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September 10

Physics Practise Exam Question - Can't Get it!

I'm currently doing a past paper for a physics exam. Would anyone be able to enlighten me on how to answer this!? Thanks guys.

Advertising for a plasma screen television that operates on a 240V electricity supply has a page of fine print which is reproduced below.

• Operating cost for electricity consumption = $30.00 per year • cost based on 2000 hours of operation • cost based on electricity charged at $0.15 per kWh

(a) What is the power consumption of this television? (b) Given your answer find the current drawn. 220.233.20.37 (talk) 03:13, 10 September 2013 (UTC)[reply]

We don't do homework for you (doing sample exams is a form of homework - pointless if we do it for you), but we help if you are stuck. As you have provided no evidence that you have made a start but got stuck, I'll give only an outline. As you have pricing data, you can work out the energy in kilowatt-hours consumed in 2000 hours of operation. You can then convert the kilowatt-hours in 2000 hours to watt-seconds in one second. Why one second? Your knowlege of SI should tell you that. The second part is a trivial relationship of power, voltage, and current. Ignore power factor for a question at this level. [0.417]1.122.160.213 (talk) 03:43, 10 September 2013 (UTC)[reply]

... unless you are specifically asked to work in SI units (joules), you can stay with hours. There's no need to make things more complicated by working in seconds. You need to find out the total number of kWh used over a year, then just divide by the number of hours to give an answer in kW. You could convert this to watts, which is how most most power consumptions are expressed. For (b) you just need an equation connecting power, current and voltage (as mentioned above). You probably should know this equation, so I won't give it here. Dbfirs 08:01, 10 September 2013 (UTC)[reply]

Of course, this being a advertising statement, the result will be a lower limit ("will under no circumstances use less than..." ;-). --Stephan Schulz (talk) 08:43, 10 September 2013 (UTC)[reply]

Oh, yes, the calculation gives the average current. In practice there will be a higher current at switch-on, and, as mentioned by 1.122.160.213, there are the complications of power-factor, but at this level is it safe to assume a purely resistive load (or corrected power factor), and that the average value of RMS current is being asked for. I hope the OP has worked out the answer by now. Dbfirs 16:51, 10 September 2013 (UTC)[reply]

Astable oscillator/multivibrator with logic gates

Hello,

How are astable square-wave oscillators built using NOT gates (or other gates configured to act as NOT gates) typically "initialized" and powered? I'm aware that, in practice, the ICs containing the gates require power, but is this enough to start the oscillator going? Is its initial output state (high or low) random? It seems that, if for no other reason, there should be source and ground connections to set the logic levels.

Some examples:

Diagram (located under the heading "NAND Gate Astable Multivibrators" on this page)
Part (b) of this figure (located on this page)

Any information on this subject would be appreciated! 142.20.133.199 (talk) 14:08, 10 September 2013 (UTC)[reply]

The resistors bias the gates into the active region. While logic gates are designed to give an all or nothing output, that is, an output that is clearly logic 0 or ligic 1, there is always, unless there is internal positive feedback to give a snap (termed "schmitt") action, where the gate functions as a more or less linear amplifier. Since the capacitor(s) or crystals return the output to the input so as to reinforce any change (positive feedback), any small change that arises, whether form thermal noise, supply rail noise, or ranmp up upon switch-on, will be self amplified until the output is driven hard into clipping. The capacitor(s) or crystal then charges to the clipped voltage. This stops the resinforcement, upon which there is gate input change in the other direction (toward the bias point), which snaps through again hard over in the other polarity. And so it goes on....

Because amplification is the key to getting oscillation started, lowering the resistors until the linear loop gain is less than unity will result in no oscillation if the power supply ramps up slowly upon switch-on, although if the circuit is then "shocked" into limitting (by say momentarily shorting the output to ground), oscillation may then start. The same applies to the traditional 2-transistor, 2-FET, or twin triode astable multivibrator oscillator. Biasing to give at least a small linear gain is key to reliable starting. Powering up is not alone sufficient for oscillation. For slow power up (meaning theh power supply ramps up slowly compared to teh oscillator capacitor(s), startup is somewhat random due to its origin in noise. If the power supply is snapped on rapidly, the oscillator will always start in the same output condition (logic 0 or 1), but which it is depends on the design of the gates and production variation. This is because all practical gates will pull more strongly in one direction or the other.

1.122.160.213 (talk) 15:25, 10 September 2013 (UTC)[reply]

The idea behind these devices is to create a kind of logical paradox - so a NOT gate could (theoretically) have it's output connected to it's input - so the NOT gate is asserting that the output is NOT the input - and the bit of wire going from output to input says that output and input are identical. Mathematically, you have a paradox.

