# Search results

1. ### Electric field inside and outside a sphere (Not Gauss)

Excuse me sir, for the late reply. I think I didn't understood it first, then forgot about it, now I stumbled across it and still haven't figured it out ρ . A . d = σ . A ρ . dx = σ Is this what you were implying? I don't see how R sin θ dθ equals the thickness of my disk, since R sin θ...
2. ### I Directional derivative: identity

Hi all, According to wikipedia: Can someone explain to me with a mathematical proof the following: $$\frac {\partial f(x)} {\partial v} = \hat v \cdot \nabla f(x)$$ I don't get this identity except the special example where the partial derivative of f(x) wrt x is a special kind of a...
3. ### Electric field inside and outside a sphere (Not Gauss)

I think the solution is just wrong because the last step isn't correct? Edit: Looks like the solutions manual from Zangwhil contains errors. This is from a site I found on the internet (check problem 4d) https://pa.as.uky.edu/sites/default/files/Phy416G-HWSol4.pdf but now dσ = 2πρR sin θdθ...
4. ### Electric field inside and outside a sphere (Not Gauss)

Find E(r) inside and outside a uniformly charged spherical volume by superposing the electric fields produced by a collection of uniformly charged disks. a+b) Given equations, sketch of problem This is the equation in the handbook for a disk (but in the exercises the z becomes x, without loss of...
5. ### I Levi-Civita: very small problem, need two steps explained

Hi, Here is the complete exercise and solution from Zangwhil Modern Electrodynamics. Since my question is about the levi-civita symbol, I thought it would be better to post it here since it is a tensor. So in the step e(ijk) D(jk) = e(ijk)D(jk) + e(kj)D(kj) So The e(kj)D(kj) part is wrong and...
6. ### I Levi-Civita: very small problem, need two steps explained

Hi all, Can someone explain me the last two steps? I don't know why suddenly there is a term with only two indices, and then in the last step you do something distributive and again three indices. Thanks in advance
7. ### I Griffiths problem 2.7 Messy Integral

Ok, so partial integration. integral (fdg) = fg - integral (gdf) f = z-Ru dg = du / (R² +z² -2zRu) ^ 3/2 For g I get 1 / ( (Rz) * (R² + z² -2*R*z*u)^1/2) fg = (z-Ru) / ( (Rz) * (R² + z² -2*R*z*u)^1/2) now the integral part - integral ( g d f) df = -R du So The integral to solve Rdu /...
8. ### I Griffiths problem 2.7 Messy Integral

I get something extremely messy. Here is the first part, the second part with integration by parts,I'll do it again tomorrow. So the first term integral (z du / (R² + z² - 2 Rz u) ^3/2 ) d(R² + z² - 2 Rz u) = -2 R z du The integral becomes -1/2R * integral ( d(R² + z² - 2 Rz u) / (R² + z²...
9. ### I Griffiths problem 2.7 Messy Integral

Hi all, "Integral can be done by partial fractions - or look it up" So second line, that's what I want to do. How to deal with this? What substitution can I use? Never encountered partial fractions with non-integer exponents. Someone give me a tip? Thanks in advance
10. ### I Evaluating Scattering Integral

Hi myself, I found out I need to include something like 1- ( stuff going in the z direction after k scattering) / (stuff that would go in z direction if there was no scattering at all), so 1 - k' projected on z axis /k = 1 - (k cos theta) / k Yes, I'm a noob. So I'll start over. :')
11. ### I Evaluating Scattering Integral

Hi, Sorry for the late reply. Hope this is somewhat more clear, because last post was a bit messy. Still haven't learned LaTex. Is this valid? I couldn't evaluatie the last integral, because of the square. Do I need to use partial fractions? Thanks
12. ### I Evaluating Scattering Integral

This is what I got uptil now, but I have to leave. Thanks for help, give me feedback if you want to, and I will post update when I'm back, probably tomorrow. Thanks for the patience anyway!
13. ### I Evaluating Scattering Integral

It's been this the whole time?, with r being k' Ps: After this I'm going to finally learn LaTeX
14. ### I Evaluating Scattering Integral

How come dk' is a volume element, it's the derivative of a vector. Most textbooks, the volume element is called a d tau or a dV My head is a mess. It's like I completely forgot how to calculus. Good news: I know I'm wrong. Really don't see it
15. ### I Evaluating Scattering Integral

