# 5614

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1. ## What is a good pictorial representation of an atom?

... because we cannot know exactly where an electron is, electron "orbitals" are pictured as a region in which the electron is most likely to be found. I think the region spans a certain number of standard deviations, usually either 2 or 3, which would result in a respective ~95.5% or ~99.7% probability of finding the electron in that space.
2. ## Electrons?

A potential difference (aka a voltage) is the force which acts on electrons and makes them move in circuits. As for their speed: electrons tend to move very quickly, their exact speed depends on their thermal energy. Using: $\frac{3}{2}kT = \frac{1}{2} m v^2$ k = Boltzman constant T = temperature (Kelvin) m = mass of an electron v = velocity a room temperature electron will be travelling at about $10^6$m/s... very fast! However in a wire (which has resistance) the fast moving electrons are always bumping into other things (other moving electrons, other electron that are still orbitting a nucelus etc.). I can't remember the numbers (I could calculate it...), but there are thousands(?) or millions(?) of collisions per second. After each collision the electron is then going to be travelling in a random direction (probably the wrong one, ie. against the current). This means a slightly different velocity for electrons in a wire would be more useful. The drift velocity of an electron in a wire can be thought of as the average speed the electron goes in the right direction. It is given by: $I = nAq v_d$ I = current n = number of charge carriers (normally electrons) per unit volume A = cross sectional area of the wire q = charge of an electron v_d = drift velocity To give you an idea: an electron travelling through a copper wire will travel at about 1mm/s... so it's actually quite slow.
3. ## Reputation!

Atheist: when I said bold, err, I kinda meant blue! I just remembered Swansont's name standing out a lot, so I said bold. It never really clicked that normally names are white and his is blue! Anyhow, cheers for the random update! And it's good to be back! Really? So if YT, who has loads of rep, gave someone else "a" rep point he would really be giving them quite a lot?
4. ## Reputation!

Fair enough YT, I suppose in that case the way it is quite hidden is not such a bad idea. Thank you for your kryptic post! So does a bold name mean expert? I had assumed it was some kind of moderator status (I know red is an admin), but then what is green? And what if an expert is given moderator status!?
5. ## Reputation!

wow! But then what's the point in a rep system if only I can see my own rep (without having to click on the person's profile page)? It makes it more about yourself feeling good than giving others an idea of who has a good rep. Personally I'm not a big fan of rep points though. The "___ expert" system we had was alright, but seemingly that does not exist anymore.
6. ## Antimatter

thedarkshade: if you consider a proton as a confined system of 3 quarks then maybe it's more (for want of a better word) obvious that it can have an antiparticle. The antiproton is made of 3 antiquarks. But alas, neutrinos have antineutrinos, so this way of thinking cannot be extended to everything. Snail: actually, that last post of mine is a bit wrong (shame I can't now edit it). Photons, Z bosons & gluons are their own antiparticles. But you are correct; W+/W- are a particle/antiparticle pair. So a W+ and a W- boson should annihilate each other but does anyone know an example of when/how this might occur? In what practical scenario would a W+/- be in such close proximity?
7. ## Reputation!

Why is it that I have a rep bar between Location and MSN (under my sig, in all threads) and nobody else has? Am I special!?! The only thing I can think of is that I didn't use my account for quite a long time. But any forum or user account change would apply globally, even if I'm not active, so I don't see how this happened... meh! *shrugs*

Even as a British student I know that Berkley has a good reputation. Other than that I can't really help. But there are league tables etc. that will give you ranks of universities, as well as a detailed breakdown of how that rank is obtained. A quick Google search will give you lots of information. I'm not sure if there are league tables just for N California, but just look through an American university league table and spot out the Cali. ones.  ah, DNA's post wasn't there when I started typing!
9. ## Antimatter

A photon is it's own antiparticle. That is, the photon and anti-equivalent of it are in fact the same thing, you cannot distinguish between them. Physically speaking an antiphoton does not exist. One way of mathematically defining antimatter is that it is what would occur if matter went back in time. An electron travelling backwards in time is mathematically identical to a positron travelling forward in time. I suppose if you were to try and extend this to a photon I might argue that a photon does not experience time, and thus saying it is going backwards through time is non-sensical. Any massless gauge bosons (force carriers) does not have an antiparticle. I think that photons, W & Z bosons and gluons are the only particles without an antiparticle. At an educated guess I would extend this to say that any massless particle will travel at c and will not have an antipaticle. True? This all also ties in nicely with Feynman diagrams. In FDs you don't need to refer to antipaticles, you can just label, for example, all positrons as electrons going backwards through time. Of course often positrons will be named, for simplicity. Also the whole no antiparticle thing would explain why photons (and other force carriers) are often drawn without arrows on them. Although there seems to be a lot of different ways of doing FDs, some do not have a time axis, in those this paragraph is not necessarily relevant.

