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Physics, Astronomy & Cosmology / Which is the shape of a planet with the maximum gravity at one point?

« on: 23/12/2009 18:03:51 »

Given a specific volum and a specific, uniform density (we could take them as unit), which shape should have a planet, so that the gravitational field in a point O (we can take it as the origin of the axis) of its surface is maximum?

Could this be the shape?
In the image I have drawn a polar plot (red, inner curve).

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Technology / How much light does a photocopier produce?

« on: 21/06/2009 11:21:48 »

Which is a typical value of illuminance (lux = lumen/m²) of a photocopier on a sheet of paper to copy?

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Chemistry / Why does aluminum react exothermically with palladium?

« on: 01/03/2009 18:44:35 »

http://en.wikipedia.org/wiki/Pyrotechnic_initiator

Palladium-aluminium

Palladium-clad aluminium wires can be used as a fuse wire, known as Pyrofuze. [9] The reaction is initiated by heat, typically supplied by electric current pulse. The reaction begins at 600 °C, the melting point of aluminium, and proceeds violently to temperature of 2200-2800 °C. The reaction does not need presence of oxygen, and the wire is
consumed. [10]

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Chemistry / Methanol attacks aluminum oxide?

« on: 22/01/2009 12:19:46 »

wiki says that CH₃OH attacks some metals, e.g. aluminum:
http://en.wikipedia.org/wiki/Methanol

One of the drawbacks of methanol as a fuel is its corrosivity to some metals, including aluminum. Methanol, although a weak acid, attacks the oxide coating that normally protects the aluminium from corrosion:

    6 CH₃OH + Al₂O₃ → 2 Al(OCH₃)₃ + 3 H₂O

The resulting methoxide salts are soluble in methanol, resulting in clean aluminum surface, which is readily oxidized by some dissolved oxygen. Also the methanol can act as an oxidizer:

    6 CH₃OH + 2 Al → 2 Al(OCH₃)₃ + 3 H₂

This reciprocal process effectively fuels corrosion until either the metal is eaten away or the concentration of CH3OH is negligible. Concerns with methanol's corrosivity have been addressed by using methanol compatible materials, and fuel additives that serve as corrosion inhibitors.

My question is: is that really true or it's more a legend? How the heck methanol attacks aluminum oxide?

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General Science / What do you think of "Chemtrails"?

« on: 21/04/2008 19:46:50 »

http://www.knbc.com/video/9265818/detail.html

http://en.wikipedia.org/wiki/Chemtrail_conspiracy_theory

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General Science / What happens if you put diesel fuel in a petrol engine?

« on: 26/12/2007 17:04:25 »

Let's say you put some diesel fuel in the tank with already some petrol,  ~ 20 - 40% diesel fuel and the rest petrol. What happens?

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Chemistry / Thiosulphate + sulphuric acid

« on: 23/09/2007 10:24:06 »

What happens mixing diluted H₂SO₄ to a water solution of Na₂S₂O₃?

Edit: Sorry, I wrote Na₂S₂O₄ as a mistake. It's corrected now.

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Physiology & Medicine / Insect punctures

« on: 02/08/2007 13:27:14 »

Why ammonia heals insect punctures?

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Physics, Astronomy & Cosmology / Does Moon exist if you don't look at it?

« on: 05/07/2007 15:35:04 »

From NewScientists 23 june 2007 "RealityCheck":

[...]

<<What they found(*) is that Leggett's formula is violated as well: even if you allow for instantaneous influences, quantum measurements do not fit with the idea of an objective reality. This is surprising because you might expect that, once any spooky "non-local" action is allowed, you could account for almost any relationship between two particles, and there would be no reason to ditch our concepts of reality. "This is not the case", says Aspelmeyer.

Although some loopholes remain - not all non-local models have been ruled out - we now have to face the possibility that there is nothing inherently real about the properties of an object that we measure.
In other words, measuring those properties is what brings them into existence. "Rather than passively observing it, we in fact create reality", says quantum researcher Vlatko Vedral of the university of Leeds, UK.>>


(*) It refers to an experiment performed by Markus Aspelmeyer e Anton Zeilinger (Nature, vol 446, p 871) to test Leggett's formula (a variant of Bell's inequality, with the additional hypothesis that instantaneous influences are allowed).

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Chemistry / Eutectics alloys

« on: 22/06/2007 19:17:34 »

Pure Sn melts at 232 °C

Pure Pb melts at 327.5 °C

Their eutectic alloy (61.9% Sn & 38.1% Pb) melts at 183 °C.

Question: if I mix 61.9 grams of pure Sn fine powder and 38.1 grams of pure Pb fine powder and I slowly increase the temperature, at which temperature the mix will start melting?

