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UPSC Physics Atomic Physics Modern physics and Sources of energy

Modern physics and Sources of energy

Category : UPSC








The basic building blocks of all the electronic circuits are the devices in which a controlled flow of electrons can be obtained. This chapter deals with the components of electronics such, as semiconductors, diodes, transistors, integrated chips. Also describes the discovery of electron, proton, and neutron and explains the latest technologies of the communication system such as Internet, Mobile Telephony etc.



An atom (size \[{{10}^{-10}}\] m) consists of a positively charged nucleus (size \[{{10}^{-15}}\]m) which is surrounded by electrons moving around it in different shells. Nucleus of an atom consists of protons and neutrons together called nucleons i.e., mass number (A) Radius of nucleus is related to mass number as \[R={{R}_{0}}{{A}^{1/3}}\] where constant \[{{R}_{0}}=1.25\times {{10}^{-15}}m\]



Electron (\[{{e}^{-}}\]) was discovered by sir J.J. Thomson in 1897 when he was studying the properties of cathode rays.

(i) Electrons are negatively charged particles with e/m ratio\[1.76\times {{10}^{8}}c/g\]

(ii) The charge   of an   electron   was   measured by R. Millikan in oil drop experiment as \[-1.6\times {{10}^{-19}}C\]

(iii) Mass of an electron is \[9.1\times {{10}^{-28}}\]gram.

(iv) Electron is approximately 2000 times lighter than hydrogen



In 1909. Rutherford discovered proton (\[{{P}^{+}}\]) in his gold foil \[\alpha -\] panicle scattering experiment

(i) Protons are positively charged particles.

(ii) The charge of a proton is +\[1.6\times {{10}^{-19}}C\](same as magnitude of an electron).

(iii) Mass of a proton is \[1.672\times {{10}^{-24}}\]gram.

(iv) The atomic number of an element represents the number of protons in the nucleus.



In 1932, James Chadwick discovered neutron (n).

(i) Neutron is an uncharged particle.

(ii) Mass of neutron is \[1.674\times {{10}^{-24}}\] gm.

(iii) The mass number is the sum of number of protons and neutrons.



The phenomenon of emission of electrons from the surface of metal when light of suitable frequency falls on it is called photoelectric effect.

The ejected electrons are called photoelectrons and the current produced due to emitted electrons is called photocurrent


Einstein’s photoelectric equation

\[\frac{1}{2}m{{v}^{2}}_{\max }=h(v-{{v}_{0}})=hc\left( \frac{1}{\lambda }-\frac{1}{{{\lambda }_{0}}} \right)=e{{V}_{S}}\]

The Einstein’s photoelectric equation is in accordance with conservation of energy.



Einstein established the equivalence of mass and energy through a relation known as Einsteins mass-energy equivalence relation.


where \[C=3\times {{10}^{8}}m/s\](speed of light in vacuum)

This relation supports both the law of conservation of mass and law of conservation of energy.


Nuclear Binding Energy

The energy required to break a nucleus into its constituent nucleons and place them at infinite distance is called nuclear binding energy or binding energy. This is the energy with which the nucleons are held together.

The difference between the rest mass of nucleus and sum of rest masses of nucleons constituting the nucleus is known as mass defect.


Binding Energy per Nucleon

The binding energy per nucleon of a nucleus is the average energy required to extract a nucleon from the nucleus.

Binding energy per nucleon

         \[\overline{B}=\frac{Total\,\,binding\,\,energy}{Total\,\,number\,\,of\,\,nucleons}=\frac{BE}{A}=\frac{\Delta m{{c}^{2}}}{A}\]



The phenomenon of spontaneous emission of radiations\[\alpha \], \[\beta \]or \[\gamma \]rays from a substance is called radioactivity.

Radioactivity is a spontaneous process which is independent of all external conditions. It is not affected by temperature, pressure, electric or magnetic field.

The number of decays per unit time or decay rate is called activity.

Activity \[A=N\lambda \]

where \[{{N}_{0}}\lambda ={{A}_{0}}\]is initial activity

Half life is the time in which activity of radioactive substance is reduced to half.

\[{{T}_{1/2}}=\frac{0.693}{\lambda }\]

Radioactive Series

  • The heavy natural nuclides can decay to stable end products by four paths. The four paths have mass number given as 4n, 4n + 1, 4n + 2 and 4n + 3 where n is integer.
  • Last element of series is stable and has a decay constant zero.