I am in my first year at Imperial College London on a physics degree. Terms 1& 2 is 6 hours per week (in 2x3hr sessions). At the end of term 2 and term 3 we then get a choice between lab work and a mathematical analysis course. If you chose the lab then, effectively, the 6 hours per week continues for the rest of the first year. I believe it is the same in year 2, although I am not certain. (sorry for posting in an old-ish thread!)
11. ## Color

If you're interested in how the eye works then maybe you'll find a page like this: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html interesting. You can Google some of the key terms you find on that page for more info too.
12. ## Energy

If you post what things from your textbook you find confusing then we can try and explain them to you in a way that you understand. That would be a lot better than such a vague initial question.
13. ## NOVA: "Absolute Zero" on PBS in USA

No one knows. If the average density of the universe is below some critical value then gravity will never get a "grip" on the universe as a whole and we will expand forever, a cold death. If the density of the universe is above this critical value then there is enough matter to allow gravity to slow, stop and then reverse the current expansion of the universe. A "hot" death.
14. ## Lost in Space

Indeed that is what I meant by "definite state". I don't think it's nitpicking, my understanding is simple to the point where it can sometimes be technically incorrect. You make a very good point. By that all I meant was that you cannot assign, for example, a "up" or "down" value to that system. It is in a superposition of both. I meant it did not have a "definite value for a chosen observable". Appologies, this is a case of sloppy wording on my part. I was basically getting at the idea of whatever the pre-big-bang universe was being some (unknown) "thing", that then expands. However I had a mental picture of a ball/balloon being blown up, so I (stupidly) described the pre-big-bang "thing" as a "ball-thing", which is wrong. I even used the term "big" merely to imply that everything we now know was all contained in the small early universe. I haven't been on these forums recently, I blame that for my poor choice of words! The big bang does not say what happens, other than that the radius of the universe tended to 0. It was just a thought path I found an interesting one, not something I have dwelled on for long. But, after the universe expanded beyond the Planck length AFAIK the first thing to be was the quark "soup". I have no idea if that could count as an observer. I do agree that the concept of an observer is not understood, especially when we probe its meaning like this.
15. ## Flat universe philosophy

Yeah, the universe includes everything, there is nothing outside. The universe itself is expanding. But at the same time: I was actually having a good talk to an astrophysicist today about inflation. One inflation model (slow roll - it is one of the main ones, I think!) says that near the beginning of the universe the universe underwent a very fast expansion (faster than the speed of light). Now I'd always imagined the "edge" of the universe to be how far light has travelled since the big bang occurred, meaning that the universe would be ct large (c=speed light, t=age of universe. speed*time = distance). However the universe is observed to be larger than ct. There are other observables, including the flatness of the universe and the horizon problem, that all indicate a faster than c inflationary epoc/era occurred during the early universe. Here's a relevant graph from Cosmological Inflation and Large Scale Structure, Liddle & Lyth: The "physical Hubble length" line is what would happen if the universe expanded at a constant ct. Note that the scale during inflation is expanded in the graph, but the "physical Hubble length" line has a constant gradient at ct (well, some logarithmic variant of that). The "physical scale" line is what inflation says happens. During inflation there was a massive, >c, inflation. It does not say that light (or any matter) went faster than the speed of light, but that the universe itself expanded. I asked why the "physical scale" line clearly has a gradient less than that of the ct line, wouldn't then light catch up with the "edge", I argued. Apparently the universe expanded so much that light may never catch this "edge" of the universe. The other thing I was told was that when the 2 lines meet for the second time, after inflation, I was told to ignore that! I don't really understand why, after inflation, the inflation model says that the universe expands at less than ct. I also don't understand why, if it is expanding less than ct, that after a finite time light will catch this "edge", the only way to avoid this is to have an infinitely fast inflation, which did not occur. Anyhow, I have this research, conversation & graph because I'm doing a presentation on the early universe this Friday and I just happened to be doing this today! Luckily we're not going into too much detail on inflation! The reason I began this whole thing with a "but" was because whilst it is the universe itself which is expanding, it is possible that the edge of the universe is beyond the furthest bit of light or matter. It's just an interesting model and thought that I only came across today.
16. ## dont tell me...

