381

Physics, Astronomy & Cosmology / Re: Does the speed that my coffee is swirling affect the notes made when my spoon hits the cup?

« on: 16/10/2015 16:10:28 »

I believe this has to do with how much of the cup is in contact with the liquid inside. When the coffee is swirling, it becomes shallower in the center and deeper at the edges, and the faster it swirls the more this difference is (even forming a vortex in extreme cases). So the faster it swirls, the more of the cup is touching the liquid, and the way that the cup vibrates is altered.

This effect will probably be most pronounced when it is filled to about a third to a half way.

382

Chemistry / Re: Could a solar powered cooling system be used to collect water from the air?

« on: 16/10/2015 16:02:37 »

I am imagining a scenario in which there is a refrigeration unit directly under some solar panels. These panels will serve two purposes in the desert: collecting solar energy, and providing shade to the unit. On average you should be able to capture about 150 usable watts per square meter (averaged over 24 hours). This would actually be able to provide for almost one standard household refrigerator per square meter.

I'm not sure how large an area you would need to generate the water required (this would likely be very high purity and sterile water, so it would be best suited for drinking, and too pure/too expensive for agriculture). Water condensation releases about 2.3 MJ per liter, and if we estimate that the heat pumps are able to move 2 J of thermal energy per 1 J of energy from the solar panels (this could be a terrible estimate, someone else can correct this if it is!), then given 150 watts (150 J/second) of power, and an endless supply of already saturated air (bad assumption in the desert, I know, I'll get there) you could expect a maximum of 12 L of water per square meter per 24 hour day. This number will decrease dramatically one you include the energy required to cool the air to it's dewpoint (and this will depend on how much moisture is actually in the air, and how hot it is) and energy required to move the air through the system (which will also depend on how much water is in the air)

Maybe you could get 4 L per day per square meter, so a 5x5 meter shed could potentially provide 5L of potable water per day to each of 20 people.

Check out this website which talks about the physics of heat pumps, and goes on to talk specifically about solar-powered refrigeration, and even has a discussion of condensation further down: http://physics.ucsd.edu/do-the-math/2012/06/heat-pumps-work-miracles/

383

Physics, Astronomy & Cosmology / Re: Is time faster than the speed of light?

« on: 13/10/2015 14:54:39 »

Time doesn't move at any speed. It's like asking if distance is faster than light.

The speed of light in a vacuum (about 300000000 m/s) is the fastest that any object, particle, or even information can move through space. (Note that speed is measured as the ratio of distance to time, so neither distance nor time can have a speed)

384

Chemistry / Re: What would happen if we increased earth's water?

« on: 10/10/2015 19:44:43 »

This is of course a function of how much we increased the amount of water, and how quickly it was introduced...

The actual amount of water on the earth does actually change with time. Ignoring for the moment chemical consumption and production of water, which is essentially a steady state that oscillates somewhat with time, water does actually leave our atmosphere, and is introduced from space. (I don't know the exact ammounts, but I think it is on the order of several kg per day either going or coming, worldwide)

Adding enough water to raise the oceans a few meters would be terrible for civilization, but I don't know, ultimately, how much effect it would have on the chemical and ecological equilibria on the planet. The oceans would be diluted to some extent, but I don't think it would be that significant. Species that the ocean is saturated in would probably stay saturated, there's more than enough CO₂, Mg and Ca to go around for that type of increase.

Increasing the oceans by a few km would have much, much more significant effects. Now the ocean will cover the entire surface of the planet, and will have absorbed almost all of the CO₂, NO_x etc. from the atmosphere.

After this point, all of the added water will be spherically distributed about the globe, so I don't think it would have any major tectonic effects, it would just compress the earth underneath as the mass of water increases.

We could theoretically add a few earth masses of water before much interesting would happen, but eventually the Earth would be massive enough to capture hydrogen and helium. Heck if added enough water, the Earth could turn into a star in a very explosive way (this would have to be a stellar amount of water)

385

General Science / Re: What would be the most likely cause of WW3?

« on: 12/08/2015 18:08:48 »

Individual terrorist acts or actual human suffering don't generally start world wars.

And WW I started how?

True, the official war started when nations (empires) declared war on each other. And yes, the world needs a certain amount of political tension for anything to turn into a large war, but given the right (wrong) circumstances, a single terrorist event can trigger a world war (for instance the assassination of a certain Archduke set everything into motion, and the actual war started one month later).

I think of it rather like a chemical reaction. There needs to be enough stored energy (tension/will to fight) for a war to happen. There also needs to be an initiation that supplies enough energy (anger?) to get the reaction (fighting) started, then as long as the fighting leads to an increased bloodlust and there is enough materiel to fight with, the war will continue. Eventually someone surrenders (or is defeated/destroyed), the war ends, and tensions begin to rise again...

386

Technology / Re: Is mid-day best for solar power?

« on: 19/07/2015 19:03:47 »

It isn't necessarily when the sun is at the maximum height. There are two real issues: 1) how directly the sunlight hits the solar panel; and 2) how much of the atmosphere the sunlight has gone through.

For the first issue, this has to do with what fraction of the sunlight gets reflected, and how much gets absorbed. If the light comes in at a 90° angle, it has the best chance to get absorbed, and the further away from this ideal angle, the more light gets reflected (wasted). This is why most solar panels are built at an angle rather than being perfectly horizontal (especially at higher latitudes, where the sun is never directly overhead).

The second issue is less critical except right after dawn and right before sunset, when the sun is very close to the horizon. In these cases, the sunlight has  a much longer path through the atmosphere than when it is higher up. This results in some losses in the intensity of the light (mostly due to scattering, but some absorption).

So to answer the title question: the solar panels can capture the most power when they are pointed directly at the sun when the sun is at the highest point in the sky. For solar panels that do not track the sun, it is slightly more complex to optimize the efficiency, and it may actually be best to have the sun hitting the panel most directly during periods when it is not quite at the highest point (especially because the sun's path through the sky is different throughout the year...)

387

Physics, Astronomy & Cosmology / Re: heat pump system to generate electricity from T>0K?

« on: 23/10/2014 19:21:16 »

There is no "stealing" going on. Heat will naturally move from a warm body to a cold body, if provided a path between the two. A heat engine can extract useful energy when it is the path between two bodies at different temperatures, but there is no energy created, and there is no energy "stolen." The hot body will give up its energy anyway, it's just an issue of putting that energy to use. Does that make sense?

Think of it as water that can be a different heights. Water will naturally flow down from a high reservoir to a lower one. This can happen with no useful energy extracted (a waterfall) or it can go through a hydro-electric power station, which will extract some of this energy for use. You cannot connect the hydro-electric power station to only one reservoir, no matter how high it is because the water has to go somewhere. It is only by making the connection between the high water and the low reservoir that energy can be extracted (for heat engine, you need both a hot body and a cold body with the engine in between). You can pump water up from a low reservoir to a higher one, with the input of energy, but this energy will always be greater than the amount of energy one will get back by letting the water flow back down. Similarly, one must expend energy to move heat from a cold body to a warmer one, and this will be more energy than can be taken back by allowing the heat to move from the hot one to the cold one.

388

Physics, Astronomy & Cosmology / Re: heat pump system to generate electricity from T>0K?

« on: 20/10/2014 22:46:56 »

Only if there is a 0 K heat sink to connect the heat pump to. The efficiency of a heat pump depends on the *difference* in temperature between the two ends. No useful energy can be extracted from a heat pump that is in equilibrium (same temperature on both ends), and it would require input of energy to move heat from a cool body to a warmer one.