Properties of\[\alpha \],\[\beta \] & \[\gamma \]-rays

(A)       Properties of \[\alpha \]-rays

(a) It is a positively charged particle \[\left( \begin{matrix}

   4  \\

   2  \\

\end{matrix}H{{e}^{2+}} \right)\] and contains a charge of \[3.2\times {{10}^{-19}}\]coulomb (exactly double the charge of electron).

(b) The mass of \[\alpha \]-particles is \[6.645\times {{10}^{-27}}kg\](It is equal to mass of a helium nucleus). Actually \[\alpha \]-particle is nucleus of helium, hence it is called doubly ionised helium.

(c) They (\[\alpha \]-particles) get deflected in both electric and magnetic fields.

(d) The velocity of \[\alpha \]-particle is very less than the velocity of light i.e., \[{{v}_{\alpha }}\approx \frac{c}{10}\], where c is velocity of light.                    

(e) The penetrating power of a particle is lowest (in comparison to \[\beta \] and \[\gamma \] particles). It is 1/100 times of \[\beta \]-particles & 1/10,000 times of \[\gamma \]-rays.


(B)       Properties of \[\beta \] rays or \[\beta \]-particles

(a) The beta particles (i.e.,\[{{\beta }^{-}}\]) are electrons contain -\[1.6\times {{10}^{-19}}C\]of charge. Actually \[{{\beta }^{-}}\]is electron and

\[{{\beta }^{+}}\]is positron.

(b) They get deflected in both electric and magnetic field.

(c) The velocity of \[\beta \]-particle varies between 0.01 c to .99 c, where c is velocity of light.

(d) The mass of \[\beta \]particle is relativistic, because its velocity is comparable to velocity of light

(e) They have both ionisation and penetration power. Ionisation power less than \[\alpha \]-particle and penetration power more than \[\alpha \]-particle and less than \[\gamma \]-ray.


(C)       Properties of \[\gamma \]-rays (or gamma radiation)

(a) They are electromagnetic waves as x-rays.

(b) They are not deflected in electric and magnetic field, it means that they are charge less.

(c) The velocity of \[\gamma \]-particle is equal to velocity of light.

(d) The ionisation power of gamma rays is less than \[\beta \] and \[\alpha \] rays but penetration power more than \[\beta \] and \[\alpha \]-rays.

(e) When \[\gamma \]-rays photon strikes nucleus then it gives rise to a phenomenon of pair production i.e.,

\[\underset{(\gamma -rays\,\,or\,\,photon)}{\overset{{}}{\mathop{hv}}}\,\xrightarrow[{}]{{}}\underset{(Pair\,\,production)}{\mathop{{{\beta }^{-}}+\,\,\,\,{{\beta }^{+}}}}\,\]


Uses of Radioactive Isotopes

In medicine

(i) \[C{{o}^{60}}\]for treatment of cancer

(ii) \[N{{a}^{24}}\]for circulation of blood

(iii) \[{{I}^{131}}\]for thyroid problem or goitre

(iv) \[S{{r}^{90}}\] for treatment of skin and eye

(v) \[F{{e}^{59}}\]for location of brain tumor


In industries

(i) for detecting leakage in water and oil pipe lines

(ii) for investigation of wear and tear, study of plastics and alloys, also in thickness measurement.


In agriculture

(i) \[{{6}^{{{C}^{14}}}}\]to study plant photosynthesis

(ii) \[{{15}^{{{P}^{32}}}}\]to find nature of phosphate which is best for given soil and crop

(iii) \[{{27}^{C{{o}^{60}}}}\]for protecting potato crop from earthworm

(iv) For pest control


In scientific research

(i) \[{{K}^{40}}\]to find age of meteorites

(ii) \[{{S}^{35}}\]in factories


In carbon dating

(i) It is used to find age of earth and fossils

(ii) The age of earth is found by Uranium disintegration and fossil age by disintegration of \[{{C}^{14}}\]



  • The X-rays were discovered by Prof. Roentgen, a German scientist in 1885. He was awarded Nobel Prize for this discovery in 1901. X-rays are electromagnetic waves.
  • The modem apparatus for the production of X-rays was developed by Dr. Coolidge in 1913.
  • X-rays are produced when fast moving electrons are suddenly stopped on a metal of high atomic number.