For a quadratic of form $ax^2 + bx + c$ the two roots can be found using the formula $x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$ (first root from using +, 2nd from -). So just take your equation and stick in the values of a,b,c into that equation. As for the complex part, if you get, for example $\sqrt{-100} = \sqrt{-1 * 100} = \sqrt{-1} * \sqrt{100} = 10i$ NB: sqrt(-1) = i I'd stick in the values, but thedarkshade and the tree got slightly different equations, and I haven't checked who is correct. Either way they both give two complex roots to your original equation.  in LaTeX should I use \i or just i for an imaginary number? Because \i ($\i$) doesn't really look like a good i, whereas using i ($i$) is a lot easier to read, but then what's \i for?
17. ## Flat universe philosophy

After the big bang the universe did expand along all 3 of those axes. Flatness / curvature of the universe is to do with how those 3 axes themselves are curved. Whilst this image shows distorted 2D planes, the 2D plane is representing the universe. The curvature, or flatness, of the universe is how the 3D (technically +time, so 4D) universe is shaped. You are familiar with the concept of curved space-time, that's what gravity does, it distorts space-time. But space-time is 4D. Including time can be confusing, so working just with 3D; we're quite happy saying gravity distorts 3D space. Just think of a rubber sheet with a mass in the middle that stretches the rubber (as is the usual example). But once again your approximating a 3D universe to a 2D rubber sheet. The reason being:
18. ## What's the next language you want to learn?

thedarkshade: I first learnt Visual Basic and then later moved on to C++, from my experience I would recommend this, although I'm sure others might disagree!
19. ## Flat universe philosophy

Are you talking about what people mean when they say "the universe is flat"? A simple overview of the shape of the universe, here: Geometry of Curved Space. Wikipedia on the apparent flatness problem.
20. ## dont tell me...

At the risk of giving the game away I will ask: are you familiar with complex (imaginary) numbers?
21. ## Lost in Space

The thing which makes a quantum system exist in a definite state is an observer. I read a book on QM / string theory which outlined some ideas, and an interesting point made was how can the universe exist? If the universe was initially some kind of big ball something, as big bang theory suggests, then this ball-thing must have been unstable (or made unstable - that's what cause the big bang). But without an external observer that ball-thing would always be in a superposition. For the big bang to have occurred there must be an external observer who forced the universe out of a superposition in the first place, to make the big bang definately occur. And who is this external observer? I don't really believe in God, and neither does the author of this book (it was a purely scientific, non-religious, book), but then who broke the initial superposition that forced the universe to exist? Maybe the universe has always existed. I don't know if there's a answer to the question, I don't think there is and I read the book a long time ago and don't remember what follows, although it probably just moved on, this was a minor side point. I don't even remember for sure which book it was. As for wave-particle duality. We don't observe that as such, initially wave was observered, then particle. By physical theory and mathematics a new and true description of matter, wave-particle duality, was arrived at. I'm not really sure if this is what you were getting at with your original post... it's some interesting thoughts though. If it's not what you meant then I appologise and can you rephrase your quesiton / idea please!?
22. ## Question Relativity and the past and future

Indeed. In a thought experiment, which are always nice with relativity, you could easily assume a "perfect" clock. But the whole point with relativity is that there is no preferred or "real" or "The" referrence frame. All frames are equally valid, and as different frames can measure different times (and each time is also equally valid) there is no "real" or universal time - it all depends on what frame you are in.
23. ## Opening file on startup

If you open your slideshow in PowerPoint and chose Save As and then change the file type to PowerPoint Show (*.pps) then when you open that file it will immediately start the presentation (full screen) and will not open PowerPoint. Then place that .pps file in the Startup folder.
24. ## SFN Statistics

I always used to want stats like this, back in the day when they weren't available! They're kinda interesting!