If I had a second story and a balloon I’d spare you guys this. But I just want to make sure.
Suppose I am downstairs, and I have a helium balloon which floats, not all the way up to the ceiling, but only four feet above the ground.
Now suppose I take it upstairs.
Does it now fall to the floor instead of floating?
Or does the floor at the second level block falling air in such a way as to create high pressure, pushing the balloon up off the ground just as it did on the bottom floor?
I think it would still float 4 feet off the ground. I doubt the pressure difference is enough to be a factor
Do you mean there is no relevantly substantial change in pressure from the 4ft point through to the 4ft+1story point?
If the pressure is basically the same at 4 ft as it as at 4ft + 1 story as well as at every point between, then there’s no reason for the balloon to float at four feet when it’s on the ground floor.
That’s what makes this interesting. If the pressure is the same on the first floor and the second floor then when the balloon finds equilibrium 4 feet off the first floor of the house, why would it find equilibrium also at 14 feet off the first floor. If the pressure is the same I’d think on the second floor it would be on the floor and in the basement it would be on the ceiling.
The reason a helium filled balloon floats upward in the first place is because of the pressure difference of between the air at the top of the balloon and that at the bottom.
But is there enough of a pressure difference between ‘ground level’ and ten feet higher then ground level for a 3 day old helium balloon to notice and react to?
Yes. Well, no. Okay, there is a pressure difference, but the thing about gas-filled balloons is that as the outside pressure decreases, the balloon expands. All things being equal, this results in the balloon having more-or-less the same buoyancy regardless of the altitude. This applies as well to a fresh helium balloon as an old, neutrally-buoyant one.
The question isn’t about pressure, guys. The balloon floats four feet above the ground because of the string.
If it were on the ceiling, the balloon would have to support the full weight of the string. On the floor, it doesn’t have to support any weight of the string. If it’s only able to support 4’ of string, that’s how high it floats, using the floor to support the rest of the string. A balloon without a string will always be on either the floor or the ceiling.
Why is it four feet off of the floor? Most of the time when I have noticed this, the ballon had fallen until some of the cord was on the ground. The weight of the balloon, gas and the part of the cord still in the air matched the weight of the same amount of air by volume.
On edit: um, what he said…
Pv = nRT, and the temperature in a room is usually stratified.
http://www.builditsolar.com/References/Measurements/strattest.htm
And there we have the correct answer.
Without the string it is impossible to make a balloon float at any constant height. They can’t control this to within hundreds of feet with hot air balloons and zeppillins, you certainly can’t control it to within inches using a 10 inch diameter balloon.
As has been noted, balloons move upwards because of the immeasurable pressure difference between the top and bottom of the balloon. You simply can’t control bouyancy to the degree that it will equalise with pressure at any specific height. Someone breathing in the same room will cause greater pressure changes than those which cause bouyancy. Far, far greater changes are caused by changes in temperature or air currents caused by people walking around. As a result you balloon will either rise or fall just because the room isn’t perfectly stable.
Now maybe you could build a perfectly sealed room with a temperature constant to less than 1/100th of a degree and set up a balloon that would hover at a constant height. But nobody has ever done so as far as I know. But if that’s what you are talking about then you can’t go into the room to take it upstairs without causing change sin air pressure that will cause the balloon to either take off or plummet. And if you could the balloon would simply crash to the ground because it is equilibrated to air pressure at ground level.
In the real world only the string allows a balloon to hover at a specified height, and that doesn’t change no matter what altitude you are at.
Sort of, but that is misleading. A helium balloon floats because the density of the balloon and gas is less than the density of the air. It will float upwards until the density of the air outside the balloon is equal to the density of the helium-balloon system, or to put it another way, until the volume of air the balloon system displaces is equal in weight to the balloon system.
So any helium balloon will ascend to the height where the balloon system weighs the same as the displaced air. But as outside pressure decreases, the balloon will expand. It is possible that the balloon will pop before it reaches its max height (due to the pressure of the gas inside).
