How a bicycle tyre is like an airconditioner

How a bicycle tyre is like an airconditioner

Hot work pumping tyres. Even the pump’s getting hot See what’s happening is I’m compressing the gas it’s going 70 psi so that’s 5 atmospheres above the atmosphere so it’s going up to 6 atmospheres, a factor of 6 I’m increasing the pressure so the temperature’s going up as well You could make a heater out of it not that I need a heater right now. Wait, you could make an air conditioner out of it if you reverse the process so you pump it get it hot and then let it cool down see it’s still bit hot.. I’ll get back to you… ok it’s cooled down now when I let it go it springs back to its original volume you’d think it would go to the original temperature but it lost some energy when it cooled down to ambient so it’s now – as it’s gone to a bigger volume – temperature’s gone down below what it was before it’s now a cooler gas when I blow that air on me it’s… so refreshing… That’s basically how your fridge works there’s a bunch of guys & girls in the back of the fridge pumping with bike pumps, letting it sit for a while til the energy comes out that’s why the back of your fridge is hot and then they let it go the air’s cooler, now they pump it back into your fridge. What? Run along then! This wheel’s a bit like an conditioner too because we pumped it up, it got hot the high pressure air went into here & then cooled down you know it’s cooled down to basically the ambient temperature 25 degrees and now if we let this gas out then it will be a lot cooler because of the energy that’s
been lost it’d be nice to use the ideal gas equation PV=nRT but we can’t because we don’t know what the volume’s changing to we know what the pressure’s changing to we don’t know what the temperature’s
changing to – there’s too many variables there’s an approximation we can do to help us out if we imagine it happens very quickly it doesn’t have any time to lose heat to the nozzle or out into the atmosphere then you can do the calculation and that says that if the pressure drops from 70 psi that’s six atmospheres, down to one atmosphere then the temperature will drop to -94 degrees that’s pretty amazing just from my pumping I’m gonna try it with a thermometer The thermometer says 25 degrees exactly, so , Orright it’s definitely cooling my finger & nozzle down Unfortunately air doesn’t hold great deal of energy It’s got a low heat capacity. Definitely getting cold though There we go we’re down to 21, so cooled it by 4 degrees that’s nice, cool the house down All I need probably is about a thousand tyres to fill the volume of the room maybe that’s why air conditioning is takes a lot of energy Ohhh, I’ve let my tyre down! F%^&!

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  1. This is a great explanation of refrigeration. I remember reading about how it works years ago and then never really being able to articulate it (the example I saw used an oven and a bowl of water or something … ) however one thing I can't understand here is why cooling the pressurised gas doesn't reduce it's propensity to spring back to its original volume. Like if I take nitrogen and compress it to a liquid form, it gets super cold doesn't it? The only thing that makes liquid nitrogen want to return to a gas is if you warm it up. If you were to keep the ambient temperature at absolute zero or something wouldn't liquid nitrogen remain a liquid?

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