Ask a physicist anything. (3)

[Gracchus]

No no, It really does get windier... If you sit outside as evening falls, the wind kicks up and sweeps through the trees and bends them all like it hasn't done during the day. Now why, people, why?

Cool air contracts, becomes denser. Warm air expands, and becomes less dense.

Thus there is a pressure differential between warm and cool air, and to equalize the pressure, the wind arises, a movement of air to from higher, to lower areas of pressure.

This is especially noticeble when the temperature changes rapidly as at dawn and dusk.

 

[Wiccan_Child]

This has been wrecking my head for a few days and I can't find an answer. Please help.

Does/Can an electron ever feel a net electromagnetic force from the quarks inside a neutron?

Yes, though the net force is zero. Any small perturbation (like a quark rip-off of van der Waals force) is so tiny as to be irrelevant. We're not even sure if they're physically next to each other, or if they occupy the same point in space.

 

[corvus_corax]

Oh, and the answer is 95 miles per hour, based on wind tunnel tests. (Taken from my handy copy of The Physics of Baseball, by Robert Adair. Bob Adair was a physics professor at Yale, the official physicist of the National League for several years, and my thesis advisor's advisor.)

Thank you and thank you Wiccan Child.

Like I said, I was just trying to get an idea of how much upward wind force it would take for that.
Do you, sfs, have the explanation behind the 95 mph explanation?
Thanks in advance.

I know, it's trivial, but I really want to know.

 

[corvus_corax]

This summer I am planning on constructing a small amateur cloud chamber approximately 6 inches across and about 4-6 inches deep.
How reasonable is it to see evaporation trails from random cosmic rays* in such a chamber? Would it be more beneficial (i.e. quicker for my son to see) to use a 'safe' radioactive source?

I know I'm being rather vague here, but any suggestions to make this amateur (i.e. less than $30) cloud chamber "better" in any way would be appreciated (even if my 10 year old son and I have to depend on something other than cosmic rays*).


Yeah it's a completely amateur science project, but my son and I live for such things



Thanks in advance



*ETA cosmic ray particles, forgive me for my initial common usage of the term

 

[Wiccan_Child]

This summer I am planning on constructing a small amateur cloud chamber approximately 6 inches across and about 4-6 inches deep.
How reasonable is it to see evaporation trails from random cosmic rays* in such a chamber? Would it be more beneficial (i.e. quicker for my son to see) to use a 'safe' radioactive source?

I know I'm being rather vague here, but any suggestions to make this amateur (i.e. less than $30) cloud chamber "better" in any way would be appreciated (even if my 10 year old son and I have to depend on something other than cosmic rays*).


Yeah it's a completely amateur science project, but my son and I live for such things



Thanks in advance



*ETA cosmic ray particles, forgive me for my initial common usage of the term

If memory serves, most cosmic rays are deflected before they get to the Earth's surface, so I'm not sure an amateur project could detect them. That said:

"
Gosmic Ray Studies with Geiger
Gounters
One of my favorite experiments is making cosmic ray
measurements in near space. Aware Electronics manufactures
an inexpensive, lightweight Geiger counter designed
for laptop use. The RM-60 Geiger counter is about the size
of a deck of playing cards and takes its power from a PC
serial port. Its power and data cables are easily interfaced
to flight computers.
ifr" *ini-ul weight, volume, and power requirements
of the RM-60 make it the ideal Geiger counter for cosmic
ray studies. The detection of a cosmic ray is signified with
a short duration, five-volt pulse. Collecting cosmic ray data
entails recording the current altitude from the CPS receiver,
then the number of pulses over a specific time span.
Most of the cosmic rays detected on the ground are
secondary and not the original - or primary - cosmic rays
from outer space. This is because primary rays create
showers of secondary rays when they slam into nitrogen
and oxygen molecules high in the atmosphere. However,
near space is high enough that Geiger counters will begin
detecting primary rays. Since the RM-60, combined with
the flight computer, can detect a single cosmic ray, my
missions into near space can detect a single atom that
originated in another star"

From here.

A fun thing you can do is find the Moon by measuring cosmic rays - a blob in the sky with next to no rays is the Moon's shadow. Coolbeans!

 

[Maxwell511]

Do White Holes exist?

 

[Wiccan_Child]

Do White Holes exist?

Depending on who you ask, yes, no, yes and no, and maybe. Some say they're the endpoint of a black hole, some say they're what happens to a black hole when spacetime gets turned on its head (event horizons do that), some say white holes are black holes and that a black hole's Hawking radiation is the white hole's emission, etc.

In other words, no, they don't exist

 

[pgp_protector]

Do White Holes exist?

Yes, here's a Blue Rock with a White Hole

 

[catzrfluffy]

Yes, here's a Blue Rock with a White Hole

A druse, geode, vug...

That has many names!

 

[canehdianhotstuff]

It's actually going to be windier during the day. Winds are weakest at dawn usually. You need a pressure gradient to get wind, not just a pressure change. Gradients are more likely to form during day when the sun is out heating up different places by different amounts.