The result should be that the gate continually changes it's mind about what the output should be and because the speed of the logic gate is finite, it ought to oscillate at a frequency that's the reciprocal of the delay through the gate.

The practical problem is that logic devices are not the perfect 0/1 binary devices we kinda imagine them to be - so in practice, the circuit settles down to some voltage between zero and three volts (or whatever the logic level for '1' is) and doesn't oscillate at all.

So practical multivibrators are all about reinforcing that essential paradox by using multiple gates and that kind of thing. Getting the device started is another ikky problem since it's essentially indeterminate which state it would initially power up into. Hence you need a solid way to initialize the device to a known state before it starts vibrating. Then of course, you usually want control over how fast the device oscillates - and that drags you back into ikky analog territory. For those reasons, it's hard to view these devices as strictly digital contraptions - and you have to fall back to understanding them as strictly analog devices. SteveBaker (talk) 15:46, 10 September 2013 (UTC)[reply]

Thank you both for your comments. Do you think another method of generating a square wave (e.g., an appropriately configured 555 timer) might be a better choice? From what you've said, it seems like using the logic gate approach introduces a number of factors that aren't easily accounted for. 142.20.133.199 (talk) 19:18, 10 September 2013 (UTC)[reply]

What is "better" depends on the circumstances. A 555 timer will deliver a cleaner better defined squarewave and its frequency is more easily predicted by calculation based on the R and C values. But the one- and two gate circuits will often be quite good enough. A Mercedes is a better car than a small Toyota, but if what you do is deliver pizzas, what are you going to buy? A given circuit design might have, after all other functions have been taken care of, have one or two NAND, NOR, or invertor gates, or even a flipflop that can be wired to perform an invertor function, left over. In such cases it will be cheaper to use the left over gates to make the clock oscillator, and not use a 555. As a practicing electronic engineer, I can tell you that success in large scale and even medium scale manufacturing involves shaving a few cents here and there wherever you can, without affecting performance too much. Another factor in reducing cost is to not have too many different parts in your inventory. If you design in a part, say a 555, and yours is the only one of many products your company makes that uses a 555, you may loose friends in the procurement section.

There are many occasions that require better frequency stability and accuracy than what astable multivibrators (whether gates or 555's) can provide, but the stability and accuracy required does not justify the cost of a crystal. In such cases an oscillator tuned by an inductor-capacitor combination will do the job. One can make a very good LC oscillator in the Colpitts configuration using a NAND, NOR, or invertor gate.

In a factory production environment, adjustments are avoided like the plague. Adjusting adjustments, whether done by a human or by some robotic machine, are expensive. Often crystal oscillators are used for that reason alone. But if you are building a one-off for a lab special or a home project, it may be no big deal to use a crummy 2-gate oscillator with a 30% frequency error on first switch-on, and adjust it as necessary.

An advantage of LC oscillators over astable multivibrators is that they are quite immune to interference. I can remember a timing oscillator based on a two-gate multivibrator that worked perfectly on the bench. But when installed it ran erratically at around 2 to 3 times the correct frequency, as hash from nearby DC electric motors made the gates "flip" early in each cycle. Changing to a colpitts LC circuit solved the problem completely. Never use a 2-gate astable oscillator in a switch mode power amplifier for the same reason, unless you can thoroughly shield it and thoroughly test a lot of prototypes. The switching spikes from the power stage are likely to couple into the oscillator and drive it nuts.

121.215.54.95 (talk) 00:41, 11 September 2013 (UTC)[reply]

Those are good points to consider (also, I've never previously encountered a Colpitts oscillator, so I'll look into that). This is for a one-off circuit, so thankfully economic considerations aren't a priority, although your point about using leftover gates rather than adding another IC makes sense even in that context. I think I'll try the oscillator based on logic gates, see if it's sufficient for my application, and then perhaps try a circuit using a 555 timer or a Colpitts oscillator if not. Thanks for your help! 142.20.133.199 (talk) 16:02, 11 September 2013 (UTC)[reply]

See Colpitts oscillator (and electronic oscillator) for our relevant articles. Tevildo (talk) 21:29, 11 September 2013 (UTC)[reply]

ISOLATION OF CELL COMPONENTS .

How are components of cell isolated ??? are there some techniques to isolate components of cell ??? what is the method to isolate cell ?? kindly reply me soon as possible ...(139.190.155.177 (talk) 14:25, 10 September 2013 (UTC))[reply]

This is actually an enormous question. The short version is—yes, there are techniques to isolate many different components and structures from many different types of cells. Google and PubMed are your friends here. Search using phrases like isolation of mitochondria or protocol for isolation of nuclei for your chosen organelles and components.

