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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127 views

Faraday cage in real life

In electrical engineering we talk about using a "Faraday Cage" all the time. In general we mean putting the circuit in a metal box and grounding it, or putting a EMI shield over the top of a chip. ...
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2answers
187 views

How does a magnetic field “push” charges?

I was under the assumption that a magnetic field acts similar to that of the normal force in mechanics; both affect the path of the object, but do no work. So now suppose that we have a rectangular ...
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37 views

What experiments of a high school standard are there to investigate the magnetoelastic effect?

I've ended up investigating the reverse magnetostrictive effect for coursework, which is 30% of my grade this year. I started by hitting a magnet with a hammer (on the recommendation of my teacher) , ...
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35 views

Is it possible for the phase of electric charge to change over large general relativistic distances?

Jackson provides examples of how magnetic charge and electric charge form together to create complex charge, \begin{align} \rho = \rho_e+i\rho_m \end{align} which gives rise to the complex faraday ...
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90 views

How can a magnetic field produce an emf if the force produced does no work?

In the textbook I'm reading it gives an example of where a person pulls a rectangular wire with a velocity $v$ and the left side of that wire is in a uniform magnetic field. After some ...
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89 views

Why is $\mu_0$ missing in EM formulas in Peskin and Schroeder?

In this post, $\hbar=c=1$ units are used throughout. It is well known that the action of classical electromagnetism is given by $$\mathcal S_{\text{Maxwell}} = \int ...
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1answer
120 views

Can metal rods detect underground wires?

Until not long ago, I was working in the drilling and infrastructure business. Some people I was working with used metal rods as a "quick and dirty" method of detecting underground wiring, before ...
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1answer
61 views

Is a superconductor perfectly opaque?

Because of the Meissner effect, no magnetic fields can pierce through the body of a superconductor. Since EM waves need both their electric and magnetic field components, it cannot pierce through the ...
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63 views

Motional emf and ohm's law

Suppose a thin, closed loop of wire is in motion in a static magnetic field. Let's define a few things $$\vec{F_B} = \vec{w}\times\vec{B}$$ $$\Phi = \int_S \vec{B}.\vec{dS}$$ ...
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2answers
379 views

Electric field around two charged hollow cylinders

There are two hollow cylinders with same lengths "l" as shown in the figure below. The smaller inner cylinder is negatively charged. The outer one is now induced to become positively charged. I am ...
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47 views

Probe/Sensor-design for pulsed electromagnetic field

So I have a wire/coil, acting as a sender, which has a pulsed signal as described below. I would like to build a sensor/probe that can detect the electric field at a distance, where the primary goal ...
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138 views

Energy conservation in electrodynamic system?

Consider two charged particles initially at rest in the configuration below. Let us assume the following: Starting at time $t=0$, we apply a constant force $f$ to the the bottom particle so that ...
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325 views

Magnetic fields in transformers and back emf?

i am having some trouble with understanding the magnetic fields inside transformers and how they relate to back emf. My first query is: is the magnetic field produced inside the soft iron core ...
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152 views

magnetic field of a rotating ring magnet

Can the magnetic field of a rotating ring magnet induce current in a conductor? Is there a flux linkage between magnetic field of a rotating ring magnet and a conductor?
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37 views

How did the author derive the distance the particle takes as $\dfrac{h}{cos\theta}$?

On Page 292 of Griffith's Introduction to Electromagnetism (3rd ed.) he mentions that the a basic for of a generator is one in which one pulls a square wire with one side of it in an uniform magnetic ...
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143 views

Is it true in electrodynamics, that $\vec{E} \cdot \vec{B}=0$? [closed]

I'm just trying to verify if $\vec{E} \cdot \vec{B}=0$ in electrodynamics.
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1answer
141 views

Free Electromagnetic field in Lorenz gauge

To get rid of the extra term in the QED Lagrangian we need to redefine the electromagnetic four-vector: $A^{\mu} \rightarrow A^{\mu} - \frac{1}{c} \partial_{\mu} a(x)$ where $a(x)$ is the function ...
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2answers
122 views

