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## Hot answers tagged electrons

19

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 ...

5

"The wave is actually probability in the sense that it assigns probability to the space coordinates of detecting photon at a certain time." No, the emergence of the classical EM wave from the quantum wavefunction of the photon is not trivial, because a classical EM wave is made up of many photons. In particular, it is not the case that the classical EM ...

4

What happens with static electricity on a shirt is, if it comes into close proximity to another surface, the difference in electrical potential can become greater than the breakdown voltage of air. Then, current flows through a small electric arc, which is a plasma of air molecules and electrons. Very locally, this has a much higher temperature than the ...

4

This is quite an interesting question. First - air is a (poor) conductor. See this earlier answer for some details on how well (or poorly) air conducts (especially when the relative humidity increases). Next - vacuum as an insulator. You are right that once electrons are "in space", a vacuum doesn't provide much impediment. This is why cathode ray tubes ...

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

Rutherford modeled the atom as an extremely compact positive nucleus surrounded by a uniform ball of negative charge the "size" of the atom. He included the effect of scattering by the electrons under the assumption that they acted like such a diffuse cloud of negative charge, and showed that such a cloud had negligible probability of scattering the alpha ...

3

"The wave is actually probability in the sense that it assigns probability to the space coordinates of detecting photon at a certain time. Now, the wave transports energy & momentum." As you say that you quote words from a book, then, you have to know that there are many books and many authors, each one with his/her opinion. There are four basic ...

2

Electrons can deflect a heavy alpha-particle backward if they are sufficiently energetic. There are such electrons indeed, but their concentration is very small, I guess.

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The gravitational force is a very weak force it cannot compete with the electromagnetic forces which create all observed matter. In addition to the neutrality of all matter, that there are positive charges in the nuclei with electrons around them, the framework is quantum mechanical. There are always spill over positive forces that attract and keep ...

2

The key difference is that the alpha particle is several thousand times heavier than the electron. It would be like you rolled a bowling ball at a marble and it bounced backward. As Rutherford said: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of ...

2

It is indeed the case that the Casimir force for a three dimensional sphere is repulsive. http://arxiv.org/abs/hep-th/9406048 Neither myself nor the authors of this paper know an intuitive explanation of why this is the case. It is important to remark, however, that this result actually disproves the naive and very common "mode counting" argument that ...

2

There is an example in which it is not clear which forces act there, but I am not sure if you learnt of such things. I'll try to make it simple. In the quantum physics we know to prepare sets of particles in what is called entanglements. Such an example is the so-called singlet of polarization of photons. Polarization is how oscillates the electric field in ...

2

The energy is: $$E = \frac{n^2h^2}{8mL^2}$$ Your mistake is that you have $L$ not $L^2$ in the denominator so your answer is a factor of $L$ too small.

2

It will help if you study this diagram of what a vacuum tube is If a cathode is heated, it is found that electrons from the cathode become increasingly active and as the temperature increases they can actually leave the cathode and enter the surrounding space. When an electron leaves the cathode it leaves behind a positive charge, equal but ...

2

The same question could be asked about electrons in atoms. Why on Earth do they remain bound to the nucleus instead of floating around freely? The answer to this question and to yours is that the nucleus (in the case of atoms) and the metal (in your case) is offering a quite cozy place for the electrons to be and leaving it is quite costly in term of energy. ...

1

In a lit candle, when gaseous candle wax reacts with the oxygen in the air, the atoms will be unstably excited. To be stable, the excited electrons will relax to the ground state by emitting photons with energy equal to the energy difference between the 2 states. The photons’ energy doesn’t change much, so the wavelength doesn’t change much. The chemical ...

1

Einstein was quoted as saying 'You know, it would be sufficient to really understand the electron'. We understand the electron, but we do not really understand it. The intrinsic spin, the magnetic dipole moment and the relation to the positron are not really in full grasp. We are lucky that nature does not play dice- again by Einstein, and this is how we get ...

1

A single charged object is sufficient to produce an electric field. Following Coulomb's law: $\textbf{E} = {Q \over 4\pi \epsilon_0\textbf{r}^2}\hat{\textbf{r}}$ where $\textbf{E}$ is the vector electric field, Q the charge of the object in question, $\epsilon_0$ the permittivity of vacuum or the electric constant, $\textbf{r}$ the vector position of the ...

1

this electromagnetic mass $m_{elec}$ has to be added to the standard "mechanical mass" of the sphere to give the total observed mass of the object.  Would this view be accepted by most physicists today? That is a wrong idea. Why? There is no reason (in this case) to count quantity $m_{elec}$, defined based on EM momentum distributed in the whole ...

1

I'm not actually familiar with the construction of these microscopes, but most devices using focused charged-particle beam rely on quadrupole magnets1 to maintain the control of the beam size and shape. It is worth noting that a quadrupole that focsuses the beam in one plane tends to de-focus it in an orthogonal plane, but by using sets of multiple ...

1

If you ask me , what IS an electron, I don't know. Some people might say, without being allowed to describe its properties, it's a meaningless question. My answer would be always to describe it in terms of it's properties, i.e as you say, by means of its forces. So, yes, I would agree with you, if all matter is made up of elementary partices and we can only ...

1

The answer lies in a thermodynamic argument. The diffusion is a spontaneous process that occurs when particles with random motions are not uniformly distributed. The electrons in the conduction band in the n region are much more numerous that in the p junction. They will naturally tend to balance the concentration from the n junction to the p junction since ...

1

What force keeps electrons to fall on the ground and discharge the wire? The electromagnetic force. Electrons want to get away from each other, but they want to be near the positively charged nuclei with which they form atoms. Even in a metal, where the electrons are weakly bound to the positively charged ion cores, those positive charges are still ...

1

Note that there is an effect on the electrons that is much larger than caused by direct interaction with the alpha particle. If the alpha partcle collides with the nucleus then the nucleus will change its momentum. To a good approximation this is an instanteneous change and then the so-called "sudden approximation" applies to the electron state. So, the ...

1

Yes, this is actually often used in a spectroscopic technique called REMPI -- see the image on this wikipedia page https://en.wikipedia.org/wiki/Resonance-enhanced_multiphoton_ionization There are some important physics techniques that rely on interaction with two photons -- two photon spectroscopy (http://cua.mit.edu/8.421_S06/Chapter9.pdf). Some other ...

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