Wikipedia:Reference desk/Archives/Science/2015 March 23

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March 23

Using a multimeter

I got one multimeter like this: http://image.made-in-china.com/2f0j00NMbtZHvdwquF/YX-1000A-Analog-Multimeter.jpg, but unhappily no instructions where by it. There were two cables with it (red and black). I'm aware this is one bottom of the market device, and have no interest in connecting it to AC, nor I believe it was developed for it.

There are instructions online, but they seems to be more about fancy devices, digital and all. Can someone explain a couple of basic functions of it? Like checking a 1.5V battery, and other random stuff that can be performer with stuff you find at home (batteries, broken electronics, light bulb)

These are the possible setting:

Ω (X10, X100, X1K)? I suppose this is resistance. DCmA (0.5 50 250) DCV (10 50 250 1000) ACV (10 50 250 100) There is also a wheel at the left side that cannot be seen on the picture. --YX-1000A (talk) 00:28, 23 March 2015 (UTC)

I believe you would use one of the DCV settings to measure voltage on a DC device, like a battery. I believe the number there would be the maximum on the scale, so, with a 1.5V battery you would want the 10V setting.

Also note that it's possible to damage the reader or device by using the wrong setting, so be sure you figure it all out ahead of time, rather than experimenting. StuRat (talk) 01:05, 23 March 2015 (UTC)

(after edit conflict) For DC mA, DC V, and AC V, the number associated with each range setting is the maximum value corresponding to a full-scale deflection of the needle. There are several graduated lines on the face of the meter, each is for a different type of measurement. From farthest from the base of the needle inward: (outermost) resistance, (next inward) DC voltage or current (depending on range setting), (next inward) AC voltage. I think the innermost scale is for using the meter as a VU meter, but I haven't used an analog multimeter for that purpose, so I'm not 100% sure.

For the resistance measurement ranges, the number associated with the range setting is a multiplier. Say you're measuring the resistance of something, the setting is "X10" and the reading is 4. The resistance is 4 x 10 = 40 ohms. The dial on the left of the meter is for calibration when using the resistance measurement settings. After you choose the range, say "X10" or "X100", you touch the two measurement probes together. The needle should deflect all the way to the right (or very close). The resistance should read 0 at that point. You adjust the dial so that the meter indeed reads 0. When you switch to a different resistance measurement range, you will need to fuss with the dial again. There's a mirror-like line right below the resistance scale. It's there to help you read measurements, if you see the needle aligned with its mirror image, you're reading the meter from the correct position.

--173.49.16.112 (talk) 01:24, 23 March 2015 (UTC)

Note that, to measure resistance, the meter needs a battery - one AA cell. See this YouTube video for how to install the battery (and set up the resistance measurement). Tevildo (talk) 01:27, 23 March 2015 (UTC)

Despite being a basic model, your multimeter can measure AC. Note that the measured value is supposed to be the RMS value of the voltage, based on the assumption that the waveform is sinusoidal. A meter like that will NOT measure the true RMS value of arbitrary AC waveform.

I think a proper battery tester tests batteries under load. Your multimeter doesn't have a setting for testing batteries, so it's not well suited for testing batteries (as a voltmeter it doesn't put much load on the battery under test). Fresh 1.5V batteries have a voltage higher than the nominal value (something like 1.65V in my experience, if I remember it right). If the measured voltage is below 1.5V, it's at least partially spent. It may still work for some time if the device consumes very little power (like a calculator or a clock). Generally I'd throw away anything that measures below 1.4V. You can use the resistance measurement setting as a continuity tester, to see if an incandescent bulb is burnt or not. --173.49.16.112 (talk) 03:29, 23 March 2015 (UTC)

I mostly use my multimeter in the resistance-measuring mode to perform continuity tests, particularly on circuits that aren't working, to find out which circuit element contains the discontinuity! It is also useful for checking circuit elements before inserting them in a circuit, to confirm they are working. The resistance-measuring mode is the one that requires a AA cell. Dolphin (t) 06:21, 23 March 2015 (UTC)

