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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48 views

Does free fall in a magnetic field also leads to no force acting on the falling object similar to free fall in gravity

In complete absence of gravity if a metallic object free falls in a magnetic field will it continuously feel the force of magnetic field or will stop feeling the magnetic field during the free fall.
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How electric field and magnetic field propagate through vacuum where there's no charge present? [duplicate]

As we know light travels as an electromagnetic wave...but electric fields and magnetic fields are interactions between charges...they are the forces acting on stationary and moving charges ...
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Are there sources for potential wave solutions for which the electromagnetic far-field is zero?

Are there sources for electromagnetic potential propagating wave solutions for $\phi$ and $\overrightarrow{A}$ for which the electromagnetic far-field $\,>\!\lambda\,$ in vacuum is zero $\...
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Where does the energy go to for a charged sphere with pulsing radius?

In the question A charged sphere with pulsing radius the answer says that the charged sphere does not radiate. However, compressing a sphere of charge to a smaller radius requires work, so where does ...
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Approximating an expression for a potential

In a problem which I was doing, I came across an expression for the potential $V$ of a system as follows $$V = k\left(\frac{1}{l - x} + \frac{1}{l + x}\right)\tag{1}\label{1}$$ where $k$ is a constant,...
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Radiation frequency from an accelerated charged particle

My understanding is that a charged particle, when accelerated emits EM radiation. In a situation where a charged particle is briefly accelerated in a straight line it should produce a propagating ...
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414 views

What is capacitance, in general?

In circuit analysis software capacitance can be measured between any two nodes of a circuit or of a multiterminal device. In practical terms we take $C_{ij}$, the capacitance between $i$ and $j$ as ...
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62 views

Constant Current Conundrum [duplicate]

Say we have a simple circuit with three resistors in series, each of which have the same resistance. We also have a battery with a given constant potential difference hooked up in series with these ...
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130 views

How does the hendo hoverboard achieve omni-directional movement? [duplicate]

An older thread already gave away the wonderful details of how the Hendo hoverboard achieves levitation, but how does it achieve its form of omni-directional control and movement? My idea was that ...
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1answer
123 views

Iron bar inside solenoid, do magnetic field lines repel at sides?

Based on the closed loops rule, an iron bar inside a solenoid that has its poles aligned with the solenoid magnetic field, will have magnetic field lines around it that flow in direction opposite to ...
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Canonical or mechanical 4-momentum to measure energy?

I've often seen that the energy of a particle $p$ with 4-momentum $p^\mu$ measured by an observer $O$ with 4-velocity $u^\mu = dx^\mu/d\tau |_O$ is given by $$E=-p_\mu u^\mu.$$ In general, what ...
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Non-uniform Polarization Vector in dielectrics

In Feynman's Lectures on Physics Volume 2, Chapter 10; Dielectrics, we calculate the Capacitance of a condenser when a dielectric is placed in-between, whose polarization vector is non-uniform. ...
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Non-relativistic vector first integral

In this paper the author discusses the Poincaré vector $\vec D$, and he wrote the Lorentz equation when magnetic charge is present. Do you have any suggested reading for this and how he derived the ...
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Why can't I see any bubble in my anode during my electrolysis experiment

I am doing an electrolysis experiment using this apparatus The left side is the anode and the right side is the cathode. I am using two 9v battery in series (total 18V DC) to power my apparatus. ...
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Why is divergence and curl related to dot and cross product?

I've been reading Griffith's intro to electrodynamics and I've been a bit confused on his explanation of divergence and curl. I don't understand how divergence is the dot product of a gradient acting ...
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Retarded time and corresponding position

EDIT: Added Clarification. By various methods, it's possible to determine that in the formula for Electric potential for the position of a charge in uniform motion, $V=\frac{q}{4\pi\epsilon_0\kappa \...
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Will a dipole placed inside a dipole rotate for considerably long time

I know that a current loop acts as a dipole and when it is placed in a dipole it starts rotating and does rotate until the fields go away. If I place a dipole inside another fixed dipole, will it ...
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Understanding DC motor torque [closed]

I am building an rc car from zero and i got stuck into a little problem.My dc motor torque is 20g/cm and using gears I have achieved a ratio of 125:1 so that means i now have 20*125 = 250g/cm. My ...
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Regarding the use of Ampere's Law

i wanted to calculate $\mathbf{B}$ of these two parallel wires, and i've taken a glance at the correction and i've found out this equation, i tried to understand it but in vain, where do $(x + \frac{d}...
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Where does the metric directly manifest in physics?

I am wondering where in physics does the metric have direct physical significance. The standard answer is that the metric gives us the "lengths" and "angles" of four-vectors. This is a good answer on ...
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39 views

Problem in magnetism (theoretical)

When we move a conducting rod perpendicular to its length in a magnetic field perpendicular to direction of motion of rod,is charge density induced at only the ends of the rod or throughout the length ...
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Mutual inductance circuit analysis

given the following circuit, I need to find $ I_1(t) $ and $ I_2(t) $ (forgive my microsoft paint skills ;) ) I've come up with a solution, and I wish to check with you if it is current or, ...
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Why is the 4D $U(1)$ electric 1-form symmetry a global symmetry?

I am reading the paper Generalised Global Symmetries to understand higher-form symmetries. The first example in Section 4 that the authors talk about is the free Maxwell theory in 4d, i.e., pure $U(1)$...
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Photon angular momentum, multipole transitions

Relying on many sources, I can summarize that somehow it is possible to show that a photon, in its plane wave expansion, can carry values of total angular momentum more than $\hbar$. Such photons can ...
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How come subatomic particles interact with each other while not being in contact? [closed]

Although electrons stays away from protons an atom remains electrically neutral But two atoms one positively charged and another negatively charged remain so until they comes in contact. Can the ...
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1answer
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Electromagnet as a consequence of special relativity

We all have been given examples and the visualisations that electricity and magnetism are the same thing viewed from different references and that one can be transformed into the other. In relation to ...
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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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Can there be charge separation by applying a electric field on ionic solutions?

