New answers tagged

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I think the main misunderstanding is that the n-layer is neutrally charged overall so it does not attract anything. Only in the depletion region is there an electric field. The electric fields acts to sweep electrons towards the n-layer and holes toward the p-layer. The depletion region partially overlaps both n and p doped layers. The part in the n-side is ...


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[This should be a comment, but I don't have enough reputation points to comment. So it has to be an answer. (Am I the only person who thinks this is odd?)] The collision boundary differs from the classical mechanical billiard ball model in that there isn't a rigid boundary that defines whether the particles collide or not. In the billiard ball model, if &...


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No, cannot, and Yes, it can... A) it is possible for the quantum object with spin J=1/2, to have an "intrinsic" quadrupole deformation of the internal charged distribution. The real example is a strongly deformed J=1/2 nucleus Cf-251, with half-life approx 900 years. You may ask Berkeley scientists, they know about the internal = "intrinsic&...


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Electrons and photons do in fact have a spatial extension. It is not a spatial extension of the particles themselves though. They are point-like objects. On the contrary, the wavefunctions accompanying the electron and the photon are spatially extended. Because of this there is a continuous range of possible interaction (collision) outcomes. If the particles ...


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Since the electron and photon are quantum mechanical objects, the angle at which the electron and photon move after the collision is probabilistic. As you have pointed out, they do not actually "collide" but instead they interact. You can think of this process as the electron absorbing the photon at one point, and then re-emitting one at another. ...


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To avoid the problem of questions answered only in the comments, I will post the following answer as Community wiki. Richard Myers helpfully points out that my question would be answered in most introductory QED texts including Peskin & Shroeder ch 4/5, Zee II.5 and II.6, Weinberg ch 8, Nair ch6/7, David Tong's notes ch 6.


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In a solid (semiconductor), an electron hole is the absence of an electron where one is expected. In this sense you can think of an electron hole as if it were an air bubble (the absence of water) in a body of water- a local spot where there should be water but there isn't. In the case of water in the presence of gravity, water falls down- but bubbles in the ...


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In a generator, the free electrons in a rotating coil of wire are pushed along the wire as they move through the field of an external magnet (usually an electromagnet). In an alternator, the electrons are pushed around the loops of a stationary coil by the emf produced by the changing field from a rotating magnet. The alternator (which supplies an AC ...


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This equation does not describe a thermally averaged annihilation cross section as one would expect in a cosmological context. Instead, it is used to discuss the expected rate of electronic recoils in a direct detection experiment as function of energy. The OP took the equation from this paper (Eq. 5), where it is indeed referenced in a somewhat confusing ...


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It is unlikely that the electronics of the device is responsible for that. The times of high static voltage in TVs are long gone. The only electronic thing I think could be responsible for that is improper grounding. But I think even a manufacturer of cheap TVs cares about proper grounding, unless he wants to risk expensive lawsuits filed by the relatives of ...


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It's hard to interpret this question, those are just exponentials with no context, and are then mathematical objects, not physical objects. Assuming you're using standard notation, the difference between those two plane waves is that they are travelling in opposite directions in some 1D space. In terms of elections and positions, depending on your current ...


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It looks to me like you have something specific in mind with your question, but you do not seem to disclose it. If the test charge were repelled (by like charges, which are also negative), it would run off to infinity, where its potential energy with respect to what you call the "reservoir" is minimal. Hence, all the potential energy would be lost, ...


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The conducting direction of the diode is defined by the convention that the electric current flows from the anode ("+" pole) to the cathode ("-" pole). Hence, in both cases the left picture (red path) defines the path the current takes: If we take the perspective of the electrons the diode conducted in the inverse direction.


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If you chose the opposite convention for the direction of electric current, the LED (diode) would also change its forward direction accordingly. Nothing at all would change in the result. In this sense, it doesn't matter.


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The interference pattern disappears if you mess with the electron any time after the slits. You can think of it this way ... the electron has chosen a path in advance (before it has even left the emitter), the excited electron in the emitter is already generating changes/fluctations in the EM field (called virtual forces or virtual photons, virtual ...


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The formula $v=\frac{2W}{QI}$ doesn't seem right, the dimensions of the sides don't match. velocity is metres per second that can't be got from the right hand side. Even if it meant $v=\frac{2W}{Q}\times I$, something is still not right, so best to check everything. Also check if there is another v, a capital $V$ that means voltage, perhaps the two vs have ...


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If found the following formula $$ \frac{1}{\vert \mathbf{x}-\mathbf{x}'\vert} = \sum_{k,q} \frac{r_<^k}{r_>^{k+1}} \frac{4\pi}{2k+1} Y_{kq}(\Omega) Y_{kq}^*(\Omega') $$ in this page. That's all what I need for helium.


