I was reading about the power radiated by an oscillating electric dipole. It then said that monopoles don't radiate.  But a monopole is just a charge, right? And an accelerated charge radiates EM waves. Can someone please explain.
89633
I see this sort of statement in many materials, for example :     The smallest radiating unit is a dipole, an electromagnetic point source.   and :     The simplest infinitesimal radiating element, called a Hertzian dipole…   However, none of them includes a clear explanation. Why a dipole, which is consist of 2 charges? Isn't a single non-uniformly moving charge enough?
60036
As everybody here knows, Maxwell's equation would look more beautiful if a magnetig charge were present. Beyond the aesthetics question, if a single magnetic Dirac monopole would be found, the consequence is that the electric charge would be quantized.  While, I guess, everybody should be happy with the news, my experimentalist background doesn't allow me to understand the experimental setting. I therefore ask for the following advice -- a rather theoretically oriented question.  A recent paper,     Observation of Dirac monopoles in a synthetic magnetic field. M. W. Ray, E. Ruokokoski, S. Kandel, M. Möttönen and D. S. Hall. .   claims to have observed such monopoles.   Can somebody shed light on the theoretical implicacions of this work? Concretely, should we now take care before saying that it's been proven that the electric charge is quantized?
10270
Both light and radio waves are electromagnetic waves.that means they have almost similar properties. Both are EM waves ,E and H fields are there in both waves.....but My question is "radio waves can pass through walls ,but why can't a light pass through walls or opaque medium???
53458
I understand that electromagnetic fields carry energy, and this energy curves spacetime gravitationally.  That's not my question.  I'm asking if anyone has tried to formulate electromagnetism in such a way that EM charges impart an EM geometry onto spacetime that is only experienced by EM charges.  That is, the EM geometry of spacetime would be a function of charges such that charges themselves produce EM curvature and the motion of charges produces EM torsion, and that charged objects move according to how their charge (electrical or magnetic, positive (N) or negative (S)) experiences that geometry.  For example, a positive electric charge would appear as a "hill" to other pe charges, a "valley" to ne charges, and either a left or a right hand "whirpool" to either nm or sm charges (I'm not immediately sure which would match with which) if it had some velocity.  This formulation would be directly analogous to how energy creates gravitational curvature in spacetime (and if one accepts Einstein-Cartan, gravitational torsion comes from intrinsic angular momentum), and then this resulting geometry is experienced by the energy in that region.  If Einstein is correct about gravity, would it be too much of a stretch to suppose separate metric functions for each of the fundamental forces, considering that they were a single force moments after the big bang?  I'm not currently concerned with the quantum mechanical approach to electromagnetism.  I understand that to be truly fundamental, EM has to be formulated quantum mechanically, but right now I want to limit my question to macroscopic charged objects.
35453
Since electromagnetic waves have both electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. How much is that electric charge? Can you possibly get a shock in some way?
15065
Does AC current in simple wires produce electromagnetic waves? AC current entails very rapid changes in polarity and therefore the electrons in the metal will feel rapidly changing forces which should make them accelerate before attaining a constant drift velocity. Will this rapidly changing accelerations produce any electromagnetic wave, since the acceleration should be sinusoidal if the voltage source is, and hence the emitted electromagnetic wave should also be sinusoidal? Am I correct? If yes, why do we not account for energy lossd through EM waves in simple AC circuit analysis?
76877
How can it be that, if like charges repel, they don't repel themselves? In other words, why don't charges break apart?  About the possible duplicate: I want to know about charges in general, not just that of an electron. My response to :  Thank you very much for the extensive explanation. Unfortunately it's a bit above my level (I'm a first year electrical engineering bachelor student). This is how I understand it: If the charge is the sum of multiple separate charges, there has to be an external force that keeps this charges together. However we most often think of electrons and protons as point charges. In other words, we don't think of them as if they are made up from different (smaller) parts. But the idea of electrons and protons as point charges has its own problems. I didn't know that the electric field has a mass. From the formula, I understand that $m\propto \frac{1}{r}$. This would mean that $m\rightarrow \infty$ as $r \rightarrow 0$ (or $r = 0$), which of course is physically not possible. But there is a way to calculate the radius of an electron based on the mass, the speed of light and the constant of Planck. (Unfortunately I don't know why.) You can get around this by a technique called called renormalization, which causes the integral to converge. Unfortunately, I don't understand the rest of the answer due to my level in physics. Nevertheless I am grateful for you answer. It would be awesome if you could confirm or correct my understanding.
