Wikipedia:Reference desk/Science

Welcome to the science section
of the Wikipedia reference desk.

skip to bottom

Select a section:

• Computing
• Entertainment
• Humanities
• Language
• Mathematics
• Science
• Miscellaneous
• Archives

Want a faster answer?

Main page: Help searching Wikipedia

   

How can I get my question answered?

• Select the section of the desk that best fits the general topic of your question (see the navigation column to the right).
• Post your question to only one section, providing a short header that gives the topic of your question.
• Type '~~~~' (that is, four tilde characters) at the end – this signs and dates your contribution so we know who wrote what and when.
• Don't post personal contact information – it will be removed. Any answers will be provided here.
• Please be as specific as possible, and include all relevant context – the usefulness of answers may depend on the context.
• Note:
  • We don't answer (and may remove) questions that require medical diagnosis or legal advice.
  • We don't answer requests for opinions, predictions or debate.
  • We don't do your homework for you, though we'll help you past the stuck point.
  • We don't conduct original research or provide a free source of ideas, but we'll help you find information you need.

Ready? Ask a new question!

How do I answer a question?

Main page: Wikipedia:Reference desk/Guidelines

• The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.

See also:

• Teahouse
• Help desk
• Village pump
• Help manual

September 21

Bouncing upwards from walls

In various Super Mario Maker levels I have watched on YouTube, Mario can jump seemingly arbitrarily long distances upwards if he's inside a canyon between two walls, simply by hitting each wall in turn and bouncing off it. Surely this can't be possible in real life, even discounting any damage hitting a wall does to Mario's body? JIP | Talk 02:12, 21 September 2021 (UTC)

Wall jumping is a well established video game tradition. It appears in many games.

But can it be done in real life? Well, obviously nobody can jump four times their own height like Mario does, but could it be done with normal human-scale jumps?

A single wall-jump is a pretty common parkour move, but there are a few YouTube videos that purport to show repeated wall-jumps up a narrow gap. (example) Whether these are real or not is another question. It doesn't seem impossible, but I sure couldn't do it. ApLundell (talk) 03:06, 21 September 2021 (UTC)

The laws of physics allow this. How high a human can jump in a simple jump is determined by the kinetic energy that can be reached at the instant of launch, which is limited by human physiology. In the parkour figure additional energy can be input at each bounce (assuming enough friction), so in principle the athlete can jump higher than they could do in a simple high jump. There is again a limit, but one that is more like the limit in how fast a runner can run (see Running § Limits of speed), where the issue is the delivery of a sustained burst of energy.  --Lambiam 10:21, 21 September 2021 (UTC)

Physics allows it. Suppose the horizontal component of the velocity of the jumper with mass $m$ alternates between $-v_{x}$ and $v_{x}$. At every bounce the jumper exchanges a perpendicular impulse $J_{\perp }=\pm 2v_{x}m$ with a wall. While exchanging this impulse with the wall, there is a normal force acting beteen the jumper and the wall, which allows friction to exchange some impulse in a direction parallel to the wall. This parallel impulse has a magnitude of at most $J_{\parallel ,lim}=2v_{x}m\mu$, with $\mu$ the coefficient of friction. The jumper moves back and forth over a distance of $d$, which is slightly less than the separation between the walls as the jumper has finite size. The time between two bounces is $T={\frac {d}{v_{x}}}$, giving a limiting average vertical acceleration of $a_{lim}={\frac {J_{\parallel ,lim}}{mT}}={\frac {2v_{x}^{2}\mu }{d}}$. As long as $a_{lim}\geq g$, with $g$ the acceleration of gravity, it is possible to climb the wall arbitrarily high (but as exhaustion sets in, your $v_{x}$ will drop). Plugging in some realistic numbers, suppose you can keep your horizontal speed at 3m/s, moving back and forth over 3m (bouncing once per second) and have a friction coefficient of 0.7, you can handle an acceleration of 4.2m/s². That's enough to do this on the Moon, but not on Earth. I guess a proper athlete could do this on Earth though. PiusImpavidus (talk) 10:56, 21 September 2021 (UTC)

• Wall jumping by real people can be done with some difficulty. It is done in parkour and freerunning. See this video for the process one man does to learn the discipline. --Jayron32 15:13, 21 September 2021 (UTC) Ed: Never mind. I'm obviously an idiot. Pay no attention to me. --Jayron32 16:14, 21 September 2021 (UTC)

That's the same video I posted. Except that version looks like it's been copied off youtube, posted on Instagram in a different aspect ratio, then copied off that site and re-uploaded to YouTube. (And re-compressed each time.) ApLundell (talk) 15:18, 21 September 2021 (UTC)

Is it necessary to take bullet out off human body?

