Water falling from a 50 m high fall is to be used for generating electric energy. If 1.8 × 105 kg of water falls per hour and half the gravitational potential energy can be converted into electric energy, how many 100 W lamps can be lit?

Height from which the water falls, h = 50 m

Mass of water falling per hour = 1.8 × 105 kg


Mass of water falling per sec,



m= 50 kg


Gravitational potential energy released per sec = mgh


= 50 × 9.8 × 50


= 24500 J


Electrical energy generated per sec = (Gravitational potential energy)/2


= 24500/2


= 12250 J


Power generated = 12250 J/s or W


Number of 100 W lamps that can be lit from this power = 12250/100 = 122.5 ≈ 122 lamps


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