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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70 views

Calculating electrodynamic momentum of a dumbbell (consisting of two point charges) in longitudinal motion

I'm working through a paper on momentum in electrodynamics that requires the integration below and would greatly appreciate any help. I'm pretty sure it evaluates to $2/d$ but I can't quite figure ...
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2answers
117 views

Does a magnetic field sustain a current in a loop?

A particular current produces a constant particular magnetic field. If the fact holds under scrutiny, why doesn't a magnet held within a loop sustain a particular current? Are the electron spins ...
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2answers
2k views

Potential energy from opposing magnets repelling each other with a gap of 1 mm

I have two powerful rare earth magnets, that are separated by a distance of 1 mm. I applied energy to bring them closer to each other, hence increasing the potential energy. Now, when one of the ...
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3answers
6k 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 ...
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1answer
216 views

Symmetries of a Uniform Magnetic Field

Simple question. A system with a uniform electric field everywhere in space has translational invariance in the directions perpendicular to the electric field but no translational invariance parallel ...
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1answer
1k views

How can a magnet have a greater magnetic field strength than a bigger, heavier and more powerful magnet made of the exact same material?

Here are some informations I found on the magnetic field strength (also called "magnetic flux density" or "magnetic induction", commonly denoted as B and expressed in tesla or in gauss) of an average ...
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43 views

Electrostatics and relativity [on hold]

How to get the "d‘Alembertian acting on four vector A=-four vector j" from Poission's electric static equation by Lorentz transformation?
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2answers
58 views

How does electromagnetic radiation affect the velocity of a charged particle?

I've heard that the acceleration of a charged particle releases electromagnetic waves. So let's say there is a charged electron moving forwards in a region with a downwards magnetic field. If the ...
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3answers
55 views

Electromagnets, and how $H$ is the thing we set on the dial

Let's say that I have an electromagnet, consisting of Helmholtz coils with an iron core. David Griffiths, in his "Introduction to Electricity & Magnetism" text claims that $H$ is what we set, by ...
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3answers
74 views

Doppler shift and special relativity

I have been listening to an audio course on Special Relativity. In the course the teacher repeatedly says that the "laws of physics are equally valid for all uniformly moving frames of reference". ...
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0answers
23 views

Can anyone list all the ways the gravitational force interacts with the electromagnetic at our energy scale? [on hold]

Apart from weight does the gravitational force manifest itself in our world in any other way?also, are these two different questions?
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1answer
23 views

Induction Oscillation [closed]

A bar (m=100g, l=10cm) is moving in a constant magnetic field $B$ as shown above. The resistor $R$ is $1\Omega$. The bar is attached to a spring with spring constant $10N/m$, can you derive ...
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3answers
244 views

Polarization vector of dielectric electrostatics

I know two laws for Polarization vector of a dielectric material $$\vec P = N q \vec{\Delta L}$$ Where $N$ is the number of dipoles per meter cube, $\vec{\Delta L}$ is the vector that represents the ...
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1answer
46 views

The Aesthetics of impossible or unreal physical phenomena [closed]

I know this may not be a typical question for the Physics board. I am a MA Design student working on a project to illustrate experiments on speculative/fictional physical events. My final work will ...
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0answers
8 views

Triodes: Why do mesh fineness and radius of the grid affect amplification factor?

My basic understanding is that a grid which is much closer to the cathode than the anode will be more able to create a repulsive electric field than the anode's attractive field, thus current can be ...
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0answers
16 views

Finite Value of Cross Section of (Scattering/Absorption) of Localized Surface Plasmon at Resonance

According to textbook written by Stefan A. Maier, ch.5, p.70, cross section of scattering and absorption are defined as: $$C_{sca}=\frac{8\pi}{3}k^4a^6 ...
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0answers
25 views

Gravitoelectromagnetism or graviton thory to derive equivelent formulas to relativity theory [closed]

Is it possible to change the definitions of an event happening (which currently in relativistic theory is defined by the moment you see* something - the time light took to travel to you) such that ...
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1answer
29 views

Force on a moving charge in relative frame

Consider a particle of charge $q$ moving at velocity, $v$, enters a magnetic field $B$ which is perpendicular to its motion. Now consider two observers, one stationary and the other moving at the ...
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2answers
166 views

Magnetic field by a solenoid?