In movies, doctors struggle to take out bullets in human bodies. But there is a reality case, where the man lived with a bullet in head for years. Source. Is it necessary to take bullet out off human body? Rizosome (talk) 07:14, 21 September 2021 (UTC)

• Doctors prefer to remove the bullet or bullet fragments, as they could cause infection or other trouble. They may elect to leave one in if the procedure to remove it is more risky than leaving it in. This is commonplace, and sometimes the bullet works itself out of the body on its own, like a splinter. Abductive (reasoning) 09:32, 21 September 2021 (UTC)

(ec) Sometimes it's too dangerous to try to remove them, e.g. They Survived Mass Shootings. Now They Are Living With Bullets Inside Them. (The New York Times), They Survived Mass Shootings. Years Later, The Bullets Are Still Trying to Kill Them (lead poisoning, Time) Clarityfiend (talk) 09:40, 21 September 2021 (UTC)

More on lead poisoning. https://pubmed.ncbi.nlm.nih.gov/8057396/ 41.165.67.114 (talk) 12:09, 21 September 2021 (UTC)

Did we just assume bullets are made of lead? 67.165.185.178 (talk) 12:56, 21 September 2021 (UTC).

They often are. But regardless of what they're made of, they're unlikely to be sterile when they enter the body. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:05, 21 September 2021 (UTC)

We believe that in GSW [gunshot wounds] of the spine, retained bullets do not increase the likelihood of septic complications. [1] Alansplodge (talk) 14:21, 21 September 2021 (UTC)

There are no standard medical guidelines regarding bullet removal and the full extent of the consequences of RBF [retained bullet fragments] remains unknown... Meta-analysis demonstrated BLL (blood lead level) significantly higher in individuals with RBF as compared to controls. [2] Alansplodge (talk) 14:21, 21 September 2021 (UTC)

• Theodore Roosevelt carried a bullet inside him for the last seven years of his life, after his failed assassination attempt. -- Jack of Oz ^{[pleasantries]} 22:22, 21 September 2021 (UTC)

That's a lie! Every person Teddy tried to eradicate ended up dead. Clarityfiend (talk) 10:35, 22 September 2021 (UTC)

September 22

Measure oxygen concentration

I want to test an oxygen concentrator for purity of the oxygen coming out. Any idea how to do that? It gives a slow, low pressure stream of oxygen coming out of a tube. Holding a lit match near the output makes the flame flare up a little, but it's not dramatic. Is there another simple test? Thanks. 67.164.113.165 (talk) 03:21, 22 September 2021 (UTC)

I once worked for a company where there was an instrument that measured that, for O2 and CO2. It had a thin needle to measure, and ran for a few seconds, then outputted a number. They said the instrument was worth $5,000. Maybe you can search on-line for such an instrument, but I'm not the right person to help get you started. 67.165.185.178 (talk) 07:50, 22 September 2021 (UTC).

A hand-held scuba nitrox analyser will give you the Oxygen fraction to within about 0.1% and you can probably find one for around €120. If you're handy with a soldering iron you could make your own for a fraction of that. 2A01:E34:EF5E:4640:A028:DB79:13E4:2403 (talk) 12:20, 22 September 2021 (UTC)

The article that discusses the equipment is oxygen analyser. There seems to be plenty of choice. Mike Turnbull (talk) 15:07, 22 September 2021 (UTC)

Those are indeed the general type of equipment. The specific analysers shown in the article are trimix analysers which also measure the fraction of helium, making them much more expensive creatures though. As discussed in the article, the underlying principle is to measure the current output of a galvanic cell across a load resistor, so if you're not too worried about high precision you can just use a voltmeter. 2A01:E34:EF5E:4640:A028:DB79:13E4:2403 (talk) 15:16, 22 September 2021 (UTC)

Thanks, I found a few articles about DIY nitrox analyzers for scuba, whose main ingredient appears to be an O2 sensor that costs around $80. I'll look around for less expensive ones. They have a shelf life, so maybe a dive shop could have some expired ones around. You are right that I don't need a precise measurement. 2601:648:8202:350:0:0:0:1598 (talk) 18:37, 22 September 2021 (UTC)

A good source for used cells is rebreather divers as they change their cells well before the notional expiry. A dive shop will probably be able to put you in touch.