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No, in realistic situations it is not possible to tear positively charged ions off from the surface of a conductor in significant numbers. The electrons are loosely bound in an electron sea, while the ionic cores are much more tightly bound at part of the atomic lattice. In fact, this is one simple way to design a diode, in which current will flow one way ...


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Why do we set the number of degrees of freedom of an electron in a gas to 2 rather than 3? In a magnetized plasma, most electrons are said to be gyrotropic -- their velocities can be decomposed into parallel and perpendicular (with respect to the background, quasi-static magnetic field), where the latter is assumed to be azimuthally symmetric. Thus, there ...


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You are mixing two things: What is the average kinetic energy of the system in $n$-dimensions? Thermodynamics states that $$\bar E = \frac{f}{2}k_B T$$ where $f$ is the number of degree of freedom. How do we convert between the energy unit "Joule" and the temperature unit "Kelvin"? Since $k_B T$ has the dimension of an energy, we use $T ...


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This link may help. The correct quantum mechanical solutions and the Bohr model solutions coinside, energy levels as calculated are the same. To get ionisation the electron must go to n=infinty, and in order not to be trapped back radiating a photon needs some extra energy and momentum. So ionisation is an interaction between a charged particle hitting the ...


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The reason why electrons jump from higher to lower state is because when they're in a higher state their potential energy is also higher than usual. Thus, making it more unstable. And as everything in universe wants to have a lower potential energy (more stability) the electron comes back to it's original state. Just like a ball when placed at a desk would ...


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I suppose you are asking why can't we pass light through solids and liquids right? One reason could be that specifically when you are doing the emission spectrum you need the sample to be in gaseous as well as in the ground state. The ground state is to ensure uniformity in experimental values and, you know, scientists are probably more interested in the ...


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The "orbit picture", like your drawing, is incorrect. You have to imagine electrons as clouds of charge around the nucleus and those clouds don't move in orbits. You could find this answer useful. In this regard, metals and non-metals look the same. The difference arises when you consider the (quantum mechanical) problem of finding the energy ...


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There are no experiments or main stream theories in which the electron is not a point particle. That being said, there are no experiments or main stream theories in which the electron's behavior is not predicted by a wave function.


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My absolute favorite example of the electron acting as just a particle is the bubble chamber. Is it possible to produce images of pair production in home-made cloud chamber? As you can see on the left, the electron you are asking about is just a "scratch", leaving nothing but a track (of microscopic bubbles) behind as it spirals inwards. I believe ...


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A massive particle in its rest frame is equal parts left- and right-handed. There is a correlation between chirality and spin which appears at high momentum; chirality is a frame-dependent observable. The weak charged current (associated with the $W$ bosons) is a mechanism for interactions between the left-handed parts of matter particles and the right-...


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A simple model for the cascade theory of electronic showers can be formulated as a set of integro-partial differential equations. Let $\Pi (E,x) dE$ and $\Gamma (E,x) dE$ be the number of particles and photons with energy between $E$ and $E+dE$ respectively (here $x$ is the distance along the material). Similarly let $\gamma (E,E')dE'$ be the probability per ...


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Well most of what needs to be said already has been said. But there is something else that I think would interest you. Charge in a system is only conserved when a very specific symmetry in the system in not broken. Noether's theorem links symmetry to conservation laws. If you can find a way to break the symmetry that governs conservation of charge, then that ...


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Some experiments use some kind of medium analogous to ballistics jell. The electron travels through it and it's trajectory is traced out or left in it, so that it is easily seen. When there are electric fields the trajectory will spiral in accordance with lorents force and classical laws of electrodynamics. These are individual electron trajectories, and for ...


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Since the idea of the electron as a particle predates the idea of it as a wave by a quarter century, why not ask why electrons were thought of as particles at first? And for that, why not go to their discoverer? In 1897 J. J. Thomson performed experiments on "cathode rays", the rays emitted when a voltage is applied between metal plates in vacuum. ...


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Charges can be created and destroyed. Total charge cannot. Whenever you create an electron, charge $-1$, you must also create a positron, charge $+1$. That gives total charge $0$. Whenever you create a proton, charge $+1$, you have to create an anti-proton, charge $-1$. That gives total charge $0$. As far as we're aware, the total charge in the Universe is ...


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Some examples of electrons behaving like a particle: The photoelectric effect : As beautifully described by Einstein, electromagnetic radiation hits a material which leads to the emission of electrons. This example is perfectly based on the particle-only view and, amazingly, disagrees with the results of classical electromagnetism (The experimental ...


