The Question That Everyone Gets Wrong (Including Me)

this is a physics question that almost everyone gets wrong when asked including myself here's the problem there's a completely sealed airtight tank in this L shape here it's completely filled with water except for this little pocket of air right here the question is this when I tilt this shape and let the air pocket float to the top what happens to the overall pressure of the vessel let's say we're looking at the pressure at this point at the bottom of the tank would the pressure decrease since there's less water on top of it or will the pressure stay the same since the Tank's completely sealed and the pressure couldn't change or would the pressure actually increase for some reason so choose your answer now decrease stay the same or increase well now let's actually try it and measure the pressure all right so I have a long tube that's sealed on both ends with a pressure gauge right here and you can see at the bottom is my bubble right now we're at 0.89 bar so now let's see what happens to the pressure as we let this bubble rise to the top and I have another camera on it here and I'll put this up to the side so you can see the pressure continually as I film the bubble go up okay here we go 3 2 1 okay there goes the bubble let's going to the [Music] top okay it's fully at the top and we are now at 1.01 bar so the pressure increased the bubble is at the top there's now less water on top of this pressure gauge but there's more pressure we've been measuring the pressure at the bottom the whole time but the same should be true for the top as well let's see if the whole system increases in pressure by measuring the top pressure as well okay now the gauge is at the top we're at 77 bar there and our Bubbles at the bottom now let's make it go to the top there goes our bubble and look at that the pressure goes up again 77 to 85 bar so no matter where our pressure gauge is in the system the pressure always increases in the entire system so literally all I did was change the location of the bubble and made it float to the top and increased the pressure in the whole system why this blew my mind when I first tried it so I'll tell you that the answer I originally chose was that the pressure would stay the same I reason that it's a sealed system so the pressure can't suddenly change in a sealed system just depending on where an air bubble is in it even if it didn't increase my next answer would have been that maybe it would decrease since there's now less water on top of the pressure gauge and hydrostatic pressure increases with water height but I would have never thought that it should increase how's this happening how is a sealed system increasing in pressure when a bubble's at the bottom versus the top well since this goes against all intuition I'm going to try to compare it to a mechanical system but first life can get a lot harder than just getting phys questions wrong that's why I want to thank the sponsor for this video better help these last few years 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you more scheduling flexibility at a more affordable price so get 10% off your first month at betterhelp.com actionlab or you can click the link in the description and thanks to betterhelp for sponsoring this video now let's get back to our experiment now what I've made here is a mechanical analog of that system I have a spring and this is going to represent the air because the air is springy and then I have a weight because it's way more dense than the air and also it's not springy like the air it's completely solid and incompressible so when we don't have a defined volume our water can compress the air but now we're going to fix the volume so this is our fixed volume now so now my springs on the bottom and I put my water in like this which is my weight and it's going to push it down a little bit and that entire system fits exactly inside of this bottle it's barely touching the top there so we want to know what the difference in force is when it's upside down like this with the spring on the top and the weight on the bottom versus when it's like this with the spring on the bottom and the weight on the top so what I have here is a little Force gauge and if it gets pushed on you can see the weight change or the force on it okay look at this odd little Contraption this looks like a bomb or something so in our case it's going to be that more force is negative that's the way I've arranged the force sensor there and you can see that when I push on it it doesn't change anything because it's the actual force on the inside of the lid there so this is the force on both of the lids cuz it's pushing on the top and the bottom so it says zero right now but then we turn it over and it says4 kg so by flipping it over suddenly there's more force on both of the lids here there's more force on this side and this side so first when it's upside down like this with the bubble or the spring on the bottom we get around 350 G now we're going to turn it upside [Music] down so now you can see when I turned it upside down the volume wanted to increase but we can't let that happen because we said it's a defined volume it's a fixed volume so I have to push down on it now to make it the fixed volume so because we had a fixed volume the force on the lid increased when it's upside down when the spring is on bottom then the spring is the only thing pushing on the container but when we flip it over then the force on both of the lids increases by exactly the elastic force of the spring which is equal to the weight of the load which is masstimes gravity now let's look at our water system again when the bubble is at the bottom it's under high pressure since there's a bunch of water on top of it the volume is now fixed and water is incompressible so that means that the volume of the bubble has has to stay fixed as it rises we can assume this is an isothermal process and apply Bo's law which says that if the volume is fixed then the pressure is also fixed so the bubble carries the high pressure it had at the bottom of the container to the top of the container so the pressure at the top used to be zero gauge pressure but now this high pressure bubble is there pushing down on the water below it so the pressure of the whole tube increased by the pressure of that bubble which was density time gravity time the height of the wall water column even when we explain exactly how it works it still seems like it shouldn't work like this how does a sealed system increase in pressure when nothing is changing except where a bubble is where did that extra energy come from where was it stored well if we look at the potential energy of the water column notice that when the bubble is at the bottom the column of water is higher off the ground so it has more gravitational potential energy when the bubble is at the top the whole column of water shifts down a bit so it now has lower gravitational potential energy so that potential energy got turned into higher pressure so we're converting potential energy to pressure when we're changing where the bubble is this is the same case with the mass on a spring when the Mass is at the top it has higher potential energy than when it's at the bottom it's actually very important to know about this effect when you're dealing with long tubes that could have bubbles in them for example picture a long oil pipeline that has a bubble in it that suddenly FL floats to the Top This can cause very large pressure spikes in the whole system because of how tall the pipes can be when drilling for oil this effect can even cause larger pressure spikes than expected in things like volcanoes that have a large molten reservoir on top of a bubble of gas that suddenly moves to the top causing the pressure of the whole volcano to increase suddenly I love questions like this because they're so counterintuitive that they make you have to learn more about the fundamentals in order to understand it better also I'm one of the featured creators at open sace this year in San Francisco so if you're available you can come meet me there and you can also apply to set up your own booth where if you've made anything that's really cool that you want to show off you can do it for free there so come check out Open sauce in San Francisco it's June 14th to 16th and thanks for watching another episode of the action lab I hope you enjoyed it if you haven't subscribed to my channel yet I hope you consider subscribing so that you can see all my videos and not just the ones that go slight more viral and thanks again for watching and we'll see you next time [Music]

Channel: The Action Lab

Views: 763,958

Keywords: Physics question, the action lab, surprising physics

Id: 0syJ8L0fRp8

Length: 10min 20sec (620 seconds)

Published: Fri Apr 26 2024