Mr 6 hours and I still have no clue, can you please hold my hand and solve it with me. I mean, just say what I have to do, I will solve it, but give me instructions. I mean what's the deal with the dirac function and the dk', it's supposed to be a volume element and the dirac term, why isn't...
16. ### I Evaluating Scattering Integral

Wait wait, I am extremely confused. I just have to integrate over the k' first?
17. ### I Evaluating Scattering Integral

dk' is a vector. Then I have an integral comprising of 3 terms each involving unit vectors. I really don't have a clue.
18. ### I Evaluating Scattering Integral

I don't see it. dk' is not a volume element?
19. ### I Evaluating Scattering Integral

Hi, I really don't have a clue to solve this. I tried something like the dirac function identity: But then I saw it's dk' not dk' and couldn't got it straight. Can someone help me with this?
20. ### Tsampirlis Chapter 1 Inner Product

To come back on my question. Part of the confusion arose, because I've forgotton or looked over the meaning of the ≡ character. They were referring to the elements of the matrix. Stupid of me, but it's clear now.
21. ### Bloch electrons, periodicity

I think a Bloch wave is not lattice periodic. If you plot real part or imaginary plot it's not lattice it's not periodic, but the complete Bloch squared, so the probability density of finding electron is lattice periodic.
22. ### Tsampirlis Chapter 1 Inner Product

So what he meant is e1 e2 times e1 e2 e3 e3 and ordinary matrix multiplication?
23. ### Tsampirlis Chapter 1 Inner Product

http://math.stackexchange.com/questions/47486/matrix-representation-of-inner-product Is it also called Sylvester's Law On Inertia? I will look it up, thanks, will let you know if I get it or not.
24. ### Tsampirlis Chapter 1 Inner Product

Apparently not, but in the video I posted it is defined as 2x2 matrices and I don't have an example for a 3x3 matrix. I have notion of Linear Algebra, never encountered matrix representation of an inner product.
25. ### Tsampirlis Chapter 1 Inner Product

So my error is, I don't understand the notations? Lower indices are not always columns? If he defines the upper indices as rows and columns lower indices why isn't it guv = eu . ev instead of writing everything lower index
26. ### Tsampirlis Chapter 1 Inner Product

So when I saw eu and ev I thought two 1x3 matrices, since see notation above, which is not right, right?
27. ### Tsampirlis Chapter 1 Inner Product

So suddenly the lower indices represent columns in the case of μ and rows in the case for ν? And it's just plain matrix multiplication. First thing Samparlis did was define this in the book:
28. ### Tsampirlis Chapter 1 Inner Product

But in the book they define lower indices as the number of colums, so I thought it were two 1x3 matrices?
29. ### Tsampirlis Chapter 1 Inner Product

Apparently I have never heard of a matrix of an inner product. Should I follow this?
30. ### Tsampirlis Chapter 1 Inner Product

Hi all, The basis vectors are defined as 1x3 matrices, how can the result be a 3x3 matrix? How can the result of a dot product be a 3x3 matrix, I'm stumbled, how can I evaluate this? A inner product returns a scalar, and now it returns a 3x3 matrix, please help. Thanks.
31. ### Bloch electrons, periodicity

? Real part of a bloch wave is not periodic with the lattice, but a bloch wave squared is. Correct?
32. ### Bloch electrons, periodicity

Is it because the blue part is only the real part? Another picture If you would square the total Bloch wave (both real and imaginary part, not individually but the total (Abs(Real + Im)))^2 in the bottom would it then have the same periodicity as the top picture, the lattice? Because I've...
33. ### Bloch electrons, periodicity

Wow, I still don't get Bloch's Theorem. I thought I knew it, but the second form always confuses me. ## \psi(r+R)=\exp(ikR) \psi(r)##. The probability density is the same, because the absolute factor of the phase turns 1. I just don't see where the probability density's are the same. If you...
34. ### Quantum QM books in order from easy to intermediate to hard

So far I got: Griffiths => easy, ok it's not so easy,not really what I ment, but it covers the basics without going much into the real deeper algebraïc stuff, emphasis on calculus, that's what I mean with easy, I mean a decent, solid book for undergraduate level. But suppose I tackle Griffiths...
35. ### Uncertainty Principle And Collapse Wavefunction

