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Question About Boyle's Law

05/21/2007 3:34 AM

Dear Friends,

In Thermodynamics as per the Boyle's law for gases, Volume and Pressure are inversely proportional to each other, but in practice as you increase the pressure of air in a balloon its volume increases! How? Pls suggest an explanation.

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Guru
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#1

Re: BAMCHAK Jabalpur

05/21/2007 4:17 AM

The pressure on the inside must be in balance by the sum of the pressure on the outside and the elastic force created by stretching the balloon.

As the outside pressure is constant the balloon will stretched to accommodate / balance the additional inside pressure.

Boyle's law still applies. (on the closed system) The inside is at a higher pressure than the outside and it is compressed, that is the volume is smaller than it would be if not restricted.

If the inflated balloon is submerged in water (or pressure chamber) the volume will decrease and the pressure will increase.

If the same balloon is placed in a vacuum the inside pressure will decrease and the volume will increase.

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#2

Re: BAMCHAK Jabalpur

05/21/2007 6:00 AM

In the balloon you are increasing the quantity of gas while increasing the pressure. Boyle's Law is applicable only for a fixed quantity of gas in which if you want increase the pressure you have to compress the volume or vice versa i.e. if you expand it pressure will decrease.

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#3
In reply to #2

Re: BAMCHAK Jabalpur

05/21/2007 9:00 AM

Well said!

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#12
In reply to #3

Re: BAMCHAK Jabalpur

02/04/2010 11:53 AM

How come you are using BAMCHAK Jabalpur. If I am correct it is a hotel in Jabalpur

pramod.sharma@ceeindia.org

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#4

Re: Question About Boyle's Law

05/21/2007 3:08 PM

You are confusing the term "air pressure" with the Medium Pressure of the Boyle's Law.

If you increase the pressure OF AIR inside a ballon you are INTRODUCING more air mass into the ballon, so.... at constant pressure (pressure of the medium or atmosphere) over the mass of air, you will have an increase of volume as the mass of air increases.

if you take the ballon (closed), with a certain amount of air inside, and put it in a closed case and start to vaccum the air of case the ballon will increase in volume as you reduce the pressure over the air INSIDE the ballon by taking away the air (therefore pressure) AROUND the ballon.

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#5

Re: Question About Boyle's Law

05/21/2007 3:50 PM

rainbopaints:

Boyle's law may be expressed mathematically as:

P1V1 = P2V2 = k (a constant)

But that is only true if the temperature is held constant.

The combined gas law (Charles's law, Boyle's law, and Gay-Lussac's law) may be expressed mathematically as:

(P1V1)/T1 = (P2V2)/T2 = k (a constant)

According to the combined gas law, as you increase the pressure within a balloon and the volume increases, then the temperature also increases ... so that (PV)/T remains constant.

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#7
In reply to #5

Re: Question About Boyle's Law

05/22/2007 10:11 AM

Recall that all these laws are derived from PV=nRT

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#9
In reply to #5

Re: Question About Boyle's Law

05/22/2007 6:22 PM

Only true for ideal gas.

Just the thought

my 2 cents.

ki, USA

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#6

Re: Question About Boyle's Law

05/22/2007 1:08 AM

Boyle's Law is applicable only in rigid body containers. To increase the pressure of air in a balloon, you increase the volume of air in it. Since the balloon is elastic, it expands.

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#8
In reply to #6

Re: Question About Boyle's Law

05/22/2007 2:41 PM

Or you did mean "in enclosed container" when you said "rigid body container"?

Boyle's Law: V=k/P or PV=k under condition of constant quantity of medium (or air) as well as constant temperature.

where: V=Volumn; P=Pressure; k=constant

A rigid body container means constant Volumn. In this case you have to increase the quantity of the medium to increase the pressure, which is not the case that Boyle's Law applies to.

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#11
In reply to #6

Re: Question About Boyle's Law

05/23/2007 8:57 PM

This is what hendrik noted above and most folks missed.

Plus it seems some think that we are adding air to the balloon, not squeezing a tied off balloon.

Who has more fun than people?

Engineers with time on their hands.

milo

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#10

Re: Question About Boyle's Law

05/22/2007 7:10 PM

Always remember, that when increasing the mass in a closed system, the preasure increases directly in proportion to k, and inversely to the volume; however in a rigid, closed system the volume remains the same, and the temp. rises. Think about this, in a balloon, the pressure changes in a logirythmic proportion to the volume, but both increasing, assuming T remains the same. however, just as in a rigid enclosure, the balloon will also have a bursting point.

ex. in rigid systems PV=kM as mass increases

in expanding systems P to e = kVM as mass increases, let's say V expands at a constant rate, and assuming T remains constant, P will rise, at an increasing rate as V expands, to the bursting point.

Also, and very importantly, even a rigid system, such as a steel container, V will also expand slightly, even though, to the naked eye, it appears to remain the same; simple materials stress and strain analysis.

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