International System Of Units, 7 Basic Units And Derived Units, Important Topics For JEE 2025
Shrivastav 3 Jun, 2024
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International System Of Units : Welcome, young scholars, to the realm of measurement and units! In the fascinating world of science, precise measurements are essential for understanding the properties of matter and the forces that govern our universe. To ensure consistency and accuracy in measurements, scientists around the world use the System of International Units (S.I. Units). By the end of our journey, you'll have a deeper understanding of how measurements are standardized and expressed in the language of science.
All such quantities which we come across during our scientific studies are called Physical quantities. Evidently, the measurement of any physical quantity consists of two parts.
1. The number
2. The unit
A unit is defined as the standard of reference chosen to measure any physical quantity.
International System Of Units : S.I. Units
Different types of units of measurements have been in use in different parts of the world e.g. kilograms, ponds etc. for mass; milk, furlongs, yards etc. for distance. To have a common system of units throughout the world, French Academy of Science, in 1791, introduced a new system of measurements called metric system in which the different units of a physical quantity are related to each other as multiples of powers of 10. E.g. 1 kg = 10 3 m, 1 cm = 10 –2 m etc. This system of units was found to be so convenient that scientists all over the world adopted this system for scientific data.
International System Of Units : 7 Basic Units
|
Physical Quantity |
Symbol |
S.I. Unit |
Symbol |
|
Length |
l |
Metre |
m |
|
Mass |
m |
Kilogram |
Kg |
|
Time |
t |
Second |
s |
|
Electric Current |
I |
Ampere |
A |
|
Temperature |
T |
Kelvin |
K |
|
Luminous intensity |
I u |
Candela |
cd |
|
Amount of the substance |
n |
Mole |
mol |
- Meter(m) : Imagine a journey from one point to another, measuring the distance along the way. The meter is the fundamental unit of length in 1/299, 792, 458 of a second.
- Kilogram (Kg): Picture a balance scale holding the weight of an object. The kilogram is the fundamental unit of mass, defined as the mass of the international prototype of the kilogram, a platinum-iridium alloy cylinder kept at the international Bureau of Weights and measures in France.
- Second(s): Think of a clock ticking away the moments of time. The second is the fundamental unit of time, defined as the duration of 9,192, 631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.
- Ampere(A): Imagine the flow of electric current through a wire, powering electrical devices. The ampere is the fundamental unit of electric current, representing the flow of one coulomb of charge per second through a conductor.
- Kelvin(K): Visualize the temperature scale starting from absolute zero, where all molecular motion ceases. The kelvin is the fundamental unit of temperature, with 0K representing absolute zero, the coldest possible temperature in the universe.
- Candela (cd): Picture a beam of light illuminating the darkness. The candela is the fundamental unit of luminous intensity, representing the brightness of a light source emitting monochromatic radiation at a frequency of 540 terahertz with a radiant intensity in that direction of 1/683 watt per steradian.
- Mole(Mol): Think of a collection of atoms or molecules, counting them in groups. The mole is the fundamental unit of amount of substance, representing the number of elementary entities (atoms, molecules, ions, etc) in a substance, with one mole containing Avogadro’s number of entities. Approximately 6.022 × 10 23 .
The units of all other physical quantities are derived from those of above basic quantities. The units thus obtained are called the derived units. Some common physical quantities and their derived units are shown below.
International System Of Units : Physical Quantitates and their derived units
|
Physical Quantity |
Description |
Unit |
Symbol |
|
Area |
Length square |
Square metre |
m 2 |
|
Volume |
Length cube |
Cubic metre |
m 3 |
|
Density |
Mass per unit volume |
kilogram per cubic metre |
kg m –3 |
|
Force |
Mass × Acceleration |
Newton |
Kg ms –2 = N |
|
Pressure |
Force per unit area |
Pascal (newton per square metre) |
Nm –2 = kg m –1 , s –2 |
|
Work, Energy |
Force × Distance |
Joule |
Nm = kg m 2 s –2 |
The S.I. System recommends the multiples such as 10 3 , 10 6 , 10 etc. and fraction such as 10 –3 , 10 –6 , 10 –9 etc. i.e. the powers are the multiples of 3. These are indicated by special prefixes. These along with some other fractions or multiples in common use, along with their prefixes are given below in Table and illustrated for length (m)
International System Of Units : Prefixes with the base units
|
Prefix |
Symbol |
Multiplication Factor |
Example |
|
deci |
d |
10 –1 |
1 decimetre (dm) = 10 –1 m |
|
centi |
c |
10 –2 |
1 centimetre(cm) = 10 –2 m |
|
milli |
m |
10 –3 |
1 millimetre (mm) = 10 –3 m |
|
micro |
μ |
10 –6 |
1 micrometre( μ m) = 10 –6 m |
|
nano |
n |
10 –9 |
1 nanometre (nm) = 10 –9 m |
|
pico |
p |
10 –12 |
1 picometre (pm)=10 –12 n |
|
femto |
f |
10 –15 |
1 femtometre(fm) = 10 –15 m |
|
Atto |
a |
10 –18 |
1 attometre (am) = 10 –18 m |
|
deka |
da |
10 1 |
1 dekametre(dam) = 10 1 m |
|
hecto |
h |
10 2 |
1 hectometre (hm) = 10 2 m |
|
kilo |
k |
10 3 |
1 kilometre(km) = 10 3 m |
|
mega |
M |
10 6 |
1 megametre(Mm) = 10 6 m |
|
giga |
G |
10 9 |
1 gigametre(Gm) = 10 9 m |
|
tera |
T |
10 12 |
1 terametre (Tm) = 10 12 m |
|
peta |
P |
10 15 |
1 petametre (Pm) = 10 15 m |
|
exa |
E |
10 18 |
1 exametre (Em) = 10 18 m |
Important Unit Conversions
1. Length: 1 foot = 12 inches
1 inch = 2.54 cm
1 Å = 10 –10 m
2. Mass: 1 Ton = 1000 kg
1 Quintal = 100 kg
1 kg = 1000 g
1 gram = 1000 milli gram
1 a.m.u. = 1.67 × 10 –24 g
3. Volume: 1 litre = 1 dm 3 = 10 –3 = 10 3 cm 3 = 10 3 mL = 10 3 cc
1 millilitre = 1 cm 3 = 10 –6 m 3
= 1 cc
4.
Energy:
1 calorie
=
4.184 joules
1 joule = 10 7 ergs
1 litre atmosphere (L-atm) = 101.3 joule
1 electron volt (eV) = 1.602 × 10 –19 joule
5. Pressure: 1 atmosphere (atm) = 760 torr
= 1.01325 bar
= 760 mm of Hg
= 76 cm of Hg
= 1.013 × 10 5 Pascal (Pa)
= 1.013 × 10 5 N/m 2
6.
Temperature:
°C + 273.15 = K;
Q1. Convert 2 atm into cm of Hg.
Sol. 2 atm = 2 × 76 cm of Hg = 152 cm of Hg
{1 atmosphere = 76 cm of Hg}
Q2. Convert 2 dm 3 into mL
Sol. 20 dm 3 = 20 litre = 20 × 1000 mL = 2 × 10 4 mL
{1 dm 3 = 1 litre = 1000 mL}
Q3. Convert 59°F into °C.
Sol.
°C =
International System Of Units FAQs
Q.1 : What are S.I. units, and why are they important?
Q.2 : What are the seven basic S.I. units, and why are they considered fundamental?
Q.3 : What are derived S.I. units, and how are they related to the seven basic units?
Q.4 : Why is it important to use S.I. units in scientific measurements?
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