Sources of Energy (Class - X)

Sunrays are focused at a point using concave reflectors. The solar radiations falling on the concave reflectors are focused at a point. Thus, large amount of heat radiations concentrated at a point increases the temperature of that point to a high value. Hence, option (1) is correct.

The following are the drawbacks of solar cell panel:
(i)    It is costly to set up.
(ii)   It is a low power source.
(iii)  It is associated with pollution.
Option 4 is the correct answer. 

Huge amount of wood is required to produce charcoal for commercial and domestic purposes. So, the residue left after burning wood in a limited supply of oxygen is called charcoal. Hence, option (3) is correct.

The element used for interconnection of cells in a solar panel that adds to the cost is silver. Hence, option (3) is correct.

(i)    Fossil fuels are used at a very fast rate.
(ii)   Fossil fuels are the remains of plants and animals. They pollute the environment as they are not completely burnt.
(iii)  Fossil fuels take a very long time to form as they are the remains of plants and animals which are deeply buried.
Option (3) is correct.

Chemical energy is the basic source of energy in thermal power plants. Chemical energy is produced by the burning of coal in thermal power plants and then converting it into electrical energy.
Hence, option (4) is correct.

Ocean Thermal Energy can be utilised when the temperature difference between the water at the surface and water at depths up to 2 km is 20^oC or more.
Hence, option 3 is correct.

Nuclear energy is the result of bombarding heavy nuclei with neutrons. Hence, option (2) is incorrect.

Many thermal power plants are set up near coal or oil fields because the transmission of electricity is easier than transporting coal or petroleum over to some distance.
Hence, option (1) is correct.

Biogas mainly contains methane. Carbon dioxide, hydrogen and ethane are not the main constituents of biogas. Hence, option (2) is correct.