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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Thinking of the Faraday field-strength tensor as a 2-form

Background I'm familiar enough with the Faraday tensor $F_{\alpha\beta}$ to know that it's is a 2-form. Hence, at each point $P$ in spacetime $V$, it's a multilinear map $$F: T_PV\times T_pV\to\mathbb{...
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Dipole moments in QFT

We say that an electron has a dipole moment (let's arbitrarily focus on magnetic dipole moment), which we can calculate classically and also add quantum corrections. Suppose we measure the electron's ...
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Repulsion b/w two flat cylindrical magnets

Given 2 identical 'flat' cylindrical permanent magnets, $A_1$ and $A_2$. The radius, height and weight of each magnet is denoted by $R$, $H$ and $W$ respectively. (magnets are flat implies that $R$ > $...
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Can electricity be generated using a rotating electromagnet and fixed metal plates enclosing it (as opposed to rotating permanent magnet)?

I was wondering , if this be possible , can we make an adjustable electricty generator? If not, why so? Appreciate any responses .
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How does alternating current provide energy? [duplicate]

In my head, direct current makes complete sense; the electrons carry energy around the circuit to something being powered losing its potential and then return to the battery or whatnot to have their ...
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Intuition of Maxwell's Equations [duplicate]

Is there an intuitive explanation for Maxwell's equations? I know they are axioms but is there a logical understanding of why instead of mathematical. Both forms don't explicate the scientific ...
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48 views

Why are there many turns in the primary coil of a step-up electricity transformer?

Since the voltage in a coil is proportional to the number of turns, the ratio of voltages is equal to the ratio of turns. So if you need a big increase in voltage, what stops you from having one turn ...
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16 views

Effect of a current carrying wire on a point charge

Given a point charge $q$ near a wire carrying current as shown, what would be the effect of the magnetic field produced by the wire on this point charge $q$? I think the sign on $q$ is important so ...
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Direction of current induced in a loop present in a magnetic field

So I have a loop, in a magnetic field directed into the plane that the loop is lying in as shown in the figure. Let's assume that the loop is being moved towards right with uniform velocity. ...
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How is magnetic energy supplied?

Enthusiastic layperson here, curious about magnetism: Scenario 1: Imagine that I lift a piece of iron with my arm from a tabletop a fixed distance upwards, then place it back on the table. In ...
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Chemistry without electrons?

This question on the behavior of a universe with massless electrons got me wondering: what would atoms look like if electrons were not merely massless, but rather the lepton fields just didn't exist ...
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84 views

How is Electromagnetic segmented-capacitor propulsion system even possible?

The alleged invention - patent US10006446B2 - suggest allowing the creation of lifting force (4) on electrically charged rotating plate when the magnetic flux is turned on and directed towards the ...
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Difference between Capacitor and a battery [duplicate]

Capacitor stores electric charge and battery also stores electric charge. SO whats the difference between them?
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Removing “electric dipole moment” from non-relativistic Dirac equation

I have found many sources (c.f. Schwartz's QFT book section 10.4) that try to obtain the non-relativistic limit of the Dirac equation by first "squaring it" so that it looks somewhat like the Klein-...
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Potential drop across inductors in DC circuits

The potential drop across an inductor is equal to $L(di/dt)$ where $L$ is inductance coefficient. But according to this in a DC circuit there should be no potential drop across the inductor (because $...
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MCS Lagrangian and Euler-Lagrange

I'm trying to solve the Euler-Lagrange equation for the MCS Lagrangian density as given by Kharzeev in this article (Eqn. 7): $$ \mathcal{L}_{\textrm{MCS}} = -\frac{1}{4}F^{\mu\nu}F_{\mu\nu}-A_\mu J^{...
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Maxwell equations in curved spacetime with cosmological constant

I am aware of Maxwell Equations in Curved Spacetime. But how do these equations change if the cosmological constant is not assumed to vanish?
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Permitivity Constant of Ionized Gas

If I have let’s say an ionized gas, composed purely of singly charged positive ions; what would be the permitivity of the material? Is it infinite, as is the case with a perfect conductor, or am I ...
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Criteria for electrostatic approximation in microscopic/kinetic plasmas

I started pondering about the electrostatic approximation in relation to plasmas, where a particle's motion $\mathbf x(t)$ is governed by the Lorentz force: $$ m\ddot{\mathbf x}=q(\mathbf E+\dot{\...
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How to incorporate refractive index in transfer matrix method

I need to determine the TE and TM reflections at the interface between a uniaxial crystal and air, using a matrix-based method, such as the TMM. I can do so using the Fresnel equation with ease, but ...
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A steady line current moving at a steady velocity can produce non-transverse electric fields. What about moving line charges?

A steady, straight current that is stationary can exert a Lorentz force on a moving charge that is parallel to that current. Conversely, a stationary charge can experience a Lorentz force that is "...
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Do emitted photons have a well defined frequency or just a spread as per HUP?

I have read this question: Conservation of Energy in photon exchange between two atoms where Kurshal Shah in a comment says: As per the energy-time uncertainty relation, the emitted photon ...
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What is the real difference between tunneling ionization and multiphoton ionization (MPI)?

Ionization is when an EM neutral atom gaing Em charge by losing or gaining an electron. Tunnel ionization is ionization due to quantum tunneling. In classical ionization, an electron must have ...
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What is the real problem in the free body problem?

