# Search results

1. ### Polymer random walk question

I understand the mean square end-to-end distance of a polymer of stepsize b and number of units N is given by \left\langle R^{2}\right\rangle = \sum_{i,j=1}^{N} \left\langle r_{i} r_{j}\right\rangle + \left\langle \sum_{i \neq j}^{N} r_{i} r_{j}\right\rangle And that the cross terms drop...
2. ### Kinetic theory question

The molecular flux (number of collisions per unit area per unit time) in Kinetic theory is given by F=\frac{1}{4} n \bar{c} where c bar is the average molecular speed and n is the density of molcules in the gas phase (molecules/m^3) I was wondering about the origin of the 1/4 term ...
3. ### Probability / Energy question

Homework Statement - Initial conformational potential energy = 2500 J - NVT Metropolis Monte Carlo simulation, T = 300 K - New conformational potential energy = 5000 J What's the probability this will be accepted within the Metropolis scheme? Homework Equations Acceptance...
4. ### Dipole Moment interaction

LOL, ok probably talking to myself here. Still playing around with this, taken it further, although I'm pretty sure my final answer here is wrong... http://voy.trekcore.com/working.jpg [Broken]
5. ### Dipole Moment interaction

OK, I've got a little bit further (this seems brutal!) I've been able to see that the separations of the charges mentioned above are: q1 & -q2: \sqrt{(d+rcos\theta)^{2}+(rsin\theta)^{2}} -q1 & q2: \sqrt{(-d+rcos\theta)^{2}+(rsin\theta)^{2}} Simplifying: q1 & -q2...
6. ### Dipole Moment interaction

Homework Statement Consider two dipoles with moments u1 and u2 arranged as in the following diagram. Each dipole is depicted as two charges of equal magnitude separated by a distance d. The centre-to-centre separation of the two dipoles is the distance r. The line joining the two dipole...
7. ### Unit cells (crystal structure)

I'm trying to clarify for myself the difference between a Unit Cell, a Primitive Unit Cell and a Conventional Unit Cell. As far as I know, Primitive Unit Cells contain only one lattice point and are the smallest possible unit cell Unit Cells are the crystal's smallest building block, and...
8. ### Entropy at different temperatures

^ Well the E/T term in the entropy would automatically go to infinity at 0K...
9. ### Entropy at different temperatures

I'm having difficulty with this problem: Consider a two state system consisting of N distinguishable and indeppendent particles where each particle can occupy one of two states separated by an energy E. What is the entropy of the system at: (A) T=0 (B) T=infinity I'm assuming this...
10. ### Bravais lattices

Homework Statement A crystal has a basis of one atom per lattice point and a set of primitive translation vectors of a = 3i, b = 3j, c = 1.5(i+j+k) where i,j,k are unit vectors in the x,y,z directions of a Cartesian coordinate system. What is the Bravais lattice type of this crystal...
11. ### Damped Driven Harmonic Oscillator

Homework Statement I'm trying to follow through a derivation involving the equation of motion for the displacement x(t) of a damped driven harmonic oscillator. m\frac{d^{2}x}{dt^{2}}+\gamma x + \beta \frac{dx}{dt}=F_{0}cos(\omega t) Where cos(\omega t) = \frac{1}{2}\left( e^{i \omega...
12. ### 1 dimensional charge distribution

Homework Statement (Lead in to question: "Assume that charges +q, -3q, 2q lie at positions -2a, 0, +2a along the x-axis respectively.") I've calculated dipole/quadrupole moments about the origin as well as the exact potential at x=+10a, however I'm confused by this next part to the...
13. ### Integration change of variables

Yep, that works good, thanks for your help nicksauce!
14. ### Integration change of variables

^ I can, I was just seeking assurance that I'd calculated that L^3 prefactor correctly after substitution...
15. ### Integration change of variables

hmm, if that was the case, wouldn't the equation take the form \Delta E_{3}^{(3)} = \frac{2}{L}\frac{V_0}{L^2}\frac{L^{3}}{(3\pi)^{3}}\int^{3\pi}_{0}\phi^{2}sin^{2}\phi d\phi (Note the L^3/... prefactor) ??
16. ### Integration change of variables

[SOLVED] Integration change of variables Homework Statement An electron is confined in an infinite one dimensional well where 0 < x < L with L = 2 x 10^-10m. Use lowest order perturbation theory to determine the shift in the third level due to the perturbation: V(x) =...
17. ### Perturbation Theory energy shift

