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Thermal Physics

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11 Jan 2008 19:21:26 IST

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on applying heat,why does rubber contract instead of expanding?

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Gaurav |spideyunlimited| Ragtah

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11 Jan 2008 19:26:56 IST

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are you talkin about the eraser rubber or the natural rubber sap?

bhaswati choudhury

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11 Jan 2008 19:38:40 IST

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natural rubber sap....

Ashwin R

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11 Jan 2008 19:49:57 IST

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how come you deleted the question

Gaurav |spideyunlimited| Ragtah

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11 Jan 2008 19:55:09 IST

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In rubber, the molecules are neatly arranged and are cohesive in nature.. but when we heat it the entropy of the rubber increases thus the rubber molecules tend to become tangled up thus creating more randomness and it becomes contracted to observe.

Learner .

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11 Jan 2008 19:57:00 IST

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Natural rubber sap is isoprene,right?

Its structure is something like

-[CH2-CCH3=CH-CH2]-

When its heated(or cooled),its catenating capacity reduces.Thus,it can no longer exist as a long chain...since chains are broken,it loses its shape.Also,it becomes soft and sticky-so the molecules are drawn close to each other..thats why it contracts.

It doesnt really have much to do with the physical interpretation of thermal expansion.

Gaurav |spideyunlimited| Ragtah

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11 Jan 2008 20:00:06 IST

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nahi yaar i read it,, as long as it is in the solid state it will tend to contract on heating since if it expands the molecules will be placed in order instead of in a tangled mass. but after it reaches its melting point, the rubber starts to turn into thin slivery liquid

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Blazing goIITian

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11 Jan 2008 20:04:04 IST

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Ok..thanks!! :-)

But its chains do tend to break.

If its is cooled excessively, then also it becomes brittle and sticky.Is that too related to entropy?

Ashwin R

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11 Jan 2008 20:07:03 IST

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i am jus extending learner's answer
isoprene (natural rubber)contains double bonds ....when heated(in presence of sulphur ) they get vulcanised to become stronger...
because of formation of cross links between carbon atoms...
if it was helpful....
plz rate us

Gaurav |spideyunlimited| Ragtah

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11 Jan 2008 20:09:21 IST

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no i dont think so .. just the contraction of the shape etc. is directly related to entropy. for what ur saying now, the reason mentioned by u would be the explanation.

Learner .

Blazing goIITian

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11 Jan 2008 23:18:04 IST

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Well,is the brittle nature of isoprene on cooling below freezing point also attributed to the same thing(entropy change).If yes,please explain how.

But if it isn't,then what could be the possible reason?

Also, I was thinking about this...could the increase in disorder lead to the breaking of chains and decrease in catenation...seems probable to me!

Thanks!! :-)

Ashwin R

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13 Jan 2008 10:11:33 IST

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ya i was jus extending learner's answer....
ya.... you may be right about entropy thing....
rubber need not be isoprene(but if isoperene....the answer i gave is possible).......
in general the answer you gaveis right.

Ronde vous Silvouz plait

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22 Jan 2008 20:59:18 IST

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