Beyond that, the Wikipedia Reference Desk isn't here to do your homework for you, but we will often try to help you if you get stuck with specific questions (and you show us how you've tried to answer them yourself). TenOfAllTrades(talk) 15:07, 10 September 2013 (UTC)[reply]

(ec) We don't even know what kind of cell you're talking about here! A prison cell? An electrochemical cell - like in a battery? A plant or animal cell? An Excel spreadsheet cell? A mobile phone reception area? A part of a spy network? All of those things might reasonably need to be "isolated". The best guess might be a plant/animal cell - but I'm far from sure that's what we're being asked here. SteveBaker (talk) 15:33, 10 September 2013 (UTC)[reply]

Oops. That's a good point, Steve—I'm looking at the question through the lens of my own current work. It could just as easily be a question about an electrochemical cell, couldn't it? TenOfAllTrades(talk) 20:54, 10 September 2013 (UTC)[reply]

We are not talking about space, but something emerged from no existence and exist that the space

We are not talking about space, but something emerged from no existence and exist that the space ? Even if nothing is opposite thing, why the two found why such a system exists? — Preceding unsigned comment added by 37.238.82.114 (talk) 20:50, 10 September 2013 (UTC) [reply]

I understand that you are probably not a native speaker of English, but this question is very hard to understand. Will you put a little more effort into explaining what you want us to answer? --Trovatore (talk) 20:56, 10 September 2013 (UTC)[reply]

That's probably the same guy from Baghdad who asked "Why did not remain nothing nothing?" on September 6, and possibly another, similar question. In short, he's seeking a logical explanation for why or how the Big Bang would have occurred. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:06, 10 September 2013 (UTC)[reply]

Using Google Translate, translate back to Arabic:

نحن لا نتحدث عن الفضاء، ولكن شيئا ظهر من لا وجود له وجود أن الفضاء؟ حتى إذا ليس هناك ما هو الشيء المعاكس، لماذا وجدت اثنين من سبب وجود مثل هذا النظام؟

Click on some of the words to choose an alternative translation back to Arabic that looks a bit better, e.g. this one:

نحن لا نتحدث عن الفضاء، ولكن لا شيء ظهر من غير موجودة، وهناك تلك المساحة؟

حتى إذا لم تكن هناك ما هو الأمر المعاكس، لماذا وجدت مدة من لماذا لا يوجد مثل هذا النظام؟

Then translate this back to English and you then click on the words that don't make sense to get alternative translations, e.g. I got this:

"We're not talking about space, but nothing emerged from the non-existent, and there is that space? Even if there were not, the opposite is something, why any duration of the Why is there no such a system?"

Count Iblis (talk) 00:14, 11 September 2013 (UTC)[reply]

I’m not convinced there could even be such a thing as absolute “nothing”. And if there really IS such a thing as “nothing” then even God couldn’t exist there. Answer the question “Where did god exists before there was "somewhere"?” The answer is "Nowhere". Nothing can “exist” and “not exist” at the same time, it’s the law of non contradiction. Positing some "supernatural" realm where God could exist is purely ad hoc reasoning, and it's not "nothing". Vespine (talk) 03:41, 11 September 2013 (UTC)[reply]

The short answer is that nobody knows. — kwami (talk) 05:40, 11 September 2013 (UTC)[reply]

I'm not 100% sure what exactly you're asking, but there are some articles that might be related (and might exist on other language forms of Wikipedia, which might serve you better). Our article Ex nihlo goes into detail about various positions related to "creation from nothing". Parmenides may be of interest to you, as might Nothing comes from nothing. On a more physics related side, Zero-energy universe, Vacuum genesis, and Hartle–Hawking state; relating to the last one, [1].Phoenixia1177 (talk) 08:14, 11 September 2013 (UTC)[reply]

Asking what happened before the Big Bang is like asking what land lies above the horizon. Asking where the Big Bang came from is like asking what is beyond the horizon. The horizon - like the singularity of the Big Bang - is a limit on what we can see from our perspective. From the first microsecond to the next is the same as from the first billion years to the next billion. Wnt (talk) 16:32, 11 September 2013 (UTC)[reply]

Why do you love God if he does not love you and you're a nobody for him

we do notanswer requests for personal opinions

The following discussion has been closed. Please do not modify it.