When the ground is cold, like in the morning, there is a local inversion that isolates ground winds from higher up winds, but by late afternoon when the ground is heated up, air is rising faster (higher surface wind) and mixing more with the higher atmospheric winds. Coreolis affect is a major part too. Coriolis effect - Wikipedia, the free encyclopedia
it is one of the driving forces that sends lower pressure areas towards higher pressure ones.
I think you simply notice the wind more at night because it is generally quieter.

 

[canehdianhotstuff]

And also, wind will have a directional change between day and night near large bodies of water. Ground heats faster during day than water, thus the air over the water blows in to replace the rising warm air over the land. At night the opposite happens. Water releases its heat and the air over the land sweeps out to sea. This is why swimming at night is usually warmer than the day. Water is releasing heat rather than taking it in.

 

[catzrfluffy]

Weather is interesting.


Why does a sudden drop in pressure signify a storm?

 

[Wiccan_Child]

Weather is interesting.


Why does a sudden drop in pressure signify a storm?

As I understand it, it means that a cold front (big chunk of cold air) is hitting a hot front (big chunk of hot air) in the immediate vicinity. That is, if you feel the drop in pressure, you're on the boundary of the two fronts. When/where the two fronts meet, you get a storm.

 

[canehdianhotstuff]

Basically. Think of it like a vacuum cleaner. In terms on storms and extreme cases like hurricanes, there is usually a center vortex that is sucking up air, thus creating the lower pressure. Tornadoes and the immediate area have very low pressure as well.

 

[sfs]

This summer I am planning on constructing a small amateur cloud chamber approximately 6 inches across and about 4-6 inches deep.
How reasonable is it to see evaporation trails from random cosmic rays* in such a chamber? Would it be more beneficial (i.e. quicker for my son to see) to use a 'safe' radioactive source?

Most of the cosmic ray flux that reaches the ground consists of muons. The muon flux is ~1/cm[sup]2[/sup]/minute. Not a lot, but in principle enough to see. You should get a lot more from a radioactive source, but I have no idea what regulations would be involved in using one.

 

[Wiccan_Child]

Most of the cosmic ray flux that reaches the ground consists of muons. The muon flux is ~1/cm[sup]2[/sup]/minute. Not a lot, but in principle enough to see. You should get a lot more from a radioactive source, but I have no idea what regulations would be involved in using one.

Household smoke detectors have a small source of alpha particles - though that only amounts to helium nuclei. I doubt muons would be emitted from that.

 

[canehdianhotstuff]

Amercium-241 if I recall. Emits alpha particles and gamma radiation on its way to Neptunium-237

 

[pgp_protector]

Most of the cosmic ray flux that reaches the ground consists of muons. The muon flux is ~1/cm[sup]2[/sup]/minute. Not a lot, but in principle enough to see. You should get a lot more from a radioactive source, but I have no idea what regulations would be involved in using one.

Looks like you can get small amounts easily enough.
Radioactive Isotopes : United Nuclear , Scientific Equipment & Supplies

$50
Lead-210 Needle Source : United Nuclear , Scientific Equipment & Supplies

 

[sfs]

Thank you and thank you Wiccan Child.

Like I said, I was just trying to get an idea of how much upward wind force it would take for that.
Do you, sfs, have the explanation behind the 95 mph explanation?
Thanks in advance.

I know, it's trivial, but I really want to know.

I don't know of an intuitive explanation for that value; it depends on the details of how force is transmitted from the air as it moves past the baseball, which involves both air flowing directly over the ball surface (including irregularities and stitching) and the formation of vortices.

I think you can get some idea of the forces involved by considering an idealization, in which a column of air exerts its full force on a baseball; imagine a baseball that just fits in a tube, with air blowing up the tube at 95 m/h. (Somehow the baseball has to absorb the air that hits it, which is hardly realistic, but this is the best I can come up with at the moment.) The force of gravity on the ball is 145g x 980 cm/sec[sup]2[/sup] = 1.4x10[sup]5[/sup] dynes. (A baseball has a mass of 145 g.) The force exerted by the air equals the momentum change that occurs when the air molecules are absorbed by the baseball. The density of air = 1.2x10[sup]-3[/sup]g/cm[sup]3[/sup], the area of the tube = 44 cm[sup]2[/sup] (since a baseball has a diameter of 3 inches), so each cm of air column has a mass of 5.3x10[sup]-2[/sup] g. The column of air moves at 95 mph = 4.2x10[sup]3[/sup] cm/sec, so the flux of mass is 2.2x10[sup]2[/sup] g/sec striking the ball. The total momentum transferred = dm/dt * v, or 9x10[sup]5[/sup] dynes, or about six times what is actually needed to float the ball.

So according to this crude model, roughly 1/6th of the momentum in the moving air is transferred to the ball.

 

[Chesterton]

Another weather question: when a hurricane forms in the ocean, it's drawing moisture up from the ocean beneath it, right? So does the storm produce salt-water rain? If not, how would the water get desalinated?

ETA: Ah, I should have Googled first, instead of right after posting. I learned that the evaporation process leaves the salt behind, so the rain is fresh water, and the ocean water underneath a storm gets saltier.