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  4. Two litre of water at initial temperature of 27° C is heated by a heater of power 1 kW. If the lid of kettle is opened, then heat is lost at the constant rate of 160 J / s. Find, the time required to raise the temperature of water to 77° C with the lid open (specific heat of water 4.2 kJ / kg )

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Q. Two litre of water at initial temperature of $27^{\circ} C$ is heated by a heater of power $1 \,kW$. If the lid of kettle is opened, then heat is lost at the constant rate of $160 \,J / s$. Find, the time required to raise the temperature of water to $77^{\circ} C$ with the lid open (specific heat of water $4.2 \,kJ / kg$ )

ManipalManipal 2014Thermal Properties of Matter

Solution:

Rate of heat gain $=1000-160=840\, J / s$
$ \therefore $ Required time $=\frac{2 \times 4.2 \times 10^{3} \times(77-27)}{840}$
$=500\, s =8 \,\min 20 \,s $