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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Spherical Conductors in Dielectric - Potential

I have computed the electric field of the following system between the two spheres, but my calculations for the potential do not make sense. The potential should be proportional to $\frac{1}{r}$ but I ...
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1answer
45 views

Moving magnetic fields?

I was reading through Feynman Lectures on physics vol II when I came across this paragraph which I don't quite seem to understand: Since electric and magnetic fields appear in different mixtures ...
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18 views

Ampere's Law and frame

We have following situation. What will happen with frame if frame turns on 90 degrees? Let's assume that we have homogeneous magnetic field.
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34 views

Faraday's Law magnetic field rate of change from induced current

A rectangular wire loop has a magnetic field traveling perpendicularly through it with a field strength of $2T$. The resistance of the loop is $2\Omega$. A change in magnetic flux induces a constant ...
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23 views

Presentation of Electric Field Strength as a function of r; voltage

I am trying to generalize the electric field strength equation as a function of radius from the anode for Gas Electron Multiplier (GEM). I think you can think this as a parallel plate capacitor (...
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1answer
144 views

Interaction of magnetic fields with the magnetic field produced by current?

In this picture, the magnetic field from the permanent magnets exerts a force on this wire, and the direction and magnitude is described by $F = iL X B$ But doesn't the wire also generate a circular ...
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68 views

Distance of the electric field generated by a Slayer Exciter ( Variation of tesla coil )

I'm currently trying to create a really basic simulation of a slayer exciter for a demonstration I have to give at my college. The thing is we want to know how far does the electric field produced by ...
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3answers
1k views

Showing Lorenz gauge is satisfied in retarded potential - vector calculus

I am trying to show that $\nabla\cdot \vec{A}=-\mu_0 \epsilon_0 \frac{\partial V}{\partial t}$ $V=\frac{1}{4\pi\epsilon_0}\int \frac{\rho(\vec{r}',t_r)}{r}d\tau'$ $\vec{A}=\frac{\mu_0}{4\pi}\int \...
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1answer
92 views

Maxwell's Equation

I was reading a book and I found the following equation. I'm not good at physics and I couldn't understand where does it come from, can anyone explain me why is it valid? $$ \mathbf E = -\nabla\phi - \...
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1answer
226 views

Why does a function $\psi(v)$ appear when a Lorentz transformation is applied on Maxwell's equations?

In his 1905 paper, Einstein applies Lorentz transformation on Maxwell's equations, and relates the electric force $(X,Y,Z)$ and magnetic force $(L,M,N)$ in an inertial frame $K$ with spectime ...
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36 views

If I know how many atoms I have in a film, can I know the total magnetic moment?

Say I have a Co film and I know its volume so I know the total number of atoms in it. Using this plus the knowledge that each atom has a magnetic moment of 1.7 Bohr magnetons, would the total magnetic ...
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58 views

Index of refraction for travelling EM wave problem

Let $n_1$ be the index of refraction of the first medium and $n_2$ be the index of the second medium. When $n_2>n_1$, then for an incident angle of $90^{\circ}$ we get a refracted light at a ...
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1k views

What direction does the evanescent wave point?

On wikipedia they say that the evanescent wave has a magnitude of: $$E=E_0 e^{\alpha y-j\beta x}$$ Where $j=\sqrt{-1}$, but what direction does this vector point in and why? (let us say the boundary ...
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2answers
172 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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3answers
154 views

Do Magnets ever lose their magnetism, and if so how long does it take? [duplicate]

I was talking with a friend about a design for a perpetual motion machine that used magnets, but he said that magnets eventually lose their magnetism. I don't want anyone to go into how Thermodynamics ...
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3answers
935 views

Extract energy from magnets

Is it possible to "extract" energy from a magnet, making it lose its magnetism? Or, to put in another way, is magnetism a form of energy? (I am not talking about potential energy in a magnetic field). ...
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1answer
49 views

Can we produce a wave by oscillating current? [closed]

If Radio waves can be generated by using oscillating current so if we changed the properties of current to produce less wavelengths or light waves - I'm not asking about the possibility of producing ...
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3answers
26k views

How does Hendo hoverboards achieve the self-propelling motion? What is the MFA?

From what I understand of the idea behind Hendo hoverboards, they use four disc shaped hover-engines to generate a self-propelling motion. I also understand that it uses Lenz's law and eddy currents. ...
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1answer
119 views

Why does transformer winding need large numbers of turns?

In transformers there are large number of turns in winding for both primary and secondary sides around the core. So, my question is why can not we just put one or two turns in winding as we need to ...
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36 views

Action functional of Born-Infeld model

I have a Born-Infeld action functional like this $$I[A,\phi]~=~\int b^2(\sqrt{1+(|\bigtriangledown\times A|^2)/b^2}-1)+|D_A\phi|^2 + b^2(1-\sqrt{(1-|\phi|^2)^2/b^2} ).$$ Have any books or notes talk ...
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1answer
71 views

A confusion with solenoidal windings

I have seen in solenoids, toroids, transformers, electromagnets and machines that the core is pretty insulated. Over this the winding coil is wrapped but it is a bare coil (turns are not insulated to ...
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1answer
108 views

Maxwell equations in 2+1 D

I have a problem with the Maxwell equations in (2+1) dimensions using differential form. Following J. Baez "Gauge Fields, Knots and Gravity" page 93 (or any other book), the equations are \begin{...
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1answer
46 views

Integral Represation of Four Current with proper time

I have a question about the four current in covariant representation. the four current is defined as $$ J^{\alpha} = \binom{c\rho}{\vec{j}} $$ and i'm having a point charge, $$\rho(\vec{x},t)=e\...
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1answer
108 views

Rotating dielectric cylinder in magnetic field?

