Q. 21

# One day in the morning, Ramesh filled up 1/3 bucket of hot water from geyser, to take bath. Remaining 2/3 was to be filled by cold water (at room temperature) to bring mixture to a comfortable temperature. Suddenly Ramesh had to attend to something which would take some times, say 5-10 minutes before he could take bath. Now he had two options: (i) fill the remaining bucket completely by cold water and then attend to the work, (ii) first attend to the work and fill the remaining bucket just before taking bath. Which option do you think would have kept water warmer? Explain.

The rate of heat energy loss is directly proportional to the difference in temperature. In the first scenario, mixing the cold water with the hot water overall brings the water to an intermediate temperature which isn’t too hot. The difference between this intermediate temperature and the surrounding temperature is less. Hence the heat loss would be less. In the second scenario, the temperature difference between the hot water and the surrounding is considerable and thus, the heat loss will occur rapidly. This will cool the water down.

So, the first option will keep the water warmer.

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