TNPSC Free Notes Physics In English – Nature of Universe & General scientific laws

இந்தக் கட்டுரையில், TNPSC குரூப் 1, குரூப் 2, குரூப் 2A, குரூப் 4 மாநிலப் போட்டித் தேர்வுகளான TNUSRB, TRB, TET, TNEB போன்றவற்றுக்கான  முறைகள் இலவசக் குறிப்புகளைப் பெறுவீர்கள்.தேர்வுக்கு தயாராவோர் இங்குள்ள பாடக்குறிப்புகளை படித்து பயன்பெற வாழ்த்துகிறோம்.

Nature of Universe & General scientific laws

Solar System
Sun and the celestial bodies which revolve around it form the solar system. It consists of large
number of bodies such as planets, comets, asteroids and meteors. The gravitational force of
attraction between the Sun and these objects keep them revolving around it.
The Sun:
 The Sun is a medium sized star, a very fiery spinning ball of hot gases. Three quarters of
the Sun has hydrogen gas and one quarter has helium gas. It is over a million times as big
as the Earth.
 Hydrogen atoms combine or fuse together to form helium under enormous pressure. This
process, called nuclear fusion releases enormous amount of energy as light and heat. It is
this energy which makes Sun shine and provide heat. Sun is situated at the centre of the
solar system. The strong gravitational fields cause other solar matter, mainly planets,
asteroids, comets, meteoroids and other debris, to orbit around it. Sun is believed to be
more than 4.6 billion years old.
Formation of the Sun:
At the time of the Big Bang, hydrogen gas condensed to form huge clouds, which later
concentrated and formed the numerous galaxies. Some of the hydrogen gas was left free and
started floating around in our galaxy. With time, due to some changes, this free-floating
hydrogen gas concentrated and paved way for the formation of the Sun and solar system.
Gradually, the Sun and the solar system turned into a slowly spinning molecular cloud,
composed of hydrogen and helium along with dust. The cloud started to undergo the process of
compression, as a result of its own gravity. Its excessive and high-speed spinning ultimately
resulted in its flattening into a giant disc.
Planets:
 A planet revolves around the Sun along a definite curved path which is called an orbit. It
is elliptical. The time taken by a planet to complete one revolution is called its period of
revolution.
 Besides revolving around the Sun, a planet also rotates on its own axis like a top. The
time taken by a planet to complete one rotation is called its period of rotation. The period
of rotation of the Earth is 23 hours and 56 minutes and so the length of a day on Earth is
taken as 24 hours. The planets are spaced unevenly. The first four planets are relatively
close together and close to the Sun. They form the inner solar system. Farther from the
Sun is the outer solar system, where the planets are much more spread out.

 Thus the distance between Saturn and Uranus is much greater (about 20 times) than the
distance between the Earth and the Mars. The four planets grouped together in the inner
solar system are Mercury, Venus, Earth and Mars. They are called inner planets.
 They have a surface of solid rock crust and so are called terrestrial or rocky planets. Their
insides, surfaces and atmospheres are for medina a similar way and form similar pattern.
Our planet, Earth can be taken as a model of the other three planets. The four large
planets Jupiter, Saturn, Uranus and Neptune spread out in the outer solar system and
slowly orbit the Sun are called outer planets. They are made of hydrogen, helium and
other gases in huge amounts and have very dense atmosphere. They are known as gas
giants and are called gaseous planets. The four outer planets Jupiter, Saturn, Uranus and
Neptune have rings whereas the four inner planets do not have any rings. The rings are
actually tiny pieces of rock covered with ice.
Mercury:
Mercury is a rocky planet nearest to the Sun. It is very hot during day but very cold at night.
Mercury can be easily observed thorough telescope than naked eye since it is very faint and
small. It always appears in the eastern horizon or western horizon of the sky.
Venus:
 Venus is a special planet from the Sun, almost the same size as the Earth. It is the hottest
planet in our solar system. After our moon, it is the brightest heavenly body in our night
sky.
 This planet spins in the opposite direction to all other planets. So, unlike Earth, the Sun
rises in the west and sets in the east here. Venus can be seen clearly through naked eye. It
always appears in the horizon of eastern or western sky.
The Earth:
The Earth where we live is the only planet in the solar system which supports life.Due to its right
distance from the Sun it has the right temperature, the presence of water and suitable atmosphere
and a blanket of ozone. All these have made continuation of life possible on the Earth. From
space, the Earth appears bluish green due to the reflection of light from waterand land mass on
its surface.
Mars:
The first planet outside the orbit of the Earth is Mars. It appears slightly reddish and therefore it
is also called the red planet. It has two small natural satellites (Deimos and Phobos).

Jupiter:
Jupiter is called as Giant planet. It is the largest of all planets (about 11 times larger and 318
times heavier than Earth). It has 3 rings and 65 moons. Its moon Ganymede is the largest moon
of our solar system.
Saturn:
Known for its bright shiny rings, Saturn appears yellowish in colour. It is the second biggest and
a giant gas planet in the outer solar system. At least 60 moons are present – the largest being
Titan. Titan is the only moon in the solar system with clouds. Having least density of all (30
times less than Earth), this planet is so light.
Uranus:
Uranus is a cold gas giant and it can be seen only with the help of large telescope. It has a greatly
tilted axis of rotation. As a result, in its orbital motion it appears to roll on its side. Due to its
peculiar tilt, it has the longest summers and winters each lasting 42 years.
Neptune:
It appears as Greenish star. It is the eighth planet from the Sun and is the windiest planet. Every
248 years, Pluto crosses its orbit. This situation continues for 20 years. It has 13 moons – Triton
being the largest. Triton is the only moon in the solar system that moves in the opposite direction
to the direction in which its planet spins.

**************************************************************************