Properties of X-rays

(i) They are not deflected by electric or magnetic field.

(ii) They travel with the speed of light.

(iii) There is no charge on X-rays.

(iv)  X-rays show both particle and wave nature.

(v)  They are invisible.


Applications of X-rays

Following are some important and useful applications of X-rays.


Scientific applications: Various diffraction patterns are used to determine internal structure of crystals.


Industrial applications: Since X-rays can penetrate through various materials, they are used in industry to detect defects in metallic structures in big machines, railway tracks and bridges.


In radio therapy: X-rays can cause damage to the tissues of body (cells are ionized and molecules are broken). So X-rays damages the malignant growths like cancer and tumors which are dangerous to life, when is used in proper and controlled intensities.


In medicine and surgery: The cracks or fracture in bones can be easily located. Similarly intestine and digestive system abnormalities are also detected by X-rays.


Harmful Effects of Radiation

  1. The losing of hair quickly and in clumps occurs with radiation exposure.
  2. Radiation kills nerve cells and small blood vessels and can cause seizures and immediate death.
  3. The thyroid gland is susceptible to radioactive iodine. Insufficient amount of radioactive iodine can destroy all or part of the thyroid
  4. Blood’s lymphocyte cell count will be reduced and an increase long term risk for leukemia and lymphoma due to high level of radiations.


Safety Precautions while Handling Radioactive source or Radiation

  • The workers should be asked to wear lead lined aprons or special film badges. These badges can absorb nuclear radiations and can be tested to find the amount of radiation to which a particular worker is exposed.
  • The radioactive contamination of the work area should be avoided at all costs.



Nuclear Fission

Nuclear fission is the disintegration (Splitting) of a heavy nucleus upon bombardment by a projectile, such that the heavy nucleus splits up into two or more lighter nuclei of comparable musses with an enormous release of energy. 200 MeV per fission of \[{{U}^{235}}\] nuclei.

\[_{92}^{235}U+_{0}^{1}n\to _{53}^{141}Ba+_{36}^{92}Kr+3\left( _{0}^{1}n \right)+200\,\,MeV\]           

Ii is the principle of atom bomb (destructive use).

Nuclear reactor has been devised for this purpose.


The main parts of nuclear reactor are

(a) Nuclear fuel: \[{{U}^{233}},{{U}^{235}},P{{u}^{239}}\] etc.

(b) Moderator: Graphite, heavy water (\[{{D}_{2}}O\]). To slow down the neutrons (or slow down the nuclear reaction).

(c) Control rods: (Cadmium, boron). To absorb excess neutrons. It controls the chain reaction.

(d) Coolant: (water etc.) To remove the heat produced in the core to heat exchanger for production of electricity.


Nuclear Fusion:

It is the fusion of two or more light nuclei to form a heavy nucleus with a release of huge amount of energy.

The nuclear fusion reaction, which is the source of the energy of sun star are proton-proton cycle.



The materials whose conductivity lies between conductors (\[{{10}^{2}}-{{10}^{8}}s{{m}^{-1}}\]) and insulators (\[{{10}^{-11}}-{{10}^{-19}}s{{m}^{-1}}\]) are called semiconductors (\[{{10}^{5}}-{{10}^{-6}}s{{m}^{-1}}\])

There are two types of semiconductor.

Intrinsic Semiconductor: These semiconductors are pure in which the thermal vibrations of the lattice have liberated charge carriers (i.e., electrons and holes). In intrinsic semiconductor, the number of electrons are equal to the number of holes. i.e., \[{{n}_{i}}={{n}_{e}}={{n}_{h}}\]     e.g. Si, Ge,

Extrinsic Semiconductor: They are impure semiconductors in which traces of impurity introduces mobile charge carriers [which may be + ve (holes) or-ve (electrons)] in addition to those liberated by thermal vibration.

Again there are two types of extrinsic semiconductors

(i) N-type semiconductor

(ii) P-type semiconductor.


