# Search results

1. ### What is the meaning of a basis?

Hi! There is a concept I don't understand and would love to have is cleared... What is the meaning of a lattice with a basis? What do I need it for? Say I have a honeycomb structure. (fig 1) and a basis as mentioned there (did I understand it right? is it the basis? ) why does it become...
2. ### Transistor circut analysis

Hi there! I'm having trouble understanding the transistor circuit analysis. Hope you could help me :) First I need to find the active region of the transistor. What I saw in the solutions was an assumption that the transistor is in saturation region and then: VBB=IbRb+Vbe+IeRe...
3. ### Mathematica Mathematica help

Hi! I hope this is the right forum... I am having trouble with Mathematica: Maybe someone could help out of his experience ? I created w[x]:=RandomReal[NormalDistribution[0,1],x] I got it's plot. Now I want to integrate it: I wish to receive another graph, which is the plot of the...
4. ### Notation problem

Hi! I can't understand something in field theory and need your assistance: I wish to understand why a particle of mass m^2<0 can't exist. For a massive particle, in its reference frame, one would write: p_\mu=(m,0,0,0). I understand that. But for m=0, why is: p_\mu=(p,0,0,p) And for m^2<0...
5. ### Understanding definitions

Hi guys! There is something I would like to get your help with... I am looking at the equation: W^{\mu}=-\frac{1}{2} \varepsilon^{\mu\nu\lambda\sigma}M_{\nu\lambda}p_{\sigma} Which is, if I understand correctly,a Casimir Operator. Now, I wish to look at a particle in its rest reference...
6. ### Commutators, etc.

Hi you guys! I am having a hard time understanding some stuff in this context. I was not able to find any guidance in books or anything. say I want to calculate: [W^2,p^\alpha]=0, W=\frac{m}{2}\varepsilon^{\mu\nu\lambda\varepsilon}M_{\mu\lambda}p_\delta How do I do that? I can't...
7. ### Why change the coordinates?

Hello! I have a question regarding a method I saw every now and then: Say I have a system containing of two masses, attached to one another by a spring. Each is attached to a wall by another spring. Now I wish to know the eigen vectors and eigen values( \omega) of movement. I get: \ddot...
8. ### How to find primitive function?

Hello! I am having trouble understanding the whole primitive function thing in complex analysis. How do I check if a certain function, say [itex] \frac {sin(z)} {z} [/tex] has a primitive, say in C/{0} ? What is the intuition? My intuition is that 0 is the only singularity, so if it is...
9. ### General question about E-L in revativity

Hello! I ask this question in this forum because I wish to be given a general explanation. Hope it is OK given this metric: ds^2= \frac {dt^2} {t^2}- \frac{dx^2} {t^2} I wish to calculate the geodesics. S=\int{ \frac {d} {d\lambda} \sqrt{\frac {1} {t^2} \frac {dt^2} {d\lambda^2}-...
10. ### Finding Geodesics What I wish to understand, is how to solve

Finding Geodesics What I wish to understand, is how to solve this one: given this metric: ds^2= \frac {dt^2} {t^2}- \frac{dx^2} {t^2} I have to calculate the geodesics. S=\int{ \frac {d} {d\lambda} \sqrt{\frac {1} {t^2} \frac {dt^2} {d\lambda^2}- \frac{1} {t^2} \frac {dx^2}...
11. ### Another gravitation problem

Hello! I need to find the relation between E^{2}-B^{2} and F_{\mu\nu} F^{\mu\nu} Actually, I need to use this relation to determine that the first is a scalar. What I can't understand is how these notations match the formal definition: If I multiply a matrix by another (same size) I should...
12. ### Gravitation question

Hello! I fail to understand this question... I don't even know how to approach it... Given: m\frac{d^{2}x^{\mu}} {d\tau^{2}} =e{F^{\mu}_{nu}} \frac{dx^{\nu}} {d\tau} I have to define u^{\mu}= \frac {dx^{\mu}} {d\tau^2} and obtain: u^0= cosh(\frac {eE\tau} {m}) u^0(0)+ sinh(\frac {eE\tau}...
13. ### Infinite potential well- Delta potential inside

Hello again. Thank you guys. You have been great help... I have another one: Given a potential well- 2a is it's width, and in the middle - there is a delta potential: V(x)= \frac {\hbar^2} {2m} \frac {\lambda} {a} \delta(x) I am looking for the odd solution to this problem. I thought...
14. ### Angular Mumentum: Question for my exam tomorrow

Hello! My exam is tomorrow... Help will be much appreciated... Say l=0 is given. My hemiltonian is \bar {H}=\frac {1} {\hbar} \Omega (\vec {n} \cdot \vec {L})^2 And we use "base states" that are L_z eigenstates, |1 m> n is a general direction unit vector. Now, I am looking for...
15. ### Potenitial well

Hello again! Say I have a potential well, between 0 and a. I also know how the wave function looks like for (t=0): \psi(x,0)= \frac {2bx} {a} for 0<x<\frac {a} {2} and \psi(x,0)= 2b(1- \frac {x} {a} ) for \frac {a} {2} <x<a Now, I wish to find the wave function of a general time...
16. ### Confused about wave function

Hello! There is something I can't understand... \psi(0)= \frac {1}{\sqrt 5}|1>+ \frac {2i}{\sqrt 5}|2> I need to find <H> for \rightarrow \psi(t) For E_n= \hbar \omega (n + \frac {1}{2}) , \hat {H} =(a^\dagger a+1/2) These make me understand that this problem is one dimensional. (+1/2 in...
17. ### Retarded time approximation

Hello again! Facing some problems (my exam is taking place tomorrow... help is needed. Many thanks in advance!) I need to find an approximation for a retarded time. I don't understand how. This is what my lecturer wrote: sin(\varphi-\omega t)=exp(i\varphi'-i\omega(t-r/c)-i\omega(r'cos\theta...
18. ### Potential between two parallel planes

Hello ! I have an exam this Wednesday... your help will be appreciated... There are two types of questions I can't figure out how to answer... the first is this one: Find the potential between two parallel planes. The first is in x=0, the second in x=L. \Phi(x=L)=\Phi_0 |sin(ky)| and...
19. ### Two dim. Oscilator

Hello! I am new here... I am a physics student, seeking for help. There is something I just can't seem to understand... Your help will be much appreciated... In my exercise, I have the following Hamiltonian: H=\frac{p_x^2}{2m}+\frac{p_y^2}{2m}+\frac{1}{2}m \omega^2 (x^2+y^2)+\lambda xy I was...