Suppose we have a NaCl salt solution. Since Na+ and Cl- ions exist freely in solution, it makes sense that putting the solution in a strong electric field (or a changing magnetic field) will mean ...
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Constructive Interference of Electromagnetic Waves

So I was wondering if Electromagnetic wave has the same property of interference as normal waves. I understand that both the electric and magnetic parts of the wave would have to be in the same ...
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1answer
31 views

Dipole moment for dielectric materials vs conducting materials

Let's assume we have a polarized charged sphere which is uniformly polarized (it is one of the Griffiths example). Since it is uniformly polarized, I assumed the material is dielectric. What would be ...
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2answers
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Does magnetic moment change under inversion symmetry?

Since magnetic moment can be view as a small electric current circle. Pictorially, when apply inversion operation, the current direction is reversed, so I think the $\vec{m}\to -\vec{m}$ under ...
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What is the maximum and minimum of antenna gap of dipole antenna?

Consider the picture below is a design of half wave length ($\lambda$) dipole antenna. I read some explanations saying that it is the best design to get the optimum transmit power. Or it is cut to $0....
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Evaluating an Integral with a Cross Product

Problem: A infinite thin spherical shell with radius $r_0$, centered around the origin, is homogeneously charged with surface-charge-density $\sigma$ and rotates with the constant angular verlocity $\...
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2answers
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Is electric current reference frame dependent?

$$EMF=\frac{dW_{external}}{dq}$$ $$dW_{ext}=\vec{F}_{net}.d\vec{s}$$ $$\vec{F}_{net}=\vec{F}_{inertial}+\vec{F}_{non-inertial}$$ $$dW_{inertial}+dW_{non-inertial}=dW_{external}$$ After reading the ...
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From where does the radiation come from in an inverse-Bremsstrahlung process?

I'm trying to know more about Bremsstrahlung and inverse-Bremsstrahlung processes. If you know any good references that treat theses topics with a good mathematical and physical basis, please let me ...
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'Electrostatic field of a circuit' What does it mean?

I've recently gotten into electrodynamics, I've studied electrostatics & magnetostatics, so far I know these facts: Electrostatic fields are produced by static charge configurations Magnetic ...
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1answer
155 views

What is happening from electro-magnetic point of view when a proton and an anti-proton annihilate?

Purely electro-magnetically before a proton (+) and an anti-proton (-) annihilate each other /assume they do/, they form a dipole. What happens after that? In other words, what does an external ...
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Any metal or chemical or any thing which repels Iron

I on a project, i required a metal/chemically treated metal/Chemical/anything which repels iron and steel like two same side magents do. I searched in the internet and didn't found anything which ...
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1answer
291 views

Squaring the E&M (Maxwell) field strength tensor

In Section 3.4 of Schwartz's "Quantum Field Theory and the Standard Model", the square $F_{\mu\nu}^2$ of the field strength tensor $F_{\mu\nu} = \partial_{\mu} A_{\nu} - \partial_{\nu}A_{\mu}$ is ...
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Dipole moment for dielectric shell with specific boundary conditions (different charge densities)

To find the dipole moment in a dielectric sphere, I normally use the bound charges density which helps me to find the potential differences and then negative gradient of that potential can give me the ...
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1answer
55 views

Electric Field in a uniform time-varying Magnetic field

Suppose a homogenous Magnetic Field $\vec{B}$ in vacuum that varies with time, but always points in the z-direction. This induces a curl in the Electric Field $\vec{\nabla} \times \vec{E} = -\frac{\...
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115 views

Why does an inductor oppose the change in current (magnetic field)?

May I get a physical interpretation on this question? What is happening in the inductor when the current is running through it and what is physically happening when the current starts changing?
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What is a permanent magnet and how do I calculate the magentic force exerted by a permanent magnet?

So far my understanding of a permanent magnet is: Since elements like iron have atoms with orbitals that have many electrons with the same spin, these cause the metal to have a magnetic field, which ...
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What are the differences and advantages of the integral and differential forms of Gauss's law? [duplicate]

What are the differences and advantages of Gauss's Integral and Differential Equations? Thank you so very much!
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192 views

One terminal of an ideal voltage source connected to earth->Massive current will flow?

Assume we have a "somewhat" ideal voltage source like a DC power supply powered by mains. Take just one terminal and form a conductive path between it and earth ground. Assuming that no conductive ...
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Why does the electric field of an infinite line depend on the distance, but not on an infinite plane?

I understand that with an infinite plane, as you get closer, the infinitesimal contributions to the electric field become greater in module. The direction of the vectors become less perpendicular to ...
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AC currents for self-inductance of a wire

I've been currently reading some stuff on self-inductance of a wire. There are different regimes where different approximations are used. For the AC currents the notes state that an approximation can ...
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118 views

Does electric and magnetic field interact?

Can electric field or magnetic field interact with an Electromagnetic radiation? And can 2 electromagnetic radiations interact with each other? Rather than the interference effect.
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Deriving Heaviside-Feynman formula for the electric field of an arbitrarily moving charge from Lienard-Wiechert potential

I've been trying to derive this (which Feynman warns takes a lot of work) for a couple of days now, without success. My current best derivation which however doesn't give the right answer is: First, ...
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1answer
140 views

Fall of a square conductor loop perpendicular to a homogenous magnetic field

This is my homework. I am not asking you to complete it for me - I just need a push in the right direction so I can complete the rest. The question is translated from German, so it reads a bit awkward....