• Physics Discussion
    973 replies, posted
That's why people ask how much juice is left in something when they talk about charge.
Is that why when I tip over an ethernet cable, all my internets fall out?
[QUOTE=Chris220;41117847]Is that why when I tip over an ethernet cable, all my internets fall out?[/QUOTE] No shit, I thought we had already established that it was a series of tubes.
I plan on taking physics next year. Idk if I should do AP physics though...
[QUOTE=CaptainMagmar;41122887]I plan on taking physics next year. Idk if I should do AP physics though...[/QUOTE] Do you like science and math?
Reading [I]Foundations of Mechanics[/I] by Abraham and Marsden. It is actually astounding how deeply you can study just classical mechanics. My junior mechanics class made me feel like that shit was simple and boring.
[QUOTE=JohnnyMo1;41144615]Reading [I]Foundations of Mechanics[/I] by Abraham and Marsden. It is actually astounding how deeply you can study just classical mechanics. My junior mechanics class made me feel like that shit was simple and boring.[/QUOTE] I think the stupidest part of high school physics is that they don't make you derive anything. They just say, "This is the equation to use, now go."
Yeah, if only they introduced you to first order differential equations so you can solve all of basic kinematics & the simple harmonic oscillator.
[QUOTE=Number-41;41151336]Yeah, if only they introduced you to first order differential equations so you can solve all of basic kinematics & the simple harmonic oscillator.[/QUOTE] Harmonic oscillators made no sense until I learned calculus, and was able to take derivatives and see for myself that the math worked. Well, kinematics too, but I had no idea why trig was involved with oscillators until calculus.
That's true. Maybe they should combine high school Physics & Math subjects into one, motivating the use of calculus through physics (and trigonometry with vectors or something like that). Maybe you won't get as far, but the understanding would be so much more thorough...
I went to the university library today and I was planning on getting 3 physics books but when I got there I was like, "Wait, I'll never read 3 books at once." So I got 4 books. I think I have a problem pls help
I was reading "Understanding Physics" by Isaac Asimov, and I got stuck at this. [img]http://img153.imageshack.us/img153/7083/1daw.png[/img] I might just be an idiot, but I can't seem to figure out what his easy rearranging was. Sorry if this is completely trivial and I'm missing something. [editline]5th August 2013[/editline] I figured it out. Someone who published this book decided to throw (1/2)mv^2 in the denominator, in order to not make sense and confuse the shit out of beginners in physics. [editline]5th August 2013[/editline] Also, post more in here guys, sheesh.
I'm taking AP Physics this year after only doing chemistry the past couple of years
Hydrodynamics question, more specifically the continuity equation in integral form. You can take in the time derivative inside (that's what I assume) because the integration borders are constant (fixed volume element), also [URL="http://en.wikipedia.org/wiki/Divergence_theorem"]divergence theorem[/URL]: [; \frac{\partial}{\partial t}\left( \int _V \rho dV \right) +\int _S \rho \vec{V} \cdot \vec{dS} = \int _V \frac{\partial \rho }{\partial t}dV + \int _V \nabla (\rho \vec{V}) dV = \int_V \left(\frac{\partial \rho}{\partial t}+\nabla(\rho \vec{V}) \right)dV=0 ;] The last one having to hold for any volume, thus you arrive at the differential form of the continuity equation: [; \frac{\partial \rho}{\partial t}+\nabla(\rho \vec{V})=0 ;] Can I do all of this? 'Cause my textbook does the whole infinitesimal volume element shebang without touching any integral theorems, but I don't want to do it like that because lazy... [I]Also fuck that zero...[/I]
I'm in Physics 11 this year. I'd just like to know what I should expect through the year. I figured this would be the best place to check. I live in Nova Scotia if that helps
Easy baby physics with lots of assumptions and lots of formula memorization
Hydrodynamics, more specifically inviscid compressible flow around M=1. All those goddamn tiny thermodynamic formulas. Never again.
I have a real thing about physics and astronomy, and i'm only a couple of years away from university, the little big problem i have is that i barely know about this stuff, so i need to actually get into this, any leads?
