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8 votes
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Currently self-studying QFT and The Standard Model by Schwartz and I'm stuck at equation 1.5 in Part 1 regarding black-body radiation

It just a clever use of the geometrical series: $$\frac{1}{1-q} = \sum_{j=0}^\infty q^j$$ which is valid for any real number $q<1$. Here $q = e^{-E_n\beta} \equiv e^{-\hbar\omega_n\beta}$. As long ...
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4 votes
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Probability of observing harmonic oscillator at a particular position

When dealing with continuous distributions, one should make difference between probability and probability density. Probability of hitting any specific point of a continuous distribution is zero, ...
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3 votes

Is it true that the Poynting theorem $P=E\times H$ is quite valid for DC circuits?

Yes, Poynting's theorem $$-\frac{\partial u}{\partial t}= \vec{\nabla}\cdot\vec{S}+\vec{J}\cdot\vec{E}$$ or, equivalently, in integral form $$-\frac{\partial}{\partial t}\int_V u\ dV= \oint_{\...
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2 votes

Electric Power and Energy

Total power produced is not a concept. It would be like asking for "total speed" of a car. You don't total rates. You could have an average speed over a period of time, but not a total. ...
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2 votes
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Moving body is collided with a spring system.Why there is a difference in maximum compressed distance for different observers?

Your derivation of the difference in compression between the two frames is incorrect; after all, in Galilean relativity, spatial distances between points at a given time are the same between all ...
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1 vote

Dispersion of $E$ and $p$

No, $∆E^2$ is not $2\Delta E$: $$E^2 = p^2c^2 + (mc^2)^2$$ $$\delta(E^2) = \delta(p^2c^2) + \delta [(mc^2)^2]$$ $$ 2E.\delta(E) = c^2(2p.\delta(p)) + 0$$ $$\delta(E) = \frac{pc^2}{E} \delta(p)$$ Hope ...
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1 vote

Is it true that the Poynting theorem $P=E\times H$ is quite valid for DC circuits?

If something is stationary, it is a special case of being time-varying (with time derivative equals zero). So there is no need to repeat the proof of Poynting's theorem in the narrow sense of ...
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1 vote
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How can we say potential or chemical energy is part of an object?

In a detailed microscopic model of a system of particles, we can distinguish energy of the microscopic EM field (EM energy), and the other energy (due to mass and kinetic energy of massive particles). ...
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1 vote

Non-Work energy conversion

Work is a more general concept than a force acting through a distance. The mechanics branch of physics primarily addresses work for a point particle, or a rigid body, for which there is no change in ...
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