N-type Semiconductor

When a pure semiconductor (Si or Ge) is doped by pentavalent impurity (P, As, Sb, Bi) then four electrons out of the five valence electrons of impurity take part in covalent bonding, with four silicon atoms surrounding it and the fifth electron is set free. These impurity atoms which donate free \[{{e}^{-}}\] for conduction are called as donor impurity (\[{{N}_{D}}\])). Here free increases very much so it is called as n- type semiconductor and impurity ions known as “immobile donor positive ions”. Free \[{{e}^{-}}\] called as majority charge carriers and holes called as minority charge carriers.


P-type Semiconductor

When a pure semiconductor (Si or Ge) is doped by trivalent impurity (B, Al, In, Ga) then outer most three electrons of the valence band of impurity take part, in covalent bonding with four silicon atoms surrounding it and one electron from semiconductor makes hole in semiconductor. These impurity atoms which accept bonded \[{{e}^{-}}\] from valence band are called as acceptor impurity (\[{{N}_{A}}\]). Here holes increases very much so it is called as p- type semiconductor and impurity ions known as “immobile acceptor negative ions”. Free \[{{e}^{-}}\] are called as minority charge carries and holes are called as majority charge carriers.



When a P-type semiconductor is suitably joined to an N-type semiconductor, then resulting arrangement is called P-N junction or P-N junction diode.


Forward and Reverse Biasing

Forward biasing: If we apply a voltage V such that P-side of the P-N junction diode is positive and N-side is negative then it is called forward biasing.

Reverse biasing: If we apply a voltage V such that P-side is negative and N-side is positive then it is called reverse biasing.


P-N Junction Diode as a Rectifier

Rectifier is a device which converts ac to unidirectional pulsating output. In other words it converts ac to dc. It is of following types.

(i) Half wave rectifier

(ii) Full wave rectifier


P-N Junction diode as an Inverter

It is a device which converts direct current (d. c.) to alternating current (a. c.)



Transistor in general is known as bipolar junction transistor. It is a current operated device. It consists of three main regions

Emitter (E): It provides majority charge carriers by which current flows in the transistor. Therefore the emitter semiconductor is

Base (B): The based region is lightly doped and thin.

Collector (C): The size of collector region is larger than the two other regions.


Transistors are of Two Types

N-P-N Transistor: If a thin layer of P-type semiconductor is sand witched between two thick layers of N-type semiconductor is known as NPN transistor.

P-N-P Transistor: If a thin layer of N-type of semiconductor sand witched between two thick layer of P-type semiconductor is known as PNP transistor.



Transistor as an Amplifier

A device which increases the amplitude of the input signal is called amplifier.


Transistor as a switch

Transistors can be used in an electronic circuit as a simple switches. A transistor conducts current across the collector emitter path only when a voltage is applied to the base. When no base voltage is present, the switch is off when base voltage is present, the switch is on.

Transistor as an Oscillator

Oscillator is a device which delivers a. c. output wave form of desired frequency from d. c. power even without input signal excitation.


Transistors are used in variety of applications such as

  • High current transistors are used in car power inverters.
  • Audio equipment uses transistors.
  • Transistors are used in hearing aids.



Zener diode: A properly doped crystal diode which has sharp breakdown voltage is known as Zener diode. It is always connected in reverse bias condition. It is used as a voltage regulator. In forward biased case, it works as a simple diode.

Photodiode: A junction diode made from light or photosensitive semiconductor is called a photodiode.

Light emitting diode (LED): When a junction diode is “forward biased” energy is released at junction in the form of light due to recombination of electrons and holes. In case of Si or Ge diode, the energy released is in infra-red region.

In the junction diode made of GaAs, In P etc. energy is released in visible region such a junction diode is called “light emitting diode” (LED).

LEDs are used in various real life projects such as

  • Solar powered LED street light.
  • LED based automatic emergency light.
  • Displaying dialed telephone numbers on seven segment display, etc.



An integrated circuit (IC), sometimes called a chip or microchip, is a semiconductor wafer on which thousands or millions of tiny resistors, capacitors and transistors are fabricated. An IC can function as an amplifier, oscillator, timer, counter, computer memory or microprocessor.


