# Search results

1. ### Direction of Photo Electron Emission

I was looking for information on how the photo electrons are emitted when under X-ray radiation. In this ancient review paper here http://journals.aps.org/pr/pdf/10.1103/PhysRev.30.488 [Broken] they state that the most common angles for non polarized X-ray beams (of various energies) range...
2. ### Relative biological effectiveness (RBE) of photon and electron

The title was too long, original question was: Why is the relative biological effectiveness (RBE) of photon and electron the same ? I was suspecting the charge of electron to possibly have some effect... Also, in biological matter, would there be a difference between photon damage and...
3. ### Why only delayed neutrons matter ?

Hi all, I've been seeing this again and again, that delayed neutrons are the ones that are allowing thermal nuclear reactors to work etc. etc. What I do not understand is how are prompt neutrons controlled and how delayed ones are allowed to "do their thing" no problem ? Wiki says...
4. ### The concept of parity and it's significance

Hi, I am revising my intro to nuclear physics class and was wondering why is parity so important ? I mean OK, I get it, physicists are crazy about symmetries, but I do not understand what does it physically mean... I was wondering if someone could possibly talk me through its importance and...
5. ### Arc-length integral with curve giving extremal value

Homework Statement For whoever does not want to read the attached problem: Firstly, I need to express the arc-length from given x=r\cos\theta, y=r\sin\theta z = f(r), \text{ where } f(r) \text{ is an infinitely differentiable function and } r=r(\theta) \text{ i.e. parameter is } \theta I...
6. ### Method of characteristics and shock waves

Hi all! I just wanted to ask if anyone is finding the usage of method of characteristics difficult ? I sort of feel, that it is a very simple approach to solving PDE's, but I get easily lost, when for instance, we have to keep switching back and forth between variables and such. When it comes...
7. ### Infinitesimal Canonical Transformation

Hi all! Another questions which is due to the gaps in my calculus knowledge. In these notes: http://people.hofstra.edu/Gregory_C_Levine/qft.pdf in the line above eq. (1) where it says that notation P is now unecessary, is it because \partial{ (p+\delta p)} is much smaller than p+\delta p...
8. ### Why is change of variables in the proof of Noether's Theorem legit ?

I have looked up a few derivations of Noether's Theorem and it seems that chain rule is applied (to get a total derivative w.r.t. q_{s} ( = q + s ) is often used. What I do not understand is why this is legitimate ? If we start with L=L(q,q^{.},t) how can we change to L=L(q_{s}...
9. ### Uncertainty principle in terms of expectations values in Dirac notatio

Homework Statement Show that (\Delta A)^{2} = \langle \psi |A^{2}| \psi \rangle - \langle \psi |A| \psi \rangle ^{2}\\ \phantom{(\Delta A)^{2} }=\langle \psi | (A - \langle A \rangle )^{2} | \psi \rangle , where \Delta A is the uncertainty of an operator A and \langle A \rangle is the...
10. ### Lagrange multipliers for finding geodesics on a sphere

Homework Statement Find the geodesics on a sphere g(x,y,z)=x^{2}+y^{2}+z^{2}-1=0 arclength element ds=\sqrt{dx^{2}+dy^{2}+dz^{2}} Homework Equations f(x,y,z)=\sqrt{x'^{2}+y'^{2}+z'^{2}} where x'^{2} \text{means} \frac{dx^{2}}{ds^{2}} and not d^{2}x/ds^{2} The Attempt at a...
11. ### Finding geodesics on a cone of infinite height

Homework Statement Find the geodesics on a cone of infinite height, x^{2}+y^{2} = \tan{\alpha}^{2}z^{2} using polar coordinates (x,y,z)=(r\cos{\psi},r\sin{\psi},z) with z=r\tan(\alpha) The Attempt at a Solution I am not sure with how should I expres the element dz^{2} ? When it is a...
12. ### Variation of S with fixed end points

Not really a homework question, just the notes are confusing me. Homework Statement Let S be a functional. (Given without proof) If S is differentiable its derivative \delta S is uniquely defined as \delta S = \int_{x_{0}}^{x_{1}}\frac{\delta S}{\delta \gamma} \delta \gamma dx where...
13. ### Method of Characteristics/Fan of characteristics

Hi all, I expect that this might be rarely taught (as I have not found much on the web). But maybe some of you have encountered this. I attach the problem At the end the author says to take the limit as t goes to 1. But to me, it does not make sense to look at something that is ε<x<=ε...
14. ### Sketching the solution to the IVP (Characteristic curves)

Homework Statement Sketch the solution to the IVP u_{t}+uu_{x}=0 \\ u(x,0) = e^{-x^{2}} Homework Equations Monge's equations \frac{dt}{d\tau} = 1 \\ \frac{dx}{d\tau}=u \\ \frac{du}{d\tau}=0 The Attempt at a Solution I think I do not really need the Monge's equation, but I have no...
15. ### Diffraction grating, angular with of the intensity peaks to the first maximum

Homework Statement A diffraction grating is ruled with 500 lines/mm and is 1 mm in width. Calculate the angular width of the intensity peaks to the first minimum in the Fraunhofer diffraction pattern of this grating, when illuminated with light of λ = 600 nm. Homework Equations n-...
16. ### Transport theorem, final integral

