# Tag Info

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It is a standard exercise in quantum electrodynamics to find the angular dependence of the differential cross section. Which more or less means how probable it is for the photons to scatter at a certain angle, given the energy of the incident particles. So assuming the spins of the electron-positron pair is averaged, and that you don't care about the photon ...

11

One has to make clear that the watches we are using now are no longer using radium , because of radiation danger awareness. Radium dials are watch, clock and other instrument dials painted with radioluminescent paint containing radium-226. The 1900s (decade) were the peak of radium dial production, as radiation poisoning was then unknown; subsequently, ...

7

Even if the laser had perfectly reflecting, i.e. lossless, mirrors at either end of the cavity, and both ends were sealed so no light could escape it would still require a continual power input. That's because excited atoms/molecules can decay by mechanisms that don't involve a photon e.g. collisional de-excitation. The lost energy goes into heating up the ...

4

An atom can be positively charged as well as negatively charged. The charged form of an atom (or molecule), which has either more or fewer electrons than protons, is called an ion. It cannot be both at the same time, though, and that is because you are viewing positive and negative charge as relative things ("atom A is negatively charged in atom C's view"), ...

3

The short, simple, and intuitive explanation is that in a superconductor state, electrons are paired (BCS case) because there is an effective interaction between them. To destroy such a pair and produce free electrons you need to invest a minimum of energy, which is this energy gap $\Delta$. This produces an excitation (2 free electrons), remember that SC ...

3

If by the exclusion principle you mean that the total wave function is anti-symmetric under particle exchange, then yes.

3

In physics (as in all natural sciences) there are never any proofs. We write theories and believe them as long as there are no experimental results contradicting them. You can prove that a theory is wrong, but we can never be sure that it is correct. However, your question can be answered in a convincing way by looking at Fermi-Dirac and Bose-Einstein ...

2

The most fundamental description of the electron we have at the moment is using quantum field theory. This describes the electron as an excitation of a single quantum field that spans all of time and space. It also neatly explains how electrons can be created and destroyed: add a quantum of energy to the quantum field and it appears as a newly created ...

2

It turns out there is a good way to estimate this. In PET (positron emission tomography), the patient is injected with a radioactive tracer that emits positrons (hence, the name). These positrons annihilate with electrons, and emit two 511 keV photons. Detecting these photons, one can estimate the location of the radiation. Doing this many times over, you ...

2

Yes ... but let's be careful to understand that the sensation of touch is a psychophysical phenomenon. The electrons at the surface of an object "push" against the electrons at the surface of your fingers. The electrons never touch each other. Your skin deforms a bit, and the nerves in your fingers detect this deformation, and send a signal to your brain. ...

2

The blue glow is cause by free electrons (i.e. those not bound in atoms) colliding with some residual gas molecules in the tube. Even though it's under vacuum, there is still some gas present (perhaps even designed this way, to create the lighting effect). In a collision, the gas molecule can absorb some energy from the electron and undergo an electronic ...

2

A cathode ray tube where the electrons are randomly ejected and are not constrained in space the Pauli exclusion seems irrelevant.. In structured beams though, things may be different: The Pauli Exclusion Principle places a fundamental limit on the brightness of an electron beam. Developing a cathode which can reach this limit is useful for achieving ...

2

The electron is an elementary particle in the underlying building blocks of matter organized in the elementary particles table of the standard model of particle physics. Elementary particles are point particles. The standard model is a precis of a very large number of measurements (data) fitted by mathematical models of theoretical physics. A point ...

2

Now, from Coulomb's Law we can find a vector for the electric field due to this electron at all points in this space. When you read about Coulomb's law, you can see that it describes the force between two charged particles. When you set them down such that they have no initial relative motion, they move together or apart in a linear fashion. But note that ...

2

This is a tough one. I'm going to take the liberty of rewording the question, and then use analogies to hopefully give some kind of answer that gets us part of the way there with this arguably obscure but worthy issue. How did the electron change from simply flying past the proton to adopting a quantised orbit around it? Because it was attracted towards ...