Uses of ICs

(i) In cars (automotive controls) televisions, computers microwaves, laptops, MP3, play stations, cellular phones aeroplanes, space crafts etc.

(n) In switching telephone circuits, data processing, military equipment’s.

(iii) In digital watch’s, scientific calculator and computers (flip-flops, logic gates, temperature sensors etc.)



Communication means transmission of information. Every communication system has three essential elements- Transmitter, medium/channel and receiver.

Transmitter, converts the message signal produced by the source of information into a form suitable for transmission through the channel and receiver receives transmitted signal.


Modes of Communication

There are two basic modes of communication:

(i) Point-to-point communication mode: In this mode, communication takes place over a link between a single transmitter and receiver as in telephone.

(ii) Broadcast mode of communication: In this mode, there are a large number of receivers corresponding to a single transmitter. Radio and television are most common examples of broadcast mode of communication.



Modulation and Demodulation

Modulation: The original low frequency message/information signal cannot be transmitted to long distances. So, at the transmitter end, information contained in the low frequency message signal is superimposed on a high frequency wave, which acts as a carrier of the information. This process is known as modulation.                                              

Demodulation:  The process of retrieval of original information from the carrier wave at the receiver end is termed as demodulation. This process is the reverse of modulation.  



Ground Wave or Surface Wave Propagation

The radio waves which travel through atmosphere following the surface of earth are known as ground waves or surface waves and their propagation is called ground wave propagation or surface wave propagation.

The ground wave propagation is suitable for low and medium frequency i.e., up to 20 MHz only.

Sky Wave Propagation

The sky waves are the radio waves of frequency between 2 MHz to 30 MHz                                               

The highest frequency of radio waves which when sent straight i.e., normally towards the layer of ionosphere gets reflected from ionosphere and returns to the earth is called critical frequency.     


Space Wave Propagation                                

The space waves are the radio waves of very high frequency i.e., between 30 MHz to 300 MHz or more.




The internet is a worldwide, publicly accessible series of inter connected computer networks that transmit data by packet switching using the standard Internet Protocol (IP).

File transfer: In computing, file transfer is a generic term for referring to the act of transmitting files over a computer network. While the term "file transfer" is often linked to the File Transfer Protocol (FTP), there are numerous ways to transfer files over a network. Servers which provide a file transfer service are often called file servers.


There are 2 Types of file transfers:

“pull-based” file transfers where the receiver initiates a file transmission request.

“Push based” file transfers where the sender initiates a file transmission request.

Some protocols for file transfer may provide both of these, and they are often referred to as “uploading” or “downloading”, from the client’s perspective.


The World Wide Web (commonly shortened to the www) is system of interlinked hypertext documents accessed via the internet. The World Wide Web was created in 1989 by Sir Tim Berners- Lee, working at CERN in Geneva, Switzerland and released in 1992.


Mobile Telephony

The central concept of this system is to divide the service area into a suitable number of cells centred on an office called MTSO (Mobile Telephone Switching Office). Each cell contains a low-power transmitter called a base station and customers. When a mobile receiver crosses the coverage area of one base station, it is necessary for the mobile user to be transferred to another base station. This procedure is called handover or handoff. This process is carried out very rapidly, to the extent that the consumer does not even notice it. Mobile telephones operate typically in the UHF range of frequencies about 800-950 MHz.






Everything we do is connected to energy in one form or another, Energy, defined as “the ability to do work.”

Various forms of energy includes: Biomass energy - energy from plants. Electricity, Geothermal energy, Fossil Fuels - Coal, Oil and Plural gas. Hydro power Ocean energy, Nuclear energy. Wind energy etc.

Energy sources are divided into two groups — renewable and non-renewable Renewable and non-renewable energy sources can be used to produce secondary energy sources including electricity.



A source of energy is that which can provide adequate usable energy at a steady rate over a long period of time. A good source of energy possesses the following characteristics: (i) large amount of work per unit volume or mass (ii) be easily accessible, (iii) be easy to store and transport, and (iv) be economical. So evaluation criteria for good source of energy are:

Capital costs, Operating costs. Efficiency, Is it renewable?. Energy storage requirements, Pollution, Environmental modification, Levelized cost to the consumer. Feasibility on large scale, Unit capacity. Proper ignition temperature,

High calorific value etc.


