# Questions tagged [electromagnetism]

The classical theory of electric and magnetic fields, both in the static and dynamic case. Also covers general questions about magnets, electric attraction/repulsion etc. Distinct from electrical-engineering.

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### Do electrostatic fields really obey “action at a distance”?

In an electromagnetic theory class, my professor introduced the concept of "action at a distance in physics". He said that: If two charges are at some very large distance, and if any one of the ...
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### Do stars remain electrically neutral? [duplicate]

How electrically neutral do stars remain through their lifetime? As an example, I could imagine processes such as coronal mass ejections leaving the Sun in a slightly charged state. Are there such ...
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### Electromagnetic Momentum

My book says : The fact that electromagnetic radiation of energy carried momentum was known from classical theory and from the experiments of Nichols and Hull in 1903. This relation is also consistent ...
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### Induction and electromagnetic fields

I've got a few questions on induction and electromagnetic fields. My current understanding of induction and electromagnetic fields is that, when electricity/current flows through a wire, it creates an ...
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### Eddy current induce EMF

Q1 Suppose there is a wire having AC current so as ac current is alternating it creates back EMF whether that back emf produces current? If yes then that current is same as eddy current or not? And ...
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If $B$ is magnetic field and $E$ electric Field, then $$B=\nabla\times A,$$ $$E= -\nabla V+\frac{\partial A}{\partial t}.$$ There is Gauge invariance for the trnasformation $$A'\rightarrow A+\... 2answers 352 views ### Relating Poyntings theorem to Lenz and Faraday's law? In system's similar to a motor, where the armature begins to accelerate simultaneously there is induced -\epsilon to reduce the applied current(hence the applied power P(t) is also reduced), or ... 3answers 2k views ### Shielding magnetic field without mu-metal I need to shield my device from magnetic interference, including earth magnetic field (if you move device around, it might be enough to cause slight currents i guess) and magnetic field caused by ... 2answers 1k views ### Is there no electric field inside a conductor? I came across this statement while studying electric currents and I am confused: "There is no electric field inside a conductor. Hence no current can flow through it". Is there a fallacy in this ... 3answers 507 views ### Satellite Power Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. Since an orbiting satellite is passing through the Earth's magnetic field would a voltage be ... 6answers 4k views ### What's the physical meaning of the statement that “photons don't have positions”? It's been mentioned elsewhere on this site that one cannot define a position operator for the one-photon sector of the quantized electromagnetic field, if one requires the position operator have ... 1answer 2k views ### What different approximations yield Gravitoelectromagnetism and Weak Field Einstein Equations? This question is inspired by this answer, which cites Gravitoelectromagnetism (GEM) as a valid approximation to the Einstein Field Equations (EFE). The wonted presentation of gravitational waves is ... 5answers 2k views ### The problem of self-force on point charges Allow me to preface this by stating that I am a high school student interested in physics and self-studying using a variety of resources, both on- and off-line, primarily GSU's HyperPhysics website, ... 2answers 6k views ### How can length contraction result in electron circular motion in a magnetic field? If you ask around about magnetic fields, you will read seemingly-authoritative articles which say magnetism is a consequence of length contraction. This is widely taught and is repeated in answers ... 4answers 6k views ### Can a third type of electrical charge exist? Upon reading my book on physics, it mentions that there are only two discovered types of electric charges. I wonder if there could be a third type of elusive charge, and what type of effects could it ... 3answers 28k views ### How does Hendo hoverboards achieve the self-propelling motion? What is the MFA? From what I understand of the idea behind Hendo hoverboards, they use four disc shaped hover-engines to generate a self-propelling motion. I also understand that it uses Lenz's law and eddy currents. ... 2answers 2k views ### Is Newton's universal gravitational constant the inverse of permittivity of mass in vacuum? Is it possible to consider Newton's universal gravitational constant, G, as inverse of vacuum permittivity of mass?$$\epsilon_m=\frac {1}{4\pi G}$$if so, then vacuum permeability of mass will be:... 2answers 3k views ### What is a gauge in a gauge theory? As I study Jackson, I am getting really confused with some of its key definitions. Here is what I am getting confused at. When we substituted the electric field and magnetic field in terms of the ... 5answers 2k views ### Is there a topological difference between an electric monopole and an magnetic monopole? When we introduce magnetic monopoles, we have duality, i.e. invariance under the exchange of electric and magnetic fields. Magnetic (Dirac) monopoles are usually discussed using topological ... 3answers 2k views ### Special relativity and electromagnetism This Veritasium video explains how electromagnets can be explained by special relativity, and how the magnetic field surrounding a current-carrying wire can also be viewed as an electric field, if ... 9answers 46k views ### What's the core difference between the electric and magnetic forces? I require only a simple answer. One sentence is enough... (It's for high school physics) 6answers 6k views ### Relativity and Current in Wire If an observer is stationary relative to a current-carrying wire in which electrons are moving, why does the observer measure the density of moving electrons to be the same as the density of electrons ... 2answers 9k views ### Why are magnetic fields so much weaker than electric? In EM radiation, the magnetic field is  3*10^8 times smaller than the electric field, but is it valid to say it's "weaker". These fields have different units, so I don't think you can compare them, ... 2answers 4k views ### Why are EM waves transverse? I was reading Griffiths' Introduction to Electrodynamics, specifically the section on plane waves. I can see that if we want a transverse wave traveling in the z direction that we are only going to ... 3answers 2k views ### Shine a light into a superconductor A type-I superconductor can expel almost all magnetic flux (below some critical value H_c) from its interior when superconducting. Light as we know is an electromagnetic wave. So what would happen ... 4answers 5k views ### How can we prove charge invariance under Lorentz Transformation? We have gravitational force between two massive particles and we have electromagnetic force between two charged particles. When special relativity suggests that mass is not an invariant quantity why ... 5answers 4k views ### Derivation of Ohm's Law Is it possible to derive Ohm's law (perhaps in some appropriate limit) from Maxwell's Equations? 3answers 11k views ### What are the differences between the differential and integral forms of (e.g. Maxwell's) equations? I would like to understand what has to be differential and integral form of the same function, for example the famous equations of James Clerk Maxwell: How to know where to apply each way? Excuse the ... 3answers 5k views ### Why does the electric field dominate in light? I read a book on the wave property of light where the author mentioned that the electric field, instead of magnetic field, dominates the light property. I don't understand why. In Maxwell's theory, a ... 3answers 1k views ### Must the action be a Lorentz scalar? Page 580, Chapter 12 in Jackson's 3rd edition text carries the statement: From the first postulate of special relativity the action integral must be a Lorentz scalar because the equations of motion ... 4answers 7k views ### Application of Kirchhoff's laws in circuits with inductors As we know, the Kirchhoff circuit laws are applicable for conservative electric fields. Now it is applicable for circuits where inductors are present but the field there is not conservative. So how ... 4answers 1k views ### How any material can provide electrostatic and magnetic shielding Several metallic objects like Iron, Copper etc can provide electrostatic shielding (one should remain inside the car during thunderstorm) and several superconductors like HTS (High Temperature ... 6answers 6k views ### What causes the permittivity and permeability of vacuum? When light travels through a material, it gets "slowed down" (at least its net speed decreases). The atoms in the material "disturb" the light in some way which causes it to make stops on its path. ... 4answers 3k views ### Influence of charged particle's own electric field on itself I read this in my textbook: A charged particle or object is not affected by its own electric field. Since I find this completely unintuitive and my mind is yelling "wrong! wrong! how could a particle ... 1answer 683 views ### Effect of introducing magnetic charge on use of vector potential [duplicate] It is well known that Maxwell equations can be made symmetric w.r.t. E and B by introducing non-zero magnetic charge density/flux. In this case we have div B = \rho_m, where \rho_m is a ... 1answer 6k views ### Why are some materials diamagnetic, others paramagnetic, and others ferromagnetic? Why are some materials diamagnetic, others paramagnetic, and others ferromagnetic? Or, put another way, which of their atomic properties determines which of the three forms of magnetism (if at all) ... 2answers 2k views ### Does a nonzero Poynting vector mean that there is propagation of energy? I don't know how this "paradox" can be solved. I'm given the following system: A permanent magnet with a magnetic field given by (\hat{a} are unit vectors in the x and y directions)$$\vec{H}=H_0\...
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I have studied that Faraday's law of induction and motional emf are two different lines of thinking but are essentially same. But then, how can Faraday's paradox be explained by Faraday's law of ...
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### Is aluminium magnetic?

