Explain renewable and nonrenewable sources of energy and mention the advantage of such classification.?

ANS:


* Those sources of energy which will be available as long as the heat from sun reaches the earth are called renewable sources of energy.

* The renewable sources are generally pollution-free.

* Solar energy, hydel energy, wind energy, tidal energy, etc. are renewable sources of energy.

* Those sources of energy whose formation takes millions of years and which are limited in supply are called nonrenewable sources of energy.

* Nonrenewable sources create heavy pollution.

* Coal, petroleum, natural gas, etc. are nonrenewable sources of energy.

* The advantage of such classification of energy sources is that it helps us to decide about the conservation of available energy sources for future generation.

* It also helps us in development of alternative sources of energy accelerating the scope for the development of appropriate technology

Write a note on solar cooker.

ANS:



* Solar cooker is a device that uses solar energy in the form of heat for cooking food.

* As shown in figure, solar cooker consists of a container box whose body is made up of nonconducting plastic or fibrous material and insulated from outside to prevent heat loss.

* A plane mirror is fixed on the top of the box inb such a way that it reflects sunrays into the box.

* Solar cooker contains small containers painted black externally to absorb heat.

* The box is covered with a glass-sheet to retain the heat inside due to greenhouse effect. This heat develops about 100-140°C temperature inside the box within 2-3 hours when placed in sunrays.

* This type of cooker can be used to prepare food items like rice, dal, pulses, vegetables,etc.

* ADVANTAGES

1. There is no combustion of fuel.

2. Maintenance is negligible.

3. It is pollution-free.

4. The natural taste of the food is maintained as it conserves all nutrients.

5. During the preparation of food no personal attention is needed.



* LIMITATIONS

1. Food cannot be cooked on a cloudy day or in absence of direct sunlight

2. It takes very long time for cooking.

Write a note on Solar Cells.?

ANS:



* Solar cell is a device that converts solar energy directly into electrical energy.

* Earlier, it was observed that when solar rays fall on a thin wafer of selenium, electricity is generated.

* A solar cell made of selenium wafer converts only 0.7% of solar energy into electrical energy which is a very small output and hence impracticable for generating electricity.

* The first solar cell made in 1954 could convert about 1% of incident solar energy into electricity.

* Modern solar cells most commonly made from semimetal silicon can have efficiency of upto 25%. Silicon is easily available and is eco-friendly.

* A typical solar cell consisting of a 2 x 2 cm square piece of pure silicon can produce about 0.7 watt electricity with about 10% efficiency.

* This electricity is quite small and to get large amount of electricity several solarcells are connected together in series. This arrangement is called Solar Panel.

* ADVANTAGES:

1. Solar cells provide eco-friendly environment.

2. Solar cells can be used anywhere as a self-generating source of electricity.


* LIMITATIONS:

1. The availability of the special grade silicon required for making solar cells is very limited.

2. The cost of connecting material used in solar panels is very high as it is generally silver.

3. The current produced by solar cells is direct current (DC) and it has to be converted to alternating current (AC) for some devices. This involves high cost and loss of efficiency.

4. The conventional method of storing solar energy in storage batteries is not efficient enough.



* USES:

1. In artificial satellites.

2. In radio wireless transmission.

3. In TV transmission.

4. In traffic signals and research centres.

5. In calculators and watches.

6. In solar cars.

Write an explanatory note on WIND ENERGY.

ANS:

* Sun's energy is the main factor responsible for the movement of air. Moving air is called wind and it possesses kinetic energy.

* Windmills, invented 250 years ago by Persians, are once again gaining popularity.

* Modern windmills convert wind energy into either mechanical energy or electrical energy.

* A windmill consists of a fan-like structure mounted at some height on a strong support. Its blades are so designed that when wind strikes them, a pressure difference is created between them. This difference produces a turning effect and makes them rotate.

* The height of the windmill, the number of blades and their shape, etc. are decided on the basis of average wind velocity and local environmental factors.

* The rotational motion of the windmill is utilised to obtain mechanical work like in a water-lifting pump.

* If a turbine is connected to the windmill, electricity can be generated but the electricity generated by one windmill cannot be used commercially.

* To generate a lrage amount of electricity, a wind energy farm is established where there are several windmills erected in a large area.

* In Gujarat, wind-energy farms are located at following places : Lamba near Porbandar, Okha, Mandavi and Dhank.

* The largest windfarm in India is near Kanyakumari in Tamilnadu and it generates 300 MW electricity.

* The greatest advantage of wind-energy farms is that the electricity is generated from a renewable source and it is pollution-free.

* LIMITATIONS

1. Wind energy farms can be established only at places with continuous flow of wind.

2. The average wind velocity should be 16 km/h to enable its working.

3. It requires a large area of land and cost of installation is very high.

4. It creates noise pollution.

Write an explanatory note on hydel (hydroelectric) energy.

