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NEET-XII-Physics
33: Thermal and Chemical Effects of Electric Current
- #10An electric kettle used to prepare tea, takes 2 minutes to boil 4 cups of water (1 cup contains 200 cc of water) if the room temperature is 25°C. (a) If the cost of power consumption is Re 1.00 per unit (1 unit = 1000 watt-hour), calculate the cost of boiling 4 cups of water. (b) What will be the corresponding cost if the room temperature drops to 5°C?digAnsr: bAns : 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 = θ2 - θ1 = 75°C
Mass of water to be boiled, m = 800 × 1 = 800 gm = 0.8 Kg
Heat required for boiling water,
Q = msθ = 0.8 × 4200 × 75 = 252000 J
We know:
1000 watt - hour = 1000 × 3600 watt sec.
∴ Cost of boiling 4 cups of water`` =\frac{1}{1000\times 3600}\times 252000``
= Rs. 0.7
(b) Initial temperature, θ1 = 5°C
Final temperature, θ2 = 100°C
Change in temperature, θ = θ2 - θ1 = 95°C
Q = msθ = 0.8 × 4200 × 95 = 319200
∴ Cost of boiling 4 cups of water`` =\frac{1}{1000\times 3600}\times 319200``
= Rs 0.09
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