Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12
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# An 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. - Physics

Sum

An 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?

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#### Solution

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

Concept: Temperature Dependence of Resistance
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#### APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 11 Thermal and Chemical Effects of Current
Q 10 | Page 219
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