It's a long time since I've seen a 'wheel' on a meter, it is used to zero the resistance measurement when you short the two leads together. Most of the above advice is not misleading, tho I expect it'll need a 9V battery for resistance. It is an extremely cheap and nasty meter, but perfectly useful, and probably quite accurate. Incidentally note that the current capacity is 1/4 A DC, essentially useless.Greglocock (talk) 06:50, 23 March 2015 (UTC)

I agree a 9V battery is probably more likely for the resistance measurement. Either way though, it's generally a good idea to turn it off the resistance measurement mode when not in use as that will potentially run down the battery. Nil Einne (talk) 11:29, 23 March 2015 (UTC)

Old TV Picture "Flips" for Several Minutes After Startup

The above question about older TVs reminds me of something I experienced in the early 1980s. We had a television housed in a large wooden structure that sat on the floor. It was BIG! After a few years of ownership, I noticed that when you turned it on, the picture would flip continuously for several minutes. By "flip", I mean that it looked like the channel was changing, although it stayed on the same channel. The "flip time" started out very fast (say once per 1/2 second), then would slow down to once every several seconds, and finally would disappear entirely after several minutes. I found that I could cut down on the time it took to stabilize by turning the TV off and on repeatedly. I hadn't thought about this in a long time, but the above question jogged my memory, and now I'm curious: does anyone know what would cause something like this? Thanks! OldTimeNESter (talk) 15:42, 23 March 2015 (UTC)

If the image is rotating, with a black bar between the top and bottom, possibly with pulsating white dots in the bar, then that means the horizontal hold is slightly out, and you are seeing the horizontal blanking interval information. Non-CPU controlled CRTs can do this, especially at power-up as the timing circuit is temperature sensitive. LongHairedFop (talk) 16:26, 23 March 2015 (UTC)

It is most likely the flyback transformer failing. They don't tend to fail immediately. They fail slowly. You lose hold (the picture scrolls up and down on the screen). You then being to lose throw (the picture gets squashed). When I used to repair televisions (shortly after we ate the last dinosaur), it is about 80% replacing the power supply and 20% replacing flyback transformers. 209.149.113.207 (talk) 18:57, 23 March 2015 (UTC)

I was going to say the same thing, except that it was always the "vertical hold" going out on mine. (A "console TV" is the name of a TV that looks like a piece of furniture, BTW: [1].) StuRat (talk) 19:00, 23 March 2015 (UTC)

Yes, this is vertical hold. It controls how long the TV takes to do its vertical scan and has to be set close to the actual rate that the frames arrive at, so the TV can synchronize by using the blanking interval. Because the analog electronics that determined the time weren't entirely reliable and could perform differently while warming up, a control was provided so you could compensate. With the analog TVs we had, it was likely to need adjustment from time to time.

Horizontal hold controlled the time to scan one line of the image, and with the analog TVs we had, in practice you never needed to adjust it. I think this was because the TV was good at syncrhonizing even if it was set wrong, because if you turned the knob far enough that it couldn't synchronize, the picture immediately went very bad. The rectangular image would "tear" into a narrow parallelogram shape that would repeat as needed to fill the height of the screen, maybe 10 times. --65.94.50.15 (talk) 20:33, 23 March 2015 (UTC)

Thanks for the answers, everyone. I don't know if the TV had a vertical hold adjustment, although it probably did (I think this was standard). We threw it away a few months later, and it would be funny if it was something simple like that.