From high school, I remember that Aluminium has 13 electrons and thus has an unpaired electron in the 3p shell. This should make Aluminium magnetic. However, the wiki page of Aluminium says its non-...
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### What is the importance of vector potential not being unique?

For a magnetic field we can have different solutions of its vector potential. What is the physical aspect of this fact? I mean, why the nature allows us not to have an unique vector potential of a ...
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### How is the electostatic field propagated by the vacuum? How does a charge feel the presence of another charge when there is NOTHING between them?

Is the word "vacuum" equal to "nothing"? Imagine two charges of opposite sign, far from one another in vacuum, and moving with constant velocity - s.t. none radiates energy, none emits photons. At ...
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### Is manufacturing roughness really the only reason we don't see optical interference in thick dielectrics like windows?

I had always kind of wondered why we didn't see interference in things like windows -- we were taught that the condition is that the thickness of the film/slab/medium just has to be an integral number ...
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### Are free electrons in a metal really free

Is it an accurate statement to say that free electrons in a metal experience NO restoring force when they interact with electromagnetic waves? I understand that the electrons exist in a space filled ...
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### Showing that Coulomb and Lorenz Gauges are indeed valid Gauge Transformations?

I'm working my way through Griffith's Introduction to Electrodynamics. In Ch. 10, gauge transformations are introduced. The author shows that, given any magnetic potential $\textbf{A}_0$ and electric ...
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### CGS Units for Magnetism

Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
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### Relativistic origin of magnetic field

There is an explanation in the Wikipedia. Unfortunately the article is quite verbose and doesn't clearly explain why both positive and negative charges vary density even if only one is moving. It is ...
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### How does the speed of electrons change around a circuit?

I have been thinking about ways of teaching electronics and I'm wondering if the following is true... For starters, when we talk about voltage as energy per unit charge, is this energy manifest ...