At the O2 partial pressures you're looking at, even a used cell is really very linear so the minimal circuit is really nothing more than a load resistor and a voltmeter. You'll need to calibrate with two known gases (typically air and pure Oxygen), ideally before each use.2A01:E34:EF5E:4640:3D07:C0BA:6A0F:BFCF (talk) 20:13, 22 September 2021 (UTC)

Thanks, I'll have to figure out where to get pure oxygen in this case. I remember separating out hydrogen by water electrolysis in school, but not oxygen. Come to think of it, I have no idea where the oxygen goes when the hydrogen bubbles up. I guess there are places that sell oxygen in cans though. 2601:648:8202:350:0:0:0:1598 (talk) 21:10, 22 September 2021 (UTC)

You may be able to get hydrogen peroxide or peroxide bleach, and bubble off some fairly pure oxygen gas, if you don't want to purchase an oxygen cylinder. It may still contain water vapour or traces of atmosphere. Graeme Bartlett (talk) 12:45, 24 September 2021 (UTC)

You could also measure oxygen by capturing the gas above water, and then oxidising something that will use up the oxygen. eg rusting iron, or burning magnesium. Then measure how much volume has reduced once temperature returns to normal. Burning things that make gas will not change the volume that simply. Graeme Bartlett (talk) 12:48, 24 September 2021 (UTC)

Making steel from Fe(II) instead of Fe(III).

Steel is currently Fe(III). What would be the problem for trying to make steel from Fe(II). Need a counter-anion? Sure, then make a counter-anion. Would that work? What would be the problem for trying to make steel from Fe(0) to Fe(II)? 67.165.185.178 (talk) 07:52, 22 September 2021 (UTC).

Steel is not Fe(III). I'm not sure where you got that from. Steel contains atomic iron, Fe(0) if you will. It is an alloy of iron and carbon and sometimes other trace metals. Alloys are not ionic compounds, they are elemental mixtures. The rest of your question cannot be answered because it was based on a false premise. --Jayron32 12:01, 22 September 2021 (UTC)

On the other hand, steel ultimately comes from iron ore and as that article suggests some of the ore is indeed Fe(II) (for example siderite) while others are Fe(III) (for example hematite). Whatever the ultimate source of the iron, the process involves chemical reduction to Fe(0) by smelting or direct reduction. The standard reducing agent is carbon, normally in the form of coke, as this is cheap and readily available. A proportion of the carbon may be left in the iron, forming steel. Mike Turnbull (talk) 14:59, 22 September 2021 (UTC)

It looks like the OP has changed their question. Fe(II) is an ion and does not exist in nature unless it is part of an ionic compound, like iron (II) chloride or iron (II) sulfate, or the aforementioned siderate (iron (II)) carbonate). Ionic compounds are not metallic in nature, and would not have any of the properties of neutral metals. The oxidizing of Fe(0) to its ionic forms such as Fe(II) and Fe(III) is a common reaction, it creates rust. So, if you convert the Fe(0) to Fe(II), you don't get a different form of steel, you get a black, crumbly powder such as iron (II) oxide. --Jayron32 15:50, 22 September 2021 (UTC)

So you guys say iron(II) does not exist in nature except with ionic compounds, or did you just mean in liquid solution? I was thinking of carbon monoxide binding to the Fe(II) in hemoglobin, at a 210x greater rate than O2. If there is CO next to siderite, it won't be able to break the FeCO3 bond so Fe(II) reacts with CO like it does in the body? 67.165.185.178 (talk) 08:49, 24 September 2021 (UTC).

