Internal energy change questions

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stillcrying
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#1
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#1
For kinetic energy, potential energy and internal energy, write increase, decrease or no change:

'Water at 0 degrees Celsius changing to ice at -15 degrees Celsius.'

Does the kinetic energy decrease and potential energy increase? Where does that leave internal energy?
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Aung
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#2
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#2
Internal energy (also known as thermal energy) is the sum of KE and PE of all the molecules in that substance.

Water freezes at 0 degree.

During the freezing process, internal energy decreases. Kinetic energy of molecules remain constant, so temperature remains constant. Molecular potential energy decreases.

After freezing, internal energy decreases. Kinetic energy decreases. So temperature decreases further.
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stillcrying
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#3
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#3
(Original post by Aung)
Internal energy (also known as thermal energy) is the sum of KE and PE of all the molecules in that substance.

Water freezes at 0 degree.

During the freezing process, internal energy decreases. Kinetic energy of molecules remain constant, so temperature remains constant. Molecular potential energy decreases.

After freezing, internal energy decreases. Kinetic energy decreases. So temperature decreases further.
Thank you very much for explaining. I understand now!
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Eimmanuel
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#4
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#4
There are some inaccuracies to what Aung had said in post #2.

In general, the internal energy of a system is NOT the same as thermal energy. Thermal energy is part of the internal energy of a system. In some simple system, they are the same.
It is best that you check with the syllabus to see how thermal energy and internal energy are defined.

Temperature is related to the average kinetic energy per particle NOT the total kinetic energy of the particles in the system. In some cases where the total KE of the particles increases but the temperature can decrease.
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Aung
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#5
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#5
Idk. I thought they were the same in A level syllabus
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