# Search results

1. ### Is there magnetic filed or not?

Homework Statement Suppose that the entire region below the plane z=0 is filled with uniform linear dielectric material of permittivity epsilon. In t=0 a particle of mass m and charge e travels with velocity v in x direction and is parallel with the z=0 plane, and is distance d above the...
2. ### Photon+deuteron-&gt;p+n , E(photon)=?

Yes, I did think that these energies were the same, that's why I asked a question, why aren't they the same. Yes, you did understand correctly but I also wanted to know why is that so. And here is why: Thank you all, now I understand.
3. ### Photon+deuteron-&gt;p+n , E(photon)=?

[SOLVED] photon+deuteron--&gt;p+n , E(photon)=? Problem: What is the minimal energy of photon for disassembling deuteron on proton and neutron (photon+d --> p+n)? By how much is that energy larger than energy of binding of deuteron? Binding energy of deuteron is E=2.225MeV, mass of deuteron is...
4. ### Which processes are posible and why?

p=(E(photon)+E(electron), p1+p2=0) before, after I should have E of electron that exceeds rest mass of electron, and no momentum, so the answer should be that process is not possible. It should not be possible because photon should have some momentum (three component momentum) and since it...
5. ### Which processes are posible and why?

1. Photon runs into motionless electron and gives the electron all of its energy 2. Fast positron interacts with motionless electron producing one photon 1. I know that if that electron were in atom, photon of some energy could give its energy, and electron would jump into a higher energy...
6. ### Q-value calculation - nuclear physics

http://www.nndc.bnl.gov/masses/mass.mas03, and the missing data: u=931.50MeV p=1.00728u=938.28MeV d=2.01355u=1875.63MeV e=0.511MeV A(a,b)B=a+A->b+B Q=[(m(a)+m(A))-(m(b)+m(B)]MeV (1) This can't be more simple, but in every example that I saw in my lectures different values occur. For example...
7. ### Quark Gluon Plasma

I don't understand this one. Why wouldn't be, in principal, possible to produce force strong enough to break the bond, how does colour prevent that? Quark would carry colour anyway, as do free or bound electrons carry electrical charge, isn't that right? Yes, I had some wrong understanding...
8. ### Quark Gluon Plasma

I'm having a problem understanding QGP. Hadrons are bound states of quarks which interact by interchanging colour. Now, as I understood we haven't observed free quarks because the force rises by distance and that force is really strong. As I read in QGP quarks and gluons are not bound, you can't...

Thank you.
10. ### Q of p+p reaction.

What is the mass of deutron then? How can I calculate it, or where can I find that data?
11. ### Q of p+p reaction.

p+p -> deutron+positron+neutrino(e) The question is how much energy is released? Q=(m_{p}+m_{p})-(m_{d}+m_{e^+}+0.26MeV)=(938+938)MeV-(938+939+0.51+0.26)MeV<0 So I don't understand how can energy be relesed? I found that Q for that reaction is 1.442MeV. What am I doing wrong?
12. ### Interpretation of solution in QM

I understand now, thanks.
13. ### Interpretation of solution in QM

Ok, but how do I write my solution down as a combination of two real solutions, just like you did for (2)?
14. ### Interpretation of solution in QM

What if I wanted to plot psi(z)? I have never seen 3D plot for a wave function in one dimensional problem. I mean, when somewere wave function is large you can say that the particle is probably there, but if you have real and imaginery you can't plot just real part and talk about probability.
15. ### Interpretation of solution in QM

I'm having a problem understanding solutions of differential equation in QM: \psi''(z)+\frac{p}{z}\psi(z)+k^2\psi(z)=0 (1) I usualy use Fourbenious method, and in this case I get a 3 coefficients recursion relation which is really messy. So I do it like this: for really large...
16. ### Proof of operator relation

Nooo, it can't bee :). I spent all night trying to solve this in most complicated ways and I didn't saw this... Thank you very much!
17. ### Proof of operator relation

I'm having a problem proving this operator relation: exp(-i\phi\hat{j_{i}})exp(i\theta\hat{j_{k}})exp(i\phi\hat{j_{i}})=exp(i\theta(cos(\phi)\hat{j_{k}}+sin(\phi)\hat{j_{l}}) (1) where [\hat{j_{i}}, \hat{j_{k}}]=i\epsilon_{ikl}\hat{j_{l}}. (2) I can prove this for...
18. ### 4-vector - momentum^2

I'm having a problem understanding this: P^2=P_{\mu}P^\mu=m^2 If we take c=1. Here is what bothers me: P(E, \vec{p})=E^2-(\vec{p})^2 Now, I assume that E=mc^2, and for c=1, E^2=m^2? Is that correct? And I don't know what p^2 is, I look at it as: (\vec{p})^2=m^2(\vec{v})^2...
19. ### Second quantization

I found the Schwabl book and first chapeter is just what I need. Thank you
20. ### Second quantization

OK, I'll try to find that book, at least, now I have some idea where to look. Thank you.
21. ### Second quantization

That is the exact question. I looked a few books (read whole Griffiths and most of Shiff) and they all mention lowering and rising operators when it comes to harmonic oscillator. But I haven't found any explanation why are these operators defined in why that they are, and how would I write a...
22. ### Second quantization

Noo... that wouldn't work H=p^2/2m+V, so I can't express kinetic energy in that way... So the question stands...
23. ### Second quantization

I don't know the formal definition but I know that we work in it when dealing with more than one undistinguishable particles. Than states can be symmetric for bosons and antisymmetric for fermions resulting that only one fermion can be in one state while bosons don't have that restriction. From...
24. ### Invariancy of H(x,p) to translation and conservation of momentum

