# All Questions

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### How can steel shavings get magnetized while hole tapping?

I was tapping a hole yesterday with my uncle into a piece of soft steel. (A tap cuts threads on the inside of a hole, to accept a machine screw or bolt.) The tap was NOT magnetic! As I finished boring ...
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### Could Dark Matter form black hole?

Some people speculate that the mysterious dark matter in the universe could be tiny black holes. But on the other side, could dark matter particles attract each other by gravity and finally form a ...
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### What is this vector problem asking to find, the magnitude of the force F or the component of it that goes along the ramp?

A shopper pushes a 7.5-kg shopping cart up a 13 (degree)incline. Find the magnitude of the horizontal force, F, needed to give the cart an acceleration of 1.41 m/s$^2$. I ask this because the ...
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### I need help understanding a step in the derivation of the Schwarzschild solution

I am looking at Wikipedia's article on deriving the Schwarzschild solution. In the section "Simplifying the components", it says, On the hypersurfaces of constant $t$ and constant $r$, it is ...
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### Acceleration of particles in a pipe with variable cross section

I am currently making an exercise from Paterson,1983, IV.3.ii. It goes as follows Water flows steadily and incompressibly along a pipe whose area cross section $A(x)$ varies slowly with the ...
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### Boundary conditions for second order PDE

For a second order PDE, for example heat conduction equation $\frac{\partial T}{\partial t} = \frac{\alpha}{C_p} \nabla^2 T$, is it possible to determine the steady-state (or even transient) solution ...
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### Question about conservation of energy and force

Feynman mentions in his book, The Feynman Lectures on physics Let us now illustrate the energy principle with a more complicated problem, the screw jack shown in Fig. 4-5. A handle 20 inches ...
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### Why do light disappears the moment we switch off the source (inside the wooden box)?

I am failing to explain why light won't remain inside the wooden box in the following situation. I considered a wooden box closed from all the sides, with a bulb inside it. If we switch on the bulb, ...
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### Is quark electric charge also confined?

Since we will (maybe) never observe free quarks which has fraction charge, is it logically correct to say that the fraction (1/2,2/3 etc) electric charge is also confined. I.e. the charges that we do ...
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### Derivation of Lagrangian density for an infinite classical dielectric in interaction with the EM field

I am tasked with reading and reproducing all the steps in J.J. Hopfield's 1958 paper "Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals". Embarrassingly I am stuck ...
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Is there any possible way of reducing the radioactivity of a radioactive material?
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### Does the radius of the Universe correspond to its total entropy?

I heard a claim that due to holographic principle, the surface area of the cosmic horizon corresponds to the universe's total entropy. As such the initial state had zero surface area and later ...
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### Why can't a spin-1 particle decay into two identical spin-0 particles?

I've got this far: Suppose a spin-1 particle with $j$,$l$,$s$ decays into a system of two identical spin-0 particles with $J$,$L$,$S$. The RHS must have total spin $S=0$, so $J=L$ which must be even ...
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### What is Scale invariance?

Could anybody tell me what does scale invariance means? Is there any book or article that describes [ and gives examples ] about it.
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### What were the dimensions of the universe during Planck epoch?

Wikipedia says: It is believed that, due to the extraordinarily small scale of the universe at the time, quantum effects of gravity dominated physical interactions. But I wonder whether ...
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### Maximum liquid velocity in a siphon

According to wikipedia: Siphon, the velocity of liquid in a siphon is equal to $\sqrt{2gh}$, where h is the difference in level between the water surface and the opening of the siphon. But it also ...
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### The universe is expanding but why the galaxies have no motion?

It is said that the universe is expanding and the galaxies are moving apart. I understand that the space between every two galaxies is increasing. Doesn't this seem to imply that the galaxies will ...
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### Half-integer Spin and “natural conformal dimension”

If we consider a classical field theory for a massless particle of integer spin $s$, in a curved space-time, one finds that it is "naturally" conformal in a space-time of dimension $2+2s$ For ...
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### Speed of light and relativity [duplicate]

Suppose there are two observers $A$ & $B$ both are in motion, now $A$ sees $B$ is moving with speed $'u'$. A says that another object $'d'$ is moving with speed $c+u$ w.r.t. A in the same ...
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### Metric Expansion Of Space

I just do not understand this concept of metric expansion of space. Shouldn't the galaxies move away from each other. How can the space between them expand if the galaxies are not moving away from ...
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### Fermi energy for arbitrary dispersion relation

I am wondering how to obtain fermi energy for an arbitrary dispersion relation for electron. For instance. Energy dispersion relation : $E(k) = A( \sin^2(\frac{ka}{2}) - \frac{1}{6}\sin^2ka )$ ...
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### Schroedinger field operators and their commutation relations

I've got several questions regarding the so called second quantization of the Schroedinger equation. My professor introduced the field operators for the Schroedinger field by simply stating them as ...
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### What is the aperture in the hydraulic analogy of Ohm's law

Note: my question is duplicate of Why doesn't water come out of tap/faucet at high pressure when I turn it on? but none of the answers given there explains how continuity equation fits properly. ...
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### Fresnel distance and Geometrical limit

I read about the geometrical limit of wave theory. The source from where I read had a slightly different explanation to provide than here(The more rigorous answer is too complicated for me to ...
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### Planck's distribution and Bose-Einstein distribution?

If the application of the Bose-Einstein distribution is in blackbody radiation, then what is Planck's distribution? Are they same? How did Planck know that he should use a Bose-Einstein distribution ...
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### diffracton at the edges?

To understand the phenomenon of diffraction as an interference effects of several dipole oscillators (like in case of several symmetrical, not sawtooth, scratches in a diffraction grating), we ...
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### Lorentz Transformations

I am trying to understand Lorentz Transformations in SR. I have some doubts. 1) In the derivation for Lorentz Transformations, why do we assume that Galilean Transformations hold true when an ...
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### Estimating the number of atom/nuclei in a single quantum dot

I often read in introductions about quantum dots that depending on the fabrication method, a single dot contains about 100 - 100000 atoms. Assuming a self-assembled dot of lens or pyramid (cone) ...
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### Observations in the cathode ray tube experiement

1.One of the observations I learned was that the glass tube begins to glow with a brilliant green light. Many websites I read through refer to a fluorescent material. However, as shown in the above ...
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### Stellar Activity Cycle versus Metallicity

Our Sun exhibits sunspot max/mins on about a 11 year period. It's a G2 spectral class on the HR Diagram. We know that some stars exhibit "starspot" cycles of various intensities and periods. These ...
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### Characteristic waves in a plasma

What does "characteristic wave" mean in the context of plasma waves? For example, when propagating parallel to the plasma magnetic field, the characteristic wave is circularly polarized. Does this ...
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### Resistors in series vs parallel

I understand that for resistors in series we need to add R for each resistor to get the equivalent resistance, where R is resistance. I also understand the mathematical manipulations that show that ...