Answer :

Given-


Time taken to boil 4 cups of water, t = 2 minutes
Volume of water boiled = 4 × 200 cc = 800 cc
Initial temperature, θ1 = 25°C
Final temperature, θ2 = 100°C
Change in temperature, θ = θ2θ1 = 75°C
Mass of water required to boil , m = 800 × 1 = 800 gm = 0.8 Kg



Now heat required for boiling water,


Where


m is the mass of the water


s is the specific heat of the water


θ is the change in temperature


Putting the value in the above formula, we get




We know, to convert watt-hour to watt-sec,



1000 watt – hour = 1000 × 3600 watt sec.



Hence cost of boiling 4 cups of water




(b)


Given,


Initial temperature, θ1 = 5°C
Final temperature, θ2 = 100°C
Change in temperature, θ = θ2θ1 = 95°C



heat required for boiling water,


Where


m is the mass of the water


s is the specific heat of the water


θ is the change in temperature


Putting the value in the above formula, we get


= 0.8 × 4200 × 95


= 319200


Again converting into watt-second,
Cost of boiling 4 cups of water


=11000×3600×319200


= Rs 0.09


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