71799
I'm having trouble understanding the diagrams of elctromagnetic waves.  I have no problem with any concept in classical mechanics, and I think this can be answered without any relativity (which I don't know yet). I hope for an answer which is as intuitive as possible but any answer will be good.  So in the common diagram of an electromagnetic wave:     What exactly do the vectors represent? The wave is propagating though an eletromagnetic field. Are the lines just the field created by a particle?  We learned in physics that a charged particle creates a field in all of space, Is this a picture of changes in the field when it moves? And if so, how does it move? does it move up and down with the amplitude of this wave?  Lastly, It's not clear to me why the E-field part is not in the direction of the motion of the wave, because usually an electric field is depicted as so:    i.e it is in a direction "outwards" from the source of the charge.  This is really running through my mind for some time, so thanx for reading this!
19497
Is charge of something for (e.g.) an electron related to electromagnetic space if it exists due to energy, due to which it may have mass? I don't know about quantum mechanics or advanced particle models. Can anyone just simply give an intuitive idea?   EDIT I want to mean what actually gives electron charge if it is not assumed fundamental but result of some other physical phenomenon or it is  just the quantity defined to explain physical interactions?I think now it is clear
13359
If we ignore 5GHz WiFi, then both microwaves and WiFi create photons at ~2.4GHz but one of them will boil water in a few seconds but the other doesn't have any effect. So what's the difference?  Is it simply the number of photons created? Is that what the wattage of a microwave measures? If so, what would be the wattage of a wireless router?  Does the enclosed space have anything to do with it?   If it all has to do with power output could I put enough WiFi routers together in a room to cook a turkey (from microwaves and not waste heat)?
85464
I have read that "thanks to conservation of momentum" there is no dipole gravitational radiation. I am confused about this, since I cannot see the difference with e.m. radiation. Is this due to the non-existence of positive and negative gravi-charges? Is this due to the non-linearity of Einstein equations? How the conservation of momentum enters here?  An example of my confusion below  Q: Why I cannot shake a single mass producing dipole gravi-radiation? A: You need another mass to shake it. Q: Isn't it the same with electromagnetism?
94291
The well known Larmor formula tells us that an accelerated classical charge radiates a power proportional to the square of the acceleration. It has been  that a charge in free fall in a gravitational field does not radiate. Since gravity is locally equivalent to acceleration, one may wonder whether a charge at rest in a gravitational field radiates. The  is that it does. However this radiation is not  to an observer in the same reference frame! Apparently, the radiation does not carry energy or momentum, or it would be observable, in this reference frame and this would mean that charged matter behaves differently from neutral matter in gravity.    How does unobservable EM radiation without energy and momentum make sense (Q1)?  Does the physical origin of charge acceleration determine whether radiation is observable or not (Q2)?  Is the answer to the question in my title "It depends?"?   Note added:  I did not find the discussion at  and  sufficiently enlightening.
102911
Maxwell's equations  very nicely if we add in magnetic monopoles: we get $$\begin{align*} \partial_\mu F^{\mu \nu} &amp;= J^\nu \\ \partial_\mu \tilde{F}^{\mu \nu} &amp;= \tilde{J}^\nu, \end{align*}$$ where $F_{\mu \nu}$ is the electromagnetic tensor, $\tilde{F}_{\mu \nu} := \epsilon_{\mu \nu \rho \sigma} F^{\rho \sigma}$ is the dual tensor, and $J^\mu$ and $\tilde{J}^\mu$ are the electric and magnetic current densities respectively. We can use differential forms to make these equations even more compact: $$\begin{align*} \mathrm{d} (\star F) &amp;= J,\\ \mathrm{d} F &amp;= \tilde{J}. \end{align*}\tag1$$  It is known that QED can be generalized to incorporate point magnetic monopoles, which leads to all kind of interesting phenomena like the . (This is done by essentially just excising the monopoles from spacetime completely and only defining the gauge field $A^\mu$ away from the monopoles, so that they act as topological defects that convert the trivial principle $\mathrm{U}(1)$ fiber bundle on which the gauge field lives to a nontrivial bundle. This is straightforward when the monopole trajectories are specified by hand, but I think there are also reasonably tractable ways to give the monopoles their own dynamics.)  But what about the case of continuous distributions of magnetic monopoles? (To avoid cheating, let's say that the $\tilde{J}$ field's support is the entire region of spacetime whose dynamics we are considering, although still bounded so that the fields are well-defined.) In this case the equation $\mathrm{d}F \neq 0$ prevents us from introducing a gauge field $A$ such that $F = \mathrm{d}A$ in the first place. It's probably not at all clear how to quantize such a theory, so let's just stick to the classical case. To make things even simpler, we can ignore the Lorentz force law and treat the source currents as background rather than dynamical fields, so that we only need to worry about the EM field's time evolution. In this case the equations (1) form a perfectly well-posed system of coupled differential equations. Is there any known/possible Lagrangian or Hamiltonian whose equations of motion are given by (1)?