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In an old-school TV picture tube, electrons were shot into one end of it, accelerated, steered into specific directions, and then collided with a thin phosphor coating on the inside of the picture end of the tube. Each collision created a burst of light, building up a visible picture for the TV watchers. This process is well-modeled by envisioning the ...


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If the distance is directly correlated to the potential difference for the equation V = E*dr, how come this does not apply to a wire in a circuit? It does apply, although it should be $dV$ rather than $V$. Inside the wire $dV \approx 0$ and since $dr \ne 0$ that implies $E \approx 0$ inside the wire. Now let's say I replace this path between the particles ...


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The integral of - e.dl is the potential or ie. -* the amount of work done by a charge in moving through a distance of infinity to a point r,where dl is the path element corresponding to the line connecting infinity to your point r . if i were to calculate the potential at a point r+1 this would be lower than at point r as it moves through a shorter ...


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It does, unless the wire is superconducting. A wire of silver, copper, aluminum, gold, etc., has finite non-zero resistance. At a fixed temperature, it will behave like an ohmic resistance and there will be a small potential difference from one end to the other depending on the length of the path:$$R=\frac{\rho L}{A},$$ where $\rho$ is the temperature-...


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Feynman once asked more or less the same question (page 129 of "Quantum Field Theory" by Lewis H. Ryder): I remember that when someone had started to teach me about creation and annihilation operators, that this operator creates an electron, I said 'How do you create an electron? It disagrees with conservation of charge'. - R. P. Feynman So you ...


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The question is obviously conflating "alpha particles" and "helium atoms". This isn't a bad approximation, though, because most alpha particles will pick up extra electrons readily from their environment unless they're traveling through a pretty hard vacuum. It is conceivable that the electrons picked up by the alpha particles could be ...


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The beta particles (electrons) have nothing to do with how much helium is formed. Amount of helium = amount of helium nuclei = amount of alpha particles. They are ions but that doesn't matter. They will grab electrons from whatever they come into contact with and become neutral that way.


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This is a more complicated question than it appears on its face. A free electron has a continuous energy spectrum. This means that the photons it emits are not constrained to quantized values like they would be for a bound system. Photons are unusual in that they don't have any mass. Formally the number of photons emitted in bremsstrahlung radiation is ...


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It’s important to note that electrons can only be created and destroyed in interactions which respect the fundamental symmetries of nature and their associated conservation laws. So, charge conservation is one such conservation law; “number-of-electrons conservation” is not. (Though before radioactivity was discovered, you wouldn’t be unreasonable to think ...


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So the mechanisms which generate and destroy electrons happen to be such that they never violate charge conservation. Let's take pair annihilation for example, an electron and a positron meet and they become two photons. Before, the total charge is zero: the positron has positive charge and the electron has the exact opposite negative charge. Afterward, the ...


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When a particle is created or destroyed, energy is conserved. Energy is also a property of an electron, but we are not inclined to ask - why is energy conserved when the particle is not, because we accept the principle that the energy is, in a sense, temporarily imbued in the particle, but is otherwise independent of it. In the same way, charge is not only a ...


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Electrons can only be created and destroyed in processes that keep electric charge constant. There are three Standard Model interactions involving the electron: $\rm W^-\to e^-\bar{\nu}_e$ and $\rm \gamma\text{ (or Z)}\to e^-e^+$.$^1$ the first case, the W boson has the same charge as the electron, so no charge is created or destroyed. In the other cases, a ...


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The answer is related to conservation laws. In physics, processes can only happen if they respect certain conservation laws. The creation and destruction of electrons does not break any conservation laws; namely the energy is always conserved and transformed to another form when electrons are destroyed or created. However, the creation or destruction of ...


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It is simply wrong that during one half-period of the antenna generator, the accelerated electrons emit only one photon. They emit photons x times and the resulting radio wave is nothing more than the periodic change in the intensity of the emitted radiation. The reason why radio waves are perceptible is that the polarisation of the photons is able to induce ...


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Spin is a rather unfortunate name. Early models of the electron saw the electron as a spinning ball of charge. Unfortunately that proved not to be the case, but the name stuck anyway. Physicist aren't very good with names of particle properties e.g. quark truth, beauty, charm, color etc. None of those names make any sense and are really just whimsical. So ...


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The essence of this is owing to the time--frequency relationship that is involved in Fourier analysis. Suppose we have a process that can in principle put energy into the electromagnetic field. In photon language, this is a process that can create photons. If the process can move the field from its ground state to a state corresponding to a single photon of ...


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You are simply assuming that radiation (production of "light" by accelerating charges) produces point-like particles. This is not the case. When we have this sort of radiation the picture that best describes the output is a wave, having said that, this implies there is no localization, the wave itself is an EM wave as described by Maxwell´s ...


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