joriki
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How to count the number of cubic tree-level Feynman diagrams at $n$ points?
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7 votes

To obtain a diagram with $n$ external legs from a diagram with $n-1$ external legs, you can insert the external leg labelled $n$ into any of the $2n-5$ edges. That there are $2n-5$ edges in a diagram ...

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Can one make a ball rotate around a vertical axis using only a combination of horizontal axis rotations?
7 votes

It appears that both existing answers refer to the cumulative effect of rotations on the orientation of the ball and not to the total angular momentum achieved. As has been pointed out in comments, it'...

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Regarding Electromagnetic plane wave and Maxwell's equations
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4 votes

This is a notation from differential geometry that's unusual in physics (as far as I'm aware). The operators $\frac{\partial}{\partial y}$ and $\frac{\partial}{\partial z}$ are basis vectors for ...

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Photon frequency when transitioning from one medium to another
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3 votes

You're confusing frequency and wavelength. $1550\,\text{nm}$ is a wavelength, not a frequency. Upon entering a different medium, an electromagnetic wave changes its speed and wavelength while the ...

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Transmission and reflection
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2 votes

The clue here is that the coefficients of $\mathrm e^{\mathrm ikx}$ and $\mathrm e^{-\mathrm ikx}$ are to be evaluated at infinity. The terms and concepts you're using apply to a situation where we ...

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How does the period of a temperature fluctuation affect the depth to which the ground freezes?
2 votes

The characteristic length of the exponential decay in the ground is proportional to the square root of the period of the fluctuations, so the answer is roughly $2\text{m}/\sqrt{365.25}\approx10\text{...

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Finding the electric field for a shell of charge
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1 votes

The electric field due to a spherical shell is zero inside the sphere and equal to the field of an equivalent charge at the centre outside the sphere. Thus the field is directed outward everywhere, ...

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Thermal equilibrium - magnetization
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0 votes

a) You can get $l$ and $s$ using the shell model. The Total orbital and spin angular momentum for a closed shell is zero. Thus you only have to consider the single electron of sodium in the third ...

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