So the wavefunction can collapse to a spike, thus like a Dirac Delta Function, with 100% certainty of position?
36. ### Uncertainty Principle And Collapse Wavefunction

Discrete spectrum of eigenvalues; the corresponding operator commutes with Hamiltonian, zero uncertainty, values with 100% certainty that are always the same like energy states in Hydrogen, because energy is basically a scalar and a scalar commutes with every operator. So, u have a...
37. ### Uncertainty Principle And Collapse Wavefunction

Upon a measurement of the position, the wavefunction collapses to a spike centered at x0 https://farside.ph.utexas.edu/teaching/315/Waveshtml/img3240.png I encounter similar spike pictures numerous times, but there is an uncertainty in position , it can't be a spike right. First thing I see...
38. ### CCS Carbon Dioxide Storage Capture

Hi, What is in general the most cost-effective way of the CCS technologies: Post,pre or oxyfuel combustion?
39. ### Engineering Calculate impedance of a feedback circuit with Blackman's technique

Thanks for info. Can you post your blackmans analysis? This evening I try Vtest Itest again.
40. ### Engineering Calculate impedance of a feedback circuit with Blackman's technique

Hi all. Need to read all these posts first. Rudeman You can send your calculations also to me if you want. Donpacino. In the short introduction of Blackman in my course it was stated I had to put the input voltage to zero. You say Vbias can't be zero?
41. ### Engineering Calculate impedance of a feedback circuit with Blackman's technique

The first drawing was the only thing I got, it is not mine, but from a student who did an earlier exam and posted the questions on a board.. I suppose it are two NMOS's. So my blackman computings are wrong? :) Blackman is poorly explained in my course.
42. ### Engineering Calculate impedance of a feedback circuit with Blackman's technique

Homework Statement [/B] Calculate impedance of the node shown in this feedback circuit with Blackman and the Vtest/ Itest method. Homework Equations [/B] Zin (with feedback) = Zin(with feedback circuit disabled) * (1-RRshort)/(1-RRopen) RR = return ratio In this form blackman theorem to us...
43. ### Extracting information from a Bode Plot (PM, GM, etc...)

DC Gain => 80 = 20 log | DCGain | So DC Gain = 10^4 = 10000 Is this correct?
44. ### Extracting information from a Bode Plot (PM, GM, etc...)

Hi all, 1. Homework Statement http://imgur.com/kqZK1Vv This is the loop gain (A.F.) of unity feedback system. I need to extract DC Gain Bandwidth GBW (is GBW Gain Bandwith ?) f unity (I suppose this is unity frequency) Phase Margin Gain Margin What is the optimal PM and f2/GBW for time and...
45. ### Tight Binding Model: mathematics

I don't get how you go from the double summation to the new one with the new index etc. Oh yeah i forgot to say: in the new expression they put Xj' = 0. It's a function of the difference, Xj-Xj'. For each choice of j',the sum over j yields the same result. => I don't get it. Also I don't get...
46. ### Tight Binding Model: mathematics

From the textbook of Omar: Can someone explain me the summation of this? I simply don't get it? Thanks in advance.
47. ### MATLAB Quiverplot of r hat / r squared in matlab

Omg. Yes. I looked over it.For some reason I made the same Typing error over and over again. Y is wrong.
48. ### MATLAB Quiverplot of r hat / r squared in matlab

Hi, I want to quiverplot this function in matlab. >> [X Y] = meshgrid(-2:.2:2,-2:.2:2); >> X1 = X./(X.^2+Y.^2).^1.5; >> Y1 = Y./(X.^2+Y./2).^1.5; >> quiver(X,Y,X1,Y1) >> I got this: What am I doing wrong?
49. ### Free Particle - Wave Packet

I've read all your posts a while ago and thanks for that. But I still haven't got an answer and wasn't sure how to really explain what I want to hear. I think I want to know what it means for the free particle to composed of more than 1 wave and how would you do it in a lab theoretically...
50. ### NMOS Common source DC-bias and gain

Yes, I was also confused about the current source symbol. :) But it is one indeed. Thanks for the confirmation that my calculations are correct.