New answers tagged

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The answer to this question is a lot like the answer to Why does the comb attract the pieces of papers if they're neutral? I'm guessing that your comb, which will be negatively charged, is not evenly charged across its width. So even though you are holding the center of the comb above the vane, the electric field between the comb and the ground (plane) ...


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The four (metal) vanes of a Crooke's are attached to an axle. There is very little friction between the axle and its supports so a very small torque applied to the vanes would produce a noticeable change in the rotation of the vanes. My suggestion is that charges are induced on the vanes by the charged comb. Thus there is a net force of attraction between ...


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The answer is already as you have given it: $m_{em}=\frac{4}{3}E_{em}/c^2$ The electromagnetic mass depends on the shape you assume for the charged object. In the case above it is assumed the object is a charged, hollow sphere. In general the electromagnetic mass for a charged object producing electric and magnetic fields $E$ and $B$ is: ...


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In order for an emf to be induced in the secondary coil, the flux through it must be changing; therefore, the current in the primary coil must also be changing. If a constant voltage is supplied to the primary coil, no emf would be induced in the secondary, and therefore, the secondary voltage would be zero.


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Of course. This is how antennas work. The Maxwell equations make this possible. The metal as a conductor has electrons in what is called the conduction band, and these are free from the ions in the metal lattice. The oscillating electric field $\vec E$ or $\vec D = \epsilon \vec E$ given by $$ \nabla\times {\vec H} = \frac{4\pi}{c} {\vec J} - ...


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Because in the FIR, $\omega \rightarrow 0$ and therefore the $4\pi i \sigma/\omega$ term dominates the $\epsilon_\infty$ term. It can therefore be safely neglected. You are right that the $\epsilon_\infty$ represents the contribution to $\epsilon(\omega)$ of the bound (or dipole-like) electrons.


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Light is an emergent collective phenomenon from zillions of photons. The relation of the energy of the photon to the frequency of the wave is E=h*nu. The photon itself just has spin and energy when measured. Its wavefunction though has a complex dependence that does contain information which will build up the emergent beam with frequency nu. A detailed ...


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The motion of a charged particle in a magnetic field is the manifestation of the fundamental relationship between magnetism and electrostatic effects. Exactly why the fundamental forces are the way they are is still beyond modern day physics which is why it is hard to give a satisfactory answer to the first part of your question. It just has to be accepted ...


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You seem to have some fundamental misunderstandings about the nature of electromagnetic radiation. The electron transition describes a change in energy that leads to the emission of a single photon. The energy of a photon is inversely proportional to the wavelength of the light. So the light "gets" its wavelength from the energy it carries and not from the ...


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Anna, you are more or less right. It is pressure from matter at higher densities that can stop the gravitational collapse. It depends on the state of the matter, and in a simplistic description the equation of state. As it collapses as a hot gas after it exhausts it nuclear fuel, and maybe after a supernova explosion, it'll collapse. The first point at ...


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The oscillating fields of the electromagnetic wave just add linearly with the static electric or magnetic fields. Nothing much happens really, and the wave goes on its merry way.


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Electrostatics is the physics of current free charge distributions. Magnetostatics is the physics of stationary (time independent) current distributions. Usually magnetostatics is defined as the physics of stationary and "divergence free" current distributions, however, a zero divergence is a superfluous condition that is not satisfied in case of many ...


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It depends on whether the current is carried by a conductor or is in free space (an electron beam). In the case of an electron beam, the current will appear to have reversed in direction if you travel faster than the charge carriers, even without relativistic effects. This web page does the transformation roughly like you have attempted, using a charge ...


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The electron energy (the portion that changes at least) will be mainly due to kinetic energy (translational) and potential energy due to the potential difference between the cathode and the anode. The electron does have a "spin", but this spin isn't like that of a spinning sphere. The reason for the name spin is simply that the electron spin describes the ...


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Kinetic energy in 1D, method 1. Free electrons. Assume no potential energy at the moment. Zero temperature. \begin{equation} n_e=\int_0^{E_F}g(\epsilon)d\epsilon=\int_0^{E_F}\frac{1}{\pi\hbar}\sqrt{\frac{m}{2\epsilon}}d\epsilon=\frac{\sqrt{2mE_F}}{\pi\hbar} \end{equation} \begin{equation} E_F=\frac{\pi^2\hbar^2n_e^2}{2m} \end{equation} Kinetic energy, ...


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Why is it that an oscillating electric field transfers energy less efficiently to ions than to electrons? This is not generally true. There are multiple cases where an oscillating electromagnetic field transfers energy/momentum much more efficiently to ions than electrons (e.g., Alfvén waves do not care about electrons in many situations). In lab ...


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A typical voltmeter contains an internal Ohmic resistor with known and very high resistance $R$ (called the "input resistance" or "input impedance"), and an extremely sensitive ammeter that measures the current through that resistor. When the voltmeter is connected in parallel across some circuit elements, then ideally the internal resistor has resistance ...


