CBSE Class 10 Physics Source of Energy Notes Set 15

Sources of Energy

A source of energy supplies useful energy for cooking, heating, lighting and running machines. A good source supplies enough energy, is easy to store and transport, takes little space, and delivers energy in a controlled way.
Non-renewable (conventional) sources are exhaustible and formed over millions of years: coal, petroleum, natural gas and uranium. Renewable (non-conventional) sources are continuously supplied by nature: the Sun, wind, flowing water, tides and heat of the Earth's interior. Fast depletion of fuels because of rising population, wasteful use and lifestyle is called the energy crisis.

Fossil Fuels and Thermal Power Plants

Coal, petroleum and natural gas formed from the remains of plants and animals buried for millions of years, so the Sun is their ultimate source. Coal has four varieties by carbon content: peat about 60 %, lignite 70 %, bituminous 80 % and anthracite 90 %.
Burning fossil fuels releases carbon dioxide, which adds to global warming, and oxides of sulphur and nitrogen, which cause acid rain.
In a thermal power plant: chemical energy of fuel, heat energy (steam), kinetic energy of the turbine, electrical energy. These plants are built near coal or oil fields to save transport costs.

Hydro Power Plants

Water stored in a dam has potential energy. When released, it gains kinetic energy and turns a turbine coupled to a generator. Energy chain: potential energy of water, kinetic energy of water and turbine, electrical energy.
Advantages: cheap, no pollution, also gives irrigation and flood control. Limitations: dams can be built only at suitable places, large land areas are submerged, ecosystems are destroyed and people are displaced.

Biomass and Biogas

Fuels obtained from plants and animal products are biomass: wood, charcoal, cow dung. Biogas forms when animal dung mixed with water is fermented by anaerobic bacteria in a sealed digester. It is a mixture of methane (about 75 %), carbon dioxide, hydrogen and hydrogen sulphide. It burns without smoke and is used for cooking and lighting. The leftover slurry, rich in nitrogen and phosphorus, is a good manure. A biogas plant also reduces deforestation and helps village sanitation.

Wind Energy

Moving air has kinetic energy. A windmill converts it into mechanical energy to pump water, grind grain or run a generator. Wind energy is clean and free, but it needs a wind speed of at least about 15 km/h for electricity generation and is available only in some regions.

Solar Energy

The Sun produces energy by nuclear fusion of hydrogen into helium. The solar constant, the solar energy reaching unit area at the top of the atmosphere per second, is about 1.4 kW per square metre.
Box-type solar cooker: a wooden box with a black-painted metal tray, insulated, with a glass lid and a plane mirror reflector. Sunlight enters, black surfaces absorb it, and the heat radiation cannot escape through the glass (greenhouse effect). The temperature reaches 100 to 140 degrees C. It cannot be used at night or on cloudy days and cooks slowly, but it saves fuel and causes no pollution.
A solar cell is made of a semiconductor such as silicon and converts sunlight directly into electricity with an efficiency of about 10 to 15 %. Many cells joined make a solar panel. Uses: calculators, satellites, remote areas, street lights. Their cost is high because pure silicon and silver contacts are expensive.

Energy from the Sea

Sea waves, tides, ocean thermal energy and others. Tides result from the gravitational pull of the Moon and Sun and are highest at new and full moon. Tidal plants are built across narrow bays.
Ocean Thermal Energy Conversion (OTEC) needs a temperature difference of at least about 20 degrees C between surface and deep water. Warm surface water boils a liquid such as ammonia, whose vapour drives the turbine; cold deep water condenses it again. OTEC can run all day through the year.

Geothermal and Nuclear Energy

Geothermal energy is heat stored in hot spots under the Earth's crust. Steam from hot underground water drives a turbine. It is continuous, clean and cheap, but suitable sites are few.
Nuclear fission is the splitting of a heavy nucleus, such as uranium, after capturing a neutron, into lighter nuclei with release of energy. Nuclear fusion is the joining of light nuclei, such as hydrogen, into a heavier nucleus, and powers the Sun. The energy comes from the small mass lost, according to \( E = \Delta m\,c^2 \) with \( c = 3 \times 10^8 \) m/s. The major hazard of nuclear power is the radioactive waste and the risk of accidents and leakage of radiation.