Homework Statement I'm trying to calculate the energy shift given an electron in a 1D harmonic potential has a wavefunction \Psi_{0}(x) = \left(\frac{m\omega}{\pi\hbar}\right)^{1/4}exp\left(\frac{-m\omega x^{2}}{2\hbar}\right) The shift in E_{0} = \frac{\hbar\omega}{2} = 2eV due to...
18. ### Perturbation Theory transmission probability

I'm trying to bridge the gap between several expressions describing the insertion of a constant perturbation: a_{f}(t) = \frac{1}{i\hbar} V_{fi} \int^{t}_{0} e^{i(E_{f}-E_{i})t'/\hbar}dt' = \frac{1}{i\hbar}V_{fi}\frac{e^{i(E_{f}-E_{i})t/\hbar} - 1}{i(E_{f}-E_{i})/\hbar} so...
19. ### Dirac Equation 2!

Yep, another quick question on the Dirac Equation! I've become slightly more clued about the use of the DE now in illustrating the negative energy problem in relativistic QM as well as the existence of spin, however one thing is still puzzling me. I've read this excerpt in a text: I'm...
20. ### The Landau Gauge

Could anybody explain to me the difference between a Landau Gauge and Symmetric Gauge? I know the Landau Gauge is given by A = (0,Bx,0) producing a constant magnetic field in the z direction. I am *assuming* (process of elimination!) that A = ½B × r = (-yB/2,xB/2,0) is an example of a...
21. ### The Dirac Equation

^ Ah! Got it, i\gamma^{\mu}\partial_{\mu} = m from the original eq. Yes, I see now! Thanks so much, Dick!
22. ### The Dirac Equation

Ah, I understand now why the commutator arises, thanks Dick. However, I'm still confused about the middle term business. If I understand you right, you're saying that: (-2im\gamma^{\mu}\partial_{\mu})\Psi is analagous to: (i\gamma^{\mu}\partial_{\mu} - m)\Psi = 0 and hence should...
23. ### The Dirac Equation

[SOLVED] The Dirac Equation I'm trying to understand the following property of the Dirac equation: (i \gamma^{\mu}\partial_{\mu} - m)\Psi(x) = 0 Acting twice with (i \gamma^{\mu}\partial_{\mu} - m): (i \gamma^{\mu}\partial_{\mu} - m)^{2} \Psi(x) = 0 = [ -...
24. ### Wavepacket expansion coefficients

Sorry, I should have probably just transcribed the question as it's written here: I thought that maybe I'd need to use this relation: a_{n}(t) = \int \psi^{*}_{m}(r) \Psi(r,t) dV But that gives me a sin² integral which seems very involved for the question...
25. ### Wavepacket expansion coefficients

I thought that the eigenfunctions \psi(x)were already specific by \psi(x) = \sqrt{\frac{2}{L}}sin(\frac{\pi x}{L})?
26. ### Energy raising/lowering operators, algebra

Ha, my own stupid fault. I'd only taken one lot of aa^{+} + a^{+}a from the factorising! Thanks malawi! Been a long day hehe
27. ### Energy raising/lowering operators, algebra

\hat{x} = \left(\frac{\hbar}{2wm}\right)^{1/2}(\hat{a} + \hat{a}^{+}) \hat{p} = -i\left(\frac{\hbar wm}{2}\right)^{1/2}(\hat{a} - \hat{a}^{+}) I'm trying to demonstrate that \hat{H} = (\hat{a}^{+}\hat{a} + \frac{1}{2})\hbar w where \hat{H} = \frac{1}{2m} \hat{p}^{2} +...
28. ### Wavepacket expansion coefficients

I'm trying to get my head around the idea of expansion coefficients when describing a wavefunction as \Psi(\textbf{r}, t) = \sum a_{n}(t)\psi_{n}(\textbf{r}) As I understand it, the expansion coefficients are the a_{n} s which include a time dependence and also dictate the probability of...
29. ### Orthonormality condition proof

I'm working through a proof of the orthormality condition for a complete set of states and am struggling with one element of it: Consider the eigenstates of the Hamiltonian in the following way: 1: \int\Psi^{*}_{m}H\Psi_{n}dV = E_{n}\int\Psi^{*}_{m}H\Psi_{n}dV and 2...
30. ### Difficult integration

I'm probably missing something obvious here, but I'm trying to integrate the following expression; \intsin{^2}(kx)dx I've tried doing it by part but with no luck. Is there some specific method I need to follow, or is it one of those I can only get by looking it up?