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This answer has been hinted at in the others, but it's worth stating their collective knowledge as a succinct one liner that every physicist should know: Electric and Magnetic force only make sense in the light of special relativity if they are unified because if they were thought of as separate entities, then relatively moving observers would reach ...


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The mass of the electron is thousands of times less than that of the ions - about 1,800 times lighter than a proton. The motions move the entire ion core, so inertia tends to resist the change of motion much more than is possible for an electron. For example, see Improved Two-Temperature Model and Its Application in Ultrashort Laser Heating of Metal Films. ...


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The answer is that it depends on the field on the outside (boundary conditions) and the dielectric constant of the insulator. For a imaginary insulating sphere of vacuum in a vacuum, it should be obvious that the sphere does not affect the electric field at all. Inside a dielectric, the field will be weaker than on the outside. For a dielectric sphere in a ...


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Firstly, Bluetooth devices do in fact interfere with one another, just that the interference is not significant, and the communication protocols of the devices make it seem like they don't (i.e. my moving of my BT mouse does not move the cursor on your screen which is paired to your BT mouse) You can isolate the system by putting the two electromagnets ...


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Several answers have given a physical explanation as to why electric and magnetic forces are tightly coupled, and why you can't develop independent theories of "just electric" and "just magnetic" fields. Your subquestions (especially #1) make me think you're looking for some kind of symmetry. It turns out, there's a really nice one! All the asymmetry ...


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The classical electromagnetic effect is perfectly consistent with the lone electrostatic effect but with special relativity taken into consideration. The simplest hypothetical experiment would be two identical parallel infinite lines of charge (with charge per unit length of $ \lambda \ $ and some non-zero mass per unit length of $\rho \ $ separated by ...


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Faraday's Law of Electromagnetic Induction predicts that the relative motion of the magnet and the copper tube will result in the presence of eddy currents in the tube; . The force relationship is described by Lenz' Law. That is, the induced fields oppose the fields that created them. If this were not so, it would be easy to build perpetual motion ...


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The arguments from special relativity given in the other answers is correct. What is charge according to one observer is current according to another observer that is in relative motion to the first. But this is, from a historical perspective, somewhat backwards. This consideration is what led Einstein to develop special relativity -- the paper is called On ...


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Consider this: A charged particle at rest creates an electric field, but no magnetic field. Now if you walk past the charge, it will be in motion from your point of view, that is, in your frame of reference. So your magnetometer will detect a magnetic field. But the charge is just sitting on the table. Nothing about the charge has changed. Evidently ...


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Regarding 1) observe that there is a pattern in common - namely that there is some region (volume for Gauss and a surface for Ampere) and integral of the source on this region is equal to the integral of the field on the boundary. This is a striking similarity. 2) currents are nothing else than moving charges. So both fields are generated by charges. These ...


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You are right, the electric field and the magnetic field are distinct fields that have different properties. The reason why they are still classified as the cause for the "electromagnetic force" are the following: In higher theories, like the field theory, the electric and the magnetic field are caused by the same gauge principles. There is just "one" ...


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Charged particle is accompanied with EM radiation (has field that falls with distance as $1/r$) when it moves with acceleration. This can be shown to be a consequence of Maxwell's equations, well-verified and reliable part of physics. It is immaterial whether the charged particle is a point or an extended body. None of that depends on the value that ...


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If T2 and T1 could be computed with a mathematical relationship as function of PD only, T2/T1/PD-weighted images would have exactly the same contrast and they would give us the same information, as tissues with the same PD will necessarily have the same value of T2 and T1. T1 and T2 depends on many parameters!


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Traversable - Overlapping (actually intersecting) region would not be Traversable even if the gravity at some parts of the region may be zero. For exampple, between earth and moon, gravity will be zero at some point. That does not mean something in that region can go out of earth/moon system. As soon as an observer leaves that region, it either falls towards ...


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I agree with Lubos Motl's answer. Geologist indeed consider the magnetic property of molten iron i.e. paramagnetism and ferromagnetism. Iron is a ferromagnetic material. It has a greater degree of magnetism when compared to dia- or paramagnetic material. Now, there is a temperature called Curie's temperature when a ferromagnetic material changes to a ...


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If the event horizons overlap you get one big horizon. EM forces can not counteract gravity if the curvature is too large since the force required to counteract gravity becomes infinite at the horizon. You can see this in the equation $$F=\frac{G\cdot M\cdot m}{r^2\cdot\sqrt{1-r_s/r}} $$ which becomes infinite at the horizon $r_s$. Since from the outside ...


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The key thing about transformers is their primary winding's magnetic field only induces currents in the secondary winding(s) during that time while that field is in motion, same as an alternator or generator. The field can be put in motion by machinery as in the alternator or generator, or the field can be put in motion (as in the stationary transformer ...


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How does the matter-radiation system, on its own, goes over to the Blackbody distribution? Evolution towards equilibrium (in macroscopic sense) happens when the system matter + radiation is isolated, for example if a piece of matter is inside a cavity that slows down leakage of energy out of the system. For example, a piece of coal in a well reflecting ...