Handy Facts

  • Calorific value of a fuel is the amount of heat produced by unit mass or unit volume of that fuel.
  • Ignition temperature of fuel is the temperature at which the fuel starts burning or producing energy.



(on the basis of recycling period)

Renewable Sources of Energy

Renewable sources of energy are those which can be generated by us or which are constantly being generated by natural processes or whose supply is unlimited. Examples: The sun, wind, flowing water, etc.


Non-Renewable Sources of Energy

Non-renewable sources of energy are those which were produced in the past by natural processes, whose supply is limited and which we cannot generate again in short interval of time, these will be exhausted in future. Examples: Coal, petroleum, natural gas, etc.



The sources of energy which are extensively used by man due to their easy availability.

For example: Fossil fuels, wind energy, energy from biomass etc.


Fossil Fuels

Fossil fuels are hydrocarbon based natural resources that were formed over 300 hundred millions of years ago by the fossilization of prehistoric plants and animals. There are three major forms of fossil fuels: coal, oil and natural gas.


Drawbacks of Fossils Fuels

  • Fossil fuels are non-renewable resources and are limited.
  • Global warming is directly associated with the increase in greenhouse gases produced from the burning of fossil fuels.
  • On burning fossil fuels releases acidic oxides which leads to acid rain and thus affecting soil and water resources.
  • Environmental pollution : Burning of fossil fuels causes air pollution


Science in Action

Hydrogen is considered as a clear fuel than CNG because burning of CNG is gives out CO and \[C{{O}_{2}}\]but \[{{H}_{2}}\]does not give any harmful gases and calorific value of \[{{H}_{2}}\]is higher than CNG.


Thermal Power Plant

A thermal power station comprises all of the equipment and systems required to produce electricity by using a steam generating boiler fired with fossil fuels or biofuels to drive an electrical generator. Such power stations are most usually constructed on a very large scale and designed for continuous operation.


Hydro Power Plant

Hydro power means making electricity from water power. It is the largest source of renewable power worldwide. A quarter of our energy requirement in India is met by hydro-power plants. Hydroelectric power uses the kinetic energy of moving water to make electricity. Dams can be built to stop the flow of river. Water behind a dam often forms a reservoir.

The water from the high level is carried through pipes in order to rotate the turbine. As the turbine rotates, the armature of the generator also rotates and thus produces electricity. About 20% of the power generated in India comes from hydroelectric power stations.


Advantages of Hydroelectricity

  • This technique does not cause any environmental pollution.
  • It is one of the cheapest sources of energy.
  • Dams constructed on rivers helps in irrigation and also in controlling floods.



Biomass is a renewable energy source because the energy

it contains comes from the sun. Through the process of photosynthesis, chlorophyll in plants captures the sun’s energy and converting carbon dioxide from the air and water from the ground into carbohydrates, complex compounds composed of carbon, hydrogen, and oxygen. When these carbohydrates are burned, they turn back into carbon dioxide and water and release the sun’s energy they contain. In this way, biomass functions as a sort of natural battery for storing solar energy. As long as biomass is produced sustainably with only as much used as is grown then this battery will last indefinitely.


Bio-mass as a Fuel:

  • Wood as a fuel. i.e., firewood.
  • Animal dung as a fuel.



Bio-gas is made from organic waste matter after it is decomposed. The decomposition breaks down the organic matter, releasing various gases. The main gases released are methane, carbon dioxide, hydrogen and hydrogen sulphide. Bacteria carry out the decomposition or fermentation.



  • In industries and also as a domestic fuel for cooking, lighting, etc.
  • In pumping sets used for irrigation.


Advantages of Bio-gas                     

  • It causes less pollution, as bums without smoke.
  • High Calorific i.e., heat producing value.
  • It is a clean fuel as it leaves no residue.
  • This method is environment friendly as it is an efficient method of waste disposal and supplies energy and manure.



Along with sun, it was the air, which showed man its power. Even before the solar energy, it was the wind energy that man used for his work. Initially, it was used in two main ways; to drive wind mills on land and to drive sailboat at sea. The first use of windmills were to grind food grains and to run pumps to irrigate. Farmers have been using wind energy for many years to pump water from wells using windmills. Now with the advancement of science and technology, we have windmills generating electricity. Naturally, now this energy can be used for many more works.