No, iron (II) is not a metal. It is an ion, and ions are ONLY part of ionic compounds, whether solid or in solution. If you react iron (II) with carbon monoxide, you'll likely get a complex ion, like Fe(CO)₄²⁺, known as the tetracarbonyl iron (II) ion, which I can find some examples of in literature. This is still not metallic and still will not form as part of an alloy. --Jayron32 11:11, 24 September 2021 (UTC)

Sneezing into a respirator

After an open-mouthed sneeze into a respirator, does it need to be replaced right away or at the end of the day? Thank you. Imagine Reason (talk) 12:57, 22 September 2021 (UTC)

Maybe it depends on how bad the sneeze was? Other than that, wouldn't you just be inhaling your own germs anyway? ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:27, 22 September 2021 (UTC)

One issue could be expelled fluid messing up the electrostatic layer in the respirator. I don't know how that would mess it up more than moisture buildup from normal exhaled air already would though. They do always tell you not to wash and re-use respirators. Another issue in a medical context might be that the respirator is full of germs that you are now exhaling onto a surgery patient or something. 2601:648:8202:350:0:0:0:1598 (talk) 18:40, 22 September 2021 (UTC)

Yes, that's what I'm concerned about, whether it counts as the respirator getting wet. It is, ounce for ounce, more water than you'd expect from breath, no? It's not in a medical context. Imagine Reason (talk) 23:23, 22 September 2021 (UTC)

I'm not sure but I think the damage from washing the electrostatic layer may come from exposure to soap, rather than moisture per se. I know that during the big N95 shortage, medical people kept re-using them for weeks. There was at least some evidence that they stayed effective through re-use. Heating them in an oven (autoclave?) was also supposed to be an ok way to sterilize them. 2601:648:8202:350:0:0:0:1598 (talk) 18:44, 24 September 2021 (UTC)

Thousands of doctors and nurses were infected last year, while many places in East Asia, like Hong Kong, Taiwan, and Singapore, saw zero such infections. I've read accounts of medical professionals getting infected after reusing masks. The CDC has found that they should be discarded after five donnings because of fit. No, it's not just soap that damages respirators. Water will do. It's advised that you discard them after they get wet. My question is, what constitutes getting wet. Imagine Reason (talk) 01:27, 26 September 2021 (UTC)

September 23

Why ammeter get damaged if connected in parallel?

Whole point of Ammeter is to measure current, so why it get damaged if connected in parallel? If a bulb withstand parallel connection, why can't this measure device? I know that it should be low resistance. Rizosome (talk) 02:21, 23 September 2021 (UTC)

I googled the subject. This article contains an answer.[3] ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:47, 23 September 2021 (UTC)

It says whatever connection is, ammeter will get damage: an ammeter connected in series or connected in parallel with a electrical circuit will probably blow the fuse, which can possibly damage the ammeter and cause injury. Rizosome (talk) 04:11, 23 September 2021 (UTC)

You are right; that is a rather curious mistake in the article. Where they write that the ammeter needs to be inserted into the circuit and become part of it, they clearly mean it needs to be connected in series with the other components of the circuit. Using the formulas given in Series and parallel circuits, it is easy to see that when connected in parallel the current in the ammeter will be very high, which may blow a fuse but will likely blow the ammeter before the fuse gets a chance. If the fuse blows first, it actually protects the ammeter. When connected in series, the current in the ammeter will be slightly lower than the current in the circuit (at the place of insertion) before it was inserted.  --Lambiam 05:30, 23 September 2021 (UTC)

Please specify: ...gets damaged if connected (into what circuit?) in parallel (to which part of the circuit?). The ammeter is designed to measure current. so it needs to be in the branch of the circuit in which you want the current measured. If you connect it directly into the supply, you make a circuit of a source and a meter. Ammeters should have minimum resistance (so they do not influence much the current they measure) so the current, equal voltage divided by (tiny) resistance, gets really big. --CiaPan (talk) 07:46, 23 September 2021 (UTC)

It will likely get damaged beyond repair if connected in parallel to the whole circuit; in the simple case of a one-resistor circuit (say a lamp) in parallel to that resistor. If there are several resistors, it may be harmless to connect it in parallel to one of them, but it never makes sense to connect an ammeter other than by serial insertion.  --Lambiam 12:11, 23 September 2021 (UTC)

Kirchhoff's circuit laws seems relevant background here regarding the measurement of voltage and current in a branching circuit. --Jayron32 15:32, 23 September 2021 (UTC)

As ammeter notes, an ammeter usually has low resistance so that it does not cause a significant voltage drop in the circuit being measured; this low resistance connected in parallel with elements with higher resistance, effectively causes a short circuit, with high current flowing through the ammeter. Klbrain (talk) 23:02, 24 September 2021 (UTC)

Bushy and straggly eyebrows

I've worn a beard for more than half my life, but otherwise I'm not noticeably hairy. Except, my eyebrows are taking on a life of their own. This only started happening in my mid-60s (I'm now 70).