(209.149.113.207) you know, when I repaired Mac Classics, it was about the same percentage of power supply vs. flyback transformer. Usually, the monitor wouldn't show an image, though. I always liked replacing the transformer, because our repair manual told us to put one hand behind your back when you pry off the anode cap, so that if you accidentally complete a circuit, the high voltage won't go through your heart. Now, the transformer had a bleeder resistor, which in normal operation would drain it of charge; still, this was highly-skilled work for $6 an hour! OldTimeNESter (talk) 12:09, 24 March 2015 (UTC)

Fluorescent lamps in bathrooms

I have an odd historical question. I remember that in some houses built in the United States in the 1950s and 1960s, it was common for the lights in the bathroom to be fluorescent lamps, which would flicker for some number of seconds and then come on strong. When one finished using the bathroom, one would normally turn off the light. Why was a fluorescent lamp used? This would have seemed to make no sense in terms of energy use, since turning on a fluorescent lamp requires a large energy use. (I realize that there wasn't a whole lot of consideration given to energy conservation at the time.) It would have seemed to make more sense for the home builder to provide fluorescent lamps in the kitchen or living room, that could be turned on in the morning and turned off at night. Does anyone have a thought as to why this was done? Robert McClenon (talk) 17:19, 23 March 2015 (UTC)

Just for clarification, are you using the term 'bathroom' in its literal sense - a room with a bath in it - or as seems common in the U.S., as a euphemism for a room with a toilet that may or may not also have a bath in it? I ask, because I've seen a great deal of confusion resulting from this, and we might as well get it straight from the start.

In answer to the question, assuming that you actually mean 'a room with a bath in it', there are safety issues with light fittings in such contexts - specifically the electrocution risk inherent in accessing a light fitting from a bathtub which might be wet (or even filled) and thus liable to be effectively earthed. Florescent lamps are often fitted on walls, and can thus be sited well away from the bath itself. Generally, fitting incandescent lamps to walls is avoided because they provide an intense localised light source, which one doesn't generally want to be facing directly when e.g. using a sink. AndyTheGrump (talk) 17:31, 23 March 2015 (UTC)

It was a room with a toilet and a hand sink in it that my parents were visiting with me. I would have gone into it to use the toilet. Would it matter whether there was a tub also? I don't know whether the rooms that I am remembering were what are known in the United States as full bathrooms or as half bathrooms. I don't understand why it matters from a safety viewpoint. Either way, there was running water, and the fixture would have been away from the water. Maybe the reason was to avoid having the intense local light source of an incandescent lamp; if so, it illustrates the lack of energy conservation common sense in the 1950s and 1960s. I still think that the use of a fluorescent lamp would have made sense in the kitchen to avoid the bright light of an incandescent lamp. There were other ways to diffuse the light from an incandescent lamp, such as various sorts of lampshades. Robert McClenon (talk) 17:50, 23 March 2015 (UTC)

As far as the U.S. National Electrical Code is concerned, the presence of a bath certainly matters - see here for illustrations of what can be placed where [2]. As for what the electrical codes required in the 1950s or '60s, I've no idea, but they may be relevant. AndyTheGrump (talk) 19:14, 23 March 2015 (UTC)

In a room where a mirror is used (e.g. an American bathroom), you want as much light as possible. It is used to check your appearance in the mirror before you leave. At the time, a single fluorescent light produced a lot of light across a wide spectrum of wavelengths. An incandescent bulb produced less light and often tended to be narrower in the spectrum (I always noticed they were yellow). So, imagine looking in the mirror. You think you have on black pants and a red shirt. You walk outside in the sunlight and notice you are wearing brown pants and a purple shirt. It would have been better to have extra light in the bathroom. Extra note: My experience is that men tend to not notice themselves in the mirror while women do. So, you get a completely different response to this type of question from men (who will talk about wattage and money) and women (who will talk about use of the light). 209.149.113.207 (talk) 18:25, 23 March 2015 (UTC)

Male here, but will talk about the quality of the light, not the wattage. Fluorescent lights tend to produce harsh blue-white light. (They do have softer variations, too.) You really can't get that harsh blue-white light from an incandescent light unless it's blindingly bright. So, then, why would a woman want a harsh light ? Well, if putting on make-up prior to going someplace with harsh fluorescent lighting, you would want matching light. This could show, for example, where a vein is visible through the skin, at least until covered with a bit more rouge. StuRat (talk) 18:39, 23 March 2015 (UTC)

Well, I think that answers my question. It did have to do with the color spectrum of the light and the use of the mirror. Robert McClenon (talk) 20:43, 23 March 2015 (UTC)

