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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Maximum electric field in a circle

I have a question as follows: "Show that $E_x $ on the axis of a ring charge of radius $a$ has its maximum value at $x = \pm a /\sqrt{2} $. Sketch $E_x$ versus x for both positivve and negative ...
3
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1answer
7k views

Is there any relationship between Gravity and Electromagnetism? [duplicate]

We all know that the universe is governed by four Fundamental Forces which are The strong force , The weak force , The electromagnetic force and The gravitational force . Now, is there any ...
3
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1answer
354 views

Why is there no (time derivative of charge density) in the $B$ field in Jefimenko's equations?

I was going through Griffiths chapter on potentials and fields just to brush up on a few old things. He gets to Jefimenko's equations by this general path: Maxwell's equations Introduce scalar and ...
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1answer
369 views

How to calculate flux density in a toroidal CT that has an electrical line pass through the center?

I have seen the Biot–Savart equation, but I don't know how to apply it. Does the conductor length need to be considered finite and equal to the core length, or will the calculation be for a conductor ...
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1answer
386 views

Local gauge invariance and fields

I have one question about local gauge invariance of the spinor and scalar theories. For the scalar complex field with lagrangian $L_{0}$ requirement of local gauge invariance leads us to the ...
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1answer
203 views

$E\times B$ drift of a single particle in vacuum with unifiorm $E$ and $B$

The force on a charged particle in electric field $E$ and magnetic field $B$ is given by: \begin{equation} m \dot{v} = q ( E + v \times B) \end{equation} Then, the $E \times B$ drift velocity $v_E$ ...
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2answers
3k views

Could you use earth's magnetic field to build a hover skateboard? [closed]

In "Back to the Future" they use levitating anti-gravo skateboards without wheels which will be difficult to build, because there is no theory on how anti-gravity could be created. A possible ...
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0answers
38 views

Effects of magnetic fields on our bodies [duplicate]

I worked 20years in a very high magnetic field at an Aluminum foundry. I'm now in my 50's and I don't know or am I aware of any negative effects from this. Is there anything I should be on the look ...
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2answers
392 views

Conceptually, why does an electric field not require a charge flowing through it?

My son has been asking me a lot about electromagnetism and so I got to read up on it.One of the interesting and hard to grasp (for me at least) concepts is that an electric field will exist even when ...
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2answers
647 views

Why are magnetic field lines perpendicular to the surface of a ferromagnetic material?

It is known that magnetic field lines become nearly perpendicular to the surface of a ferromagnetic material. The quantitative proof which uses the boundary condition requires that magnetization at ...
2
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3answers
274 views

What is the interpretation of the Chern-Simons electromagnetic spin density?

Hans de Vries (who happens to be a no-longer-active physics.SE user) has an online book (referenced below) in which ch. 6 is a presentation of an object he calls the Chern-Simons current, ...
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1answer
394 views

A dielectric table is being inserted between a plate capacitor and $\triangle U<0$ how to deduce the table is attracted to the plates?

I am practicing for an exam in my Physics $2$ course. One of a previews exam questions described a plate capacitor and asked to calculate the initial energy $U_{0}$, then a dielectric table was ...
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0answers
644 views

Help making a solenoid engine [closed]

I am currently working on a school project and my aim is to build an electric motor using DC. I am looking at a design similar to this: Solenoid Engine and I have built majority of it however I am ...
7
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4answers
653 views

Why does electric field intensity $E$ can be uniquely determined by its divergence and curl? [duplicate]

My question is, the number of following equations $$\nabla\cdot E=\frac{\rho}{\varepsilon}$$ $$\nabla\times E=-\frac{\partial B}{\partial t}$$ is 4 while the number of unknown variables ...
2
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2answers
157 views

What does E/M field look like when I close a circuit?