I have AP Physics this year, so far it's pretty hard for me. I have difficulty with learning from textbook reading only, so the summer work was hell
[QUOTE=Cosa8888;41935859]I have a real thing about physics and astronomy, and i'm only a couple of years away from university, the little big problem i have is that i barely know about this stuff, so i need to actually get into this, any leads?[/QUOTE] Intro Calculus (limits, derivatives, integrals, ordinary differential equations) Linear Algebra Those two will keep you busy for at least a year and you'll be prepared as fuck. If you're done with those, there's: the lagrangian (for classical planet trajectories, shit like that) vector calculus (for electromagnetics) tensor calculus (relativity, even though special relativity doesn't really require it that much for the basic stuff, it does help your insight in the whole Lorentz transformation thing) Oh and maybe some c++ if you feel like it. But if you reach tensor calculus after all of those, you either have a lot of free time or you're a smart/quick learning fucker (and your first and second year will be a breeze). I'd say calculus and linear algebra are the most important (the other ones depend on your knowledge of those).
[QUOTE=Number-41;41938092]Intro Calculus (limits, derivatives, integrals, ordinary differential equations) Linear Algebra Those two will keep you busy for at least a year and you'll be prepared as fuck. If you're done with those, there's: the lagrangian (for classical planet trajectories, shit like that) vector calculus (for electromagnetics) tensor calculus (relativity, even though special relativity doesn't really require it that much for the basic stuff, it does help your insight in the whole Lorentz transformation thing) Oh and maybe some c++ if you feel like it. But if you reach tensor calculus after all of those, you either have a lot of free time or you're a smart/quick learning fucker (and your first and second year will be a breeze). I'd say calculus and linear algebra are the most important (the other ones depend on your knowledge of those).[/QUOTE] I'd go with vector calc before Lagrange since, at least in my curriculum, it came up before Lagrangian and Hamiltonian mechanics in my intermediate mechanics class. Tensor calc is a good thing to get a handle on. Definitely start coming to grips with it in undergrad if you want to go to grad school. If you ever plan on learning GR, some familiarity with tensors will put you way ahead. You can even find some tensors on manifolds in classical mechanics if you study it in depth enough. (symplectic manifolds)
Kind of reviving this one, but I've stumbled upon a problem and I can't seem to figure it out. And this baby physics so it ought to be easy peasy for your guys. I have a Natrium lightbulb, with the power 100W and light at a wavelength of 0.59*10^-6. The question is, how many photons gets "shot out" per second from the lightbulb. So I just tried to figure out how many J one photon had, with the E = h * c/λ , and then divided that with 100 J. But I'm not getting the correct answer [sp] 3.0 * 10^20[/sp] So I'm wondering if anyone here could help me.
[QUOTE=booster;43975488]Kind of reviving this one, but I've stumbled upon a problem and I can't seem to figure it out. And this baby physics so it ought to be easy peasy for your guys. I have a Natrium lightbulb, with the power 100W and light at a wavelength of 0.59*10^-6. The question is, how many photons gets "shot out" per second from the lightbulb. So I just tried to figure out how many J one photon had, with the E = h * c/λ , and then divided that with 100 J. But I'm not getting the correct answer [sp] 3.0 * 10^20[/sp] So I'm wondering if anyone here could help me.[/QUOTE] That looks like the correct working, Did you do 100/photon energy? E = h * c/λ 6.63e-34 * 3e8 / 5.9e-7 = 3.37e-19 J 100/3.37e-19 = 3.0e20 photons
Mmm, I remember that problem from my course in December and I tried to solve it exactly like that, but there's some reason you can't use that... Gotta find my notebook. [QUOTE=techtuts0;43975758] E = h * c/λ 6.63e-34 * 3e8 / 5.9e-7 = 3.37e-19 J 100/3.37e-19 = 3.0e20 photons[/QUOTE] Hm, I remember it being more complicated than that.
[QUOTE=techtuts0;43975758]That looks like the correct working, Did you do 100/photon energy? E = h * c/λ 6.63e-34 * 3e8 / 5.9e-7 = 3.37e-19 J 100/3.37e-19 = 3.0e20 photons[/QUOTE] Seems like I had done a big mistake. I somehow in my tired delusional state mistook wavelength for frequency, and used the formula E = h * [I]f[/I] Thanks for clearing it up. [editline]20th February 2014[/editline] Also, while the thread is alive. We talked about dark matter and all that stuff today, and the teacher said that dark matter covers our entire galaxy, "giving" it more mass. Does this dark matter cover the galaxy perfectly even like one giant invisible ellipsoid? Or is it very uneven?
