Wikipedia:Reference desk/Archives/Science/2019 March 18

= March 18 =

Can I boil water in an old tin can?
I have a chunk of used thermoplastic (the stuff that melts in boiling water so you can shape it as it cools and hardens) that I want to fix something with. I'd rather not boil the water in a cooking vessel because of crap that might be in the plastic. Can I use an old soup or tuna can, or will that create toxic vapor or whatever when I put it on the stove? Stove is gas not electric if that matters. Thanks. 173.228.123.166 (talk) 00:41, 18 March 2019 (UTC)
 * Many cans are plastic lined these days, and heating them over gas may easily burn such a liner. However if it's full of water, it won't go over 100&deg;C and so you're fine.
 * My workshop has a range of charity shop pans and Chinese supermarket ladles for just this sort of purpose. Sulphur inlay and pewter casting too (although look into fume protection and don't use the kitchen stove if you're doing lead alloys). Andy Dingley (talk) 01:00, 18 March 2019 (UTC)

PAGE ]]) 19:33, 18 March 2019 (UTC)
 * Thanks! This is just a one-off thing and I have some soup cans in my recycle bag, so I'll give one a try. 173.228.123.166 (talk) 03:47, 18 March 2019 (UTC)
 * If the can has a paper label, you might want to peel it off first. ←Baseball Bugs What's up, Doc? carrots→ 04:03, 18 March 2019 (UTC)
 * ...However if it's full of water, it won't go over 100°C... - careful here. The water will not go over 100°C but the bottom of the can will, if you heat fast enough, because the convective heat transfer between the can and the water is not instantaneous. (There might even be some Leidenfrost effect at play if the boiling is sufficiently fast.) Tigraan Click here to contact me 09:10, 18 March 2019 (UTC)
 * This is getting a little too close to "professional advice". The OP should talk to his chemistry teacher, or someone qualified who can give first-hand advice. ←Baseball Bugs What's up, Doc? carrots→ 11:57, 18 March 2019 (UTC)
 * Wikipedia has no rule against giving professional advice. Please stop acting as if it does. --Guy Macon (talk) 18:04, 18 March 2019 (UTC)
 * Yes, it does. Please stop acting as if it doesn't. ←Baseball Bugs What's up, Doc? carrots→ 19:29, 18 March 2019 (UTC)
 * It may go above 100°C, but not by much. I've successfully boiled water on a gas stove in a paper bowl without the paper even scortching, so I know it never went above 232°C. --Ahecht ([[User talk:Ahecht|TALK
 * When I was a Scout back in the day (early 1970s) one of my older school teachers (no longer in the movement) asked if we were still required to boil an egg in a paper bag up a tree. It's possible he may have been joking. {The poster formerly known as 87.81.230.195} 90.201.157.27 (talk) 00:08, 19 March 2019 (UTC)
 * "Boiling water in a paper cup" is a common grade-school science class experiment (e.g.). —107.15.157.44 (talk) 04:25, 20 March 2019 (UTC)
 * Boil water in paper bag - ScoutHelp. Alansplodge (talk) 19:35, 21 March 2019 (UTC)
 * Suggestion: Heat it outside if at all possible. Use a grill, if you have one.  Watch it heat, as you don't want the water to run dry. SinisterLefty (talk) 12:46, 18 March 2019 (UTC)

Feynman Lectures. Lecture 34. Special relativity, Aberration
Feynman in section 34-8 advises to derive the telescope angle using the Lorentz transformation. I use the Fig. 34–12. For a man at rest wave vector $$\mathbf {k}$$: $$\mathbf {k} (k_x, k_y, k_z) = \mathbf {k} (0, k_y , 0)$$ For moving man: $$\mathbf {k'} (k_x', k_y', k_z') = \mathbf {k'} (\frac{k_x+\omega v/c^2}{\sqrt{1-v^2/c^2}} , k_y , k_z) = \mathbf {k'} (\frac{0+(k_y c) v/c^2}{\sqrt{1-v^2/c^2}} , k_y , 0) $$ So the angle is: $$\tan(\theta) = \tfrac{k'_x}{k'_y} = \frac{v/c}{\sqrt{1-v^2/c^2}}$$

Is it correct? I don't understand why $$x', t'$$ are NOT the values observed from moving reference frame, but $$k', \omega'$$ ARE such values.

Username160611000000 (talk) 08:25, 18 March 2019 (UTC)

Electrical question
Suppose a garage has no electrical service and it is located 100 feet from the house. And suppose the house were to run out a 100 ft electrical line, a heavy gauge wire of adequate thickness and physical protection via conduit, in a semi-permanent fashion, and energize it only when needed. Would there be any advantage or dis-advantage from an electrical point of view, if to save wire conductor cost, the Line and Neutral wires were extended from the house, and a much shorter Ground line was well-grounded right at the garage? This would give the ground wire a shorter, presumably even better path to the ground in the case of a fault? Let's assume this is in a remote off-grid area and no electrical or fire codes are even applicable. --24.68.185.98 (talk) 22:41, 18 March 2019 (UTC)
 * Electrical codes are applicable to all installations, whether they're run from the mains grid or from local generation. I presume you're in Canada, which has rules I'm unfamiliar with in detail (I'm a Brit).
 * In UK / European practice, there are a number of earthing types recognised and some of these are used in typical practice for new installations (some are obsolete enough to still be permitted, but wouldn't be chosen for new work)
 * A typical installation (UK practice) for this would be to supply the garage over SWA "steel wire armoured" cable. This is two-core copper cable, wrapped in a protective bundle of steel wires. The steel wires would also be used as an earthing conductor. This would count as a TN-S system (earth conductor, all the way back to the supply). Your local earth rod idea would be a TT system. These are legitimate, but the difficulty with them is the need to achieve an adequately low earth loop impedance (a big enough buried earth), which also depends on the soil type and how dry the ground is. Because this is difficult, it's also a requirement that such systems are protected by an RCD. That's a good idea anyway, especially in a workshop, but here it's a requirement and has been so for a long time (the requirement to do so for TN-S hasn't been in force for so long).
 * Personally it would be a strange day before I installed this with anything other than armoured cable, using the armour as a protective conductor. Andy Dingley (talk) 23:43, 18 March 2019 (UTC)