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Hint : A key to the solution is what is meant by the complex wave 3-vector $\:\mathbf{k}\:$. This vector is not any complex 3-vector in $\: \mathbb{C}^{3}\:$ $$ \mathbf{k} \ne \left(k_{1}, k_{2}, k_{3} \right) \in \mathbb{C}^{3}, \:\:\text{that is with} \:\: k_{\rho} \in \mathbb{C} \tag{a-01} $$ but $$ \mathbf{k}=\left(k_{1}, k_{2}, k_{3} \right) ...


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Dynamo Effect : The dynamo effect is a geophysical theory that explains the origin of the Earth's main magnetic field in terms of a self-exciting (or self-sustaining) dynamo. In this dynamo mechanism, fluid motion in the Earth's outer core moves conducting material (liquid iron) across an already existing, weak magnetic field and generates an ...


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To question 2: When the electron reaches the end of a conductor, it would have to move into the air, which is an isolator. The entire conductor is at equal potential, which is much much lower than the potential at a point out in the air. So it reaches the end and stops, since it is only driven by the potential difference $$F=\frac{dU}{dx}$$ in it's rush to ...


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The problem is that they have too many solutions if the gauge is not fixed. Imagine you have some initial values, and want to solve it on the computer. Then you have to solve the equations for the next time step given the values for the previous one. But you have to compute the values for, say, four variables but have only three equations. You somehow ...


1

The magma has temperature between 700 and 1300 Celsius degrees. The Curie temperature of iron is at 770 degrees Celsius. Above that temperature, iron loses magnetism. Note that right above 770 °C, iron is still solid because the melting point is around 1500 °C. So magma almost never can be magnetic because it's just too hot for that. Incidentally, if it ...


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First, the gauge invariance means that the solutions $A_\mu(x^\alpha)$ are not unique. For every solution, the gauge transformations of it are solutions, too. That may be a problem because sometimes we want to have specific values of $A_\mu(x^\alpha)$ that answer a physical question. Second, we sometimes gauge fix because the equations simplify. For ...


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Oxygen and Nitrogen do absorb 'light' but only in the ultra-violet region of the spectrum below approx 200nm, an area invisible to our eyes but easily observed by photomultipliers and similar detectors. This absorption is caused when an electron from a molecule's ground state is promoted to one of several electronically excited states. These states have such ...


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The equations are entirely equivalent, as can be proven using Gauss' and Stokes' theorems. The integral forms are most useful when dealing with macroscopic problems with high degrees of symmetry (e.g. spherical or axial symmetry; or, following on from comments below, a line/surface integrals where the field is either parallel or perpendicular to the ...


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Your confusion lies in failing to recognize that they are exactly the same equations. Take for example Gauss's law $$ \vec \nabla \cdot \vec E = \dfrac{\rho}{\epsilon_0}$$ You can see that there $\rho$ is the charge distribution, and in general can be a funcion of the position. Now consider a volume $V$, you can just integrate the density to obtain the ...


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As is written here the two remaining equations follow from the Bianchi identity which says that the anti-symmetrized derivative is zero, ie. $$ \partial_{[a} F_{bc]} = \partial_{a} F_{bc}+\partial_{b} F_{ca}+\partial_{c} F_{ab} = 0 $$ (remember the $F_{\mu\nu}$ is antisymmetric itself!)


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All this tells you is that the fields satisfies both the inategral and the differential equations. The two are related by the mathematical identities called the divergence theorem and Stokes' theorem. So which do you apply? Well, which ever one you want! If you run into an integral, you use the integral form, and if you're ever asked for the divergence or ...


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Electromotive force is not a mechanical force, but a driving electrical force for charges or the potential energy per unit charge stored in the electrical source. It can be seen as the work that can be done by the source to drive off electrons in a circuit, provided there is no internal resistance of the source. This potential is the gradient of the ...


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In a simple DC circuits the charge carriers will drift through the bulk volume of the wire and resistor. Collisions in the resistor (and also wires) will be converted in heat and effectively transferring battery energy to the resistor and less so to the wire. There will be net charge accumulation on the surface of the wire and resistor maintaining the ...


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International Handbook of Research in History, Philosophy and Science Teaching quotes the English translation of Guisasola et al. (2008), which discusses some of the early history of the EMF. The man who coined the term "electromotive force" was Alessandro Volta, who stated that there was a force separating the charges in current flowing in a closed circuit. ...


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In general your relation is $$ \vec{B}(\omega) = (1 + \chi_m(\omega))\vec{B}_0(\omega) $$ or in the time domain $$ \vec{B}(t) =\vec{B}_0(t) + \int\limits_{-\infty}^\infty \chi_m(t,t') \vec{B}_0(t') \;\rm{d}t' $$ Only in the case of instantanous material response, i.e. $\chi_m(t,t') = \chi_{m,0} \cdot \delta(t-t') $, your equation is correct. This already ...



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