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Quizmas episode

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1339627.  Wed Dec 25, 2019 7:13 am Reply with quote

While Qi is a great program and generally very interesting, I have three issues with the Quizmas episode...

The first is the assertion by Sandi that there is "zero gravity" in space. This is not the case. Gravity permeates space and, indeed, it is the one thing stopping the whole thing from blowing apart. For example, the Sun's gravity keeps the Earth in a elliptical orbit. So the idea that there is zero gravity in space is fallacious. Rather, space craft in Earth orbit are constantly falling towards the ground but their lateral movement means they keep missing. However, occasionally gravity will win and a craft comes tumbling down.

The second is the statement "a molecule of ice has three things connecting it" (or something to that effect...) There is no such thing as a molecule of ice. It is a water molecule bound within a three dimensional lattice which was poorly depicted by the drawing. Each water molecule makes four bonds with its neighbours - two through the hydrogen atoms and two through "lone pairs" with the net result that you get a hexagonal crystalline structure the same as diamond. Further, the notion there is vibration free water on the surface of the ice is something which has been known for some time and it is this water which makes the surface slippery. (By the way, the crystalline lattice is why all snowflakes - the pretty and not so pretty ones - exhibit hexagonal symmetry. With the pretty snowflakes it is easier to see the hexagonal structure but even nasty, blowy ice crystals are hexagonal columns if you look at them up close.)

The third issue is more of a question about the size of elephant ears required to fly. The show asserts they would need to be 60ft long but, as Johnny Vegas was trying to say I believe, it is more about area - and this calculation doesn't seem to be consistent with aeronautics. A Boeing 747-400 at take-off has a differential pressure across its wings of 1.047 lbs per sq. inch. If we use this as the basis for our elephant, a 288 lb baby would only need 2 square feet of ears to take off (presuming some rigidity and shape similar to a Boeing's wings). Even if we allow for some slop, the 60 ft answer seems to be a little excessive. Was the assumption being made that the elephant would have to beat its ears like a bird beats its wings? If so, the size still seems a little excessive as a Muted Swan can weigh 15 kg or 33 lb and take off with about 4 square feet of wing area. Extrapolating that to a 330 lb baby elephant gives only 40 square feet and presuming some sort of normal wing structure, that would result in ears only about 16 feet long... so what were the assumptions made?

1339701.  Thu Dec 26, 2019 4:37 am Reply with quote

Todd wrote:
...occasionally gravity will win and a craft comes tumbling down.

The orbit is not perfect and gradually, or not so gradually, decays - or it is perfect and therefore doesn't.

Todd wrote:
...the same as diamond.

Not quite; diamond bond angles are perfectly towards the corners of a tetrahedron (cos^−1(−⅓) = 109.4712206... ≈ 109.5).

Like this

where the red portions are a sort of 'negative space' and the vertices of the yellow portions are the location of the carbon atoms@.

Water molecule bond angles are only nearly tetrahedral. The δ- electron pairs will repel each other; as will the δ+ hydrogens less powerfully. The hydrogens and the electron pairs will attract each other (think poles of a magnet and hence 'polar'). The net result of these competing forces is that the H-O-H angle is 104.5 and the other angles similarly depart from the tetrahedral.

An interstitial is formed when an extra water molecule occupies open space in the crystal lattice. The concentration of interstitials is given by

Ci = e^(Si/k)e(−Ef/kT)

[physics stuff the jist of which is that H bond lengths in ice are variable resulting in point defects.] This results in an interstitial concentration of about 10^−6 near the melting point. This is significantly higher than the vacancy concentration in ice or the interstitial concentration of metals. That is a direct result of the open structure of ice Ih, which makes the introduction of interstitials relatively easy.

A quick bit of mental arithmetic:

Mass of 1 molecule of water
18.015 grams/mol 6.022 x 10^23 mol-1 = 2.992 x 10^-23 grams

A typical snowflake made of 100 snow crystals was calculated to be about 3 mg.

So there's about 10^20 water molecules in a snowflake and consequently about 10^14 interstitials.

<E> @ No they aren't! The central carbon is at the absolute centre of the tetrahedron and the vertices are what I'm going to call a compromise position in the middle of bonds# which allow maximum spacing between carbon nuclei (in perfect pure diamond structure).

# all bond energies being equal... which they won't be but in a pure diamond probably not too far off I'd suggest.


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