Wikipedia:Reference desk/Archives/Science/2022 November 24

= November 24 =

Pumping water
Suppose I wanted to pump water at a rate of 5m3/s to a height of 5m. How much energy would that require in kW? If I doubled the height to 10m, would it require double or quadruple the energy? Mjroots (talk) 07:56, 24 November 2022 (UTC)
 * From my very rusty physics:
 * 5m3 weighs 5,000 kg, so exerts a force downwards of 5,000g N. To lift this water 5m would therefore take 25,000g J.  To do it in one second (ie 5m3/s to a height of 5m) requires 25,000g W, that is 25g kW or roughly 250 kW.  All assuming no losses anywhere in the system. To lift to twice the height would indeed require twice the input. Martin of Sheffield (talk) 08:44, 24 November 2022 (UTC)
 * That's good news, was fearing that double the increase of height would result in a quadrupling of the energy required. Mjroots (talk) 13:18, 24 November 2022 (UTC)
 * You will have to ensure that your pumps can deliver the higher pressure, 10m of water is near enough 1 bar. As the pressure increases the efficiency of the pump (particularly for non-displacement types) will reduce.  With any luck a hydraulic engineer will happen along soon. Martin of Sheffield (talk) 14:15, 24 November 2022 (UTC)
 * @Mjroots For the details, see Potential_energy. U=mgh, not accounting for any losses owing to your pump not having 100% efficiency. Mike Turnbull (talk) 18:18, 24 November 2022 (UTC)

As MoS said, 250 kW and 500 kW. I'm not a pump engineer, but I am an engineer with a lot of pumps. So you need to look at the discharge curve for the pump. A 250 kW mixed flow pump will cost a many many thousands of dollars, it is a substantial piece of gear. https://www.torishima.co.jp/en2/pdf/products/smsv_smiv.pdf You'll also need 250 kW of electricity to run it. Greglocock (talk) 22:37, 24 November 2022 (UTC)
 * 5 m3/s is quite a lot of water, so this is not a small pump. In the image I put on the right (just uploaded from my collection) you can see two pumps that can handle 4 m3/s over 5 m each, powered by 450 kW electric motors, so that's the size of pump you're talking about. PiusImpavidus (talk) 10:10, 25 November 2022 (UTC)
 * I like the cute little drain in floor, for when the pipe springs a leak with that much flow. DMacks (talk) 15:18, 25 November 2022 (UTC)
 * An amusing thought, but our Dutch neighbors are fairly competent at water handling. It's probably just intended to deal with the small amounts of water arising from normal operations, some of it perhaps condensation, or when the janitor mops the floor. Any substantial volumes would probably flow to larger drains elsewhere out of the picture. {The poster formerly known as 87.81.230.195} 90.217.47.60 (talk) 16:18, 25 November 2022 (UTC)
 * When there's a leak, best to shut down the pump quickly. It's above polder water level, so no more water will come through. It may be below river level, but there's a check valve on the outside. This pumping station, in operation since 1933, drains about 160 km2, about half of it in Germany (hence the name), so a single pump can handle a precipitation rate of around 15 mm/week. There are 4 pumps, giving some redundancy. PiusImpavidus (talk) 18:41, 25 November 2022 (UTC)
 * Is this true considering the Pump Laws head is proportional to the square of shaft speed and power is proportional to the cube of shaft speed for centrifugal pumps? If looking at the power applied to the pump shaft, wouldn't doubling the head require 23/2 or ~2.83 times the power? fiveby(zero) 17:23, 25 November 2022 (UTC)
 * I don't know these laws, but increasing shaft speed would also increase the volume pumped. Graeme Bartlett (talk) 21:59, 27 November 2022 (UTC)