Presentation based on scientific report

I was told that for these kinds of presentations its best to read your report, write a PowerPoint then use the slides as prompts rather than have a separate set of notes. I was told that using a separate set of notes would just confuse you since you wrote the report and should know it well. Is this true? 194.66.246.6 (talk) 18:14, 23 March 2015 (UTC)

It depends on the audience. There is no definition of "best" practice. I've presented at places where powerpoint presentations were banned. I've presented at places where you are required to print your notes, numbered, each on a separate card for the entire audience. I prefer to have slides in the background (not many - just a few) and give a presentation without a podium of any kind. I want to walk around. I want to walk up to the audience. In my opinion, that is best. For others, it is strange, unusual, and very scary. 209.149.113.207 (talk) 18:18, 23 March 2015 (UTC)

An oral presentation isn't the time to give detailed numeric values, those can be included in a separate handout. The oral presentation is to "get the gist across". So, a few key facts, written on the slides and then repeated by you, is a good way to go. However, you do run the risk of getting somebody in the audience who asks for a level of detail inappropriate for that format. You may then refer them to your detailed handout, and keep a copy handy for yourself so you can point out the page number, etc. I'd avoid actually reading the details, though, as that will tend to bore the rest of the audience and throw off the schedule. StuRat (talk) 18:52, 23 March 2015 (UTC)

thanks but I more meant for your own preparation. Is it a good idea to write notes for yourself? I was told it's better not to otherwise it just seems too scripted or you go into too much detail or get confused. 82.132.219.217 (talk) 18:58, 23 March 2015 (UTC)

Can you tell us 1) the size of the audience 2) the broad field of science 3) whether this is for school 4) if so what level and in what country? 5) How long is the talk? There are lots of ways give talks, but recommended methods will change depending on audience, level, and lots of other context. For example, I've given 15 minute talks to large audiences of scientists at professional meetings, and I've also presented to 10 people around a table for 2 hours. The techniques and best practices are very different. SemanticMantis (talk) 19:03, 23 March 2015 (UTC)

- It will be 15 minutes to a small group of scientists. 194.66.246.6 (talk) 19:15, 23 March 2015 (UTC)

That's not very long. 30 slides at 30 seconds each should do it. You should be able to put 30 seconds worth of spoken material onto each slide, eliminating the need to resort to notes. I suggest that the handout contain copies of those slides, along with additional details, if needed. That way, if the projector dies, or they can't see it, they can still follow along. Be sure to number the pages, so you can direct them to a particular page. And try to rehearse on an actual slide projector, so the time to flip slides, adjust the focus, etc., can all be taken into account. StuRat (talk) 19:25, 23 March 2015 (UTC)

Thanks. I was thinking 1 slide per minute just so there isn't too much text on the slides? 194.66.246.6 (talk) 19:44, 23 March 2015 (UTC)

30 slides for 15 minutes is not recommended by any sources I've found. Having been to hundreds of these talks and prepared dozens myself, my WP:OR is that 1 per minute is good, and minimal text per slide is good. SemanticMantis (talk) 20:22, 23 March 2015 (UTC)

Well, at 1 slide per minute with minimal text, that won't be enough to fill the minute, so then you are back to either memorizing additional material or using notes. I was trying to avoid that. StuRat (talk) 21:27, 23 March 2015 (UTC)

What do you mean, it "won't be enough to fill the minute"? Surely you're not suggesting that the presenter read the slides. That's the classic boring Powerpoint presentation. I've found that one slide for every 45 to 60 seconds is about right (i.e., up to 20 slides for 15 minutes). Short Brigade Harvester Boris (talk) 00:06, 24 March 2015 (UTC)

Well, not word for word, but pretty much, yes. If he has a chart showing 10 whatchamajiggers for item A and 31 for item B, he could throw in "We were amazed that item B has over 3 times as many !". StuRat (talk) 03:00, 24 March 2015 (UTC)

