Wikipedia:Reference desk/Archives/Science/2012 June 30

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June 30

given that gold foil averages 2 atoms thick...

... how many atoms, very approximately, would be in a square centimetre of 22k foil?

thanks Adambrowne666 (talk) 04:46, 30 June 2012 (UTC)

What's the source for that "given"? ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:48, 30 June 2012 (UTC)

$\rho _{Gold}=19.3g/cm^{3}$
$M_{rGold}=197g/mol$
${\frac {19.3}{197}}\approx 0.1mol/cm^{3}$
$N_{A}=6.022\times 10^{23}$
$0.1\times N_{A}=6.022\times 10^{22}$
${\frac {1}{6.022\times 10^{22}}}cm^{3}/atom$

${\sqrt[{3}]{\frac {1}{6.022\times 10^{22}}}}=2.55\times 10^{-8}cm$

${\frac {1}{2.55\times 10^{-8}}}=3.92\times 10^{7}$
It's two atoms thick so,
${\frac {1}{6.022\times 10^{22}}}\times 2\times (3.92\times 10^{7})^{2}=5.103\times 10^{-8}cm^{3}$

${\frac {5.103\times 10^{-8}}{\frac {1}{6.022\times 10^{22}}}}=3.07\times 10^{15}atoms$
$karats=24\times {\frac {M_{Gold}}{M_{total}}}$
Assuming the impurities are the same atomic mass of gold and they occupy the same size space in the lattice;
$\therefore {\frac {22}{24}}\times 3.07\times 10^{15}=2.82\times 10^{15}atoms$
112.215.36.174 (talk) 05:45, 30 June 2012 (UTC)

that's fantastic, thank you .174; the formula will look very impressive in my artist's statement. Bugs, I must admit, I can't remember where i read that factoid, and am now finding estimates on the net ranging from 2 to 300....Adambrowne666 (talk) 07:29, 30 June 2012 (UTC)

Just to let you know, the assumption that the impurities are similar to gold in atomic mass and size in the lattice is wrong, but shouldn't make a big difference. It's almost certainly copper which is 3.1x lighter in terms of atomic mass, but the small proportion of it in the gold makes it more or less negliglible. Also, I didn't calculate this in the easiest way; it is much simpler if you do it like this:

$\rho _{Gold}=19.3g/cm^{3}$
$M_{rGold}=197g/mol$
${\frac {19.3}{197}}\approx 0.1mol/cm^{3}$
$N_{A}=6.022\times 10^{23}$
$0.1\times N_{A}=6.022\times 10^{22}$

${\sqrt[{3}]{6.022\times 10^{22}}}=3.92\times 10^{7}atoms$
$2\times (3.92\times 10^{7})^{2}=3.07\times 10^{15}atoms$
${\frac {22}{24}}\times 3.07\times 10^{15}=2.82\times 10^{15}atoms_{Gold}$
I just found the first way a bit more logical as it showed the volume of the individual atom and worked out the total from there. 112.215.36.174 (talk) 07:49, 30 June 2012 (UTC)

300 would be the absolute thinnest gold leaf you'll find, I think. I have a hard time imagining going thinner without breaking it, unless you're using electroplating, but then it's not foil. Someguy1221 (talk) 07:46, 30 June 2012 (UTC)

According to gold, 1 gram of Au can be beaten into 1sqm of gold leaf. That corresponds to 110x thicker than my calcs above i.e. ~220 atoms thick. 112.215.36.174 (talk) 07:59, 30 June 2012 (UTC)

Gold foil a few atoms thick is used in science labs. I think you are right that you wouldn't be able to use it for art without breaking it. --Tango (talk) 08:01, 30 June 2012 (UTC)

You can also work it out by taking the atomic radius, multipling by 2 to get the diameter and dividing 1cm into it. Square that to get the number of atoms of 1 thickness in 1cm^2 and then multiply by 2 to get it for 2 atoms thickness or whatever. I got 2.4x10^15 atoms for 2 atoms thick using a radius of 144pm. 112.215.36.174 (talk) 08:36, 30 June 2012 (UTC)

It can be fewer. Gold foils down to 0.02 µm (2x10^-8m or 20nm) are supplied by this company. At this thickness the foil is transparent. The atomic diameter of gold is 2.88x10^-10 m so its thickness is about 70 atoms. Its density is 19.32 kg m^-3. DriveByWire (talk) 12:25, 30 June 2012 (UTC)

