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Thermal Physics

Scorching goIITian

Joined:	1 Feb 2007
Post:	297

11 Apr 2007 13:14:03 IST

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738

heat and temp.....

None

a bubble of air starts rising from the bottom of the lake. its diameter is 3.6mm at the bottom and 4mm at the surface.the depth of the lake is 250cm and the temp of the surface is 40 degrees. the temp of the bottom of the lake (given tat atm pressure = 76 cm Hg and g=980cms^-2 and relative density of mercury = 13.6 )is......

a)9.4 deg

b)10.4 deg

c)8.4 deg

d)12.4 deg

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Comments (4)

vishak p

Blazing goIITian

Joined: 9 Feb 2007

Posts: 1347

11 Apr 2007 13:18:00 IST

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here see this question is a mixture of solids and fluids and heat chapter.... the upthrust force is varying with density which varies inverselyy with temperature......so.. think we should take row inversely proportional to temperature..... hmmm.... thinking..pls... the more the upthrust force...more will be the diameter i think so...

sneha kulkarni

Scorching goIITian

Joined: 1 Feb 2007

Posts: 297

11 Apr 2007 20:38:39 IST

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can neone try this pleaseeeeeeeeeeeeeeeeeeeee

goutham harsha (1nkZ)

Blazing goIITian

Joined: 7 Feb 2007

Posts: 544

11 Apr 2007 22:10:52 IST

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It shud b an easy one..i think.....

firstly, we shud b observing that the sum involves pressure, temp and volume....

so, let these parameters at the surface be P1,V1,T1 and at da bottom be P2,V2,T2

.: P1V1/T1 = P2V2/T2 [as no. of moles is constant]

now, u know the pressure at top.....

pressure at bottom(P2) = P1+hdg where P1 is pressure at top, h is ht of lake, d is den of water

for vol of bubble, use the formula V=4/3*

*r^3

so, u will get the answer...

saurabh panda

Cool goIITian

Joined: 14 Dec 2006

Posts: 55

12 Apr 2007 10:50:35 IST

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kghedriu is right...

just what i was thinking.... press. inside the gas bubble must be equal to press. outsie gas bubble for it to exist. otherwise the bubble would either shrink or expand untill the press. becomes equal.

look we can't consider any surface tension here and say

Pi - Po = 2T/R..

therefore press. inside bubble= press. outside it. then we can isolate the bubble as a system and apply gas eq. to get the answer just as blazing iitian did. as we know the vol. and temp.

i think i am right. ur suggestion plz....

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