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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15 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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0answers
14 views

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

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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33 views

How much external field have to be applied to saturate a Mu metal shield till magnetization?

My main goal is to magnetize and demagnetize a Mu metal shield. Till now I am using a Helmholtz coils setup and I can generate 10 mT applying 2 A DC. I am using a DRV425 fluxgate and a hall probe ...
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1answer
35 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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1answer
19 views

Induction Oscillation [on hold]

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
231 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
43 views

The Aesthetics of impossible or unreal physical phenomena [on hold]

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
14 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
23 views

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

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 ...
2
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1answer
27 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
110 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
26 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
51 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
123 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
82 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
28 views

Current carrying conductor with a loop at a distance [on hold]

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
39 views

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

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
18 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
149 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
34 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
72 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
26 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
16 views

Electric field/ Gauss´s Law [on hold]

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
50 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
153 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
41 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
39 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
162 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
91 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
81 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
64 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
20 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 ...
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0answers
22 views

Is there a method to detect a magnetic field within this range: 100 fT to 1 pT

Is there a method to detect a magnetic field within this range: 100 fT to 1 pT (human brain magnetic field) I know there are SQUID magnetometers currently using in primarily Medical Researches. But ...
1
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1answer
57 views

Magnetic moment of an iron-core solenoid

I'm currently developing a Simulink model for the attitude control system (ACS) of an undergrad-developed CubeSat. The ACS uses magnetorquers for attitude actuation. The magnetorquers are iron-nickel ...
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1answer
31 views

Weak localization and constructive interference

To my knowledge weak localization is a result of constructive interference between scattering processes and their time reversed. Traveling a given path in a vector potential leads to the phase $q\int ...
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63 views

Paper challenging Einstein - Thoughts and comments? [closed]

I recently came across this paper and wondered what peoples thoughts were on it. He aims to show 'On the Electrodynamics of Moving Bodies' by Albert Einstein is incorrect in his paper "Experimental ...
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1answer
868 views

Force from solenoid

I'd like to approximate the force from a solenoid, or at the very least find a formula which is proportional to the force so that I can experimentally find the constant for my particular case. ...
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2answers
24 views

How do I choose the right value of $r$ to find where the electric field is zero?

Sorry for the long question. I'm having a difficult time trying to explain my confusion. I have a positive point charge$\ Q_1 =+q$ at the origin and a negative point charge $\ Q_2 = -2q$ at $\ x=2$ ...
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2answers
40 views

Is there any macroscopic experiment that reproduces the results of Stern-Gerlach experiments on electron spin?

In a Stern-Gerlach experiment measuring electron spin, electrons are diverted to one of two areas on a detector, rather than ending up in a continuum between those two areas. I'm wondering if there is ...