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Old Oct 19th 2018, 05:50 PM   #1
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Calculating Work and Entropy Change for an Ice Maker

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Last edited by SniffLimit; Oct 20th 2018 at 02:36 PM.
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Old Oct 19th 2018, 07:58 PM   #2
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question 2 ...........

energy extracted to reduce temp of water to 0C =4.18 x 25 = 104.5 KJ

energy extracted to change phase = 334 KJ

energy extracted to go from 0 to -5C = 5 x 2.11 = 10.55 KJ

total = 449.05 KJ This is the energy change in the water to ice.

But the question asks " what is the minimum electrical work" ... this is another example of an incompetently worded question ! ... Every electrical device for freezing water ( normally compressor) has a theoretical maximum efficiency , usually about 30%, no details are given ...the correct answer should be about 1500KJ , but the idiots who asked the question probably want 449.05 !

question 4 .... At 20% efficiency 2245KJ is required ... 625 WHrs

Last edited by oz93666; Oct 19th 2018 at 08:04 PM.
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Old Oct 19th 2018, 08:04 PM   #3
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Thanks for the help. Unfortunately, the homework closed 2 minutes after you replied, so I didn't have a chance to solve the question. Got a 20% on the homework lol.

I had to have entered 449.05 kJ at least 20 times and it was wrong every time. No idea what the right answers were. I think the 449.05 was the Q output, but no idea how to get the W input from that. Maybe the Q / COP or something, but I tried that probably 15 times and didn't get it correct either.

Last edited by SniffLimit; Oct 20th 2018 at 12:47 AM.
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Old Oct 20th 2018, 04:29 PM   #4
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Originally Posted by SniffLimit View Post
Thanks for the help. Unfortunately, the homework closed 2 minutes after you replied, so I didn't have a chance to solve the question. Got a 20% on the homework lol.

I had to have entered 449.05 kJ at least 20 times and it was wrong every time. No idea what the right answers were. I think the 449.05 was the Q output, but no idea how to get the W input from that. Maybe the Q / COP or something, but I tried that probably 15 times and didn't get it correct either.
Dude... if you're coming to this website for help with your exam as it is happening, then something is seriously wrong with your study schedule. Go and see your teacher for help.
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calculating, change, entropy, ice, maker, work



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