First slide

Change of state

Question

An electric kettle takes 4 A current at 220 V. How much time will it take to boil 1 kg of water from temperature $20^{0} C$ ? The temperature of boiling water is $100^{0} C$

Easy

A

6.3 min
B

8.4 min
C

12.6 min
D

4.2 min

Solution

Heat evolved due to Joule's effect is used up in boiling water.
$VIt = ms ∆ t or t = \frac{ms ∆ t}{VI}$
We are given I = 4 A, V = 200 volt, m = 1 kg
$∆ t = {(100 - 20)}^{0} C, s = 4200 J / {kg}^{0} C$
$t = \frac{1 \times 4200 \times 80}{220 \times 4} = 6.3 \min$

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