Suppose that we have a charged capacitor with two pins: $ C_+ $ and $C_-$. Suppose that we have a long wire with fixed geometry, that is already connected to the pin C+. Let the ...
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2answers
3k views

On the atomic level how do permanent magnets work? [duplicate]

How do magnets work on the atomic level? Is there current inside the magnet?
5
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1answer
123 views

Is there a better alternative to assuming $a=0$ and then calculating acceleration? (Exam question: radiated energy after Coulomb scattering)

My classmates and I are having some debate about how to solve this problem which came up on one of our comprehensive exams. A full, step-by-step solution is not needed, an outline is enough. A ...
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3answers
537 views

The magnetic field of a magnetic monopole

Let us define the magnetic field $$\vec{B} = g\frac{\vec{r}}{r^3}$$ for some constant $g$. How can we show that the divergence of this field correspond to the charge distribution of a single magnetic ...
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0answers
185 views

Solving the source free Maxwell equations for plane waves

I've been trying to solve the maxwell equations: $$\nabla\cdot\vec{D}=0,\quad \nabla\cdot\vec{B}=0$$ $$\nabla\times\vec{E}=-\frac{\partial \vec{B}}{\partial t},\quad \nabla\times\vec{H}=\frac{\partial ...
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1answer
186 views

Magnetic moment and other invariants of Lorentz System

It's a standard fact that the quantity $\mu = v_{\perp}^{2}/B$ is approximately conserved in the system $d_{t}\mathbf{v} = \gamma (\mathbf{v}\times\mathbf{B})$. The paper Magnetic Moment to Second ...
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1answer
129 views

Energy conservation in plane wave

In plane wave the H and E are in phase. So the pointing vector disappears regularly every pi. How is energy conservation validated ? Is it through the uncertainty of energy and time ?
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1answer
277 views

Are electrodynamics problems in the complex plane relevant to real life?

This is a question I asked in Maths SE, and it was suggested I ask it here. This is a direct copy of that question. I have been reading Tristan Needham's excellent Visual Complex Analysis. The end of ...
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3answers
751 views

If electromagnetic induction generates a potential difference in a loop, where are the “high” and “low” potentials?

In AP Physics, I learned that when a changing magnetic flux generates an electromotive force around a loop, and emf is measured in volts, where are the "high" and "low" points to measure the potential ...
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1answer
209 views

Can an AC magnetic field be measured by a hall probe?

Many hall sensors on the market have a datasheet that explain the proportional relationship between the analog voltage that is outputted and the relative gauss/tesla reading (for example: 2.5mV = ...
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73 views

Can modern times physics explain Hutchison phenomenon?

I saw this video and trying to understand the phenomenon. But as you can see the electro-magnetical device lifts the non-polar material.(1:35~2:00) How can it be possible?
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1answer
135 views

Inductor rise time and inductor wire length

So here is an interesting question about inductors or coils in general. Suppose you had a inductor which was 12 inch's in diameter and say 12 inch's in length and has 100 turns of wire. The total ...
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3answers
1k views

There is no such thing as magnetism?

Here's an interesting "proof" that there is no such thing as magnetism. I know the answer but I love this so much I had to ask it here. It's a great way to confuse people! As we all know, $$\nabla ...
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1answer
6k views

What causes electron to orbit the nucleus in an atom? [closed]

What causes the electron to orbit the nucleus? Which is the force that causes it to do so? Is it related to the Electro - Magnetic force? .
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1answer
138 views

How is momentum conserved when a magnet attracts a metal?

Suppose your have any magnetic object and no external force acts upon it, and the object comes near a metal which causes an impulse (think that will happen). However, the magnetic force is internal to ...
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1answer
836 views

Calculate Magnetic Flux Density between Two Disc Magnets

If I have two disc-shaped magnets (radius r=0.05m, width w=0.03m, Remanance Br=1.06 T) separated by a distance d, how can I calculate the magnetic flux density somewhere between them? I found this ...
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0answers
167 views

Is it possible to have a magnet which looks like a hollow cylinder and who's north is interior and south is exterior? [duplicate]

Is it possible to have a magnet which looks like a hollow cylinder and who's north is interior and south is exterior? What then happens as the cylinder gets longer? Would that affect the strength of ...
2
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1answer
551 views

Why are magnetic fields only produced by moving charges? [duplicate]

