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In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

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Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#131

Post by MereInterest »

lIl-IIIl wrote: post #109
nielsbot wrote: post #99

Earlier quoted context omitted.

My question exactly.. I hope someone can chime in :) EDIT: Thinking a bit more... I suppose it's a reasonable assumption that the molecules (mostly) wouldn't stack on top of each other. They all want to get lower and perhaps the resistance to the oil spreading out is much lower proportionately that the gravitational force encouraging the oil to flatten

But if I spill some oil on my plate it doesn't look a molecule thin to me. Why is it different with water?

There’s a couple of possible reasons. First, you probably spilled more oil onto the plate. In the experiment, 0.81 milliliters of oil spread out until it covered a circle with diameter 84 cm. Most spills would be more than a mL of oil, and most plates are much smaller.

Second, most plates aren’t flat. If you have an area of the plate that is at a lower elevation than the rest, the oil would pool up in that area.

Third, even if you fill the plate with water, you could have elevation changes due to surface tension of the water. If the water is concave up, the oil would float upward and form a ring around the edge. If the water is above the surface of the plate, held in just by surface tension, the oil would float upwards and form a bubble at the center of the plate.

Fourth, you could have something else on the plate that acts as an emulsifier. Whether a bit of egg, some pasta water, leftover detergent, these would break up the oil and prevent a film from forming.

The easiest way to have a flat surface is to do the experiment in the center of a much larger body of water, since any effects from the surface tension would be at the edge.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#132

Post by wholinator2 »

db48x wrote: post #110
SyzygyRhythm wrote: post #39

Earlier quoted context omitted.

For that to happen, you would have to be very unlucky: all of your measurements would have to be 2e, 4e, 6e, etc. If a 3e or 5e sneaked in there, you'd realize that the charge was e, not 2e. With enough measurements, you can be confident that you've hit all the expected multiples of the quantum.

Not quite so. They did end up measuring a multiple of the fundamental electric charge. The experiment really measured 3e, 6e, 9e, etc. It turns out that the electron and proton have an electric charge 3× bigger than that of a quark. Since the experiment didn’t generate any free quarks, nobody noticed for years. Even today the mistake persists and school children are taught, unironically, that down quarks have ±⅓ of t…

No need to nitpick, the original comment states perfectly accurately that he was measuring the fundamental charge _of the electron_, from which the constant e is derived. We've been using e to mean _electron_ charge for a very long time. Where do you get the idea that constant has anything to do with quarks?

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#133

Post by MereInterest »

komali2 wrote: post #81
happytoexplain wrote: post #13

Earlier quoted context omitted.

It feels intuitive that a thin fluid on a low-friction surface (like water) would spread out "as much as possible" given enough time. There certainly may be confounding factors, but it seems like a reasonable thing to pin as an "assumption" in a hypothesis. I.e. he didn't have to "know" - assumptions are OK, and I don't feel like this one is huge .

I'm confused, the blog wrote "known amount of water," so was it a closed little area like a bathtub? If you added a ton of oil wouldn't it spread out as much as possible aka 600 molecules thick or whatever? Or did he pour it into a huge lake or something?

My understanding is that the actual body of water was larger, but that the oil would only spread out to one molecule of thickness. So you start with a larger area of water, and measure the diameter of the resulting oil slick.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#134

Post by MereInterest »

jjk166 wrote: post #35
bqmjjx0kac wrote: post #4

> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecule. How did he know that the film of oil was one molecule thick? It feels like a huge assumption to me, but maybe this blog post left something out.

If there were multiple layers of molecules then the film would spread out over a wider area. With repeated experiments it would be clear that films are always an integer multiple of this thickness and never thinner.

Except that you could have part of the surface covered in 1 molecule, and another portion covered in 2 molecules. Since you never directly measure the thickness, this would produce the same apparent thickness as a uniform 1.5 molecule thick layer.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#135

Post by db48x »

wholinator2 wrote: post #132
db48x wrote: post #110

Earlier quoted context omitted.

