Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.
If you mean for "small production runs", then it really depends on a lot of factors. If you need literally one circuit board and have access to a CNC machine, it will probably be quicker to use that. But if you need 20 boards, it might be quicker to print all 20 and etch them in one go than to mill 20 boards on the CNC machine. Secondly, depending how "automatic" your CNC machine is, starting the run and letting it go "overnight" for example, to mill 20 baords will be a lot "quicker" for you (counting only your labor time) as compared to the time and effort required to set up a chemical etching batch and the clean up etc involved, even if the end result will finish "sooner", so which do you prefer? Vespine (talk) 02:21, 18 April 2016 (UTC)[reply]
Just thought of another significant factor to consider is that chemical etching will not drill your board for you. So if you have a lot of through-hole components, manual drilling can be very time consuming and also introduce a significant chance of making a "mistake" potentially wrecking a board. Presuming the CNC machine can also drill the holes, but I'm pretty sure that's almost a given. Vespine (talk) 03:52, 18 April 2016 (UTC)[reply]
Just in the last year 3D printing of multi-layer circuit boards has got good enough to be used for fast prototyping or small runs and one can buy machines to do it. Dmcq (talk) 11:26, 18 April 2016 (UTC)[reply]
When using Windows and your computer crashes and you have to reboot:
When it comes to the screen where you choose to logon or restart or shutdown, should you restart so it shuts down properly and restarts again, or just logon? Thanks. Anna Frodesiak (talk) 08:11, 18 April 2016 (UTC)[reply]
It actually depends on what caused the computer to crash in the first place. Sometimes a proper restart is beneficial, while in other circumstances it doesn't matter. 81.132.106.10 (talk) 09:55, 18 April 2016 (UTC)[reply]
Newer versions of Windows (XP and after) recover better so that logoff and log on clean up a lot. But if in doubt, reboot, because Windows probably needs a reboot twice a month anyway to sort out installs and other problems. Graeme Bartlett (talk) 09:59, 18 April 2016 (UTC)[reply]
@Anna Frodesiak: for future questions, the computing, rather than the science desk might be the better place to ask such questions.
@Anna Frodesiak:, when staring an operating system, serives are started an open files like databases or logfiles. When not shuting down properly, these files are not finished in wirting and the filesystem needs to synchronize all copes of it's tables of contents. Windows writes a copy of it's configuration, called the registry when logging on. In the start procetude, immedialtely after die BIOS, Windows listens for a key like F8 oder the spacebar. It allows the user to boot using the last known good configuration. This is the recent copy taken after the last logon. When updates were or other software was installed, those settings were lost, but the files are still stored on the drive. A resinstall of that software fixes any problems, if the installer works propperly. Upgrades may be a problem, due an old configuration ist applied to the new file versions. When using an autologon on windows, the last known good configuration is not possible, due being overwritten in the start procedure. --Hans Haase (有问题吗)22:05, 20 April 2016 (UTC)[reply]
I could get why the flow of current exist only for a short time as said in the video Principles of Electricity when we connect two different beakers that have metal strips immersed in them using a wire.The reason is the electrons from the zinc electrode goes through the wire to the copper electrode.But I couldn't get why there is a continuous flow of current when the beakers are joined together and why the electrodes(zinc and copper) become negatively and positively charged.Could anyone help me.JUSTIN JOHNS (talk) 09:01, 18 April 2016 (UTC)[reply]
Recall that when the zinc strip first entered the acid (at 6:01 in the Principles of Electricity video) some zinc atoms went into the solution, leaving their electrons behind so the zinc gained a certain negative charge (more so than the copper strip which being "less negative than" is the same as "positive relative to" the zinc). The flow of electrons stops when electrical balance is reached (at 6:18) where the solution is more positive than the zinc. However when the beakers are joined, the solution around the zinc receives electrons from the solution around the copper. The continuous flow of electrons from zinc to copper is exactly matched by a continuous flow of electrons through the solution in the opposite direction.
The linked video reflects its date, which was the end of WW2, when it teaches the Bohr model of the atom and mentions "the 92 known chemical elements" that seem to conclude with Uranium (number 92). In fact Neptunium (number 93) had just been isolated in 1944 and Plutonium (number 94) had been isolated in 1940, but wartime secrecy prevented its anouncement until 1948.
