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Dr. Bob is Robert Hsiung, MD,
bob@dr-bob.orgShown: posts 1 to 13 of 13. This is the beginning of the thread.
Posted by alexandra_k on September 2, 2014, at 20:39:43
so...
if you have a tube shaped like a U with a selectively permeable membrane (allowing water but not solutes) along the line of symmetry..
then if you stick some solutes into one side... the water level will rise on that side.
because the water will pass through the membrane but the solutes can't... and the water will reach equilibrium (where the movement through the membrane in both directions occurs at the same rate) when there is the same amount of water on each side of the membrane. so one side of the membrane will have y amount of water + solutes (and a higher water level) while the other side of the membrane will have that same amount of water with no solutes (and have a lower water level).
and osmotic pressure is the amount of pressure you need to apply to the side with the higher water level to squash it down / force the water back through the membrane until the water levels on both sides are even.
ok.
so my question / problem...
if you take the same tube with the same selectively permeable membrane... and instead of sticking solutes into it like sugar or salt or whatever... you stick a great big rock... then... won't the rock displace the water and the water level be the same on each side of the tube? the... atmospheric pressure... will result in the water level being the same on each side... so... the side with the rock it it would have less water in it.
is this right?
so, what's up with that, then? is it something to do with atmospheric pressure being greater than osmotic pressure? why for a great rock but not for itty bittier solutes??
?
Posted by alexandra_k on September 2, 2014, at 23:04:17
In reply to osmosis question, posted by alexandra_k on September 2, 2014, at 20:39:43
or maybe it won't do that. maybe the water level won't be the same in both compartments. i just... that seems very surprising to me. i mean... lakes aren't like that... with nets half way to keep the fishies on one side... i can't see the water level changing in only half if all the swimmers must stay over one side...
that just seems bizarre...
Posted by SLS on September 3, 2014, at 10:54:03
In reply to Re: osmosis question, posted by alexandra_k on September 2, 2014, at 23:04:17
> or maybe it won't do that. maybe the water level won't be the same in both compartments. i just... that seems very surprising to me. i mean... lakes aren't like that... with nets half way to keep the fishies on one side... i can't see the water level changing in only half if all the swimmers must stay over one side...
>
> that just seems bizarre...Wouldn't the pressure applied across the membrane depend upon the surface areas of the openings in the tube and the weight of the water rather than the height of the displaced water? The height of the water column containing the rock should be higher.
That's the best that I can come up with. Too much for me.
- Scott
Posted by alexandra_k on September 3, 2014, at 17:04:18
In reply to Re: osmosis question » alexandra_k, posted by SLS on September 3, 2014, at 10:54:03
> > or maybe it won't do that. maybe the water level won't be the same in both compartments. i just... that seems very surprising to me. i mean... lakes aren't like that... with nets half way to keep the fishies on one side... i can't see the water level changing in only half if all the swimmers must stay over one side...
> >
> > that just seems bizarre...
>
> Wouldn't the pressure applied across the membrane depend upon the surface areas of the openings in the tube and the weight of the water rather than the height of the displaced water? The height of the water column containing the rock should be higher.
>
> That's the best that I can come up with. Too much for me.
>
>
> - Scottpossibly. i didn't think of that.
i said about dividing the U shaped tube along it's line of symmetry... but of course it's got 2 lines. one of them through the middle so you end up with two J sort of shapes... the other one... if you partition it through the middle from left to right and end up with two U.
perhaps that makes a difference... perhaps the water level will be different in the first way of dividing but not the second.
i wish i had a U shaped tube... dammit...
i suspect you may well be right that the shape matters. to minimise the surface area or something... otherwise... why not use any old container?
Posted by alexandra_k on September 3, 2014, at 18:14:09
In reply to Re: osmosis question, posted by alexandra_k on September 3, 2014, at 17:04:18
i think height is important...
something something i'm supposed to know (but i don't feel like i understand it properly) about measuring atmospheric pressure with reference to the height a column of mercury will reach.
so you have this dish of mercury open to the air... and if you stick a small tube with a closed end (e.g., a narrow test tube) into the dish then the mercury will start to climb up the tube... the idea being that the atmospheric pressure of the air molecules bouncing on the mercury in the open dish... is greater than the gravity pressure down on the mercury in the tube... and the mercury climbs up the tube...
to a height of 270 millimeters. at sea level. so 270mm (millimeters) Hg (mercury) is atmospheric pressure at sea level.