For myself, I tend to go a lot closer to 15 slides for 15 minutes than 30, though I suppose it depends on the material being presented. Dragons flight (talk) 19:44, 23 March 2015 (UTC)

I don't think there is any one answer. I've given many scientific presentations over the years and almost never write any notes. The key facts are included on the slides, and I rely on memory for other things. By contrast, my wife (who is also a scientist) writes incredibly detailed notes when preparing presentations. That is what works for her. Either way, I would suggest that it is very important to practice your talk repeatedly. By practicing the details of what you are going to say you can work on getting the flow right and making it all sound natural and polished. Whether you choose to prepare notes or not, I'd say that practice is the best way to get comfortable with what you are going to say, build confidence, and make it sound good. Good luck. Dragons flight (talk) 19:44, 23 March 2015 (UTC)

↑↑↑ This. Wise words. Short Brigade Harvester Boris (talk) 00:12, 24 March 2015 (UTC)

• Here are selection of suggestions for how to prepare a scientific presentation [3] [4] [5] [6]. Some mention having a set of notes, some don't. Use of separate notes is ultimately a personal preference. If you're afraid you'll get lost without them, use notes. If you are concerned that you will appear distracted, write notes and don't use them. Powerpoint has a "presentation mode" that allows you to view notes on each slide while it is up, though they are not shown to the audience [7]. SemanticMantis (talk) 20:21, 23 March 2015 (UTC)

Better yet, become familiar enough with your material that you don't need notes. Don't worry if you forget some little detail.

Here are the suggestions that I give to my class; almost all of these apply to professional scientific presentations as well. Short Brigade Harvester Boris (talk) 00:06, 24 March 2015 (UTC)

• Having done literally hundreds of these, here's my OR. 30 seconds a slide is definitely far to quick. My personal rule is about 1.25 slides/minute, but I'm a fairly fast presenter. 15 minutes as a rule of thumb means 12 minutes of talking with 3 minutes of questions, so I would normally go for about 15 slides. NEVER more than 5 lines of text on a slide, and try and have a relevant image on most of them to keep the attention. Don't listen to Sturat who seems to be suggesting you just read out your slides, that's the single biggest mistake you could make. Don't write out notes, but know you subject well enough that the text on your slides just prompt you what to say. On every results slide, put a one line take-home message about what it's showing, then repeat those in your conclusions in the end. Drive your message home! Judicious use of animation (only some appear/disappear, no flying about)can make a more complicated slide a lot easier to follow. Anyway, just my 2p. Fgf10 (talk) 08:04, 24 March 2015 (UTC)

• I agree on not more than 5 lines per slide, in fact, I might reduce that a bit. But as for not "reading the slides", I've got to disagree there. Some people may not be able to see, although the handout with a copy of the slides will help there. Of course, you don't have to read each slide verbatim, instead you should summarize it. One of the most annoying things is when the presenter puts up a slide and says "This leads us to this conclusion !", and I can't see what the slide says before it changes. The presenter should orally present all the relevant info, with the slides as a supplement, not the entire presentation itself.

• As for the number of slides per minute, I like the style where there's a list of items, but only one is highlighted, and the presenter only talks about that one. Then, the next slide has the next item on the list highlighted, and he then talks about that item. So, each slide is on the screen for a shorter period, this way. (Note that the handout need only contain the list once.) StuRat (talk) 17:26, 24 March 2015 (UTC)

If people can't see, you haven't made your font large enough. No reading of slides, capital sin number one, which is essentially what you're saying anyway, "instead you should summarize [sic] it". All you need on a slide are the main points or figure and a take-home message. The focus should be on the speaker, not the slides. Though with many poor speakers it's on the slides anyway, which will then generally be poor as well. I would also strongly advise against the 'highlighted list' type of presentation, as it's far more confusing to the listener than a properly designed presentation. Fgf10 (talk) 18:18, 24 March 2015 (UTC)

As for people not being able to see, they might be behind somebody taller, might have forgotten their glasses, the projector may be blurry, etc. So you shouldn't count on people reading your slides.

