How does buoyancy work?

I know this isn't the right group for this question, but I want an answer an electrical engineer can understand.

I understand the simple model of buoyancy. If something weighs less than the medium it displaces, it floats.

The question is, "why?" If you stick a ping pong ball under water and let go, it floats. What's the mechanism? The water has no idea what's inside the ball. We have water molecules banging into the ball on all sides.

Is there a simple explanation, in layman's terms, of what's actually going on to force the ball up?

I really want to understand how convection works in a chimney, but water should be easier to grasp.

The simple answer is that the system is rearranging itself to obtain the lowest energy configuration possible. If the ball is too heavy this is with the ball at the bottom of the tank. Otherwise the ball will displace water and raise the water level in the tank against gravity until the net force it experiences is zero. That is once it has displaced its own weight of water it floats. Pushing it down or raising it from this equilibrium position increases its potential energy.

The ping pong ball is accelerated by the vertical pressure difference across its diameter so if it is lighter than the water it displaces then upwards it goes. Various modern executive bubble toys exploit the behaviour of viscous oils and air bubbles a revamp of the lava lamps popular in the 1960's.

Warm air rises because it is less dense and the heavier cold air falling can thereby lose some potential energy.

As you say, there are water molecules banging into the ball on all sides. But consider a horizontal slice of the water, one centimetre thick, at some arbitrary depth. It is being banged into by water molecules from above and from below, but it doesn't go anywhere, despite its weight, so its weight has to be balanced by a difference between the net forces generated by the molecules above and below it. The implication is that the rate at which molecules hit from below is greater than the rate at which they hit from above by an amount sufficient to create the required net force.

This is just another way of saying the pressure rises as you go deeper into the water.

So now if we go back to your immersed ping pong ball, it is apparent that the forces generated by the molecules banging on the ball are not balanced, because the forces on the lowerhalf of the ball are greater than the forces on the upper half. Thus there is a net upwards force on the ball.

So how much net force is there? Well, if the pingpong ball were not there, then in its place would be a sphere of water. Since the water wouldn't go anywhere, the net force on that sphere must be equal to its weight. Thus the force on the pingpong ball is equal to the weight of the water that would have been there had the ping pong ball not been.

Sylvia.

The reason why is gravity tugs one way ALL THE TIME. The molecules of one gaseous or liquid medium push against each other with a force based on their masses.

So liquids or gasses introduced into this would rise or fall to whichever place their masses would put them with respect to the other members of the matrix.

SOLID items, or solid items with a hollow inside and a gas of a particular pressure level filling that, will rise or fall to a particular place in the matrix based on what the mass of that exact same volume it displaces of the material it happens to be in at the time.

So a one cubic foot displacing solid element in our sea level air has a zero point if it weighs the exact amount one cubic foot of sea level air weighs. If it weighs less, it will get pushed upward by the air around it (a balloon with Helium in it), until it reaches an equilibrium point that matches the weight it has against the matrix it is in (pretty high up).

If it is in water, same rule applies. If that one cubic foot item weighs less than one cubic foot of sea surface, sea level water, it gets pushed UPWARD, and rides on said surface (in this case). as we cross into the realm where the one cubic foot device weighs more than the water it is in, it sinks (think submarine) toward the center of the earth, by gravity, and the fact that the molecules of water around it are pushing up less than said gravitational pull is pulling down.

In free space, it merely changes the way said one foot cube would 'slosh" through a given medium, based on relative velocities. as there would be no gravitational attractor in space. Down here, we merely get to examine how things move through slurries, and from that we can understand gravity. They found the Higgs Boson particle... maybe they'll see the gravity particle next.

Ever see that surfer dude's multi-dimensional layout of the sub-atomic particles?

Anyway... gravity is the answer. and molecular pressure.

In other words, it only sinks a small bit because that small bit of volume, in the form of water, would weigh the same as the ping pong ball.

So, if it displaces one teaspoon (for the sake of the analogy), then the weight of one teaspoon of water and one ping pong ball would balance on a scale (be the same).

On the Moon, it would sink far less. And the water wouldn't last long either! (unless you were aboard The Eagle).

You may want to think about that last statement a little longer....

Jeroen Belleman

About the sinking, the water, or the Eagle?

The sinking, specifically.

Jeroen

Thanks, guys, for the inputs. Those are the classical arguments. I accept them as true and useful models for everyday use.

But, I'm still not satisfied.

Throw a sheet of thin plywood into the swimming pool. If you put sufficient force on it, it sinks. Now turn it vertical and push it down. The force to submerge it should not change. But the direction of force applied by all the molecules banging on the side do not obviously have a net upward force.

