# Search results

1. ### Testing Error on 96 physics gre?

On problem number 77 on the 1996 Physics GRE test that is now available for practice, I believe there are two answer choices that are completely equivalent. The test is here: http://www.stanford.edu/group/sps/files/GR9677.pdf [Broken] I think answer choice (d) reduces down to choice (b)...
2. ### Changing The Shape of a potential well

This question was posed to me a while ago, but I never fully understood how to solve it. You have a particle in the first excited state of the infinite square well (with the origin taken to be at the center. You "abruptly" change the shape of the potential well to be that of the harmonic...

Am I banned for asking that? I understand if so

That is a very good question. Thank you

I am reading the section in Griffiths' Quantum about adding spins together. It says if you have a particle of spin s1 and another of spin s2 then the possible composite spins are s1+s2, s1+s2 -1, s1+s2-2, ... |s1-s2| that rule (though not proven in this text) has seemed straight forward to me...
6. ### News Black people in China?

Okay that document says there are about 1.1 billion Han. I am assuming none of them are black So I have narrowed it down to: < .2 billion black people in China So far I am winning
7. ### News Black people in China?

OK then let's see who can make the most clever use of resources we do have. How far can you narrow your estimate. I will start # of Blacks (African) in China < 1.3 billion who can improve this
8. ### News Black people in China?

I am hoping for a number or some sort of decent estimated range
9. ### News Black people in China?

I can't find this information online. I was wondering if anyone could tell me about how many black people reside in China. There are 1.3 billion people in China
10. ### Finite Spherical Potential Well

I don't understand that reply. I got it by solving the schrodinger equation, so it must satisfy it, no?
11. ### Finite Spherical Potential Well

This is more a qualitative question than a specific homework question, but a homework problem got me wondering about it. I was solving the finite potential well. V(r) = 0 \hspace{1cm} r \geq a V(r) = -V_0\hspace{1cm} r < a I am trying to solve for the ground state energy. When I find...
12. ### LaTeX code for Quantum

I have been using LaTex lately for my Quantum assignments. I am not too good with it yet. I was wondering if anyone knows how to get the proper font for the quantum numbers I want the lowercase loopy L that is almost always used for angular momentum, but I can't figure out how to type it...
13. ### Having Trouble with this DEQ

I am trying to solve x^2y'' + xy' = 0 I know that two solutions that work, by inspection are y_1 = c_1 and y_2 = c_2ln(x) where c_1 and c_2 are just arbitrary constants. However I was hoping I would be able to find a more systematic way to solve it. I can get the...
14. ### Integrating through singularities

These replies are all helpful. Thanks for the feedback.
15. ### Integrating through singularities

I have done principle parts, but even though sin(x)/x converges to 1 in the limit that x approaches 0, if we wish to integrate directly through the point x=0 how can we ignore the fact that it is undefined at 0. It is a removable discontinuity, but it is still a discontinuity. Isn't...
16. ### Integrating through singularities

I am a little bit confused about dealing with integrals around singularities because my professor seems to treat some situations more rigorously than others. We talked this integral and said \int_{-\infty}^{\infty}\frac{1}{x^3} dx = undefined This seems a little bit unintuitive to me...
17. ### Explicitly Deriving the Delta Function

The convergence factor seems to work, except when I integrate the delta function you defined I don't get 1, I get some constant with pi. Maybe I am doing the integral wrong.
18. ### Explicitly Deriving the Delta Function

When working with fourier transforms in Quantum mechanics you get the result that \int_{-\infty}^{\infty}e^{-ikx}e^{ik'x} = \delta(k-k') I understand conceptually why this must be true, since you are taking the fourier transform of a plane wave with a single frequency element. I have also...
19. ### Quantum Mechanics - Ehrenfest's Theorem

I see where I was going wrong, thanks for the help.
20. ### Quantum Mechanics - Ehrenfest's Theorem

The Potential term is zero, I think because it is in an infinite well where the potential at the walls (-L/2 and +L/2) is defined to be infinity and the potential at -L/2<x<L/2 is defined to be zero.
21. ### Quantum Mechanics - Ehrenfest's Theorem

We have to apply Ehrenfest's theorem and I don't think it was ever explained well to us. I have read that expectation values of measurable quantities behave according to classical physics equations ie. M\frac{d\left<x(t)\right>}{dt} = \left<p(t)\right> I think I must be applying this idea...