Regarding highlighting, that's a tried and true method of calling attention to an item. What's your objection to it ? The alternative is to use a pointer, but that's not going to be as visible from the back of the room. StuRat (talk) 05:55, 25 March 2015 (UTC)

Yes, and people might also be blind, so I'll have to provide Braille handouts. Or they could be dead, so I'll make sure to have a medium on hand as well. You design your talks for normal circumstances. Can't and shouldn't cover every possibility. People can come talk to you afterwards. Highlighting is fine, and often even required, in pathway diagrams etc, you were speaking of highlighting items in a list, rather than using multiple slides earlier. Highlighted lists make people read ahead and not pay attention. I've been in 500+ seat lecture theatres and worked just fine with pointers, you just need the right ones. Pointer use can be minimised if not entirely eliminated with proper slide design anyway. By the sounds of it, I'd much rather sit through one of my talks the one of yours. Suspect it's also a difference in field, I'm in neuroscience, you're in computing? I find the harder sciences generally give far worse presentations than we do in biology. Fgf10 (talk) 11:30, 25 March 2015 (UTC)

If you know your material well enough, just pretend that you're explaining it to your friends in the pub and use the slides as a prompt. Talking directly is far more attention-grabbing than reading notes. Alansplodge (talk) 21:58, 26 March 2015 (UTC)

Where do the electrons go to?

If you power a light with an electrical generator and disconnect the light, what happens to the electrons that were powering the light?--Fend 83 (talk) 18:54, 23 March 2015 (UTC)

If the generator is running with no load connected, then it will maintain an electric charge on each of its contacts. With a DC generator, one contact will remain positive and the other negative—just as happens with a battery. No current flows, so the electrons aren't moving. With an AC generator (sometimes also called an alternator), each contact will alternate between positive and negative in opposite phase. In that case the electrons just flow back and forth along the path from one contact to the other through the generator. --65.94.50.15 (talk) 20:39, 23 March 2015 (UTC)

I just had this explained to me differently in the case of AC electricity. Apparently in an open (non-working) circuit one wire from the alternator at the generating station is live, with a potential alternating flow. The other wire, which goes to ground--not all the way back to the generating station--is by itself dead. The live wire's potential current originates at the alternator located at the power generator, where the alternator too is grounded. When the circuit is closed at the consumption end, the potential AC power then flows from the alternator, down the live line, through the bulb, or whatever, into the "dead" (but now live) line which is grounded near the consumer, with the earth itself completing the circuit. For this reason the live wire alone cannot electrocute you unless you yourself are grounded, or you also touch the grounded "dead" wire and the complete your circuit. μηδείς (talk) 00:42, 24 March 2015 (UTC)

You're assuming a remote generating station with the ground forming one side of the circuit; I was assuming a local generator, as the question seemed to imply that. --65.94.50.15 (talk) 04:56, 24 March 2015 (UTC)

... and that's not how it works with remote electricity generation in the UK. I'd be surprised if it's common anywhere. See Three-phase electric power. Who "explained" this to you, Medeis? Dbfirs 09:50, 24 March 2015 (UTC)

Better never assume that you are not grounded when in presence of an electrical wire. Generators even remote will not abdicate their bites. --Askedonty (talk) 10:29, 24 March 2015 (UTC)

I was specifically addressing the fact that at the household end, the two lines you see in the cord are not both live by themselves. One line carries the AC into the house, t is always live. If you touch it and then are grounded the electricity will flow though you. (IP 64 had said that "each contact will alternate between positive and negative") but this is not precise--you cannot be shocked by the dea line unless you touch the live line. You can be shocked by the live line regardless of the dead line, as long as you are gronuded

If you touch the dead line but nothing else, nothing will happen. The second line only carries a current if it is (1) grounded, and (2) the circuit is connected between the to lines with a switch, bulb, human body, or other conductor. I was not interested in the details of the upstream activity like distribution lines and how plants deal with a varying consumption load. μηδείς (talk) 21:46, 24 March 2015 (UTC)

Yes, that's all true except the claim that the second line has to be grounded to carry current. It is grounded only because distributors choose to implement protective multiple earthing Dbfirs 22:53, 24 March 2015 (UTC)

Connecting the circuit between the live wire and the dead with your body would only be dangerous if there were a large floating ground, able to accomodate the flux of the live line, to "accept the electrons". With absolutely no grounding of any sort at all there should still be no current, correct?