Why do you love God if he does not love you and you're a nobody for him Why live then account you on life and your situation and you did not ask him to create you, He does not need you and you do not need him before create you. So What?Why?37.238.93.248 (talk) 21:09, 10 September 2013 (UTC)[reply]

Your basic premise runs counter to standard theology. God loves everyone, and He does "need" us, which is why we were created. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:14, 10 September 2013 (UTC)[reply]

Requests for people's personal reasoning for their relationship with their God do not belong at the reference desks. μηδείς (talk) 21:18, 10 September 2013 (UTC)[reply]

It does belong at the Humanities desk, but the people reading this one won't be as likely to know. There is scarcely an argument in relation to God that hasn't been proposed, named, rebutted, and most likely fought over at some point in history. Wnt (talk) 21:23, 11 September 2013 (UTC)[reply]

That we live in a computer screen on the face likening or simulator or why we discovery if that was all you discover is the information illustrated in space , So How crash this screen that was all of this illusion.

That we live in a computer screen on the face likening or simulator or why we discovery if that was all you discover is the information illustrated in space , So How crash this screen that was all of this illusion. Controlled by the laws of the invisible So, Why try to discover the image from nothing embodied in a space — Preceding unsigned comment added by 37.238.93.248 (talk) 21:27, 10 September 2013 (UTC)[reply]

Please clarify your question. 163.202.48.126 (talk) 07:23, 11 September 2013 (UTC)[reply]

It sounds like you might be talking about the Simulation hypothesis, or something related to it. Your question makes me think of the following related topics as well: Allegory of the Cave, Brain in a vat, Cogito ergo sum, René Descartes, and Gnosticism- if this is a relation, perhaps taking a look through them could help to clarify what you are asking:-) The phrase, "information illustrated in space" brings to mind, Holographic principle.Phoenixia1177 (talk) 08:22, 11 September 2013 (UTC)[reply]

Yes, and in layman's terms, perhaps he is thinking of The Matrix. StuRat (talk) 13:04, 11 September 2013 (UTC)[reply]

My guess is by vote. Those running the simulation probably recognize they're doing something ethically dodgy, but if the denizens get together and make a clear statement calling for its discontinuation the ethics committee might get off their ass and do something about it. It occurs to me that with society racing toward a Singularity (the absolute disparity between rich and poor) this might be an interesting political tactic - probably more legal though perhaps less effective than ordinary biowar. Wnt (talk) 18:54, 11 September 2013 (UTC)[reply]

I think by "screen" they are referring to a theory I saw touched upon briefly during a space/physics documentary of sorts (I watch a lot of those.) Basically the theory was that all of reality was just a projection... though it didn't elaborate much further on that. Sounds a little more farfetched than the theories presented on the simulation hypothesis page (or maybe it's on there? I didn't have time to read through it all.) As for how the screen could "crash" and the illusion be lost, I don't think anyone can really know. --.Yellow1996.^(ЬMИED¡) 03:26, 12 September 2013 (UTC)[reply]

That sounds a lot like the Holographic principle, see the second paragraph of the lede there. I guess this, see the 2nd section, could be a "screen crash", kind of reminds me of a BSOD:-)Phoenixia1177 (talk) 04:53, 12 September 2013 (UTC)[reply]

Ah - thanks for those links, Phoenixia. And perhaps if the operators are running older technology, we could even experience a Snow Crash. ;) --.Yellow1996.^(ЬMИED¡) 03:25, 13 September 2013 (UTC)[reply]

Hydrolysis of PET with 30% ammonia

Some month ago I poured a few centiliters of 30% concentrated ammonia in an empty coca-cola bottle. I didn't know that concentrated ammonia hydrolize PET, so after a few days the bottle turned opaque and a white powder was formed. I verified that the white precipitate was terephthalic acid (not ammonium terephthalate) and I tried to get more using 36% H₂SO₄ as the hydrating agent. I waited for more than a week buth the PET is still intact. So my question is: why does ammonia corrode the PET, being a weak base, and the sulfuric acid doesn't although is a strong (and more concentrated) acid? Bokuwa (talk) 22:43, 10 September 2013 (UTC)[reply]

I'm not sure, but perhaps you're getting sulfonation reactions instead of hydrolysis? Also, sulfuric acid is a very efficient dehydrating agent, so maybe the water is not even getting to the PET in the first place because the acid is tying it up. FWiW 24.23.196.85 (talk) 06:34, 11 September 2013 (UTC)[reply]

I don't think so: the acid is mixed with 64% of wather by weight. This means that there are 40 mol of wather per 3,63 mol of sulfuric acid. I read the article about that acid and there isn't any explanation about the dehydrating proprieties of H₂SO₄, so I have no certainty but I think that in these proportion the acid can't act as dehydrating by itself, and it might occur hydration reactions. Furthermore, my first decomposition reaction of PET was made with 94% sulfuric acid; did I dehydrate that compound? And what did it become?

I haven't tried other concentrated acids yet, but I will, to verify all the hypothesis. Thanks for answering :D Bokuwa (talk) 19:31, 11 September 2013 (UTC)[reply]