At 183 °C?

Or at the lowest melting point compound of the two (that is Sn) = 232 °C ?

It's a real conundrum for me.

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Chemistry / Does bleach always...bleach?

« on: 12/06/2007 16:45:30 »

What would happen to a coloured shirt put in the bleach?
It looses its colour, of course.

What would happen to a white shirt?
Of course, it would whit...one moment! My shirt has become orange! How is possible?

In some parts only, and it doesn't go away! []

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Chemistry / Can potassium and sodium hydroxide (KOH and NaOH) react with glass?

« on: 09/05/2007 21:52:26 »

In a chemistry book I have it's said water solutions of KOH reacts with glass less than those with NaOH. Why?

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Physiology & Medicine / Communicating Emotions

« on: 11/03/2007 10:54:56 »

How can I communicate to someone a strong and deep inner sufference if they don't notice any physical/organical problem in me? How can I make them understand it?

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Chemistry / CuSO4 + Aluminum

« on: 03/09/2006 11:09:39 »

Why an acqueous solution of CuSO4 doesn't react with an aluminum foil, but, if we add table salt (NaCl), the reaction starts immediately?

Cu++ + Al +3H2O --> Cu + Al(OH)3 + 3H+

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Chemistry / Red Flares on the Water

« on: 29/08/2006 18:16:27 »

Some days ago, during a firworks exhibition, they shooted fires to a river, that then kept burning on the water's surface with very brilliant and long-lasting red light.

What are they made of?

(I'm not wondering about the flame's colour: it could be Ca, Li, Sr).

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Physics, Astronomy & Cosmology / Wien's displacement law: a (mathematical?) paradox

« on: 22/08/2006 16:00:33 »

Wien's displacement law relates a blackbody temperature T with the wavelenght l of its maximum emission: l(max) = a/T, where a is a constant. Let's calculate a.

Spectral radiance as a function of frequency v:

R(v) = [8(pi)h(v/c)^3] * 1/[exp(hv/kT)-1]

as a function of wavelenght l:

R'(l) = (8(pi)hc/l^5) * 1/[exp(hc/lkT)-1].

You can obtain this last formula from the previous one, writing:

Integral(0;+infinity)R(v)dv = Integral(0;+infinity)R'(l)dl

Now we derivate the radiance, to obtain the point of maximum:

dR(v)/dv = 0 --> 1-(1/3)hv/kT = exp(-hv/kT) -->

1-(1/3)hc/lkT = exp(-hc/lkT)  (1)

dR'(l)/dl = 0 -->

1-(1/5)hc/lkT = exp(-hc/lkT) (2)

Solving for l equations (1) and (2) gives two different values of l !

(1) gives (numerically, Mathcad): hc/lkT = 2.82 -->

l(max) = 5.11*10^-3/T

(2) gives (numerically, Mathcad): hc/lkT = 4.97 -->

l(max) = 2.90*10^-3/T

The values taken for computation:
k = 1.38*10^-23 J/°K
h = 6.63*10^-34 J*s
c = 3.00*10^8   m/s

Why two values for a?

Since the blackbody colour depends on l(max), this question could also be put:

which is the real blackbody's colour?

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Physics, Astronomy & Cosmology / Sound Spectrum of a Cavity

« on: 19/08/2006 03:11:20 »

Is it possible to make a sound analogous of a blackbody?

That is, a cavity which absorbs perfectly all sound frequencies (at least in a broad range) converts them into some form of energy (for example heat, but not necessarily), put itself in energetic equilibrium and then re-emits sound frequencies according to its energy?

If that would be possible, at least theoretically, which would be the sound spectrum of the cavity? Would it be independent of the material and dimension of the cavity, as in the luminous cavity?

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Physics, Astronomy & Cosmology / What Really is a photon?

« on: 08/08/2006 16:29:57 »

Hello to everyone,

I hope this subject hasn't already been covered; if so, I ask you to forgive me.

A Laser emits a weak beam of low frequency (I will explain later why low frequency) light, so that a revelator "ticks" once every, let's say, minute. Most physicists explain: "one single photon is emitted from the Laser every minute, travels to the revelator and it's revealed".

Question: How can we say that, in this case, a spatially localized particle travels from the source to the revelator? A way to prove it would be to put...another revelator in the middle of the route, so destroing the photon (here is why low frequency and hence low energy); so we are left with the same question: What is the photon from the source to this new revelator?

From my point of view, and I've thought a lot about it, the "photon" doesn't really exist from the source to the revelator; the "photon" actually is "The Tick of The Revelator".