When you say "dead" do you mean the "neutral" wire? If so, then connecting between live and neutral with your body is possibly even more dangerous than connecting between live and earth. With absolutely no grounding, each wire carries equal and opposite potentials, but I still wouldn't risk touching either wire, especially with high voltages, because it is difficult to completely insulate a generator from the ground, and there might just be some accidental connection to the ground through some appliance connected to the system. It doesn't take much current to kill you. Dbfirs 21:59, 25 March 2015 (UTC)

By the dead wire I mean the wire that is not in a circuit with the generator until the switch is closed. At this point, we should probably drop it, because I am almost totally ignorant regarding the subject, so anything I say is going to come across at best as a diversion, if not actual gobbledygook. μηδείς (talk) 18:25, 26 March 2015 (UTC)

Electrons power things when they move. When you disconnect the light they just stop where they are, so nothing happens to them, they just don't move anymore. (This is simplified a bit, if you want more details ask. In particular be aware that electrons move very little when powering things.) Ariel. (talk) 00:57, 24 March 2015 (UTC)

Fred (to expand a bit on what Ariel said), the bulb is not powered by an excess of electrons, but by a flow of electrons, which constitutes electricity. So if you suddenly disconnect a bulb from its power source there are no "excess" electrons in the bulb (or, in the generator) that have to go anywhere. Relatedly, an electrical generator does not produce electric charge or extra electrons; it produces an electric potential difference that causes electrons in the wire and the bulb to flow, which powers the bulb. Let us know if that answers your question. Abecedare (talk) 01:02, 24 March 2015 (UTC)

(EC) I think the confusion, for me at least, comes from the fact that VOLTAGE is not CURRENT, electrons flow with CURRENT not VOLTAGE, so even though the voltage is changing in AC, if there is no CURRENT, the electrons aren't "flowing". Voltage is the POTENTIAL difference. So I believe the comment by 65.94.50.15 In that case the electrons just flow back and forth along the path from one contact to the other through the generator. is incorrect. I THINK, please correct me if I'm wrong. Vespine (talk) 01:13, 24 March 2015 (UTC)

When the voltage at a point changes, that means the number of electrons there is changing, which means they are going somewhere and that's a current. In an AC system the voltage at any one point is changing continuously. So yes, electrons are flowing—just not as many of them as there would be if the light bulb was still there. --65.94.50.15 (talk)

Maybe the hydraulic analogy will help. Replace the wires with pipes, the electrons with water, the generator with a water pump, and the light bulb with a watermill. -- BenRG (talk) 06:27, 24 March 2015 (UTC)

I have the impression that the hydraulic analogy is misplaced here. If you are pumping water, and the pipes are not connected to anything, the water will run loose at the end. However, I don't believe the electrons will "fall" at the end of a cable that's not connected to anything. 14:36, 24 March 2015 (UTC)~ — Preceding unsigned comment added by Senteni (talk • contribs)

Picture it as a pipe with a cap on the end, then. Just as a bare wire would not lose electrons right away, a capped pipe would not spill water. If you keep pumping more water into the pipe, however, pressure builds up until a weakness is exploited, and the pipe springs a leak, shooting a jet of water out into the room. Likewise, in a wire, electrons can continue to build up, generating a voltage (voltage = electron pressure) until a high enough voltage builds up to overcome the resistance of the air, at which point a jet of electricity (AKA a spark) shoots out. Analogy works perfectly. --Jayron32 14:40, 24 March 2015 (UTC)

Even better: imagine a sort of "balloon" cap at the end of the water pipe. This models the capacitance of an unterminated wire. If you pump water into the pipe, it will collect in the balloon. The balloon will stretch under pressure - storing water.

