# All Questions

140,118 questions
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### How can beta plus decay be possible?

$\beta +$ decay is where a proton gets turned into a neutron and a positron and a neutrino. However, a neutron is heavier than a proton, so obviously this reaction is endothermic. So then, why does ...
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### Electric current dipole moment

Electrical Current Dipole of dipole moment $\mathbf{p_{EC}}$ has unit of $A\cdot m$. My knowledge of dipole moments unfortunately stopped in high school where I learned that dipole moment $\mathbf{p}$ ...
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### Probability density during equilibration

An ensemble of states $x$ are initially drawn from a non-equilibrium probability density $P_0(x)$. Over time they will evolve towards the equilibrium distribution $P_\mathrm{eq}(x)$. What I want to ...
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### Muzzle Velocity of a Bullet out of a Barrel

We want to know the speed of a bullet, in feet per second, for a given round. We know the weight of the bullet, volume of the casing, amount of gun powder, length of the barrel, and diameter of the ...
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### Radio wavelength to penetrate 8 ft of seawater

I am trying to penetrate about 8 ft of seawater with a radio wave. What wavelengths will work? I understand that low wavelengths penetrate seawater the best, but I am looking for a frequency that does ...
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### How do I understand Kinetic energy formula? [duplicate]

$$\frac{mv^2}{2}= Kinetic Energy$$ Can you explain me? What is purpose of $v^2$, $mv^2$, I am trying to understand the formula.
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### Threshold energy for particle production

For you smart redditors, this is the following problem: Particles are being accelerated (close to the speed of light) and collide on a resting target. Now I have to show that the necessary energy (...
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### Quantum description of Raman effect

In the classical description of Raman effect the object of study is the electric polarizability of the system. Since I'm interested in learning the quantum description of the Raman effect and in ...
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### Writing a two variable function$f(x,t)$ in terms of Dirac-Delta $δ(x)$ function and a function $P(t)$?

How to write a two variable function $f(x,t)$ in terms of Dirac-Delta $\delta(x)$ function and a function $P(t)$? For example; I read something in a book. You can find the following picture. But ...
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### Fermi level in metals and semicoductors

As we read about Fermi energy, it is the energy of the highest occupied electrons. But if we look at the energy level diagram of semiconductor the Fermi level is situated somewhere between the valence ...
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### Why don't all electrons contribute to total orbital angular momentum of an atom?

There are 47 electrons in a Silver atom, but talking about its orbital angular momentum we only take the outermost valence electron which occupies the 5s orbital. Why don't the remaining inner 46 ...
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### Boundary condition for Hamilton-Jacobi PDE

For separable solutions to Hamilton-Jacobi PDE (say in 2D), we treat the Hamilton's principal function $S$ as $$S= W(x) + W(y) - E*t$$ and treat the separate parts as constants and find $W(x)$, $W(y)$...
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### Conductance Charged Box

A box with thick and conductive sides with dimensions: Lx, Ly, Lz- while LZ>>Lx,Ly. The long dimension of the box [Lz] can be viewed as infinite so Phi(x,y,z)=Phi(x,y) Given that: Phi(0,y)=-Phi(Lx,...
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### $\mu \rightarrow e\gamma$ in the $R_\xi$ gauge: trouble with momenta and Dirac matrices

My first ever question on stackexchange! Sorry if it is clumsy... I am trying to follow the computation of $\mu \rightarrow e \gamma$ in Cheng and Li and am confused about the second and third ligns ...
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### Inducing coil current by falling?

Typically, if a rod or coil is moved at a constant velocity through a constant magnetic field, a motional EMF will be induced by Faraday's Law but no current because there is no net emf or no closed ...
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