Homework Statement Show that \frac{d}{dt}\int \rho r^{2}\phi dr = \int \rho r^{2}\frac{d\phi}{dr} dr Homework Equations Fundamental theorem of calculus The Attempt at a Solution So I follow the derivation from the textbook and I think I get the rather sneaky rearrangement of...
17. ### Principal axes for a disc

Homework Statement I do not understand how the vector (1 -1 0)T represents the axis tangent to the disk at Q. The Attempt at a Solution Tried thinking in terms of simple vector addition, but just got another vector in the radial direction... I mean, (1 -1 0)T is parallel to a tangent...
18. ### Compression/longitudinal wave equation

This is not really a homework problem, I do not understand the concepts. The attachment below is as informative as the slides provided are. The course seems to be based on H.J. Pain's book "waves and vibrations". I know that the information provided is limited, but there is little I can...
19. ### Is a factor group by a non-trivial normal subgroup is always smaller than the group ?

This is not really a homework questions, rather a concept based one. I am studying from Fraleigh's ''Intro to abstract algebra'' and in chapter 15 it states, that for a group G and normal non-trivial subgroup of N of G, the factor group G/N will be smaller than G. I am not sure how he counts the...
20. ### Classify the group Z4xZ2/0xZ2 using fund.thm. of finetely gen. abl. grps.

Homework Statement Clasify the group Z4xZ2/{0}xZ2 using the fundamental theorem of finitely generated abelian groups. Homework Equations FTOFGAG: In short it states that every finitely generated abelian group G is isomorphic to a direct product of cyclic groups of the form...
21. ### Homomorphism of a cyclic subgroup is a cyclic subgroup ?

Homework Statement Let \alpha:G \rightarrow H be a homomorphism and let x\inG Prove \alpha(<x>) =<\alpha(x)> Homework Equations α(<x>) = α({x^{r}: r ∈ Z}) = {α(x^{r}) : r ∈ Z} = {α(x)^{r}: r ∈ Z} = <α(x)>. I do not understand how can we take out the 'r' out of a(x^{r}) to...
22. ### Proof: Cosets equal or disjoint

Homework Statement Two left cosets aH, bH of H in G are equal if and only if a^{−1}b ∈ H. This is also equivalent to the statement b ∈ aH. Proof: Suppose that aH = bH. Then e ∈ H. So, b = be ∈ bH. If aH = bH then b ∈ aH. So, b = ah for some h ∈ H. But, solving for h, we get h = a −1 b...
23. ### Differentiation tricks/shortcuts

This is not a specific HW/CW question, just a gap I have and want to fill. I came from a school in which calculus was only introduced in the last year so I learnt only the basics. Now, I see more and more stuff like taking an expression, say A=B+C and simply making it to a dA=dB+dC. The...
24. ### Superposition of two wavefunctions

[SOLVED] Superposition of two wavefunctions Homework Statement The problem is more of complex number arithmetic more then conceptual : Homework Equations |\psi|^{2}=\psi\psi^{*} The Attempt at a Solution I simply used the equation given above, but instead of getting 2Re{...} I...
25. ### Fictious force: Cylinder on an Accelerating Plank

Homework Statement Problem is described in the picture I do not understand how can \alpha^{'}R=a^{'}. The dimensions do not seem correct. Angular velocity x distance from the origin = tangential velocity, is that correct ? How can this equal acceleration then ? 3. Attempt to solution I...
26. ### Normal Subgroup Conjugate of H by element

Homework Statement Let H be a subgroup of group G. Then H \unlhd G \Leftrightarrow xHx^{-1}=H \forall x\in G \Leftrightarrow xH=Hx \forall x\in G \Leftrightarrow xHx^{-1}=Hxx^{-1} \forall x\in G \Leftrightarrow xHx^{-1}=HxHx^{-1}=H \forall x\in G...
27. ### Studying Career, def. of studying hard, knowledge gaps etc.

Hello all, Firstly, sorry for the long post, I was just wondering if someone is in a similar position and would like to hear their thoughts and how they deal with certain things. If it is too long, just skip to the questions I have under each paragraph. I was never spending much time...
28. ### Deflection angle of a comet traveling around the Sun

Homework Statement A comet is approaching the Sun from a vast distance with velocity V. If the Sun exerted no force on the comet it would continue with uniform velocity V and its distance of closest approach to the Sun would be p. Find the path of the comet and the angle through which it is...
29. ### Three spring two mass system, compression of the middle spring

Homework Statement I decided to revise my mechanics course and came across a problem involving a system with three springs (say, different k's) and two masses (say different). The s1-m1-s2-m2-s3 system has the outer springs connected to walls of infinite mass. I was interested in what...
30. ### Mathematics classes for MHD/Plasma Physics(maybe Fusion)

Hello all, I am wondering if I could get some advice from people who have done MHD/Plasma Physics (or even better, went to do something like 'MSc in Fusion Energy' which is offered by York University). Ideally I would like to work somewhere with fusion energy (although it is vastly dominated...