Analogously, if you have a copper wire and you apply voltage to it, electrons will accumulate (in the wire itself! But - perhaps slightly concentrated at the unterminated end). As electric current flows into the capacitor, the wire begins storing up electrons, but as they accumulate, the voltage keeps increasing. If you applied DC current forever, eventually, your "balloon" will "fill up" - the wire can not handle an infinite amount of excess charge. At that point, the wire will spark. This is called dielectric breakdown, and it's analogous to a balloon popping under extreme pressure. The material simply cannot contain so much stored energy, and it "breaks."

In the case of alternating current, the direction of electron flow keeps changing - so it's similar to filling a balloon, and then reversing the water flow to drain it again, and then repeating.

Nimur (talk) 15:38, 24 March 2015 (UTC)

... just to clarify Jayron's and Nimur's analogy, the build up of pressure to the point of dielectric breakdown of air will occur only if charge is constantly added as in a Van de Graaff generator. It will not happen with a fixed voltage supply. Dbfirs 23:06, 24 March 2015 (UTC)

Right: the idealization I described was that of a constant current, current source power supply. Most real, general-purpose power supplies that you will encounter behave more similarly to a constant voltage voltage source power supply; but in reality, a real power supply is never capable of delivering perfectly constant current or voltage when connected to a pathological load. Electronics engineers sometimes use test-equipment that can very nearly model an ideal power supply for some range of operations. Nimur (talk) 02:38, 25 March 2015 (UTC)

How do humans generate electricity, how do they store it, where does it go to after being used?

How do humans generate electricity, how do they store it, where does it go to after being used?--Fend 83 (talk) 18:56, 23 March 2015 (UTC)

Do you mean how do we generate electricity in wires, or in our own bodies ? StuRat (talk) 19:33, 23 March 2015 (UTC)

The proton pump in the mitochondria is the first electrical energy storage, but it is transferred to other locations via chemical energy and a range of other ion pumps, channels, cotransporters, antiporters etc.

The electrical energy is generally stored in plasma membrane acting as a capacitor, with a resting potential; an activation potential action potential can reverse that briefly to send information but is generally an energy-expensive breakdown in that storage.

When electrical energy stored in a plasma membrane or any capacitor is discharged, such as by letting ions through a channel, it's basically gone, leaving behind only the waste heat one would expect from that amount of energy (which is puny and I doubt anyone has actually measured it, except maybe in an electric eel or something, but that's just a guess). Wnt (talk) 20:50, 23 March 2015 (UTC)

I made a redirect for activation potential > action potential. You can measure the total human electrochemical waste heat quite easily, it's basically what raises endothermal body temperature above ambient temperature, but I won't go into the details. μηδείς (talk) 21:00, 23 March 2015 (UTC)

You shouldna done that ... it was a brain fart, sorry! Wnt (talk) 22:37, 23 March 2015 (UTC)

It's not a problem, we create redirects for common mistakes and misspellings all the time. μηδείς (talk) 00:44, 24 March 2015 (UTC)

Chemistry

Explain the following observations as fully as you can: (a)The mass spectrum of a certain element consist of a number of closely-spaced lines.The intensity of one of these lines decreases over a period of time as two new lines simultaneously appear.One of the new lines corresponds to a relative mass of 4. (b)When 1cm³ of concentrated (10 mol\dm³) hydrochloric acid is added to 1dm³ of water the pH of the latter decreases by approximately 5 units,but when 1cm³ of concentrated hydrochloric acid is added to 1dm³ of an aqueous solution containing both ammonia and ammonium chloride the decrease in pH is very small. — Preceding unsigned comment added by 188.164.1.183 (talk) 21:00, 23 March 2015 (UTC)

Please do your own homework.

Welcome to Wikipedia. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. DMacks (talk) 21:28, 23 March 2015 (UTC)

I'm in a charitable mood, so I'll encourage the OP to study alpha decay and chemical buffers. Let us know if you have trouble understanding those. Wnt (talk) 22:35, 23 March 2015 (UTC)