The thing I'm trying to get my brain around is convection. If I have a 20 foot chimney and build a fire in the firebox, air gets sucked in the box and goes up the chimney. If I cut a hole in the bottom and stack it on top of a similar chimney (close up the front), does the same amount of air get sucked into the bottom (cold) fireplace? My intuition says it does. And that means that if you're interested in ventilation, it doesn't matter where the heat gets into the pipe. Solar collector should work for that. And the distance to the space to be ventilated should be irrelevant???

OK, so now, put an S-curve in the pipe, kinda like a P-trap. Can you make a passive thermostat out of it? Suck air only when the sun shines?

Quit being so lazy--draw a picture and do the arithmetic.

Edgeways, the bottom edge of the plywood is now 8 feet below the top edge, instead of 1/4 inch, so the pressure differential is 32 times larger. That acts over an area of 4 feet x 1/4 inch, i.e. 32 times less than the area of the flat side. Net force is unchanged.

Cheers

Phil Hobbs

...snip...

Uh, what? Well, no the vector of the force on the long sides now are opposing and so cancel out. But the unit area force on the bottom, small edge is much greater because it is at a greater depth. So the total force is the same. In the general case you have to integrate the forces over the entire surface. The rectangle of the plywood makes that easy. Do you know how to express this as an equation? It will be area (the bottom edge of your rectangle) times the unit area force (proportional to the depth).

In the chimney case, as long as the temperature of the hot air being sucked in does not change a taller chimney will create a greater force because of the higher difference in air pressure from the top to the bottom just like in the water.

I like Sylvia's way of looking at it. How much does the air in the chimney weigh vs. the air that would otherwise be in the chimney weigh. That is the total force pushing the air up the chimney.

How fast it moves is another matter. A longer chimney will have more resistance but that may or may not be directly proportional to the length of the chimney. Air resistance is a complex subject.

I'm not sure what you are thinking here. What would the S-curve in the pipe do? Are you putting water in it, or cold air, or what? Any sort of gas would just diffuse away I expect. Even a little water would create a huge barrier to air movement. That's how a water trap works on your sink. A water trap could work if it were shallow enough. But it wouldn't really act as a thermostat to maintain temperature in the room. It would just allow differences in pressure to be equalized with a minimum of air movement.

I should take my own advice! 96/0.25 is of course 384 times. Point remains.

Cheers

Phil Hobbs

(snip to the real issue)

OK, now think about why the air does that. Hint; buoyancy and convection are about *density*, not *mass*.

Also, crack a window somewhere in the house or the chimney won't draw. Remember, this is a circuit and the fire is your potential source. ;>)

IOW the chimney is now 40' tall. Fine.

Intuition will usually get you into trouble in technical topics. What does your intellect say will be different with a longer chimney? Think resistors.

Now you've built a fire on top of the chimney. My intuition mentions carts and horses.

Wait, we've moved from heating (what fireplaces are usually designed for) to just moving air? How do you plan on coupling the heat from the solar collector to the chimney?

An air version of a lambda diode? Interesting.

Mark L. Fergerson

In the case of your chimney, when the smoke inside the chimney is warm enough, then the pressure differential from top to bottom will be less than the pressure differential of the surrounding atmosphere, and smoke will be pushed up the chimney from below (i.e., the chimney will 'draw').

This is why you want to start a small fire at first: until you get that warm column of smoke through the whole chimney, the pressure differential won't be working for you, and the thing won't draw.

I used to have a friend who lived in a cabin in the woods, and depended on his stove for heat. The thing was built so that the chimney came straight out of the back of the stove for about four feet, went through the exterior wall, then went up. If he just built a fire in the stove it would never draw. If he stuffed a small wad of newspaper back in the elbow of the chimney and lit it, then the chimney would start drawing, he could light his fire, and the thing would work fine until everything cooled down again.

Mikey, You must be a leftist... science be damned, your answer _must_ be correct >:-} ...Jim Thompson

Convection works because, other things being equal, warm air is less dense than cold air. Since it's less dense, the pressure drop with height is less than the pressure drop for the same height in cold air.

With a chimney, you then have the situation where the pressure at the bottom has to equal the pressure of the outside air at the bottom, and the pressure at the top has to equal the pressure of the outside air at the top, but the pressure change from the bottom to the top is different. The apparent impossibility is resolved by accelerating the air.

The effect is thus dependent on the height of the column of warm air, and is consequently very dependent on where the heat is introduced.

Well, if the convection is solar powered, then it will only occur when the sun is shining, regardless of the shape of the pipe. When it exists, the effect depends on the difference in altitude of the inlet and outlet. The only effect of an S-curve is to lengthen the pipe, and thus increase the frictional forces that slow the air down.

Sylvia.

For a "layman engineer," generic calculations involving integrating the hydrostatic pressure outside the submerged body should be easy to grasp. Analyzing simple shapes (cylinders, cubes etc.) can make it trivial.

See:

Laws of physics..... Leftists like to change Laws.

Cheers

No, water will trickle out of the hand..