I don't think the answer is that simple, as you cannot simply assume that 100W is directly converted to just photons. A large part will go to heating and ionizing the gas, etc. I don't know exactly how these things work, but I do know that no lamp is 100% efficient. Anyway, we don't need to know how it works to figure out the amount of photons it emits per unit time. I think you better look up the efficiency of the lamp, which is in in lumen/Watt. Lumen is the amount of energy emitted per unit time in every direction (well, it's a bit more complicated, but it boils down to energy per unit time, Wikipedia is being a bit vague) You then calculate the amount of lumen it emits (efficiency*100W), which corresponds to the amount of energy emitted in all directions per second. Once you know that, you can divide that energy by the energy of a single photon of wavelength λ, which is hc/λ. In short, with P being the power of your lamp, mu the efficiency (I'll take the optimistic value of [URL="http://en.wikipedia.org/wiki/Sodium-vapor_lamp#Low-pressure_sodium"]200 lumen/Watt[/URL]), 589.3 nm the wavelength of each photon and hc some constant [URL="http://www.wolframalpha.com/input/?i=h*c&dataset="]Wolfram Alpha[/URL] can supply me with, I get: [IMG]http://quicklatex.com/cache3/ql_d7be0fa614349a18909bf643a7a52754_l3.png[/IMG] which is [IMG]http://quicklatex.com/cache3/ql_24232fccff1e2a255300063ba7ee2088_l3.png[/IMG] photons per second. Note that I had to convert lumen to Watts, which introduces a factor 1/683. Dimensions (W being : [E/(E*T)]*[E]/[E]=1/[T], informally "dimensionless amount per unit time", or photons per second, check. Does it make sense? -it scales with the efficiency: more efficient lamps emit more photons -longer wavelength photons have less energy so per unit energy, more photons are emitted -if you increase the power, more photons are emitted Protip: always check your units and make sure your answer makes physical sense (change a variable and see how it affects the result). I found more photons than when you assume 100% efficiency. First because I converted nanometers to meters wrongly, then because I forgot 1 lumen is not 1 Watt.
[QUOTE=Number-41;43976087]I don't think the answer is that simple, as you cannot simply assume that 100W is directly converted to just photons. A large part will go to heating and ionizing the gas, etc. I don't know exactly how these things work, but I do know that no lamp is 100% efficient. Anyway, we don't need to know how it works to figure out the amount of photons it emits per unit time. I think you better look up the efficiency of the lamp, which is in in lumen/Watt. Lumen is the amount of energy emitted per unit time in every direction. You then calculate the amount of lumen it emits (efficiency*100W), which corresponds to the amount of energy emitted in all directions per second. Once you know that, you can divide that energy by the energy of a single photon of wavelength λ, which is hc/λ. In short, with P being the power of your lamp, mu the efficiency (I'll take 200 lumen/Watt), 589.3 nm the wavelength of each photon and hc some constants Google can supply me with, I get: [IMG]http://quicklatex.com/cache3/ql_9858f6aec6eecb45399c93b2c26cc627_l3.png[/IMG] Dimensions: [E/(W*T)]*[W]/[E], check. Does it make sense? -It scales with the efficiency:more efficient lamps emit more photons) -longer wavelength photons have less energy so per unit energy, more photons are emitted -if you increase the power, more photons are emitted[/QUOTE] Yeah, the question in my book was very simplified. Nullifying any "reality" component. So your example gives a good view over what a real situation would look like. Also, what is the software you use for those sexy looking calculations?
It's LaTeX markup coupled with [URL="http://www.quicklatex.com/"]QuickLaTeX[/URL], basically a sort of "math HTML" that works extremely well. It's not that hard for simple stuff like this and you can create beautiful documents with it. It's been ages since I used Word. For numerical calculations I use Desmos Graphing Calculator (which when copy pasted immediately is in LaTeX so you can throw it in QuickLaTeX instantly, fucking brilliant)
[QUOTE=booster;43976010]Also, while the thread is alive. We talked about dark matter and all that stuff today, and the teacher said that dark matter covers our entire galaxy, "giving" it more mass. Does this dark matter cover the galaxy perfectly even like one giant invisible ellipsoid? Or is it very uneven?[/QUOTE] It's actually thought to be more like an incredibly large invisible sphere that engulfs the entire galaxy, getting denser as you move toward the centre, something akin to this: [IMG]http://i.imgur.com/ut7ppdK.gif[/IMG] Basically it's because dark matter doesn't self-interact or interact with other stuff. If I remember rightly, normal matter collapses faster because the dust/gas interacts with it self, losing energy by friction/collisions allowing it to collapse inwards more readily under gravity. If the galaxy is rotating, this collapse can be staved off by conservation of angular momentum within the plane of rotation, leaving the normal matter in a rotating disk, and the dark matter in a giant spherical halo.
I got to derive e=mc^2 today. I'll have to go look at it again if I want anything to cement though. Pretty neat considering how famous it is.
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