Simple, everyday science

[El_Machinae]

Diffusion takes time (well, distance, actually). For practical purposes, angles of incidence are fine. However, in perfectly reflective aspect, it's something to consider.

If some diffusion didn't happen, then in space objects in a shadow would be perfectly black.

 

[Truronian]

Seems I had a different understanding of diffusion when it comes to light. I had thought it to be the property of light to eminate in all directions from a source; decreasing in intensity as it gets furthur away. I've never encountered the phenomenon your describing, is it a product of Quantum mechanics or is it a factor even in the Newtonian model of light?

 

[Bozo Erectus]

Bozo, could you please explain?

Like El said, that its a mystery in the same way gravity is. Funny that two such fundamental things as electricity and gravity still havent been nailed down.

 

[El_Machinae]

Seems I had a different understanding of diffusion when it comes to light. I had thought it to be the property of light to eminate in all directions from a source

The mirror becomes a source, once you hit it. There is a practical limitation to diffusion, so theoretically, a big enough room would eventually diffuse the light so much that a camera could be between photons. But that's BIG.

 

[Truronian]

The mirror becomes a source, once you hit it. There is a practical limitation to diffusion, so theoretically, a big enough room would eventually diffuse the light so much that a camera could be between photons. But that's BIG.

Even if the mirror is perfectly smooth? I'm not convinced.

 

[El_Machinae]

You're thinking of refraction (which is assumed away), yeah, a smooth mirror prevents refraction.

I can't explain it much better. I'm pretty sure that this is a Quantum Mechanics kinda thing. Of course, don't take my word for it. I think that the diagrams you've designed beats the poster's puzzle.

 

[Truronian]

You're thinking of refraction (which is assumed away), yeah, a smooth mirror prevents refraction.

I can't explain it much better. I'm pretty sure that this is a Quantum Mechanics kinda thing. Of course, don't take my word for it. I think that the diagrams you've designed beats the poster's puzzle.

Hmm. I'll ask my Physics teacher, he'll relish the distraction, and I was the poster

 

[El_Machinae]

Ask your teacher to think of it in QM terms, not basic terms. People can get stuck on forumulas, thinking they're the answer, when formulas are just rough approximations of more complicated (but more true, but not TRUE) formulas.

 

[Pontiuth Pilate]

Hey, back onto the straw, I think I may have figured out what you were talking about. Here's a question to show if I understand.

The 40' drinker could pull water up the straw, as long as there is still water in the cup, right? Because the air pressure at ground level would still be 'pushing' down on the water surface at ground level. It's only once all the water is in the straw, that it gets tough, and the last bit is impossible. Am I right?

Close but no

It is not the vacuum in your mouth that sucks up the water, but the pressure of the surrounding atmosphere that forces water up the straw. The atmospheric pressure actually means "how heavy the air is." At ground level the atmospheric pressure is 15-ish pounds per square inch which means every square inch of ground has above it a column of air, stretching to the edge of space, which weighs 15 pounds.

Basically, on one end of the straw you have a vacuum and on the other end you have a constant 15 psi (pounds per square inch) of pressure. Thus, water wants to go up the straw. When the straw has some water in it, there are two forces at work at the opening of the straw. The first is that constant 15 psi pushing up, and the second is the weight of the water that is already in the straw, pushing down. Let's say your straw is exactly one square inch in diameter. Clearly, when your straw has 15 pounds of water in it, no more can go into the straw because the weight of the water already in the straw exactly counterbalances the atmospheric pressure. No matter how hard you suck, no water is going to go in that straw! Instead of straws, think of scales. When the weight on each side is equal, the scale balances.

No matter what size straw you use, you will find that the atmospheric pressure will support no more than 15 pounds of water per square inch... or 10 meters or 34 feet, about.

Now to answer your other question, if you carefully sucked up just FOURTEEN psi of water into your straw, then took the straw out of the glass (being careful not to let the water escape from the straw), you could then suck those 14 psi of water up any height you like - all the way up the Empire State Building. As long as you are generating a vacuum in your mouth, the "scale" is not "balanced" and the water will move up the straw.

 

[El_Machinae]

Ah! Thanks! I was forgetting (even though it was my first instinct) that the water had weight while in the straw. Thanks again!

 

[Bozo Erectus]

Bumped for a new question:

Ive noticed that when I leave lighters out on the balcony, in the cold, they quickly stop working. At best I get a small flame that doesnt last very long. When I take the same lighters back in, and theyve warmed up, they work perfectly. My working theory is that the cold makes the gas in the lighters too dense to flow smoothly to the spark.

Anybody have a clue as to whether this is correct?

 

[ArneHD]

Bumped for a new question:

Ive noticed that when I leave lighters out on the balcony, in the cold, they quickly stop working. At best I get a small flame that doesnt last very long. When I take the same lighters back in, and theyve warmed up, they work perfectly. My working theory is that the cold makes the gas in the lighters too dense to flow smoothly to the spark.

Anybody have a clue as to whether this is correct?

Have you tried it with different types of lighters? It could be a design flaw, but Butane has a boiling point of -0.5C which means that in sub zero temperatures become liquid.

 

[Ayatollah So]

Butane is probably your lighter fuel. Its viscosity is very low either way. Instead, the evaporation rate of the butane off the cold lighter surface is probably too low to give you a good flame. Now I just got an email saying ArneHD replied while I was typing, let's see if we agree...

 

[Ziggy Stardust]

Does that mean that shaking a lighter really does help make it work, since you force some molecules out of the liquid, into gasseous state? edit: probably more important you increase the surface area, so it's easier for molecules to escape.

I do that all the time with near empty lighters and am convinced it works, but a mate sais I am fooling myself.

 

[Ayatollah So]

Remind me how lighters work: do they have a little straw that sucks liquid from the bottom? If so, are you talking about ones that still have enough liquid to cover the straw opening?

Anyway, shaking the lighter increases the heat transfer from your hand to the butane (both liquid and gas). The warmer the butane, the higher the pressure in the lighter will be, because more liquid will turn to gas. I'm thinking that's the main point. If you increase the surface area of the liquid/gas interface, that helps drive the evaporation process toward its temperature-based equilibrium, faster, so that could boost the effect.

 

[Ziggy Stardust]

Yep, indeed, it's a little straw so I was quite of the mark. One of these. There is just a small amount of it left at the bottom. That's when most lighters start burning with smaller and smaller flames.

I think you're right. Increased presure so more liquid (which turns to gas right away) is pushed through the straw is probably right. This is also the reason that the lighter didn't work well in the first place. Because it's almost empty, there's less stuff in it, so less presure.

Works for me

edit: Everyday science Question:

If you pull the plug in a filled bathtub, does it empty faster when you are still in it, or does it drain faster when there's no one in it?

edit 2: You're not sitting ON the hole of course. Water is free to flow in both scenarios.

 

[brennan]

If you pull the plug in a filled bathtub, does it empty faster when you are still in it, or does it drain faster when there's no one in it?

With you in the bath you are displacing water, this would mean you are increasing the pressure at the bottom and the gravitational potential energy of the water overall. I guess those would both lead to an (extremely minor) increase in the flow.

 

[Ziggy Stardust]

Yep. I see that was far too simple.