An electric kettle takes 4 A current at 220 V. How much time will it take to boil 1 kg of water from room temperature 200C? The temperature of boiling water is

# An electric kettle takes 4 A current at 220 V. How much time will it take to boil 1 kg of water from room temperature 200C? The temperature of boiling water is

1. A

6.4 minutes

2. B

6.3 minutes

3. C

12.6 minutes

4. D

12.8 minutes

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### Solution:

Electric power consumed by kettle, P = 220 x 4 W
Heat required,

$\begin{array}{l}\mathrm{H}=1000\text{\hspace{0.17em}\hspace{0.17em}}×\text{\hspace{0.17em}\hspace{0.17em}}1\text{\hspace{0.17em}}\left(100-20\right)\text{\hspace{0.17em}}=\text{\hspace{0.17em}\hspace{0.17em}}1000\text{\hspace{0.17em}}×\text{\hspace{0.17em}}80\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{cal}\\ =4200\text{\hspace{0.17em}\hspace{0.17em}}×\text{\hspace{0.17em}\hspace{0.17em}}80\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{J}\\ \mathrm{P}=\frac{\mathrm{H}}{\mathrm{t}}\text{\hspace{0.17em}\hspace{0.17em}}⇒\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{H}=\mathrm{P}\text{\hspace{0.17em}\hspace{0.17em}}×\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{t}\\ \therefore \text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}220\text{\hspace{0.17em}\hspace{0.17em}}×\text{\hspace{0.17em}\hspace{0.17em}}4\text{\hspace{0.17em}\hspace{0.17em}}×\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{t}=4200\text{\hspace{0.17em}\hspace{0.17em}}×\text{\hspace{0.17em}\hspace{0.17em}}80\text{\hspace{0.17em}\hspace{0.17em}}⇒\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{T}=6.3\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{minutes}\end{array}$  