Advantages of Wind Energy

  • It is a renewable source of energy.
  • It does not cause pollution.
  • It does not require any recurring expenses.


Disadvantages or Limitations of Wind Energy

  • It requires high maintenance and large area for installation.
  • These farms can be established in the areas where wind blows for most part of the year.



Solar Energy

The energy obtained from the sun is called solar energy. The inner temperature of the sun is very high (\[{{10}^{2}}K\]). Energy of the sun reaching every year on earth is about \[1.6\times {{10}^{8}}KWh\] (Kilo watt hour). Value of energy used by all the living beings on the earth is \[7\times {{10}^{13}}KWh\] per year.


Solar Cell

Solar cell is such a device which converts solar energy into electric energy. Solar cells are also known as photo or photo election cell (PV cell) because it works on the principle of photo-voltaic effect. Solar cells can be found on many small appliances, like calculators, and even on spacecraft. They are made of silicon, a Semi-conductor. Silicon (Si) is abundant in nature but availability of the special grade silicon for making solar cells is limited. A single cell of \[4\,\,c{{m}^{2}}\] silicon develops a voltage of 0.5-IV and can produce about 0.7 W of electricity when exposed to sun.

The group of solar cells connected in specific pattern to produce desired potential difference and magnitude of solar cell panel.


Applications of Photoelectric Cells

  • In television cameras for telecasting scenes and photo telegraphy.
    • In production of sound in motion pictures.
    • To switch on and off the street lights automatically.
    • To control temperature in furnaces and chemical reactions.
  • In fire and burglar's alarm, to open and close the doors automatically and in counting devices.


Solar Cooker

A solar cooker is a device that uses the energy in sunlight to generate sufficient temperatures to be able to cook food. Swiss naturalist Horace de Saussure was known to have been experimenting with solar cookers as early as 1767. Three basic solar cooker designs exist:


  • Parabolic Reflector • Box Cookers • Panel Cookers


Handy Facts

The earth receives about 1.4 kW of direct solar radiations per square meter per second is called solar constant.



Tidal Energy

Tide arise due to the gravitational pull of mainly the moon on the water and spinning earth. The tide moves a huge amount of water twice each day, and harnessing it could provide a great deal of energy. Tide is the alternate rise and fall in the water level of oceans and seas.

Advantages: (i) It is an inexhaustible and renewable source of energy.

(ii) It does not cause any environmental pollution.

(iii) It does not produce any harmful waste.


Wave Energy

Kinetic energy exists in the moving waves of the ocean. Waves are a powerful source of energy. That energy can be used to power a turbine. There are several methods of getting energy from waves, but one of the most effective works like a swimming pool wave machine in reverse.


Ocean Thermal Energy

The energy from the sun heats the surface water of the ocean. In tropical regions, the surface water can be \[40{}^\circ \] celsius or more degrees warmer than the deep sea water. This temperature difference can be used to produce electricity.

The energy available due to the difference in the temperature between the water at the surface and water at depths is called ocean thermal energy (OTE).


Geothermal Energy

Energy present in the depth of the earth is called geothermal energy. Temperature in the earth at a distance of 10 kilometres is about \[120{}^\circ C\]and it increases to \[300{}^\circ C\]at the depth of 320 kilometres. It is evident that temperature increases with depth. Melted liquid, magma is present in the depth of earth. It is surrounded by various layers of soil, sand and water. Whenever there is some passage, it comes in contact with water present between these layers and converts this water into the steam of sufficient pressure. This vapour pressure can be used for production of energy.


Science in Action

In Iceland, virtually every building in the country is heated with hot spring water. In fact, Iceland gets more than 50 percent of its energy from geothermal sources. In Reykjavik, (population 115,000), hot water is piped in from 25 kilometres away, and residents use it for heating and for hot tap water.

Nuclear Energy

Nuclear power is an alternative energy source that can be obtained from either the splitting of the bigger nucleus of atoms (nuclear fission) or the combining of the lighter nuclei of atoms (nuclear fusion).

Atom bomb is based on the principle of nuclear fission and

Hydrogen bomb is based on the principle of nuclear fusion.





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