That makes an unstated assumption about the orientation of the crystal. The distance between the layers is only 2.03x10^-10m, so there could be up to 98.5 atoms of thickness in 0.02µm if you assume optimum packing. 112.215.36.172 (talk) 00:05, 1 July 2012 (UTC)

science

is students perusing engineering from computer science branch can participate in u.p.s.c exams or they can give i.e.s exams? as computer science is not a core branch so to give such exams what should be the plan of action for such students?Divyajharia (talk) 07:40, 30 June 2012 (UTC)

Your question isn't very clear. Is "u.p.s.c" the Indian Union Public Service Commission? And is "i.e.s" the Indian Engineering Services? You probably have a tutor that can help you easier than we can. If you want our help, you'll need to ask your question more clearly. What are you studying? Where are you studying? What exams do you want to take? Why do you want to take them? --Tango (talk) 08:08, 30 June 2012 (UTC)

What is this machine

What might this machine be for? Roadsworth88 (talk) 17:38, 30 June 2012 (UTC)

A (1940's era ?) device for measuring the flow rate (of water ?) between Croydon North (UK Parliament constituency) and Mitcham and Morden (UK Parliament constituency), presumably. StuRat (talk) 17:51, 30 June 2012 (UTC)

Indeed, the river Wandle flows just south of a sewage treatment plant in Beddington Park, so it's important to be able to tell if things are threatening to back up, and drop a trouble ticket if they do. 75.166.192.187 (talk) 17:55, 30 June 2012 (UTC)

More modern I think. Seems to show flow rate of water (or sewage?) and the total quantity (calculated by integrating the flow rate. Integrator round the back!)--78.150.226.117 (talk) 17:59, 30 June 2012 (UTC)

Dial on right looks like pressure meter.--78.150.226.117 (talk) 18:01, 30 June 2012 (UTC)

Agreed. It must have something to do with the Wandle Trust. 75.166.192.187 (talk) 18:02, 30 June 2012 (UTC)

<nitpick> It's almost certainly not measuring flow between Croydon North (UK Parliament constituency) and Mitcham and Morden (UK Parliament constituency), since Parliamentary constituencies of the UK parliament are almost never used for any purpose other than electing MPs and local councillors. It's much more likely that the areas are the County Borough of Croydon and the Municipal Borough of Mitcham. Having Mitcham mentioned as a separate entity, as opposed to a part of the London Borough of Merton, would place the machine's date as between 1915 and 1965. </nitpick> - Cucumber Mike (talk) 11:06, 1 July 2012 (UTC)

Unless the Metropolitan Water Board and their successor, Thames Water, have their own area boundaries. Alansplodge (talk) 16:56, 1 July 2012 (UTC)

How to fix scrolling/tiling interference

I recently purchased a Panasonic Plasma TV 720 DPI 600 Hertz. After hooking it up to my digital low-def cable service I got a low hum in the audio and lines of lighter and darker interference traveling in horizontal bands up the screen taking maybe 5-7 seconds to go from the bottom of the screen to the top. No interference originated from the DVD signal, only from the cable signal when routed directly or indirectly into the TV through the AV (red/white/yellow) input. The RF (co-ax) input was clear. I had the cable guy visit. Depending on which of the three cable boxes we used the problem was more or less pronounced. We tried analog TV's, and the problem disappeared. I finally took the Panasonic TV back and traded it in for a Samsung plasma 720 dpi which is getting the same exact video problem, but now through both its RF and AV inputs. I have scheduled the cable guy back out. Is there any way to identify this type of rolling interference and what causes it? Can I assume it is an interaction between the signal source and the TV, i.e., interference on the cable line which shows up only on a certain type of set? Will ferrite cores potentially correct this sort of problem? Will it help to call 911? μηδείς (talk) 19:27, 30 June 2012 (UTC)

Assuming you tried different red/white/yellow cables, from the tests which have been done so far, it sounds like the interference comes from the cable boxes, apparently a defect in their design. Assuming they don't have another model of cable box for you to try, this leaves the following options:

1) Use an HDMI cable to connect to the cable box, if it has an HDMI output. Same with S-Video, etc.

2) Use the RF (co-ax) input. Picture won't be as clear, though.

