# All Questions

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### Global and local symmetry for Isospin/Strangeness etc

Why some symmetries $\big[SU(3),SU(2)$ and $U(1)\big]$ of the Standard Model are local, and some others remain global, like Isospin and Strangeness. Is there a fundumental reason for that? Doesn't it ...
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### Electronic spectroscopy question in a more physics oriented view. What is the operator that describes relaxation pathways?

Recently I was studying spectroscopy in Symmetry and Spectroscopy: An Introduction to Vibrational and Electronic Spectroscopy p.357-358 as shown in the image http://i.stack.imgur.com/LSLyZ.jpg The ...
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### The commutator between observable and unit radius vector

As I encounter the commutator relating to unit radius vector, I am quite confused. I have just started the learning of quantum mechanics and all I know about the commutator is based on two identities: ...
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### Expectation of energy?

Suppose we have a hydrogen atom in a specific state $\psi_{nlm}$. Of course, the state is given by $$\psi_{nlm} = Ce^{-r/na}r^l L_{n-l-1}^{2l+1}(2r/na)Y_l^m(\theta, \phi)$$ I am being asked to ...
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### Jumping from a moving train

If you were to jump from a moving train at 20 km/h, would it be better to run in the direction of movement before jumping to maintain inertia or in the opposite direction of the train ?
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### Induced charges, electric potential

Consider a solid spherical conductor with charge Q on it, a charge q is placed at a distance r from the centre of sphere. Find the electric potential at the centre of sphere due to induced charges on ...
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### What are the limitations of recording very faint sounds?

Is it possible to record very faint sounds like, say, the bending of a strand of hair? Part of the problem, I imagine, would be the need to find a way to isolate the target sound from other sounds. ...
$\mathbf{Q:}$ While teaching "Real Gases", my professor remarked last day that "Liquid phase is a highly compressed gaseous phase." But he did not explain the reason behind it and left it as food for ...
How to derive the formula for potential energy per unit volume of a simple harmonic wave in a string? My book just states the formula as $\frac{1}{2} \rho v^2 (\frac{dy}{dx})^2$ without the proof. \$\...