Ampere's law of circular path when “bulging” out

The picture shows a use of Ampere's law. A circular path is chosen. $$\oint \vec B \bullet \mathrm{d}\vec l=\mu_0 I_{encl}$$ When using Ampere's law we are talking about the current enclosed. That ...
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12 views

magnetization of a infinite crystal lattice

Cosider a infinite crystal lattice with right face end at infinity and left face end at origin.my question is, consider this crystal is made of material which can be magnetized and if you magnetize ...
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80 views

Perfect conductor and magnetic field

Inspired by this question. Can current be induced in a superconductor? Magnetic flux of a magnet cannot enter a perfect conductor. Again moving electrons ,protons produce magnetic field(and thus ...
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2answers
166 views

Moving electric charges

I just wanted to double-check these three statements, as I'm not entirely sure I understood them completely: 1) A stationary electric charge (let's say a proton) produces electric field. 2) A moving ...
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1answer
117 views

Orbital magnetic moment versus Biot-Savart law

In atomic physics, the fine structure of spectral lines assigned to atomic hydrogen has always been explained by considering an orbital magnetic moment of atomic electron. Still this concept is ...
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1answer
64 views

What is the action of maxwellian electromagnetism?

What is the Lagrangian formulation of classical electromagnetism? Specifically I want to know the action in classical electromagnetism.
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115 views

I want to know why light even moves at all.

Never mind in a vacuum, why and how does light even move at all? What propels it?
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50 views

How does one show specific thickness and wavelength determine full transmission of electromagnetic waves?

How does one show that thickness and wavelength determine the full transmission between two different dielectric media if the boundary condition equations between two dielectric media are independent ...
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31 views

Why is permittivity or permittivity negative in certain metamaterials?

I read some papers and I think the explanation is that the driving electric (magnetic) field leads the response by π, thus making the effective permittivity (permeability) negative. But my question ...
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2answers
157 views

Classical EM : clear link between gauge symmetry and charge conservation

In the case of classical field theory, Noether's theorem ensures that for a given action $$S=\int \mathrm{d}^dx\,\mathcal{L}(\phi_\mu,\partial_\nu\phi_\mu,x^i)$$ that stays invariant under the ...
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2answers
115 views

Momentum around an accelerated electron

Assume that an electron is accelerated along the +x-axis. The electron will radiate electromagnetic energy and momentum in every direction. But it seems to me that the EM momentum it radiates in ...
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1answer
72 views

Does current flow if the current density $J=\sigma f=0$?

A current density $J$ is proportional to the force per unit charge and is of the form $J=\sigma f$. In cases of a conductor where $\sigma=\infty$ and hence $f=0$, is it that current is moving so ...
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58 views

Acceleration in relativistic particle [closed]

Suppose that a relativistic particle of mass at rest $m_0$ and charge $q$ moves under the influence $F$. Show that the acceleration $a$ of the particle can be expressed as follows: $$a = \frac1mF + ...
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156 views

Why is there a magnetic field around a magnet if there are no charges moving? [duplicate]

From what I understand, the magnetic force is just a relativistic effect of the electric force, and I understand how this is can be the case when considering the magnetic field generated by a current- ...
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1answer
58 views

Confusion regarding inductors

I understand that a change in flux linkages of an inductor results in induced emf across it, the magnitude and direction of which can be determined by Faraday's and Lenz's laws. The magnitude of this ...
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0answers
92 views

How can I calculate the Teslas and resistance of an electromagnet?

I'm trying to build a small electromagnet to use in a generator. Right now, I have a coil of insulated 16 AWG copper wire wrapped around a large iron nail with a diameter of 1.5cm. The length of the ...
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1answer
133 views

Why can't the electron enter the nucleus?

Actually, there exists an attractive force between an electron and the protons inside the nucleus, but the electron cannot be attracted towards the nucleus! What force balances that attractive force? ...
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165 views

(Capacitance of a battery?) Finding the charge accumulated on a rod

We all know the expression for the potential difference across a conducting rod moving through a region with a uniform magnetic field: $\epsilon = B l v$ Now my question is different. Is it ...
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57 views

How can we assume that we're in electrostatics if we are talking about current?