The video is not specific about the "chemical solution" involved, in fact a variety of soluble salts, acids or bases dissolved in water give an electrically conducting solution that works as an electrolyte; nor does it explain that copper, zinc or other metal electrodes will be negative or positive relative to one another depending on their relative electronegativities. AllBestFaith (talk) 11:11, 18 April 2016 (UTC)[reply]
Could you explain a little bit more about "The continuous flow of electrons from zinc to copper is exactly matched by a continuous flow of electrons through the solution in the opposite direction." as you have told.I couldn't really get that sentence.JUSTIN JOHNS (talk) 11:16, 18 April 2016 (UTC)[reply]
Your video is so out of date that it is leading you down the wrong track completely. It is now generally accepted that there is no flow of electrons through a wire when an electric current passes along it: it is the charge which flows, not the sub-atomic particles. If you want to know what electricity is and how it works, look at something up to date. 81.132.106.10 (talk) 11:50, 18 April 2016 (UTC)[reply]
Please ignore the above comment. Electric current really is a flow of subatomic particles and this view hasn't changed since 1945. There is nothing wrong with the video, and it even states that its picture of the Bohr model is only a symbol and not what an atom really looks like. --Heron (talk) 19:47, 18 April 2016 (UTC)[reply]
It's not that the electrons travel fast. It's that the movement of one near the end of a conductor displaces an adjacent one, which displaces an adjacent one, etc, etc. The effect is that an electron at the far end is displaced very rapidly. The movement of each individual electron is quite slow. Kind of like the water in a garden hose. When the tap is turned on, water flows out immediately but the water at the tap takes a few seconds till it flows through. Akld guy (talk) 09:06, 19 April 2016 (UTC)[reply]
The internal voltage would not be external, so the included battery / cell provides the energy for the current. However this will not last forever as the chemical energy will be drained. Another way is to have a continuously variable magnetic field threading through the circuit. Unfortunately we cannot make a magnetic field that increases forever, so it is much more common to use a variable field and get alternating current. Graeme Bartlett (talk) 09:40, 19 April 2016 (UTC)[reply]
Do you mean to say that the current will last only for a few seconds since it stops when the all the electrons from zinc rod are transfered to the copper rod?Can we make the process continue forever by using a salt bridge or any other solution?JUSTIN JOHNS (talk) 09:53, 19 April 2016 (UTC)[reply]
A zinc battery may last for quite a few days until all the zinc is dissolved. If you want it to keep going you need a continuous supply of material. This can happen in a fuel cell. A fuel cell could use hydrogen and oxygen to make electricity and water. Graeme Bartlett (talk) 10:38, 19 April 2016 (UTC)[reply]
The reason why it seems like so little happens when an EMF is applied to an open circuit is that it takes a really, really small movement of charge to make a big difference in voltage. How much exactly depends on capacitance, but let's put it this way: one mole of electrons, as might be generated from a molecular-weight's worth of grams of some substance giving up or accepting one electron, contains a faraday unit of charge (Faraday constant), i.e. more than 100,000 Coulombs. And the largest usual value for a capacitor, according to our article, is about 1 mF = 1 mC of charge moved / volt of difference. Electrochemical reactions typically are on the order of a volt, so that means that with good technology you might make a system where you can move 1/100,000,000 of a coulomb of charge before the voltage that creates puts a stop to it, or maybe consume about one millionth of a gram of some material with MW 100. However, there are no capacitors described in this circuit, and so we're looking at parasitic capacitance, less than picofarads, which is to say, a billion times less than that. So unless I fouled up in my math/logic (very possible) I'm seeing we usually expect something on the order of a million billionth of a gram of the battery's substance to get used up in order to create enough of a charge difference to prevent any more from reacting. Wnt (talk) 11:30, 19 April 2016 (UTC)[reply]
Maybe a useful analogy is a metal pole. If you poke on something with the metal pole, you know that the far end would not put any pressure on it unless the metal atoms were being pressed together by the force of your thrust. But you certainly can't *see* the compression; and what's more, you have no real idea how much the pole compresses per force applied, yet it has no practical effect on how much force you put on the far end unless it's practically a wet spaghetti noodle. Wnt (talk) 11:35, 19 April 2016 (UTC)[reply]
The video Principles of Electricity did not point to the impedance or internal resistor of a power source. Batteries do not contain supraconductors. Also the chemical process in the battery is limitted. Depending on the load, the voltage of the source dropps. On a constant load the voltage also keeps constant while the source delivers energy. During this time a continuous flow of electric current is present. It changes when to load is beeing changed or the battery gets empty. --Hans Haase (有问题吗)22:54, 20 April 2016 (UTC)[reply]
lol, now you have me wondering whether the resistance of a metal is different when there's not any current flowing in it. ;) Wnt (talk) 16:01, 22 April 2016 (UTC)[reply]
Why aren't there fungi that are sexually transmitted?
