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Q.

A kettle with 2 litre water at 27°C is heated by operating coil heater of power 1kW. The heat is lost to the atmosphere at constant rate 160J/s, when its lid is open. In how much time will water heated to 77°C with the lid open? (specific heat of water =4.2kJ/C  ·kg )

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a

6min2s

b

14min

c

8min20s 

d

7min

answer is A.

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Detailed Solution

Net power available per second,

P=1000-160=840J/s

Heat required,

Q=msΔθ=(2)(4200)(77-27)=420000J

Therefore time required =8min 20sec

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