Ions are produced when an EM neutral atom gains EM charge by gaining or losing electrons, by collision with other charged particles or photons. The study of such collisions is of fundamental ...
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$B$ field inside a Magnet

From Lorentz Force law there are experimental proofs for defining the direction of the magnetic field $B$ outside the magnet. But when the interior of magnetic material is concerned how is the ...
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1answer
26 views

Poynting vector flux on a closed surface

What does the flux of the Poynting vector through a closed surface represent? I know that the flux of the Poynting vector on a certain surface represents the power flowing through it. But if the ...
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24 views

Magnetic horn function

I wonder how a magnetic horn works. I can imagine that a magnetic field will distract particles due to their charge but what I don't understand is, how to isolate antiprotons? The charge is probably ...
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Magnetic Detection of Magnetite

I have a tool with a very thin and faded coating of what seems to be magnetite black oxide ($\mathrm{Fe_3O_4}$), I’m trying to see if this coating is magnetite, while a chemical test is possible, a ...
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If a constant current can create constant magnetic field , why do we need changing magnetic field to induce current?

The electrons only going from negative to positive are able to create a magnetic field, why does the magnetic field have to keep changing to induce a current? If I pass the same magnetic field through ...
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Orientation and sign convention in 2D electrodynamics using differential forms

I've been following this paper for a treatment of electrodynamics using differential forms. In particular, they demonstrate that Maxwell's equations expressed using differential forms are form-...
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1answer
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An electrodinamic identity: starting point [closed]

With this request, I would like to ask you kindly how you can prove this identity. I thank you for those who can help me. \begin{equation} \overline{\nabla} \times (\overline{\nabla} \times \...
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Naive question - Is Magnetic generator (like this) not a 'perpetual motion machine'? [duplicate]

Perhaps a question that is too naive to be on Physics.SE but I let the experts here decide (and downvote at will), but there seem to a certain company that claims to have created what it calls a "...
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2answers
124 views

H field Vs B Field inside a magnet

I was studying electromagnetism and was going the basic definition and trying to understand their importance. I could not figure the difference between The H- Field and B- Field at first. So I ...
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How does the Earth's magnetic field protect it from the solar wind?

Several sources (see linked questions at the end) say that earth's magnetic field shields us from the solar wind. The general consensus is that without earth's magnetic field, the atmosphere would be ...
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Direction of EMF and Induced current through the circuit

When the circuit shown above is moved with a constant velocity to the left I know that B is going to have the higher potential. And I believe that the current flows clockwise that is in the sense of ...
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Magnetic Field Lines Vs Magnetic Vector field

I am studying electromagnetic theory and when I started researching the history of conventions used in magnetic interactions I could not get them. The basics of how they modelled the magnetic ...
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55 views

Permanent magnet attraction from Maxwell's equations / Lorentz force [duplicate]

Edit: My question is different because I am not asking how permanent magnets come about. Rather, I am asking why (taking permanent magnets as a given) they attract or repel each other. I.e. What ...
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208 views

Compact expression of Maxwell's equations: is there a miss minus sign?

The compact form of Maxwell's equations: $$\boxed{\square\, \boldsymbol{\mathsf{F}}=\mu_0 \boldsymbol{\mathcal{J}}} \tag{1}$$ where the current density quadrivector is given by the relation $\...
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Best books for Electromagnetism? [duplicate]

I would like to know some titles of the best books in Electromagnetism. Some poeple recommended me to read Griffiths. My big deal is to become a theoretical physicist in Particles/Fields and ...
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Understanding of Gauss law using vector fields

I was going through the conventions and terminologies followed to describe the magnetic interactions. I understood that the field lines are just a simpler representation of the magnetic interaction ...
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1answer
50 views

Does protons moving parallel to each other exert magnetics force

If two protons are moving parallel to each other in same direction with equal velocities then do they exert magnetic force on each other.? As protons are moving they should produce magnetic field and ...
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Newton's third law in magnetic in magnetic fields

Say I have a charged particle moving through a magnetic field perpendicular to it. It will experience a force, but according to Newton third law Every force has an equal and opposite reaction. So ...
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Time derivative of dipole moment

While deriving the relation, $$\int_V \bar{J}d \tau = \frac{d\bar{p}}{dt} $$ in this question. We have, $$\frac{d\bar{p}}{dt} = \frac{d}{dt}\int_V\rho \bar{r} d\tau \\ \implies \int _V \frac{\...
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Poynting vector flux in a tube (geometrical optics)

Let's consider the following tube (defined as the solid described by two arbitrary surfaces dA1 and dA2, and by all the rays which connect their boundaries): I read in a textbook that, because of ...
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How to recover the potential field from Green's function and Poisson's equation for a point charge

I first find the Green's function for the following PDE in $n=3$ dimensions, where $k:=|k|^2$. $$\nabla^2G(x,x')=\delta^3(x-x')$$ Upon Fourier transforming both sides, and inverting, I find that $$G(x,...
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What do you see when you are inside a transparent conducting material?

Lets say that there exists a transparent conducting material. Lets carve a cavity in the meat of the conductor big enough for a person to stay in. There exists an external electric field. But, We ...
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112 views

Why doesn't a magnetic field accelerate moving charges?

As a magnetic field of intensity, $\mathbf{B}$ applies a force of $\mathbf{F}= q\mathbf{v} \times \mathbf{B}$ on a charge $q$ moving with velocity $\mathbf{v}$, this force $\mathbf{F}$ should be ...
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Electromagnetic waves according to Maxwell

If a variable Electric field creates a variable magnetic field and VICE VERSA (according to Maxwell's equations), then why don't we enter a loop where E vector and B vector keep creating one another ...
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Electromagnetism on 3 torus

We all know Maxwell equations in 3+1 spacetime, where the "space" is $\mathbb{R}^3$ and time is $\mathbb{R}$. Moreover, it is easy to construct (using differential forms) the corresponding theory in a,...
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Electric field from the intersection of two spheres with different charges

This question is repeated a lot, but I for two spheres with different charge densities, sphere one with radius a centered at the origin and charge density $\rho_1$, and sphere two with radius centered ...