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Old Jan 20th 2013, 09:22 AM   #1
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radiation blackbody

before the invention of the electric freezer people used the techniquer of freezing water at night to form ice by alllowing the water to raidate its energy to night sky since the night sky acts asa black b0dy , the ambient temp is 6 degrress and the temp of the sky of -23 , if a mass of 4.5 g of water is used in a container having an area of 9cm^2 and a dept of 5mm and radiation from the water is 90% of a blackbody determine the time taken for the water to freeze given the following data.|
initial water temp = 6 degrees
specific heat capacity of water- 4190 kg^-1JK-
latent heat of fusion of water= 3.33 x 10^5 Jkg-1

the answer is got is 5.7 hours, but the answer my teacher says is 5.9 can anyone help me
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Old Jan 21st 2013, 10:09 AM   #2
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It would help for you to show us what you tried here - otherwise how can we tell what you're doing wrong?

However, one thought does come to mind - the heatr transfer under radiative cooling is equal to the difference of temp of the water in K to the 4th power minus the temp of the sky to the 4th power. As the water cools from 6 degrees to 0 degrees the rate of heat flow is reduced. I wonder if you simply used 6 degrees as the temp of the water throughout the cooling process? Perhaps your teacher used an average temp of 3 degrees to determine the heat flow for this phase?
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