• Why does this happen only at such a time in men's lives?
• Why does it happen to some men and not others?
• Why does it not happen to women (not that I've ever noticed, anyway)?

One other thing: I quite like the look of prominent bushy eyebrows on a senior gentleman, but mine are more straggly than bushy, so I need to keep trimming them. Is there anything I can do to promote bushiness rather than straggledom? -- Jack of Oz ^{[pleasantries]} 21:44, 23 September 2021 (UTC)

Re your third bullet point – many women in Western and other cultures routinely pluck, wax, thread and otherwise groom and modify their eyebrows to a far greater extent than men. {The poster formerly known as 87.81.230.195} 90.200.67.3 (talk) 23:52, 23 September 2021 (UTC)

Here is an article addressing some of these issues.  --Lambiam 09:30, 24 September 2021 (UTC)

• Just your eyebrows? Hahaha. Wait till your ears start getting hobbity and your nose sprouts wings. My ears have four different hairy places. Top of my head's a cue ball, mind you. --jpgordon^{𝄢𝄆𝄐𝄇} 00:22, 25 September 2021 (UTC)

  No, I have developed ear and nose hair as well, but they don't need regular trimming. -- Jack of Oz ^{[pleasantries]} 20:18, 25 September 2021 (UTC)

  Why fight it? Embrace your inner Andy Devine! ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:16, 25 September 2021 (UTC)

  I'm not fighting it per se. I'd be very happy to wear bushes above my eyes, but nature has seen fit to give me unsightly weeds. -- Jack of Oz ^{[pleasantries]} 21:23, 25 September 2021 (UTC)

For what it may be worth (there are a couple of relevant links), here's an old ref-desk thread on the old-age-hairiness question. Deor (talk) 14:55, 25 September 2021 (UTC)

I once mentioned to a friend that I trim my eyebrows. He was aghast. So for a couple of months I stopped trimming my right eyebrow, and the hairs ended up long enough that I could pull them over my eye down to the top of my cheek! I was in my mid-30s at the time. I'll turn 44 in a few days. I'm tempted to let them grow again to see how long they get. nagualdesign 19:29, 26 September 2021 (UTC)

September 24

Why can't I feel sucking when standing behind table fan?

If front side of the fan feeling like blowing, why cant it gives me the same sucking feel when standing behind? Is it fan blade design is the factor here?Rizosome (talk) 02:25, 24 September 2021 (UTC)

The fan imparts linear momentum in a specific direction to the air that is blown out. It does not do that to the air at the other side in anticipation of it going to be sucked in. The air is equally being sucked in at the backside of the fan from all directions that lead to the spinning blades, which means that most of it comes in sideways.  --Lambiam 09:23, 24 September 2021 (UTC)

There will still be a low vacuum behind the fan. Try dangling a piece of paper near there, for example. 2601:648:8202:350:0:0:0:1598 (talk) 18:46, 24 September 2021 (UTC)

Deflection of a dangling piece of paper will merely show the presence and direction of the horizontal component of a local airflow. There is definitely no vacuum behind the fan, unless it is operating in a vacuum – in which case there is no airflow at all. To the extent the pressure is locally lower, this is almost entirely due to Bernoulli's principle. With an upright table fan operating in normal conditions, one can easily demonstrate the airflow behind the fan. This will show it as converging in from all directions. If a source of smoke is available, it may give the best demonstration.  --Lambiam 11:12, 25 September 2021 (UTC)

Is the airflow turbulent, thus rendering Bernoulli inapplicable? Imagine Reason (talk) 12:15, 25 September 2021 (UTC)

It is true that the velocity of the air approaching the fan is slower than the air departing from the fan. Wikipedia has an article on the matter. See Momentum theory which also addresses the difference in pressure upstream and downstream from the fan. Dolphin (t) 14:09, 25 September 2021 (UTC)

@User:Lambian, are you implying that the air flow becomes symmetrical between back and front as Re tends to zero? catslash (talk) 17:04, 25 September 2021 (UTC)

