Lakhmir Singh Physics Class 10 Solutions Page No:142
Question 39: A large coal-fired power station produces 2000 MW of electrical energy. A wind turbine with 33 m blades can produce 300 kW.
(a) How many turbines would be needed to replace the power station ?
(b) Why, in actual practice, this number of turbines could not replace the coal-fired power station ?
Solution : (a) Electrical energy produced by coal-fired plant = 2000MW = 2000x106W
Electrical energy produced by wind turbine = 300kW = 300x103W
No. of wind turbine required = 2000×106/300×103 = 6666.6
So, 6667 wind turbines would be needed to replace the power station.
(b) In actual practice, this no. of wind turbines could not replace the coal-fired power plant because the efficiency of wind turbines keeps changing due to changes in wind speed but the efficiency of steam turbines used in coal-fired power stations remains the same.
Question 40: In a solar water heater, why is the storage tank placed at a higher level than the solar panel containing coils ?
Solution : This is because hot water, being lighter and less denser, rises to the top.
Question 41: In many applications, solar cells are connected to rechargeable batteries. Why is this so ?
Solution : The electricity made by the solar cells during day time is stored in rechargeable batteries so that it can be used later on, for example, at night.
Question 42: (a) Solar cells are used to provide the electric current to charge the batteries of a car driven by an electric
motor. Describe the energy changes which take place.
(b) What differences would you expect in the charging of car batteries (i) in bright sunlight (ii) on a cloudy day (iii) at night ?
Solution : (a) Solar cells convert solar energy into electrical energy.
While charging the batteries, electrical energy is converted into chemical energy.
During the use of batteries, chemical energy is converted into electrical energy; then electric motor converts electrical energy into kinetic energy (which drives the car).
(b) (i) Batteries are charged quickly.
(ii) Batteries are charged very slowly.
(iii) Batteries are not charged at all.