Mirrors.
What is the process of converting glass to mirrors, and mirrors back to glass? I'm assuming mirrors are made from glass as a precursor. 67.165.185.178 (talk) 21:20, 24 November 2022 (UTC).
 * The typical mirror I've seen is just a piece of glass with a thin coating of silver on the back. It's the silver that provides the reflection. You might find out more in the Mirror article. ←Baseball Bugs What's up, Doc? carrots→ 23:18, 24 November 2022 (UTC)
 * Also commonly aluminium - slightly less reflectance but cheaper and less liable to tarnish.--Phil Holmes (talk) 08:36, 25 November 2022 (UTC)
 * If the reflective metal tarnishes, does that actually make a difference? Won't the glass itself protect the "working" surface of the metal? --174.89.144.126 (talk) 08:49, 25 November 2022 (UTC)
 * Not in practice. I would guess that gases get into the mirror from the side and tarnish the silver.  If you do an internet search on tarnished mirror you will see plenty of examples.  I took a photo of one of our mirrors with tarnish.  See the thumbnail image. Tarnished mirror.jpg--Phil Holmes (talk) 12:08, 25 November 2022 (UTC)
 * Have you read our articles mirror, silvering and Tollens' reagent? Martin of Sheffield (talk) 08:44, 25 November 2022 (UTC)
 * The 1st 2 articles. So if I just took a piece of silver or aluminum and put it behind glass, it is now a mirror? 67.165.185.178 (talk) 13:37, 25 November 2022 (UTC).
 * Basically, yes. The glass is a transparent support and seal, the actual mirror is the bright untarnished metal that reflects the light.  Just putting the bulk metal behind glass though will not produce a good mirror, there may be surface imperfections and without an adequate seal air will get in and tarnish the reflective surface.  Fifty years ago it used to be (maybe still is) a standard school chemistry experiment to make up silvering solution and apply it to a microscope slide.  You ended up with a fragile mirror.  Industrially the backing is then built up to protect the microscopically thin metal layer from scratches and wear. Martin of Sheffield (talk) 13:56, 25 November 2022 (UTC)
 * Then what's used to glue the metal and glass? Can I try this with aluminum foil? 67.165.185.178 (talk) 14:00, 25 November 2022 (UTC).
 * You don't need our permission. And you could pull out some foil and see if it reflects properly. The glass isn't needed except to provide sturdiness. ←Baseball Bugs What's up, Doc? carrots→ 14:25, 25 November 2022 (UTC)
 * See Silvering. Most aluminium foil these days seems to have a pattern embossed on it, so you will not get a good mirror that way.  Try it as an experiment if you want, sandwich the foil between two sheets of glass or glass and some MDF, but frankly it will be cheaper and easier just to buy mirrored glass. Martin of Sheffield (talk) 14:31, 25 November 2022 (UTC)
 * MDF is the glue? Because I was thinking the glue must also be transparent. 67.165.185.178 (talk) 15:06, 25 November 2022 (UTC).
 * No. The sandwich consists of glass, foil and something to keep the foil pressed against the glass, such as another sheet of glass, MDF or anything reasonably flat.  There is no glue.  Read the articles quoted and when you've understood them, then come back here. Martin of Sheffield (talk) 15:14, 25 November 2022 (UTC)
 * Ah, so silver is the glue. Via a CVD process. 67.165.185.178 (talk) 15:49, 25 November 2022 (UTC).
 * No, there is no spoon glue. Unless you lay the whole thing flat, you need something to clamp the layers together though, which could be just your hands. The only thing that is important is to keep the aluminium smooth, without wrinkles. For most metals, any object of the material with a smooth polished surface works as a mirror; no glass is needed. --Lambiam 19:24, 25 November 2022 (UTC)


 * Note, OP, that you have introduced an artificial situation. Actual mirrors are created by the methods described in the two articles linked by above, where the deposited reflective material bonds to the glass at an atomic level (creating a very even and reflective surface). It was you who introduced the ad hoc comparison of placing a piece of pre-existent aluminium behind a sheet of glass: while this corresponds to the broad set-up, it is not how useful mirrors are actually made.
 * If you actually want to make a mirror at home, use chemicals as described: the process is not difficult – amateur astronomers have been silvering their own telescope mirrors for centuries. In that instance the silver is deposited on the front surface of the glass, not the rear, so it tarnishes quite quickly and has to be regularly re-done. Nowadays, however, most amateurs send their mirrors to professional companies who use vacuum deposition of Aluminium, which tarnishes much more slowly – this is likely beyond the capabilities of home hobbyists. {The poster formerly known as 87.81.230.195} 176.249.29.80 (talk) 08:31, 28 November 2022 (UTC)