I'm not trying to imply that. A table fan is essentially a propeller, which generates thrust (a force in one specific direction) by exerting force on the air in the other direction. If you mount the fan on a wheeled platform, it may act as a propeller and push the system backwards. But under normal operating conditions, friction keeps the fan from sliding off the table. The force on the air is only imparted near the spinning blades and gives it momentum in the opposite direction of the (reactive) thrust. The momentum carries the air stream forward over possibly a considerable distance. There is no operating principle behind the fan to give the air momentum in a single direction. The air flow cannot be expected to tend to become symmetrical until the blades stop spinning.  --Lambiam 22:01, 25 September 2021 (UTC)

@User:Lambian, if the flow is not symmetric when inertia can be neglected, then momentum is not the cause of the asymmetry. catslash (talk) 20:32, 27 September 2021 (UTC)

If you put a box fan in a window, facing outward, and open some other windows, the fan will pull some air through the house. Just not with the kind of force imparted outward by the blades. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:53, 25 September 2021 (UTC)

If you stand behind a fan, or place your hand behind it, you will block the airflow, so it will pull air from elsewhere. But if you stand back and blow smoke at it you'll see that it does indeed pull air predominantly from the rear. nagualdesign 19:23, 26 September 2021 (UTC)

From the rear, but not preferentially in a direction parallel to the axle of the fan. If the source of the smoke is to the side, it will move in sideways. But it is blown out in a jet of air with a preferred direction before it disperses.  --Lambiam 10:04, 27 September 2021 (UTC)

September 26

How a hard fruit can improve dental health?

If Sugarcane is a hard fruit, why it recommded for dental patients? Because of their hardness, isn't that make dental conditon worse? Rizosome (talk) 03:57, 26 September 2021 (UTC)

It's not a fruit and I don't see evidence that it's "recommended for dental patients". --184.144.99.72 (talk) 06:37, 26 September 2021 (UTC)

Google "sugar cane effect on teeth" and you'll see mixed opinions. ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:24, 26 September 2021 (UTC)

In general, eating whole foods will improve dental health, simply because most people's diet include a substantial amount of non-whole foods which have adverse health effects including for dental health. Homo Sapiens like all other animals has evolved under the condition where it was exclusively eating whole foods at a time where there were no toothbrushes, no toothpaste, no dentists, etc. Count Iblis (talk) 09:59, 26 September 2021 (UTC)

Eating more sugary foods, like fruits, led to an increase in rotten teeth. Eating whole foods won't save you from dental decay. Imagine Reason (talk) 12:04, 26 September 2021 (UTC)

See here:

An “apple a day” is also great for teeth too. Although not a substitute for brushing and flossing, eating an apple or other fibrus fruits like oranges, carrots or celery can help clean your teeth and increases salivation, which can neutralize the citric and malic acids left behind in your mouth. And while sugary apple juice may contribute to tooth decay, fresh apples are less likely to cause problems. This is because chewing the fibrous texture of apples stimulates your gums, further reducing cavity-causing bacteria and increasing saliva flow. And more saliva is good, because it decreases acidity in your mouth and washes away particles of food that lead to decay. Be sure your diet includes citrus and other fresh fruits rich in vitamin C, such as apples, pears, strawberries, pineapples, tomatoes and cucumbers -- all rich in vitamin C.

Count Iblis (talk) 13:36, 26 September 2021 (UTC)

Have you looked at the credentials for a dentistry school vs. that for an paleontologist? Imagine Reason (talk) 16:44, 26 September 2021 (UTC)

Or a biologist. [4] Imagine Reason (talk) 16:50, 26 September 2021 (UTC)

Or a random Wikipedian? 2603:6081:1C00:1187:DD45:E1CD:EA6A:4AD0 (talk) 17:36, 26 September 2021 (UTC)

Which is why we don't provide medical device. --OuroborosCobra (talk) 22:19, 26 September 2021 (UTC)

September 27

Why bulbs are sell in terms of watts instead of amperes or voltage?

Why bulbs are sell in terms of watts instead of amperes or voltage?