Either way, if the balloon is at equilibrium at 4 feet above sea level, and then you go upstairs (10 feet), the balloon will drop to the floor absent any air currents, etc. The air at 14 feet is less dense than the air at 4 feet, and even though the difference is small, it is enough. (Obviously, the air at 4’5" or so is less dense than the air at 4’, which is why the balloon hovers at 4 feet.)
The OP is correct. The balloon will fall to the floor, absent any other factors.
Of course, this also depends on the temperature of the air in each room, and yes, the string does add weight to the balloon system that might be partially taken up by the floor, and there might be thermals off the carpet, etc etc. But a balloon that was neutrally buoyant at 4’ would fall it 14’.
ETA: Blake, you are incorrect. Airships and hot air balloons can control their altitude by changing their density. That was how it was done for most designs. Modern airships generally fly heavier than air. The Goodyear blimp doesn’t have a string. You are correct about the difficulty of experimentally showing this in such a small scale test, but the principles are clear.
If you tie a piece of cardboard to the string, you can snip off bits of cardboard to get it neutrally buoyant.
To the OP: The answer to your question depends on the design of the balloon envelope. A latex toy balloon is stable, due to the elastic envelope. When the external pressure decreases, the internal gas can’t expand to fully compensate because the envelope must stretch more, which creates more pressure difference between the inner and outer sides.
If it were a mylar balloon, and not full, then the internal pressure is at equilibrium with the external air. In this case the helium will expand to maintain the same buoyancy at any level. This why high altitude research balloons are launched with only a small bubble of helium in the top of the envelope.
The third case is a non-elastic (say mylar) balloon that is full at the lower level. In this case, the internal gas can’t expand. The pressure difference will increase even more than in the first (elastic envelope ) case, and the balloon will try to fall to the 4’ above the ground floor level.
In real balloons, the envelope won’t stand hardly any pressure, so helium must be vented to prevent bursting.
Wait, is this true?!
I always assumed the balloon’s height was determined by helium volume and pressure.
In fact, I’m pretty sure I’ve seen partially deflated balloons floating below the ceiling but without the string touching the floor. But maybe I’m confabulating.
If what you said is true, then I retract the question.
That’s not inconceivable, but it would be caused because of electrostatic interaction with the ceiling, not because of the pressure of the balloon.
It’s effectively impossible to get a balloon to hover by setting a constant pressure. Ambient air pressure is just too variable compared to the tiny pressure differences that cause the balloon to be bouyant in the first place.
It’s like expecting a car to drive straight over a rough road by welding the steering column in place. It can’t work because the variation in the environment will jolt it off course and you need the steering to bring it back on course. Same with balloons. You need to be able to adjust pressure to compensate for turbulence if you want to maintain height.
This is simply not true. A properly weighted mylar miniblimp, example is at darn near neutral bouyancy, and will float near the ceiling or the floor of a room. Over the course of a day, it will spend considerable time between the floor and the ceiling, possibly floating free, or else jammed up against a wall or a piece of furniture.
There’s no need for a string laying on the floor to get the weight just right.
Temperature gradients, and hence pressure gradients are the culprit here.
That doesn’t make sense, what your saying (I think) is that the balloon will float four feet high on the ground floor AND four feet high on the second floor, right…
So using that logic, if you took the balloon into a house that was two stories high, but only had one floor (so no upstairs, just a really tall room) it would float at both 4 feet AND 14 feet?
Besides, I think the String Theory plays in to this.
Anyone care to gather some empirical data?
I’ll agree that temperature gradients are one factor in the equation.
That said, I’ve never seen a balloon hover except when there’s a string involved. The string is a pretty significant “brute force” method, whereas temperature gradients would require a lot of delicate balancing.
Assuming you’re right about this type of balloon, what’s it do in the situation I described in my OP?
Squink, you have the wrong link for blimps. Here’s the right one.
Quoth Joey P:
You don’t know how much it tickles me to see someone referring to String Theory in a context where that’s actually the correct terminology.