Thanks I got it now!
25. ### Second quantization

Write momentum, kinetic and potential energy, and two particle interaction in second quantization. That is the question that I need to answer for my exam, but I don't have any idea what second quantization is, except that you can solve harmonic oscilator by using ladder operators. I can't find...
26. ### Invariancy of H(x,p) to translation and conservation of momentum

I don't understand your answer. First, I don't understand why I need this: H(x+a)=exp(iap)H(x)exp(-iap) Second, how do I expand H in Taylor series (and why?) and how to connect that with [p,H]=0, and how does that prove the connection between hamiltonian invariantcy to translation and...
27. ### Invariancy of H(x,p) to translation and conservation of momentum

I have two similar questions. 1. Show the relationship between invariancy of hamiltonian to translation and conservation of momentum. (In QM) 2. Show the relationship between invariancy of hamiltonian to rotation and conservation of angular momentum. (In QM) I have no idea how to prove this...
28. ### Proof that x and p are hermitian

I did it, thanks. Now that I have seen your example it was not hard for momentum.
29. ### Proof that x and p are hermitian

I understand what you're saying but I haven't been able to proove it.
30. ### Proof that x and p are hermitian

Proove that position x and momentum p operators are hermitian. Now, more generaly the proof that operator of some opservable must be hermitian would go something like this: A\psi_{n}=a_{n}\psi_{n} Where A operator of some opservable, \psi_{n} eigenfunction of that operator and a_{n} are the...
31. ### Momentum operator and comutators

Yes, of course, I understand now! Thank you!
32. ### Momentum operator and comutators

I'm having trouble understanding the folowing: (I'll write h for h/(2Pi)) Momentum in x direction is represented by operator p=-ih\frac{d}{dx}. So comutator [x,p]=xp-px=x(-ih)\frac{d}{dx}-(-ih)\frac{dx}{dx}=0+ih*1=ih. Now here comes the part that I don't understand. I'll calculate [x,p^2]...
33. ### Another Bar Pendulum Question

Moment of inertia around a center of mass for perfect rod of length 2k is: I_{0}=\frac{1}{12}m(2k)^2 How did you got this equation: d_{1} + d_{2} = \frac{4\pi^2(d_{1} + d_{2})}{T^2} ? Fixed walue of T? What two walues of d?
34. ### Which is better for measuring g? Bar Pendulum or Simple Pendulum?

Yes, a small iron ball at the end of very thin rope. How precise result do you want anyway? I really can't see the way that you could get a better result using physical instead mathematical pendelum. There are much more parameters that you can't assess precisely.
35. ### Which is better for measuring g? Bar Pendulum or Simple Pendulum?

If I remember correctly I got 9.90+-0.07m/s^2 with mathematical pendulum. I think that's a better way; lower air resistance, easier to define dimensions. Oscilations out of vertical plane should not bother you, if you are careful the vertical oscilations are small, and don't affect final result.
36. ### De Broglie wavelength of a photon

It would be very useful if you studied theory once again before trying to solve any more problems, becoase if you have problems on this level it means that you haven't quite got the grasp of meaning of things and you will not be able to make any progress.
37. ### Rotating rod

I have successfully solved the problem by my self, so no need for an answer.
38. ### De Broglie wavelength of a photon

You probably didn't converted energy from eV in Joules. And the formula is: E=\frac{p^2}{2m_{e}} (2 is missing in yours). I got the result \lambda=5.48\cdot{10^{-10}}m.
39. ### De Broglie wavelength of a photon

Try finding it form: E=\frac{p^2}{2m}
40. ### Rotating rod

Is this ok: E=\frac{1}{2}(\dot{x}_{cm}^2+\dot{y}_{cm}^2+\dot{z}_{cm}^2)+\frac{1}{2}I_{cm}(\dot{\theta}^2+\dot{\phi}^2) ?
41. ### Rotating rod

Homework Statement The rod of length 2a swings and it rotates \dot{\phi}[/tex]. Find kinetic energy. The Attempt at a Solution I know how to find kinetic energy if we have rotation just in xy plane, but I'm having a problem to understand how to do it with rotation around y axis. If we have...
42. ### Classical mechanics - sphere attached to rod

It seems that I wrongly understood theory that solves such problems. Thank you for the explenation. Nevertheless I would like if you could check if I did it correctly: The rod: E_{rod}=\frac{1}{2}m\dot{r}_{cm}^2+\frac{1}{2}I_{cm}\dot{\theta}^2 E_{rod}=\frac{1}{2}m(\dot{x}_{cm}^2+\dot{y}_{cm}^2...
43. ### Classical mechanics - sphere attached to rod

Homework Statement I need to find equations of motion of system on picture. It is easy when one finds Lagrangian so I am not asking you to calculate it, but I'm not sure if I correctly calculated the kinetic energy, so If anyone can verify, I would be greatfull. The rod has mass m, length 2a...
44. ### Solutions for Griffiths and recommendation for book on problems in QM

I'm reading a book, Introduction to Quantum Mechanics by D.Griffiths, and at the end of each chapter there are problems to be solved, nice problems, but I can't find the solutions, and I don't really see the point of solving the problem if you can't check the solution. In the preface of book...
45. ### Looking for supplementary physics book!

If you want a book on specific topics, than find a Schaum on topic you are interested in. (I can't imagine a topic not coverd in Schaum) My recomendation on general physics, collection of briliant problems... Irodov - Problems in general physics. You'll find all kinds of problems there...