Parasites, viruses, and bacteria seem to be the main agents of STDs. What about fungi? Why aren't there fungi that are sexually transmitted? 140.254.77.156 (talk) 17:25, 18 April 2016 (UTC)[reply]
This is pretty much semantic. You can get lice from someone else's towel, or a cold from kissing your girlfriend. Most things like C. albicans can be sexually transmitted, but they are not exclusively so, nor primarily treated as if the partner may also need treatment. I.e, chaste virgins can still have yeast infections. μηδείς (talk) 21:05, 20 April 2016 (UTC)[reply]
If you put one rooster with ten hens in a coop, then would they reproduce? Can you safely put two or more roosters in one coop, or will they become aggressive and territorial? 140.254.77.156 (talk) 17:40, 18 April 2016 (UTC)[reply]
You need to be careful putting a stranger rooster with an established group of hens. They sometimes gang up on him and attack - thought to be reason behind the phrase "hen-pecked". I would strongly advise against putting 2 roosters together - They will almost certainly fight, perhaps fatally. DrChrissy(talk)17:46, 18 April 2016 (UTC)[reply]
Some roosters might get along, others might not. No simple answers.I don't think this is something that there are generally applicable answers for. There's just far too much lore and science of chicken rearing to cover in a short post, but I'll sketch out some ideas and provide some refs that will help. For the breeding - it depends on how happy they are, what their lodgings are, what time of year it is, all sorts of things. For roosters fighting - it depends a lot on how much space is available (to roost, to nest, to forage), and likely what breed. Lots of other factors too (age at introduction, time spent in coop, etc.) Some chickens are more territorial than others. If you want to raise chickens, I suggest they start by reading a book, something like this [1]. Also you can read and post at a chicken-specific forum, like this one [2], or this subreddit [3]. Here [4] is the WikiHow category on chickens, and here [5] is a wikibook on raising chickens, both of those together will be a decent (free) primer, but raising chickens is also something that just requires some practice and experimentation to get good at. Finally I've WP:OR seen at many multiple rooster set ups in back yards; it can be done without aggression if there is enough space and you have the right breeds. SemanticMantis (talk) 18:34, 18 April 2016 (UTC)[reply]
How hard is it to change fields within engineering as a graduate? For example if your original major was in civil but your thesis was related to biomedical but you now work in civil engineering project management whilst volunteering for a mental health charity. How hard would it be to get into biomedical engineering then? 2A02:C7D:B907:6D00:A568:CD48:3581:1FB1 (talk) 20:22, 18 April 2016 (UTC)[reply]
It depends, particularly given your "as a graduate" caveat. First, how much do the two disciplines have in common? For example, here's the current plan of study for a B.S. in civil engineering from my college. Note that the majority of the coursework for junior and senior years are prefixed "CE". Someone changing over from electrical engineering would have none of those courses in their regular plan of study . Even the "technical electives" probably wouldn't permit crossover from one to the other (similarly, your "civil engineer with a thesis in biomed" is highly suspect, even as a thought experiement). "As a graduate", though, makes for a different question. If you're talking career, then it's a matter of "can you pass an interview?" The precise prerequisites are as varied as the jobs in question, and much of the matter is now a matter of your abilities as a candidate rather than your paper qualifications. Plenty of people are working in engineering fields for which they do not have the specific engineering degree. — Lomn20:53, 18 April 2016 (UTC)[reply]
In the U.S., Engineers generally need to pass two exams to be fully practicing engineers: the general Fundamentals of Engineering Examination, passed shortly after one graduates with an engineering degree from college (university), and a second Principles and Practice of Engineering Examination which is specific to the engineering discipline you are working in. Hypothetically, one could (on their own) learn what is needed to pass a second PPE exam, and simply take the second exam to be qualified to work in the new discipline, I suppose. I'm not sure how common or feasible that is, generally you're employed in a specific field, and work as an unlicensed "apprentice" before taking the PPE exam, since your on-the-job training generally prepares you for taking the PPE. --Jayron3201:06, 19 April 2016 (UTC)[reply]
Yeah, the relevance of professional engineering licensure is highly dependent on both field and role, and "unlicensed apprentice" misstates the nature of a large portion of engineering work. The distinction is (within the US, at least) one of whether you are certified to sign and seal documents for engineering work that does not involve interstate commerce. Lack of licensure does not prohibit one from performing the work, nor does it prohibit one from signing and sealing in cases where the work is interstate in nature (see engineering licensure in the US with regard to automotive and other applications). So no, the FE/PE is not a general requirement to be a fully practicing engineer, but rather a potential requirement for a narrowly-tailored area. — Lomn15:41, 19 April 2016 (UTC)[reply]
Easy. YOU are what matters. Not whichever papers you chose at uni. I've designed straw processing gear, crankshafts, undersea monitoring systems, steering systems, yacht hulls, electronic controllers, and every bit on the dirty side of cars. And buildings. Greglocock (talk) 03:55, 19 April 2016 (UTC)[reply]