Not quite so. They did end up measuring a multiple of the fundamental electric charge. The experiment really measured 3e, 6e, 9e, etc. It turns out that the electron and proton have an electric charge 3× bigger than that of a quark. Since the experiment didn’t generate any free quarks, nobody noticed for years. Even today the mistake persists and school children are taught, unironically, that down quarks have ±⅓ of t…

No need to nitpick, the original comment states perfectly accurately that he was measuring the fundamental charge _of the electron_, from which the constant e is derived. We've been using e to mean _electron_ charge for a very long time. Where do you get the idea that constant has anything to do with quarks?

https://i.kym-cdn.com/photos/images/original/000/992/402/c35...

But seriously, it really is time to fix all the chemistry textbooks so that the charge of the electron is +3 instead of −1.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#136

Post by nobrains »

danielam wrote: post #128
hn_throwaway_99 wrote: post #2

I love articles like this. I feel like too often in science education (at least my science education) that laws and theories are presented as just something that you need to memorize, when in my opinion the stories of how things were originally discovered and figured out is eminently more fascinating and inspiring. Like I remember having to learn all of these biochemical pathways, but I left school with nary a clue a…

Yes, often what is taught is taught in a manner that seems mysterious in origin, as if it were revealed, certain and final, and developing a sensibility like that concerning scientific matters is not good. You could argue that the viability of science as such rests on certain articles of faith, but the particular findings of science themselves are a matter of demonstration, interpretation of demonstration and argumen…

Pragmatic or luck? Hear me out...

Why assume "that the oil formed a single layer of molecules — a monolayer" ?

That is a very fundamental assumption, and could have been wrong as well (we know it is right, because the values match with more accurate recordings, but still...)

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#137

Post by wheatgreaser »

hn_throwaway_99 wrote: post #2

I love articles like this. I feel like too often in science education (at least my science education) that laws and theories are presented as just something that you need to memorize, when in my opinion the stories of how things were originally discovered and figured out is eminently more fascinating and inspiring. Like I remember having to learn all of these biochemical pathways, but I left school with nary a clue a…

but chronological order of scientific discoveries does not imply conceptual linearity, so i kinda get why colleges and schools do not go for that kind of approach

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#138

Post by dotancohen »

lIl-IIIl wrote: post #106
owyn wrote: post #84

Earlier quoted context omitted.

I was looking at my parent's bookshelf and saw a book on Shakespeare and I recognized the author's name: Asimov! https://en.wikipedia.org/wiki/Asimov's_Guide_to_Shakespeare It's like 800 pages, I haven't read it but I think I'll keep that one. Seems like it might be hard to find another physical copy. He was definitely prolific on a number of topics.

Not surprising! "Asimov was so prolific and diverse in his writing that his books span all major categories of the Dewey Decimal Classification except for category 100, philosophy and psychology" - from his Wikipedia page.

He was also incredibly talented in phrasing ideas so that they stick in the reader's mind. I am right now sitting next to a dog Asimov named after him.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#139

Post by shthed »

wwarner wrote: post #50

In 1676 Roemer estimated the speed of light by timing the orbit of Jupiter’s moon Io, noting that as the Earth approached Jupiter, Io emerged from behind Jupiter a little earlier every day, and as the Earth traveled away from Jupiter it appeared a little later every day, with the time of day varying by 22 minutes over a year. Knowing the difference between the two distances, he reckoned that light travels that distan…

https://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_o...

https://en.wikipedia.org/wiki/Speed_of_light#First_measureme...

Another interesting thing about using the timing of moon eclipses:

> Galileo proposed a method of establishing the time of day, and thus longitude, based on the times of the eclipses of the moons of Jupiter, in essence using the Jovian system as a cosmic clock. The times of the eclipses of the moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude.

https://en.wikipedia.org/wiki/Galilean_moons#Determination_o...

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