i has been learning :)
but that should surely depend on the width of the column of mercury... i would have thought... like how capillary action for water depends on the width of the capillaries. i think. and perhaps also on the surface area of the mercury that is being bombarded by air molecules...
anyway... height is important because of potential energy... you can put a tap in the higher bit and water will flow out the tap to the lower bit. you could use the flow of water to power a motor or something...
but i guess energy went into putting the rock in there in the first place...
i really... shouldn't think about such stuff. it feels to be... nonsense. on some level. i don't know what to say.
my textbooks is quite clear that a bunch of metal (hollow or solid, of any shape) will form a farraday cage *unless a current is running through it*. dammit. i missed that last bit. got myself all into a state about how current must be conducted along the surface of a wire and not through the middle of it :(
Posted by baseball55 on September 3, 2014, at 18:53:42
In reply to Re: osmosis question, posted by alexandra_k on September 3, 2014, at 18:14:09
I'm lost.
Posted by SLS on September 3, 2014, at 21:06:57
In reply to Re: osmosis question » alexandra_k, posted by baseball55 on September 3, 2014, at 18:53:42
> I'm lost.
Me, too.
I thought a Faraday cage was something used by Houdini.
- Scott
Posted by alexandra_k on September 4, 2014, at 5:15:34
In reply to Re: osmosis question, posted by SLS on September 3, 2014, at 21:06:57
i'm lost as well.
:-/
Posted by Beckett on September 4, 2014, at 9:05:42
In reply to Re: osmosis question, posted by alexandra_k on September 4, 2014, at 5:15:34
I like this.
Of course, I'm lost, too.
There is the size of the molecule interacting with gravity. And it's different energy (not right word) reactive qualities.
Since I am truly lost, the above might be implied?
Posted by baseball55 on September 4, 2014, at 19:29:40
In reply to Re: osmosis question, posted by Beckett on September 4, 2014, at 9:05:42
I remember taking biology and somewhat understanding osmosis. But I never felt a really good grasp of the issue. Hope you can figure it out. I give you lots of credit for trying.
Posted by ClearSkies on September 4, 2014, at 20:05:15
In reply to Re: osmosis question, posted by alexandra_k on September 3, 2014, at 18:14:09
I thought a Farraday cage had to do with electrical current and being protected from it. I've seen demonstrations of them at science centres. (They can be spectacular on a large scale.)
Posted by alexandra_k on September 4, 2014, at 20:35:49
In reply to Re: osmosis question, posted by ClearSkies on September 4, 2014, at 20:05:15
the farraday cage thing was a lateral leap.
it was related only because it was something else i was having trouble with. for one of the assignments we had to say whether there was a current inside the wire and what direction the current was travelling in.
and i thought the wire would make a farraday cage since we learned that farraday cages can be solid or hollow. round or... any shape. that the electrons spread out across the surface to minimise repulsions in such a way that the net charge inside a farraday cage is zero...
so i figured that the current must travel along the surface of the wire and not inside it.
which made some kinda sense because i faintly remembered something about neurons carrying charge along their surface.. (anyway... i think they turn out to be really rather different so i best just forget about that for now)... and... anyway... it turns out that current is conducted along the inside of a wire. and apparently current isn't really about the flow of electrons, anyway, since electrons don't move very fast really and they are moving in the opposite direction anyway...
and it is all very confusing to me.
osmosis...
i only need to remember about cells and interstital fluids... about how water gets shunted about as a consequence of solutes being shunted about. about how membranes are selectively permeable to solutes... about how it takes energy for cells to maintain concentrations of solutes that are different from the external environment. how water will follow the highest concentrations of solutes... how lots of solutes in cells results in lots of water in cells results in bloating of cells results in osmotic pressure accumulating inside cells results in them bursting. that's all...
i can't tell how much i need to learn better focus... or how much it is useful to think, sometimes. probably... focus for now. i don't know.
i don't suppose i'm doing very badly, actually. it is just that i'm used to doing really very well. and i'm certainly not doing that. grump.
with the farraday cage thing... i did start to wonder... whether we were supposed to say something about vectors... that the wire was a farraday cage in the width dimension but that the current travelled lenthways between the voltage differences ? then i started to draw that... and then i realised that probably that just didn't make any sense.
Posted by alexandra_k on September 4, 2014, at 21:48:21
In reply to Re: osmosis question, posted by alexandra_k on September 4, 2014, at 20:35:49
and in neurons the current travels along the neuron even thought the voltage difference is between the inside and outside of the neuron not between one and the other end of it (i don't think) and so ak. so confused. time to go to the gym...
This is the end of the thread.
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