I am really not able to understand why is it that the current,while finding the magnetic field in the solenoid (ideal one), is not $I$ the current flowing throught the solenoid but $nI$, where $n$ is ...
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2answers
111 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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1answer
27 views

Does a correct application of the Lorentz force to find induced emf need resistance?

The emf between two points is defined as the work done per unit charge on the charge when moved (along a given and stated path) between those two points. To find the emf due to a magnetic field the ...
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2answers
52 views

How come the length of a wire does not affects on the circuit current?

Today we started learning about the electromagnetic induction. Out teacher gave us the following explanation: Suppose we have a conduction frame inside a magnetic field $\vec{B}$ going towards the ...
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4answers
9k views

Why is the conductor an equipotential surface in electrostatics?

Since the electric field inside a conductor is zero that means the potential is constant inside a conductor, which means the "inside" of a conductor is an equal potential region. Why do books also ...
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1answer
29 views

Why does the Magnetic Flux Density B, saturates in a ferromagnetic material with increasing H?

I understand that the magnetization must saturate as more and more domains are aligned. But $B$ is still directly proportional to $H$, and hence it must increase linearly with $H$. But every book ...
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2answers
124 views

Electromagnetic reaction force?

The classical (retarded) Lienard-Wiechert scalar and vector potentials describe the electromagnetic field due to an arbitrarily moving electric point charge. Thus given the motion of electron $A$ one ...
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2answers
147 views

Use of Amperian Loop to Calculate the $B$-field inside a co-axial wire

The problem asks to calculate the magnetic field at point $a$ of a co-axial cable with a wire at the center surrounded by a rubber layer, and then another hollowed cylindrical wire followed by a ...
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1answer
84 views

Constant magnetic field attenuation by µ-metal (mu-metal)

I am interested in the magnetic field attenuation by µ-metal. Specifically shielding from earth's magnetic field $\boldsymbol{H}_E$ through closed cylindrical layers of such a metal, yielding an ...
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5answers
10k views

Where do magnets get the energy to repel?

If I separate two magnets whose opposite poles are facing, I am adding energy. If I let go of the magnets, then presumably the energy that I added is used to move the magnets together again. However, ...
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0answers
30 views

Current carrying conductor with a loop at a distance [closed]

In Fig, there are an infinitely long conductive wire and a circle conductive loop C with the radius r and the number of turns n. The distance L between the wire and the loop is much larger than r, ...
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0answers
40 views

In Solution of the equation $\Box A_\sigma=0$. I find some mathematical difficulties [closed]

The solution of the equation $$\Box A^\mu(x)=0$$ has the form $$A^\mu(x)=\int\frac{d^4k}{(2\pi)^4} e^{ik^\mu x_\mu}A^\mu(k)\delta(k^2).$$ Where we take $A^\mu(k)$ of the form $$A^\mu(k)=\sum ...
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1answer
19 views

Why diamagnetic material is not repelled?

When an external magnetic field is applied to a diamagnetic materials, their atomic current loops will tend to align in such a way as to oppose the applied field. Diamagnetism is the residual magnetic ...
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1answer
166 views

Forces on a wire moving in a magnetic field?

When a wire is placed inside a magnetic field and current starts to flow in that wire, that wire experiences the Lorentz force = $IL \times B$ and beings to move. Aside from induced motional - ...
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0answers
35 views

How to calculate the potential energy of a magnetic dipole (with zero torque)

In the following problem, I would like to know how they arrived at their solution for finding the potential energy of a magnetic dipole (boxed in red). I understand I probably have to use the ...
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0answers
42 views

Deriving Ampère's Circuital Law from Ampère's Force Law?