In the textbook I am currently reading it states that to even out the flow of current throughout a circuit, the force required is of the form $f=f_s+E$ where $f_s$ is from the battery or other energy ...
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1answer
89 views

What does it signify that electrostatic mass cannot exist without inertial mass?

From Coulomb's law and Newton's second law we can state that if there is electrostatic mass (charge) at any point of space then there has to exist inertial mass also at that point of space. Otherwise, ...
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49 views

Induction in a coil of a hollow conductor

Will a mutually induced AC current in a coil of a solid conductor behave differently from that induced in a coil made from a hollow conductor. Background: A colleague and I are researching for an ...
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1answer
67 views

How to propagate a planar e/m field in free space using plane waves?

I read this great answer to this question: Numerical software to manipulate a light beam in its plane wave representation? The main thing that I am trying to clear in my head is the following: ...
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0answers
79 views

Why did Heaviside eliminate the magnetic potential from Maxwell's Equations?

Maxwell's original equations had magnetic potential, but Heaviside eliminated this variable. What was the reason for Heaviside's removal of the magnetic potential?
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1answer
295 views

What is reduced momentum in “A Dynamical Theory of the Electromagnetic Field” by James Clerk Maxwell?

I was reading Maxwell's paper titled [A Dynamical Theory of the Electromagnetic Field][1]. In part 2, section 3 ("Dynamical Illustration of Reduced Momentum"), Maxwell discusses a mechanical ...
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39 views

Electro-magnetic wave propagation in vacum

I was wondering what will the EM wave propagation including virtual particle pairs existence look like. When electric field face virtual pair it should make the lowest state energy the most ...
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99 views

How to calculate the magnetic and electric fields in an electrical system with arbitrary geometry? [closed]

What physics and mathematics are involved in calculating the fields around an electrical system of arbitrary geometry? Is it possible to describe a system in 3D, assign physical properties to the ...
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1answer
36 views

Lienard-Wiechert fields for low velocity source

I would like to use the Lienard-Wiechert E and B field expressions for a slowly moving charge where $\beta = v/c << 1$. Is there an accepted approximate form to use? Can one just set $\gamma = ...
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3answers
166 views

Would a rotating magnet emit photons?

If a magnet is rotating, around an axis perpendicular to the axis north-south axis of the magnet (which I assume to be cylindrical symmetrical), in space (so no-gravity/freefall or friction), should ...
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1answer
60 views

Magnets: can you tell which pole is which without a second magnet?

In other words, is there something fundamentally different between the "north" and "south" poles of the magnet or are they just relative terms. If there isn't anything fundamental, what is the ...
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1answer
38 views

Why are $\vec{E } \cdot \hat{n}$ and $\vec{J} \cdot \hat{n}$ needed to prove the electric field is uniform in a wire

Here is an excerpt from Griffith's Introduction to Electrodynamics third edition. In the proof, what was the rationale in deciding to do the dot product of $\vec{J}$ with a unit vector and $\vec{E}$ ...
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37 views

Can electromagnetic fields be measured from a distance?

I know that direct measurements of electromagnetic fields can only tell information about the field near the probes, but I am curious if there is a way to measure the electromagnetic field in a not ...
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2answers
103 views

Could synchrotron radiation be produced by a gravitational field instead of a magnetic field?

I know that synchrotron radiation is produced when a charged particle is accelerated radially by a magnetic field, but I was wondering if synchrotron radiation could also be produced by an uncharged ...
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1answer
60 views

Counting d.o.f. and gauge fixing $A_{\mu}$ and $\psi$ in $D$-dimensions

Setup: Let us assume we are in $D$-dimensional Minkowski space-time where $D=d+1$. Consider a free Abelian gauge theory. Then the electromagnetic field will satisfy $$\partial_{\mu}F^{\mu \nu}=0 ...