No | Physical Quantity | Unit | Symbol for the unit |

1. | Area | Square metre | m^{2} |

2. | Volume | Cubic metre | m^{3} |

3. | Density | kilogram per cubic metre | kg/m^{3} |

4. | Velocity | metre per second | m/s |

5. | Acceleration | metre per second squared | m/s^{2} |

6. | Force | Newton | N |

7. | Work, Energy | Joule | J |

8. | Power | Watt | W |

9. | Pressure | Newton per square metre | N/m^{2} |

10. | Surface tension | Newton per metre | N/m |

11. | Torque | Newton metre | Nm |

12. | Electric Charge | Coulomb | C |

13. | Electric potential | Volt | V |

14. | Electric resistance | Ohm | Ω |

15. | Magnetic induction | Tesla | T |

16. | Luminous flux | Lumen | lm |

SI Units List 3: Some derived units in SI

The SI units and dimensional formulas for more than 100 physical quantities are written in my previous article about the dimensions and dimensional analysis of physical quantities.

## Advantages of SI Units

The SI units of measurement system have several distinct advantages over all other systems in use. The main advantages of SI units are as follows:

- SI units are simpler than all other system of units.
- The SI units system is comprehensive.
*i.e.,*the seven base units of the SI system cover all branches of science, engineering and technology. - SI is a rational system of units.
*i.e.,*this system makes use of one unit for one physical quantity. - SI unit system is coherent.
*i.e.,*All the derived units can be easily obtained from fundamental and supplementary units through their multiplication or division. - The fundamental units of SI satisfy all the characteristics that a unit should have.
- SI unit system is a metric system.
*i.e.,*multiples and submultiples of the units can be easily expressed as powers of 10. - SI units are internationally accepted.

## Definitions of the SI units

The definitions of the fundamental SI units are given below:

### Metre definition

Metre (m) is the unit of length. The metre is defined as a length equal to 1,650,763.73 wavelengths of the orange-red light emitted by the Krypton-86 atom in electric discharge. Since 1983, a standard metre is defined as the length of the path travelled by light in vacuum in of a second.

### Kilogram definition

Kilogram (kg) is the unit of mass. The kilogram is defined as the mass of a cylinder of platinum-iridium preserved at the International Bureau of Weights and Measure at Sevres in France.

### Second definition

Second (s) is the unit of time. The second is defined as the time required for 9,192,631,770 cycles of radiations that cause the transition of caesium–133 atoms between two specified lower energy bands.

### Kelvin definition

Kelvin (K) is the unit of temperature. The Kelvin is defined as of the thermodynamic temperature of the triple point of water (triple point is the temperature at which the three states of water i.e., ice, water and water vapour co-exist).

### Ampere definition

Ampere (A) is the unit or electric current. The ampere is that constant current which if maintained in two straight parallel conductors of infinite length and of negligible area of cross-section and placed one metre apart in vacuum, would produce a force of 2 x 10

^{-7 }Newton/metre between them.### Candela definition

Candela (cd) is the unit of luminous intensity. The candela is defined as the luminous intensity in a direction perpendicular to a surface of square metre area of a black body kept at a temperature of freezing platinum under a pressure of 101,325 N/m

^{2}.### Mole definition

Mole (mol) is the unit of amount of substance. The mole is the amount of substance which contains as many elementary units as there are carbon atoms in exactly 0.012 kg of carbon-12.

The definitions of the supplementary SI units are given below:

### Radian definition

Radian (rad) is the unit of plane angle. The radian is the angle subtended at the centre of a circle by an arc whose length is equal to the radius.

### Steradian definition

Steradian (sr) is the unit or solid angle. The steradian is the solid angle subtended at the centre of a sphere by an area on its surface equal to the area of a square whose sides are equal to the radius of the sphere.

### Becquerel definition

Becquerel (Bq) is the unit or radioactivity. The Becquerel is defined as the quantity of a radioactive substance that undergoes one disintegration per second. Earlier, the unit of radioactivity was curie.

** **So let me conclude this topic by asking one question;

## Why do we need standard units of measurement?

Standard units of measurement are necessary because the lack of a standard unit would cause confusion and waste of time in converting from one unit to another all the time. So, for the sake of uniformity, scientists from all over the world adopted the SI units of measurement system as the standard unit system for measurement and now SI is being used as their official system of measurement in almost all countries in the world except the United States, Myanmar and Liberia.

So that’s all about the basics of measurement in physics and SI units of measurement. If you want to read further about measurement, consider reading my other related articles about the different errors that occur in the measurement, dimensions, dimensional formula and dimensional analysis of physical quantities, different charts used for measurement etc.

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