37535
I was wondering how light or any electromagnetic radiation can be "absorbed" if it consists of electric and magnetic fields. For example if there is a charge at point A, and the  light reaches point A, it will cause that charge to move depending on the value of the electric field. However, why wouldn't this electric field continue on and influence others particles everywhere else? For example, if you have a static charge creating an electric field, then if you put a charge at point A it will feel its effects and then if you bring a charge at another location B it will also feel its effects, namely it will not have been absorbed. Please shed some light on this. Thanks  edit: Does this mean the electron that absorbs this light moves in such a way that it creates an electric field of its own canceling the initial one? Or am I way off?
15872
If a radio could produce waves in the visible light spectrum, what would the result be?  This is a thought experiment that I've pondered for a few years now.  I realize there are a few/many real-world constraints, but if we lifted these constraints for the sake of thought, what could we expect?  Personally, I don't see why we wouldn't observe visible light emitting from the antenna, disregarding any light from Blackbody Radiation.
53664
This is probably a dumb question. I guess what I'm trying to ask is if radio waves travel the same speed as gamma rays, how do gamma photons carry more energy than radio photons? Do they spin faster? What other energy sources could they carry if they are moving the same speed through space?
85489
I understand how, and why, an accelerating charge emits radiation, and loses energy in the process, as well as the  formula for the power, and its derivation.  However, in classical mechanics, when we want to find the motion of a particle, the process is (theoretically) simple - find all forces on the particle as a function of position and time, plug in Newton's second law and solve the differential equation. It seems to me as if simply because a particle is charged, this process just ceases to work.  When talking about a charged particle, it is simply not enough to find all forces and solve Newton's law - now we need to somehow take into account the energy lost by the particle to radiation. However no one seems to mention how to take that into account, as a term in an equation of motion. It seems like all we know is the total power radiated by the particle - there is no concrete, complete description of how that loss affects its motion.  Of course, in some simple cases we can easily guess how the particle will be affected: for example, for an electron circling in a uniform magnetic field, it is obvious that energy loss due to radiation will cause the electron to spiral inward. You can, at least theoretically, write the Lorentz force acting on the particle, due to the uniform field, plug it into Newton's second law, and also write the Larmor formula, take that loss into account and find a complete description of the particle's motion.   However still, when we write simply Newton's second law here, with the only force being the Lorentz force, we find a circular, not spiral, solution; we have to synthetically add in the Larmor formula to truly recover the particle's motion. Contrary to classical mechanics where Newton's law is a complete description of the motion.   Why is Newton's law no longer enough, or put another way, what term am I missing that will fix the electron's motion, add an additional "force" the will cause it to spiral inward? Is there a general expression for that force? And how does one in general write the equation of motion for a charged particle (or a system of)?   I suspect the answer has something to do with the EM field changing not instantaneously but at the speed of light, since Newton's law of gravitation clearly yields stable, closed orbits and propagates instantaneously. Perhaps I should phrase my question this way:   How does the fact that the EM field propagates at a finite speed affect the way we have to write the equations of motion  for a charged particle (as opposed to the equations of motion for a massive particle, acted upon by an instantaneous, newtonian gravitational field)?   One last note: perhaps this has something to do with Maxwell's equations being Lorentz-invariant and not Galilean? However I find this unlikely to be the case since the electron doesn't have to move in a relativistic speed in order to experience this effect. In any case, this is also why I allowed myself to use Newton's second law and not its (special) relativistic expression - would using that fix the problem?