gatewaycityca
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Question relating to expansion of heated air
I've always dreamed about building my own simple engine, and lately I've been reading a lot about Stirling engines. I started to build my own
homemade one, but I was pretty sure it was never going to work because I made the cylinder too big and it would just take too much heat to make the
power piston move. I don't know hardly anything about this area of physics - my hobby has always been experimenting with electricity. So I figure I
just need to start with something simpler, and figure out how to make a better sized cylinder and just get the air to expand enough to move a piston.
I'm sure this is a really basic question, but is there a formula or something somewhere that tells how much you can expect air to expand for a given
increase in temperature? Say if you have a small cylinder, and increase the temperature by X degrees, how much would the air inside the cylinder
increase in volume? That way, I could figure out whether it would be able to move a piston of a certain size. I'm hoping to make a Stirling engine
that doesn't take a ridiculous amount of heat to work...something that could work with just a candle or a lantern or something. But I also want to
build something a little better than all the soda can Stirling engines on YouTube!
If anyone can help, that'd be great!
- Chris
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smaerd
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Of course gatewaycity
The simplest model of gasses is the "Ideal Gas" model. Which is a decent approximation for many thing. Once you understand that model you can move
forward to other ones. The other ones are more accurate because they account for the intermolecular forces of the gas (in this case water vapor).
Here's a good intro video about it(5 min) - https://www.youtube.com/watch?v=88uEsGgXJrU
When it comes to moving the piston that is related to the pressure. Pressure is force per unit area. In the case of a piston the area is the circular
section of the moving component. Here's a short link explaining that - http://www.dummies.com/how-to/content/how-to-calculate-force...
Hope this helps get you started. One you've read through that stuff, I'd be willing to help get you off the ground with the ideal gas law and derive
some basic equations if you need help. There are some thermodynamics wizards on this forum as well.
[Edited on 5-7-2015 by smaerd]
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gatewaycityca
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Thank you! This is a lot to digest! But I think I'm starting to understand. The video in the first link was very helpful. I haven't checked out
the 2nd link yet. I was reading a lot just now too. Let me see if I understand things right, and please correct me if I have anything wrong.
Okay, so the equation for the Ideal Gas Law is pV = NRT. Where "P" is the pressure in atmospheres, "V" is the volume in liters, "N" is the number of
Moles, "R" is a constant of .0821, and "T" is the temperature in Kelvins.
So let's say I have a small closed cylinder of air, I would first measure it and figure out its volume in cm3, then convert that to liters. That
tells me the "V" part of the equation. I read that 1 Mole is equal to 22.4 liters. So if I know the volume of air in the cylinder, I can also find
the number of Moles of air. That tells me the "N" part. And I would convert the temperature from Celsius to Kelvin to get the "T" part of the
equation.
Do I have that right so far?
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Fulmen
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https://en.wikipedia.org/wiki/Combined_gas_law
We're not banging rocks together here. We know how to put a man back together.
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smaerd
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Sounds good to me so far 
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