--10.SOLAR AND LUNAR ECLIPSES:

ECLIPSE:
An eclipse is a New or Full Moon that occurs near the Moon's Nodes. Because the Moon and Sun are so close to the Nodes, they are aligned perfectly enough with the Earth to cast a shadow.

SOLAR ECLIPSE:

The solar eclipse takes place when the light of the sun is partially or totally cut off from the earth by the moon which comes in between both these celestial bodies.i.e., sun and the earth.

However the solar eclipse occurs only at new moon.As moon is of the similar size to that of the sun,therefore when it passes directly between the earth and the sun it obscures it completely.

There are three types of eclipse.
1.Total
2.Partial
3.annular

The total eclipse is possibel when the apparent sizes of both sun and moon are equl to each other as the moon can completely obscure the bright disc of sun called photosphere.

The Partial eclipse occurs when the sun,moon and earth are not exactly in line.Therefore the moon covers the small part of the sun.

The annular eclipse takes place when the moon's size is too small to completely cover the sun's photosphere which therefore appears as a bright ring around the sun.


LUNAR ECLIPSE:

The passing of earth directly between the moon and the sun results in a lunar eclipse.
Like the solar eclipse the lunar eclipse also has three different types.
A total lunar eclipse occurs when when the whole moon passes through the umbra.In case of partial eclipse the entire moon passes through the penumbra and only part of it passes through the umbra. and in a penumbral eclipse the moon passes through only the penumbra.


11.DAY AND NIGHT AND THEIR VARIATION:

The rotation of the earth is responsible for the day and night varaitions.While rotating the half of the earth faces the sunlight while the other half faces away from the sun.The hemisphere of the earth that faces the sun has day time while the hemisphere that faces away from the sun has night time.
Earth completes its rotation in 24 hours and during this 24 hour duration the variation between day and night occurs.

12. ENERGY 13.SOURCES AND RESOURCES OF ENERGY:

The ability to do work is energy.The significant sources of energy are coal,gas,wood and oil.These sources area slo called the rpimary sources of energy.Electricity which is reagrded as a scondary source of energy is produced by thses primary sources.For the poduction of electricity the sources of nuclear fiision ,sunlight and water are also employed as the primary sources.

CONVENTIONAL AND NON CONVENTIONAL SOURCES OF ENERGY:

The sources of energy that are used for the objective of power generation are called conventional sources of energy.whereas the sources that are not utilized for the factor of electricity generation are included in the category of non conventional sources.However these categories change with time like once the nuclear energy was considered under a non conventional source of energy but with the modern discoveries now it is considered to be an important source of energy.

FORMS OF ENERGY:
COAL: Coal is considered to be an important source of energy .Almost 30 % of worlds power production is dependent on this form.It is a fosil fuel .The organic matter of plants is buried in rocks and soils.The pressure and heat changed this organic material to peat,lignite and then coal.

PETROLEUM:
Another essential form of enrgy is the use of petroleum.It is also a fosil fuel and a crude oil.Through the process of fractional distillation,the constituients of petroleum are seperated .Major oil producin countries are USA,SAUDI ARABIA,ALGERIA and IRAN ,KUWAIT.

NATURAL GAS:
Gas is a cheap source of energy and is an organic matter .It is also used for power generation and its major constituent elements are methane,ethane,propane and other hydrocarbons.
the natural gas is abundntly used in Pakistan especailly.

HYDROELECTRIC POWER GENERATION:
Falling water in the mountanious areas is used as a source of mecahnical energy to rotate turbines and generate electricity.The process of electromagnetic induction is used for this purpose.

SOLAR ENERGY:
The solar energy is used in photoelectric cells.When light strike certain heavy metals electricity is produced.Saudi Arabia,France and other european countries are utilizing this significant source of energy to heat buildings,power cars and other communication systems.

NUCLEAR ENERGY:
This form of energy is now being used by the countries like USA<UK<CANADA.In this source of energy is usually released in the form of heat which is first used to heat water and get steam.This steam is used to run a turbine which in eturn generates heat.

--14.ENERGY CONSERVATION:

By energy conservation it is meant that energy can neither be created nor destroyed but only converted into other forms. This principle infact, is also known as the "law of conservation of energy".

For instance a pendulum that moves to and fro with the bob changing its speed from maximum to teh lowest.In this process the kinetic energy is greatest at the lowest point of teh swing and zero at teh highet.howver the potential energy is maximum at the highest point and is zero at teh lowest.This shows that kinetic energy changes to another form of energy .i.,e potential energy.

Similarly in every production of work all forms of energy generate in this manner.Heat energy is converted to light energy in some cases whereas in some the chemical energy is changed to the potential energy