# Search results

1. ### I Lagrangian differentiation

No you didn't scare me away. I do understand what the notation represents too. However, I saw in my notes that they had differentiated quickly the klein gordon lagrangian to get the klein gordon equation. It is defnitely a notation thing though. I don't really understand the difference between...
2. ### I Lagrangian differentiation

I'm just in need of some clearing up of how to differentiate the lagrangian with respect to the covariant derivatives when solving the E-L equation: Say we have a lagrangian density field $$\mathcal{L}=\frac{1}{2}(\partial_{\mu}\hat{\phi})(\partial^{\mu}\hat{\phi})$$...
3. ### Differential operator acting on scalar fields

Yeh sorry I did mean to put that A is a scalar field. Should be a bit more explicit about the problem I'm having as I wrote that last night after spending almost all my evening on it and I had given up. Basically the problem is to do with the klein gordon current with an electromagnetic field...
4. ### Differential operator acting on scalar fields

Homework Statement I really cannot seem to be able to follow the logic of how you would use the product rule when using 4 vector differential operator. ∂μ is the differential operator, Aμ is a scalar field and φ and φ* is it's complex conjugate scalar field. I have the answer, I'd just really...
5. ### I Why does the magnetisation orient itself in the -y direction

In magnetic resonance, if we apply a 90 degree pulse in the x direction when we have a magnetisation orientated in the z direction. Why do we get the magnetisation then orientated in the -y direction immediately after the pulse? I don't understand why it would not be in the +Y direction
6. ### I Why is the spin of this state equal to one?

Why is the spin of this state s=1 then? What am I missing?
7. ### I Why is the spin of this state equal to one?

Do hunds rules not say that the S= sum of all m_s values?
8. ### I Why is the spin of this state equal to one?

I've just come across the spin states of a two electron system. There are 4 states possible and I am a little confused as to why the state below has values of s=1 m_s=0? [1/√2]{α(1)β(2)+α(2)β(1)} where α(i) and β(i) tell us if the particle has +ve or -ve z component of spin respectively. I...

Sorry I understand this now. The expansion was about z=i, but I understand you would just the distance between the place you are expanding around and the closest singularity.
10. ### I Calculus of Variations Dependent variables and constraints

If we have a function: $$f(x,x',y,y',t)$$ and we are trying to minimise this subject to a constraint of $$g(x,x',y,y',t)$$ Would we simply have a set of two euler lagrange equations for each dependent variable, here we have x and y...

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > How would you find the radius of convergence for the taylor expansion of: $$f(z)=\frac{e^z}{(z-1)(z+1)(z-3)(z-2)}$$ I was thinking that you would just differentiate...
12. ### Why do we want high Radiation Resistance in an antenna?

I just read that if we have an antenna, then if the radiation resistance in the antenna is small, then the antenna is an inefficient antenna? This seems somehow counter intuitive to me. Could anyone help explain?
13. ### I What does ALPHA_EM stand for?

Omg ofc it is! I was confused because it was about alpha particles. Thank you!
14. ### I What does ALPHA_EM stand for?

What does the term αEM mean? I'm looking at the Coulombic Potential of an alpha particle separating from a daughter nucleus and it is stated that: VC(r)=2ZαEMħc/r Im not really sure where this term derives from? Does anyone know?
15. ### Heisenburg Ferromagnetic Model

Homework Statement There are several parts to this question, however I could complete these parts. It is just an equation used in the prior part to the question that is need to solve this: If we define $$\sigma_{n}^{-}=\sigma_{n}^{x}+i\sigma_{n}^{y}$$ and with the...
16. ### Show the drift velocity is ExB/B^2

yes but what does it specifically mean by 'drift'. You said the velocity in the 3rd dimension is not a drift.
17. ### Show the drift velocity is ExB/B^2

Can I ask what it means by a drift velocity then?
18. ### Show the drift velocity is ExB/B^2

Would that be an acceleration then?
19. ### Show the drift velocity is ExB/B^2

Why are you allowed to assume the drift is in two dimensions though?
20. ### Show the drift velocity is ExB/B^2

A charged particle drifts in uniform, constant magnetic and electric fields. The electric field, E, is perpendicular to the magnetic field, B. Show that the drift velocity is given by vd = (E×B)/B2 Heres where I get to: F=e(E+vxB)=0 as v is uniform. Therefore E+vxB=0. Take vector product...
21. ### I How to prove the curl curl of a vector?

Why is this though?
22. ### I How to prove the curl curl of a vector?

I've got ∇×(∇×R)=∇(∇.R)-∇2R [call it eq.1] However I have the identity ∇×(A×B)=A(∇.B)-B(∇⋅A)+ (B⋅∇)A-(A⋅∇)B [call it eq.2] Substituting in A=∇ and R=B into eq.2 we get ∇×(∇×R)=∇(∇.R)-R(∇⋅∇)+ (R⋅∇)∇-(∇⋅∇)R which i work out to be ∇×(∇×R)=∇(∇.R)-R(∇⋅∇)+ (R⋅∇)∇-∇2R Basically I don't understand...
23. ### I What happens to the eigenvalue if an operator acts on a bra?

That's exactly it, thank you :). I didn't realize that you could separate the operators out. It's part of the derivation for finding the energy changes in perturbation theory.
24. ### I What happens to the eigenvalue if an operator acts on a bra?

I don't understand why the signs change at all.
25. ### I What happens to the eigenvalue if an operator acts on a bra?

Yeh that's part of the proof but that's not what I don't understand. Why do the operations flip the sign of the eigenvalues. I'd have thought <una|VA-AV|unb>=(ana-anb)<una|V|unb>
26. ### I What happens to the eigenvalue if an operator acts on a bra?

I'm going through a derivation and it shows: (dirac notation) <una|VA-AV|unb>=(anb-ana)<una|V|unb> V and A are operators that are hermition and commute with each other and ana and anb are the eigenvalues of the operator A. I imagine it is trivial and possibly doesn't even matter but why does...
27. ### I What is the spin quantum number of an atom in 4P state?

Sorry I've never seen this notation before so I wasn't aware of the relationship. So why do we have L=P then?
28. ### I What is the spin quantum number of an atom in 4P state?

So the angular momentum quantum number is l=1 as it's a P state? How is the value of s determined?
29. ### I What is the spin quantum number of an atom in 4P state?

That's the terminology in the question yes. I don't quite understand what 4P means though? What do you mean by term symbol?
30. ### I What is the spin quantum number of an atom in 4P state?

If an atom described by spin-orbit coupling is in a 4P state, why is the spin quantum number s=3/2?