2

Theoretically yes, the laser principle does not consume any material. There is a light source that excites the electrons in the material to higher levels, they deexcite to some intermediate one, here the avalanche of photons appears producing the laser light and leaving the electrons in the ground state. And you can repeat the process without a loss.

2

The Universe is indeed electrically neutral at the cosmological length scales which means that the total charge of the positively charged particles is equal to (minus) the total charge of the negatively charged particles. However, one must be more careful what these particles are. Electrons and protons are two dominant charged particle species. However, ...

2

Quantum mechanic predicts, that the allowed directions of the spins are quantized. This is one of the main findings of the Stern–Gerlach experiment. In a thermal beam I suppose the the spins to be equally in up and down. (There is no reason why they should not.) But "up" and "down" only correspond to a specific direction in space if there is an external ...

2

The angle is the same as long as you consider a free electron. Then they are parallel: $\vec{\mu}_\mathrm{elec}=-g_\mathrm{elec}\mu_\mathrm{Bohr}\frac{\vec{S}}{\hbar}$ with $g_\mathrm{elec}\approx 2$ (neclecting effects from quantum electro dynamics). But when dealing with bound electrons (e.g. in an atom), where the electron also has some orbital angular ...

2

We know that solar cells generate electricity by utilizing the energy of the photon, This is an every day language, electricity. It means things electrical in general every day language. but how does it generate electricity forever? What is generated when the photons hit any material, is heat, and the sun's energy is at maximum 1300Watts per ...

1

You can derive it simply by noting that each level can have $n-1$ transitions,so we have $n-1+n-2+...+1=n(n-1)/2$

1

Boron can conceivably fit a maximum of 8 electrons in its outer shell. This could be achieved through boron covalently bonding with a non-metal (as boron is a metalloid). It is in the 2nd period (row) of the periodic table, hence has 2 'shells', following the 2n2 pattern for maximum amount of outer shell electrons (where 'n' is the amount of 'shells' the ...

1

The filling order of the shells is 1s, 2s, 2p, 3s, 3p, 4s, 3d, ... Silicon, with 14 electrons, has only filled 1s, 2s, 2p, 3s and half of 3p. It hasn't any electrons in 4s or 3d (in the ground state). Although the 3rd orbital can have a maximum of 18 electrons, the shell is considered full with 8 electrons if the 4s is not filled.

1

It all comes down to the band structure. In a solid the energy levels are not the same as an isolated atom, there are bands instead of levels. So if you are thinking about an electron jumping between discrete energy levels then that is probably not going to happen. Particularly if you are thinking that an electron will de-excite emitting a photon, then that ...

1

What makes you think that the question has an answer? The Bohr model has limited validity and this was realized from the start. In essence, you're describing a transition from an unbound state of the electron (with positive total energy) to a bound state (with negative total energy). This cannot happen all by itself, as the extra energy needs to go ...

1

theoretically if its components never wore out then yes. however in practice things do wear out eventually and so no it could not be done in the same way that a perpetual motion machine can work in theory but not in practice.

1

Benjamin Franklin proposed electric fluid theory and considered electric current to be flow of a charged fluid. He meant to use positive to denote a surplus of the fluid, negative as a deficit of it. No one knows how he came up with the choice, but it became the convention and as a result lead also to the labeling of charge. I know of no fact that could ...

1

That there are two distinct types of electric charge is a metaphysical fact. But nature is indifferent to what we choose to label these charges; up / down, left / right, positive / negative, black / white, etc. Electrons will still flow to the plate in a CRT regardless of how we choose to label the polarity of the charge on the electron and plate. ...

1

there are various databases for this sort of information. In your position I would start with http://www.vamdc.eu/ which has links to other databases and is currently maintained. It may have more information on molecules than on atoms, but I hope it is useful. Another database is Gaphyor, but an issue with this database is that it does not accept new ...

1

Chemistry as we know it would not exist if electrons were Bosons, because many of the details of the chemical "bonding" of atoms are strongly affected by the requirement that the overall wavefunction of all the electrons involved be anti-symmetric. This anti-symmetrization principle (which is the basis of the "Pauli Exclusion Principle") along with the fact ...

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