Why do charged particles only produce magnetic fields while in motion?
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0answers
434 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 ...
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0answers
58 views

Conceptual way of understanding what material 'does' at critical angle

I would like a conceptual way of understanding how a material behaves at the critical angle. So why does it all reflect? I can see why it reflects in the maths, but conceptually, it doesn't make ...
5
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3answers
8k views

2 ways to generate electromagnetic wave

According to Maxwell's equations, accelerating charges emit electromagnetic radiation. According to Quantum physics, heating causes electromagnetic radiation too. These 2 radiations, are they ...
2
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3answers
566 views

Unpolarized light vs. randomly rotating polarized light?

I am confused with physical picture about unpolarized light. Is unpolarized light very fast rotating polarized light? or co-existing state of two orthogonal polarization? (or something else?) If ...
2
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2answers
538 views

Cause of electromagnetic induction?

The rate of change of magnetic flux through a surface (open) is related with the line integral over the closed loop binding the selected surface by one of the Maxwell's equation. But that means even ...
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0answers
124 views

Angular momentum contained in E&M fields

I am trying to do this question as a prep for my quals and find my solution too simplistic. I fear I am missing something important. "Iron atoms (atomic mass 56) contain two free electron spins that ...
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5answers
3k views

Does alternating current (AC) require a complete circuit?

This popular question about "whether an AC circuit with one end grounded to Earth and the other end grounded to Mars would work (ignoring resistance/inductance of the wire)" was recently asked on the ...
7
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2answers
318 views

Can a magnet magnetise an object with greater strength than it possesses?

Trivial thought ... Materials may be broadly superconductive, diamagnetic, paramagnetic, ferromagnetic. An object is magnetized by repetitive motion of a magnetic field across it's surface Say a ...
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0answers
61 views

Physical and dynamical components the four potential

I have a question regarding the four-potential and its gauge symmetry. We have a gauge freedom: $A_{\mu} \rightarrow A_{\mu} + \partial_{\mu}\chi$ Such a transformation does not alter the EM field. ...
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0answers
53 views

what are the main factors holding back magnetic energy storage systems?

Perhaps is it the magnet's own magnetic field that causes it to lose its superconductive state? It seems as though these systems could become incredible energy dense.
2
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0answers
92 views

Consistency of equation with special relativity?

The following is the equation which, I want to know, if it is valid in relativistic domain. Consider two equal charges moving in same direction with velocity $v$ and charge $q$ at a separation of ...
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3answers
132 views

What are correlated magnetic moments?

My book has the following sentence and I don't understand what correlation or lack of correlation means: At high temperature the magnetic moments of adjacent atoms are uncorrelated (to maximize ...
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2answers
820 views

No magnetic dipole moment for photon

Electrically neutral particles such as neutrinos can have nonvanishing magnetic dipole moments. Spin-1 particles, e.g., deuterium nuclei, can also have dipole moments. Googling seems to show that the ...
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1answer
117 views

What is the intensity of this light?

I am struggling with a derivation that calculates the cross sections for Mie scattering and since the incident light is considered to be a x-polarized plane wave I thought that we would have $$I_i = ...
10
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2answers
2k views

Is it possible to levitate a toy maglev train using only permanent magnets as long as the train stays in motion?

I am fascinated with magnets, and specifically the idea of magnetic levitation. I recently purchased a Levitron toy maglev train and have enjoyed finding the perfect sweet spot at which it levitates, ...
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1answer
629 views

Definition of complex permittivity

I'm not sure if this is the appropriate forum for my question as I actually am studying this as part of electrical engineering and I don't actually study physics. Nonetheless, I shall ask and if need ...
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1answer
500 views

Graphene batteries/super capacitors

A while ago, there was some news about micro-scale graphene-based supercapacitors and these devices can charge and discharge a hundred to a thousand times faster than standard batteries. Question: ...
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1answer
62 views

What would happen if you attached a wire to an electrically charged sphere?

If you have a sphere covered in electrons, and you connected a copper wire to it, what would happen? The copper wire's other end is not connected to anything and assume that the copper wire is ...