ANS:


* The electricity generated by moving the turbines of a generator using the energy of flowing water ( or falling water) is called hydroelectric energy or hydel energy.

* During rain or snowfall, the potential energy of water molecules is converted into kinetic energy. When ice at higher altitudes melts, its gravitational potential energy is converted to kinetic energy and it flows as a stream of water or as a river.

* If we store rainwaters by constructing a dam, the water has higher potential energy. When this water is allowed to fall from a height, its potential energy is converted to kinetic energy.

* This kinetic energy of flowing water can be used to rotate the turbine blades along with the armature of electric generator to produce electricity.

* The hydroelectric power station at Ukai on the river Tapi in Gujarat generates 300 MW of electricity. A hydroelectric power station has been constructed on river Narmada at Sardar Sarovar dam.

* Mini hydroelectric power plants can be constructed in hilly regions or at small dams if the water falls through the height of at least 10 meters.

* ADVANTAGES:

1. Once completed, the hydroelectric power station needs only maintenance and proper management which is not much costly.

2. A dam constructed for hydroelectric power station can also be used for irrigation and it prevents floods.


* LIMITATIONS:

1. The cost of installation (construction) of a hydroelectric power station is very high.

2. The requirement of a large area for constructing the dam leads to destruction of forests and causes a great ecological imbalance.

Explain Tidal Energy OR How can electrical energy be generated using tidal energy of the sea (ocean) ?

ANS:


* The level of water near the seacoast changes twice a day. This everyday movement of water along the seashore is known as tides. A high tide occurs on every new moon day and on a full moon day. During the tide the water level rises by a few meters.

* The energy possessed by the rising and falling water is known as tidal energy.

* a dam is constructed across a narrow opening of a sea. Due to tide, the water moves in and out through the openings and flows over the turbines fixed inside the walls of the dam. The movement of turbines generates electricity.

* Tides are not uniform and the rise and fall of water is not large enough to generate electricity on a large scale. Also, there are a very few sites where we find a narrow opening suitable for a dam.

* Wind energy is an indirect form of solar energy. Wind possesses tremendous kinetic energy and causes waves in the oceans. The height of such waves is great when the wind velocity is high. Such waves can be used to generate electricity by rotating turbines using their energy.

* The greatest limitation of this method is that the location of turbine and other equipment is in the sea. They require a lot of maintenance and replacement. This makes them costlier.

Write a note on Geothermal energy.

ANS:


* Geothermal energy is the energy obtained from the heat of the earth.

* Geothermal energy does note depend directly or indirectly on solar energy.

* The interior region of the earth below its crust is very hot and it consists of molten rocks called 'magma'. Magma is in semisolid state.

* The geological changes push this semiliquid magma towards upper surface and it gets collected at some depth below the surface of the earth. These places are called 'hot spots'. Hot spots are considered as the source of geothermal energy.

* When the underground water comes in contact of such hot spots, it turns into steam which is trapped in the space between rocks and becomes compressed due to high pressure.

* This steam can be collected by introducing after drilling holes. These pipes reach upto the depth of hot spots. The steam coming up through the pipes can be used to run turbines.

* Sometimes the steam finds its way through the cracks in the crust and comes up to the surface in the form of geysers. The steam from such geysers has the temperature of about 150 -200°C.

* Geothermal energy is an ecofriendly source of energy and the cost of electricity generated from it is almost half of that of the electricity generated nusing conventional methods.

* ADVANTAGES:

1. This energy can be used for 24 hours throughout the year.

2. The cost of production is comparatively low.

3. It is eco-friendly, i.e. pollution-free.


* A large number of geysers is found in USA and New Zealand but we have limited places in India for geothermal energy.

* In Gujarat, we have geysers at places like Unai, Tulsi Shyam in Saurashtra, and Lasundra and Tuva villages in Godhra District.

Write a note on biogas.

ANS:



* Biogas is produced by the decay of biomass and it consists of 65-75% methane, 30-40% carbon dioxide as well as traces of H2,H2S, and N2.

* Because biogas is generally obtained from the solid wastes of animal dung, sewage, crop residues, agro-wastes and poultry droppings, it is traditionally called 'Gobar gas'.

* Methane is an excellent fuel. The calorific value of biogas is 35-40 kJ/g.

* Two types of biogas plants are commonly used in India.They are :

1. Fixed-Dome Type

2. Floating Gas-Holder Type


* The floating gas-holder type plants have been prepared by the Khadi and Village Industries Commission (KVIC) and are used where biogas is obtained from the animal dung. It consists of a dome like structure, made of steel, that floats up as the pressure of biogas evolved increases.

* The fixed-dome type structure has a longer life. Its dome can be made from bricks. Thus, the cost of the plant is reduced. This type of plant is more suitable for obtaining biogas from human excreta and other bio-wastes.

