# boyles law

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• April 11th, 2008, 07:59 PM
rhysboi1991
boyles law
Today i was talking too somebody about propulsion in jet engines and i heard the name boyles law, can somebody please explain to me what boyles law is and what it is used for and if there are any equations involved can you involve them in the explanation. I am sorry if i have spelt 'Boyle' wrong.
• April 11th, 2008, 08:15 PM
Bunbury
Boyle's Law says the pressure and volume of an ideal gas are inversely proportional to each other if the gas is kept at constant temperature.

In a jet engine the gas compression is far from isothermal and then there's a chemical reaction (combustion) followed by a non isothermal expansion with work done, so Boyle's Law by itself really wouldn't be all that helpful.
• April 11th, 2008, 08:26 PM
Harold14370
Here is an article that explains Boyle's law, which states that the pressure of a gas is inversely proportional to its pressure, at constant temperature:
http://en.wikipedia.org/wiki/Boyle's_law

In jet propulsion, Boyle's law might be used to calculate how much a pressurized gas expands when it goes from the pressure in the turbine to atmospheric pressure through the jet nozzle.
• April 11th, 2008, 08:47 PM
rhysboi1991
thank you bunbury and thank you harold for ellaborating his answer
• April 12th, 2008, 12:21 PM
bit4bit
Quote:

Originally Posted by Bunbury
In a jet engine the gas compression is far from isothermal and then there's a chemical reaction (combustion) followed by a non isothermal expansion with work done, so Boyle's Law by itself really wouldn't be all that helpful.

I agree. You'd need calculus, the ideal gas law (Which won't necessarily hold), and some fluid dynamics to extend it to a jet engine situation.

With some basic situations, the ideal gas law is enough to do the job. This takes temperature into account too:

PV / T = constant = nR

n=number of moles
R=Avagadros constant

You can use this to say that at different points in time,

P<sub>1</sub>V<sub>1</sub> / T<sub>1</sub> = P<sub>2</sub>V<sub>2</sub> / T<sub>2</sub>

(Since nR is constant)

Thats more useful in describing an expanding gas within a piston for example in an engiine.