Ampère's force $d^2\vec{F_{21}}$ of current element $i_2d\vec{\ell_2}$ on $i_1d\vec{\ell_1}$ is$$d^2\vec{F_{21}}=-\frac{\mu_0}{4\pi}i_1i_2\frac{\hat{r}}{r^2}\left[2(d\vec{\ell_1}\cdot ...
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1answer
74 views

Why do electron and hole mobilities decrease with temperature?

From page 35 of "Microelectronics" by Millman Grabel Mobility $\mu$ decreases with temperature because more carriers are present and these carriers are more energetic at higher temperatures. ...
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1answer
29 views

Explanation of different magnetic field measurements due to frame of reference

If there is an stationary infinitely long wire carrying current I, and we turn on a gaussmeter, it will record a magnetic field B. Since I know the Biot-Savart law, I am satisfied with this ...
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0answers
17 views

Electric field/ Gauss´s Law [closed]

if I want to calculate the electric field, since I have 3 charged densities. The first one is -2 microC/m^2 , the second one is +4 microC/m^2 and the third one is -5 microC/m^2. What would be the ...
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0answers
52 views

Angular momentum of light in classical electrodynamis [closed]

I' reading at this time electrodynamics from Griffith's Introduction to electrodynamics. As i see, to have the conservation laws in field theory we have to consider the fields as carrying energy, ...
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2answers
2k views

The electric field of a conductive sphere containing a charge - grounded vs not grounded

Let's suppose we have a sphere but unlike theoretical ones it'll have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge $q$ ...
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0answers
20 views

Magnetic levitation manipulation

Suppose a small crystal or mineral piece with a completely known structure was held by magnetic levitation. Would it be possible to orientate the piece by manipulating the magnetic field so that a ...
0
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1answer
164 views

Why do we always consider the area of inner solenoid while calculating the mutual inductance of two coaxial solenoids?

While we know that flux linked to an area is the dot product of the area vector of the loop(and not the actual area encountering the field lines) and the magnetic field linked to it, why then do we ...
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1answer
42 views

Continuity equation for charge and current densities of an accelerated point charge

For a point charge that moves with the trajectory $ \vec r(t)$, we know that it has the singular charge and current densities $$ \rho (\vec x, t) = q \delta^3(\vec x - \vec r(t)) $$ $$ \vec J(\vec x, ...
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1answer
136 views

Are magnetic hysteresis losses relevant to alternating currents flowing in a wire?

Say we have an AC in a magnetically lossy material, like iron. Because of iron's relatively high permeability, skin effect will be more pronounced than it is in say, copper, so this iron wire isn't so ...
3
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1answer
41 views

Do two bodies spinning about the same axis in the opposite direction repel each other?

I recently read about the GEM equations, which look very much like the Maxwell's equations. Does this mean the behavior of mass is like the the behavior of the electric charge? So for example if you ...
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0answers
16 views

Is the induced EMF dependent on the conventional or actual flow of electrons?

I have been studying Faraday's Law and Lenz's Law and the flow of current to oppose change in flux linkage. My question is whether the actual flow of electrons to the top of a material makes the EMF ...
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1answer
163 views

Boltzmann Distribution of Electrons in Confining Potential

I have a particle simulation wherein many non-interacting electrons are trapped in a electric potential well. I would expect, and therefore I initialize according to this, that the electrons would ...
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3answers
92 views

What makes waves propagate?

Why do electromagnetic waves propagate? I have searched a lot about EM waves, but I am still unable to understand what is driving them. Could you explain?
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2answers
86 views

Derivation of Jefimenko's Equation in Jackson's EMT book

I have been trying to understand the derivation of Jefimenko's equation in Jackson on p.246-247 which can be seen in the photographs attached. First of all I did not fully comprehend the ...
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2answers
65 views

Can Ampere's law be applied to 3D loop

Usually in textbooks Ampere's law is just illustrated using 2D loops which forms a plane. Can the law be applied to 3D loops which cannot form a plane surface within the loop? In a 3D loop which ...
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1answer
22 views

Confusion regarding Changing flux

I have a confusion regarding where flux changes and where it is constant. When a flat coil moves between two opposite poles of a magnet, parallel to their faces we say the flux is constant and no ...