* PROCESS:

o A slurry of dung, other biowastes and water is prepared in a mixing tank.

o This slurry is fed into the digester which is a closed underground tank made of bricks.

o The digester is sealed from the top.

o The anaerobic micro-organisms decompose the biomass in the digester and produce biogas which is a mixture of fuel gases like methane, hydrogen, hydrogen sulphide with carbon dioxide and nitrogen.

o After a few days, sufficient gas is produced which is supplied to the places of use.


* ADVANTAGES:

1. It can be supplied to the consumers through pipes.

2. Its main component methane is an excellent fuel and burns without smoke.

3. It does not produce ash during burning.

4. It can be used as a fuel for domestic heating, cooking, lighting as well as to generate electricity.

5. It can be supplied to industries as a fuel.

6. The slurry left behind in the digester serves as an excellent manure for farmers.

7. It provides us with a scientific method of disposal of organic wastes.

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Explain Nuclear Fission.

ANS:


* During an attempt to produce transuranic elements, Prof. Fermi and his coworkers observed that when neutrons are bombarded on a nucleus, β-particles are emitted.

* Hahn and Strassman found that 92U235 on bombardment by a neutron produces 56Ba139 which emits β-radiation and gets converted into 57La140.

* The splitting of a heavy nucleus into daughter elements spontaneously or as a result of bombardment by a neutron is called NUCLEAR FISSION.

* There are two categories of nuclear fission: (1) Prompt fission (2) Delayed fission.

* If an atom (like uranium) is bombarded by a high-energy neutron, its nucleus splits immediately. This is prompt (rapid) fission.

* In delayed (slow) fission, the bombarded neutron is absorbed by the target nucleus (uranium) and excites the nucleus which splits later on.

* During nuclear fission, the heavy nucleus obtained after fission has atomic mass 130 - 149 u and the light nucleus has the atomic mass 85 - 104 u.

* If the nucleus of a heavy element (or its isotope) splits without being bombarded by any particle, the fission is called 'Spontaneous fission'.

* During nuclear fission, some neutrons are released. For example, in the fission of 92U235, on an average 2 - 3 neutrons are released.

* During fission, a large amount of energy is released. Let us take the following example:

92U235 + 0n1 → 35Mo95 + 57La139 + 20n1

Here the sum of the atomic masses on the left hand side is

235.124 u + 1.009 u = 236.133 u

and the sum of the atomic masses on the right hand side is

94.946 u + 138.955 u + 2.018 u = 235.919 u

the difference in the total mass of reactants and products is called 'MASS DEFECT'. The mass defect in the above example is

236.133 u - 235.919 u = 0.214 u

This lost mass (mass defect) is converted into energy. According to the Einstein's mass-energy equation when 1 u mass is converted, 931.48 MeV energy is produced.

Thus, in the above example, 0.214 u mass is converted to produce 200 MeV energy which is equivalent to 3.2 x 10-11 Ws (joule) of energy.


* The fission of 1 kg of 92U235 produces 109 W-day or 1000 MW-day energy. If only 30% of this energy is converted into electrical energy, we get 300 MW-day energy for which 2500 tons of coal are needed.

* Electrical energy is produced by using chain reaction in which the neutrons produced in previous reaction are used to split uranium nuclei in subsequent steps. This type of fission is carried out in special type of vessels called Nuclear Reactor.

# What is Chain Reaction ? Describe the ideal conditions to support a chain reaction.

ANS:



* If in a reaction the products of one step result in producing the subsequent step of the reaction then that reaction is called 'Chain Reaction'.

* During the fission of 92U235, two or three neutrons are released. If the energy of these neutrons is utilised to cause fission of other nuclei, then again two or three neutrons are released per nuclei. Thus, the process of fission continues as chain reaction.

* The reaction mentioned above will go beyond control if all the neutrons released in one step are utilised to cause fission in the subsequent step. Because the number of neutrons released will go on increasing, the amount of energy produced will also go on increasing and reach an uncontrollable stage.

* The following processes can take place after neutrons are produced in the fission reaction.:

1. At least one of the neutrons (released during previous step) hits the other 92U235 to cause fission to release more neutrons ( & energy).

2. The neutrons may be captured by the uranium nucleus (parent nucleus) without fission to take place.

3. The neutrons may be captured by the other nucleus (daughter nuclei) either in fission material or in the surrounding containers even without causing fission.

4. The neutrons may not interact with one another and the nucleus and may escape from the system.


* The fission reaction would stop if the last three processes dominate. However, the fission reaction will continue if the first process is repeated at least once.

* Thus, the ideal condition to support the chain reaction will be to lower the energies of released neutrons to thermal neutron energy range of 0.04 eV.

* This can be achieved by using special substances called moderators like graphite and heavy water which take out some of the energy of the neutron.

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