CBSE Class 10 Science 2020 · Paper 31/3/1 · Question 3

(a) Water is stored at a height in a reservoir behind a high dam, where it has potential energy. It is carried through pipes to a turbine at the foot of the dam; as it falls, its potential energy changes into kinetic energy, which turns the turbine, and the generator coupled to the turbine converts this energy into electricity. (b) $1$ MW (one megawatt) is the power of a source that does $10^6$ joules of work every second: $1\text{MW}=10^6\text{W}=10^6{\text{J s}}^{-1}$. (c) Any two: large areas of agricultural land and settlements are submerged; a large number of people (peasants and tribals; more than 1,00,000 at Tehri) are displaced and must be resettled; large ecosystems are destroyed under the reservoir, and the submerged vegetation rots, releasing methane. Other correct disadvantages also earn credit. (d) The falling water strikes the turbine blades with great kinetic energy and makes the turbine rotate at high speed; the turbine turns the armature of the generator in its magnetic field, and electricity is generated.