3) Switch cable companies (or switch to satellite) and hope new one has a better cable box model.

4) Stick with analog TVs. If your digital TV has an option to lower resolution below 720, that might be worth a try, too. StuRat (talk) 20:00, 30 June 2012 (UTC)

The Panasonic which we took back had a perfect picture with the RF cable and craptastic with the RCA. The Samsung picture has weak interference through both lines. I don't see the ultimate problem being with the boxes but with a voltage on the line itself from the pole. Our neighbours all have the same cable company but not the interference we are getting.

The cable company doesn't want to give us a box with an hdmi connection unless we pay for hidef, although I intend to have the tech let us try out a box to see if it fixes the problem. Sattellite does not offer the sports broadcasts we want. μηδείς (talk) 21:27, 30 June 2012 (UTC)

If the HDMI test is clear, then you might be able to convince them to give you that box for free, if their tech tells them that their non-HDMI box just isn't working and you threaten to find another cable company due to their failure to uphold their end of the contract (provide a clear pic). StuRat (talk) 21:39, 30 June 2012 (UTC)

At this point what I really want to know is what sort of interference we are having and what typically causes it and what can fix it, like can a ferrite core fix visual interference. Again, the interference is perfectly horizontal bands of lightening or darkening of the picture of varying widths from fractions of an inch to a few inches which travel in regular waves up the screen from bottom to top over a period of about five to 10 seconds, maintaining their spacing and speed as they rise. The article on ferrite core is not very helpful and I don't know where to begin to look under interference. μηδείς (talk) 21:27, 30 June 2012 (UTC)

What you have described is often caused by what techs call an "earth loop", which causes a small amount of AC power to get into the signal cables. The subject of earth loops is complex, especially for unqualified people, however powering the TV from separate wall outlet to the rest of your interworking equipment is a possible cause in some houses. To fixed earth loop problems you need a good knowedgable electronics tech.

Ferrite cores are designed to fix radio frequency interference, such as may be caused by motorised applicances. This means that ferrite cores will NOT address your problem.

As the cable TV tech tried different boxes, faulty boxes are unlikely. I would investigate further before changing cable companies.

There is a (very) small possibility that the power voltage is low at your house, Electronic equipment (including TV's and set-top boxes) have internal voltage regulator circuits. If the power voltage goes lower that these regulators can cope with, the result is often hum in audio and rolling dark/light bars in pictures. You can tell if voltage is low as it casues light from incadescent globes to be yellowish, but some people don't notice it. In any case many people no longer have incandescent globes. If you have a multimeter and knbow how to use it, check your power voltage at the wall outlet - this is easy to do and will rule out this being the cause. If voltage is less than 90% of nominal, call the power company. It is quite common for low power voltage from the power company to apparenetly affect only one house in a street. This is because your neighbours have different brands/models of equipment, which may be more tolerant, and/or they may have their TV's etc on a different phase.

Otherwise, of if voltage is normal, call a TV serviceman - not because your TV is suspected faulty, but because, in general, TV service techs are smarter that cable company techs, and are more likely to deduce what the cause is. Keit60.230.195.130 (talk) 04:26, 1 July 2012 (UTC)

Yes, I am fairly certain you have identified the problem. Ground_loop_(electricity) One question. The TV and (cable box and VCR/DVD) are plugged into separate sockets on the same two-socket outlet. Will plugging all the devices into one power surge strip possibly help? Or is the voltage possibly flowing out the cable rf cord rather than between two electric cords? μηδείς (talk) 20:55, 1 July 2012 (UTC)

If ALL devices in the system (TV, cable box, DVD player, etc) are powered from the 2 outlets of the same two-socket outlet, electrically that's about as good as it can be, and using additional power strip(s) in order to use only one of the 2 outlets is quite unlikely to help. The Wikipedia article has a good point - if you have an antenna system connected somewhere for free-to-air reception, then, if it is earthed/grounded, that can contribute an earth loop issue. This is quite unlikely to be a problem in a stand-alone dwelling, but if you live in a duplex or block of flats, sharing a common antenna, it is possible. If you have an antenna connected, unplug it and see if cable reception improves. Don't forget the supply voltage check. While that is a low probability issue, it is easy to check. Keit121.221.222.136 (talk) 01:36, 2 July 2012 (UTC)