If laptop chargers are sell in terms of voltage and ampere: here, but why bulbs are sell in terms of watts like hereinstead of amperes or voltage? Rizosome (talk) 04:29, 27 September 2021 (UTC)

The wattage tells you the relative brightness of the light. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:31, 27 September 2021 (UTC)

Brightness, or, rather, luminous power, is measured in lumen (unit). A 25 W incandescent light bulb will typically deliver something like 375 lm. A 25 W LED bulb will deliver more like 2000 lm. The wattage tells you the energy consumption, which for incandescent bulbs is mainly wasted on heat.  --Lambiam 09:54, 27 September 2021 (UTC)

Sure. But experience tells us that the higher the wattage of an ordinary incandescent bulb, the brighter it is. And we select wattage based on how bright we want it. 60 watts is typical. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:55, 27 September 2021 (UTC)

And how much electricity it uses. Chargers have many different output voltages, so it is important to be able to select the correct one for your appliance.--Shantavira|^{feed me} 08:31, 27 September 2021 (UTC)

• Light bulbs use the main supply, which is a fixed-voltage, as-is; while its exact source depends on the country it usually is roughly 230V of AC current at 50 or 60Hz. As a consequence, in a given country, rating light bulbs by intensity or by wattage is equivalent, and there is no voltage rating to give since you have no choice as to what voltage you use at home.

There is also another complication linked with the different technologies of light bulbs. The power a bulb consumes is always equal to voltage times intensity (root mean square-d for AC current) whatever the technology, but the efficiency by which they convert this into light depends on whether they are LEDs, incadescent bulbs, phosphor, halogen lamps etc. I used to see many bulbs marketed as "X watts, but equivalent to Y watts of incandescence" (with the Y value given much more prominence than the X value on the packaging) five years ago - I assume the manufacturers thought that a consumer trying to replace a 60W incandescent bulb would not buy an LED rated 10W "because I do not want something six times dimmer".

Laptop chargers, or any appliance that uses DC voltage from the AC supply, are a different thing entirely. They modify the current supply from AC to DC and also change the voltage. As a consequence, they have to specify their output more fully, so that (for instance) you can check if the charger you got by buying laptop A is compatible with laptop B. Taking as an example the image you linked to, INPUT: 100-240V~2.34A50-50Hz means in order

100-240V: "will accept voltages between 100V and 240V" (i.e. almost every grid system around the world)

~: "of alternating current"

2.34A: "might draw up to 2.34A from the supply"

50-60Hz: "will accept frequencies between 50 and 60 Hz"

and similarly OUTPUT: 19.5V⎓9.23A means

19.5V: "will output 19.5V"

⎓: "of direct current"

9.23A: "from which you can draw up to 9.23A"

In this case, the wattage of that charger is about 180W (19.5V times 9.23A), if used with a compatible laptop (that draws the maximum power). Tigraan^{Click here for my talk page ("private" contact)} 09:18, 27 September 2021 (UTC)

Note that in some parts of the world where incandescent bulbs are no longer available, their LED equivalents are sold with a lumen rating rather than unhelpful wattage. E.g. [5] Bazza (talk) 09:44, 27 September 2021 (UTC)

September 28

How can a petrol burn slowly in vehicle engine?

If I burn petrol, then it fire quickly. But in engine, how does petrol burn slowly? Is it Piston ring behind this? Rizosome (talk) 06:56, 28 September 2021 (UTC)

It doesn't burn slowly. It ignites within a sealed combustion chamber. Internal combustion engines are designed so that only measured quantities of fuel are injected into and ignited within each combustion cycle, thus preventing rapid combustion of the entire fuel stored in the tank. PaleCloudedWhite (talk) 07:31, 28 September 2021 (UTC)

There is no answer to 'how' it burns slowly because it does not burn slowly. It rather explodes. And a pressure caused by the explosion pushes the piston, which makes a useful force. For more explanation see Internal combustion engine (and then possibly External combustion engine, too.) For even more details, see Spark-ignition engine with Ignition system and Diesel engine. --CiaPan (talk) 11:07, 28 September 2021 (UTC)

It does not explode. There is a distinct difference between an explosion and combustion. 41.165.67.114 (talk) 11:55, 28 September 2021 (UTC)

"Combustion is produced by the reaction of oxygen and some sort of fuel at high temperatures" and "Detonation is a process of intramolecular breakdown." See https://cs.stanford.edu/people/eroberts/courses/ww2/projects/firebombing/detonation-and-combustion.htm 41.165.67.114 (talk) 11:59, 28 September 2021 (UTC)