In this example we find that the polarization in a rotating cylinder is given by an iterative method (see section 2.2 in the link) using an effective $E$-field of $\vec E_0=\omega B \vec r/c$. Why ...
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4answers
851 views

How do electrons actually move in a circuit?

Last year, we were taught about electricity, about how electrons move in a closed circuit. But as our teacher had not taught us about electric fields yet, she gave us a simplified model of motion of ...
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3answers
8k views

Does special relativity make magnetic fields irrelevant?

I've heard that special relativity makes the concept of magnetic fields irrelevant, replacing them with relativistic effects between charges moving in different velocity frames. Is this true? If so, ...
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3answers
810 views

Why do Capacitor Inductor circuits Oscillate instead of reaching equilibrium?

When the capacitor is fully charged there is a potential difference between its poles and that creates a current. This current would create a magnetic field that is changing in the Inductor (because ...
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1answer
101 views

light effects of electric current

In german high school textbooks and curriculua, the distinction of following main sorts of effects of electric current is usual: magnetic, heat, light and chemical. My question is, whether the glow ...
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1answer
55 views

Transfer of electrons and static electricity

From what I currently understand about static electricity, electrons "jump" from one object to another, which object loses electrons depends on where it falls on the Triboelectric series. For example ...
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11 views

Getting H-Plane Cut from Near to Far Field Transformation

I am using the Near to Far Field Transformation method described in http://www.keysight.com/owc_discussions/servlet/JiveServlet/download/72-15457-47956-2405/36-tkadlec_roman.pdf. The method describes ...
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1answer
200 views

Predicting lightpaths after gravitational bending

(Artwork by Dave Jarvis) In this image, we can see the bending of light by gravity. Suppose I put a big glass (it almost has zero weight) of considerable thickness near sun (near the split of the two ...
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3answers
77 views

development of magnetism in a magnet [closed]

If we cut a magnet, the magnetic field develops. But if we cut it vertically, then will the magnetic field develop?
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1answer
159 views

An apparent contradiction to $m = m_0/\sqrt{1-v^2/c^2}$ [duplicate]

Using theoretical framework of the special relativity, we can show that the quantity that we classically regard as energy does have a property of inertia. And particularly, if the total energy of a ...
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5answers
859 views

Where's the energy in a boosted capacitor?

Suppose I look at a parallel plate capacitor in its rest frame and calculate the electrostatic energy, $E$. Next, I look at the same capacitor in a primed frame boosted in the direction perpendicular ...
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53 views

Can a magnetic lock be negated or overcome by a stronger electro magnet? [closed]

Magnetic locks seem to be fairly standard on modern buildings. Is it, though, possible to negate a magnetic lock my placing an oppositely-polarized, strong electromagnet across from the magnets on the ...
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1answer
89 views

Hendo Hoverboard

What if the Hendo hoverboard had a magnetic surface connected to it under the engines so that the current could always be occurring, would it then be free to float over anything?
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18 views

Method of images for currents - assumption

According to these solutions to problem 5.17 in Electrodynamics by Jackson. The assumption is made that the field in the region $z<0$ (that is in a region of the semi-infinite medium) that the ...
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3answers
15k views

What is the use of this formula 1 Tesla = 1 Newton/Ampere/Meter? [closed]

What does Newtons/Ampere/Meter stand for? From this formula: 1 Tesla = 1 Newtons/Ampere/meter what can it be used for? To do what? Ampere/meter Is the same unit as a field's intensity H? Or what is ...
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235 views

Confusion between primed and unprimed coordinates

While deriving Ampere's law in Magnetostatics we come across a term $\nabla\times J(r') = 0$ and the reasoning we give is because current density is only a function of primed coordinates hence it's ...
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1answer
921 views

Induced voltage of a conductor in a magnetic field

A book which I referenced for Electrical Machinery states that the voltage induced in a conductor inside a magnetic field is given by $$ \mathcal{E}=(\mathbf{v} \times \mathbf{B})\cdot \mathbf{l}$$ ...
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4answers
11k views

Induced emf in AC generator

The induced emf in a coil in AC generator is given as: $$\mathbb E = NAB\omega \sin \theta $$ $\omega = d\theta/dt$ Now, when the angle between the normal of plane and magnetic field is zero ...
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1answer
101 views

Coaxial cables for cable tv transmission and normal wires for electricity transmission

Why are coaxial cables used for cable tv transmission and normal wires used for electricity transmission?
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0answers
56 views

Energy on Voltage-distance relation of Gas Electron Multiplier (GEM)?

I am thinking if there is any single graph which could somehow visualize the energy in relation to the GEM schema. So I selected the voltage-distance relation, V-r, in Figure b) in relation to the ...
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2answers
49 views

How does this incorrect model of a wire give the correct result for B-field produced?

I see in Griffith's book Introduction to Electrodynamics, 4th edition, the following model (paraphrased). Consider a wire (infinitely-long) with positive linear charge-density $\lambda$ moving to ...
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0answers
39 views

Why does inter-electron distance in wire grow with acceleration?

When you have an electrically neutral wire, and apply a voltage across it, free electrons (provided by source) flow through the wire and thus an electric current is created. Assuming an ideal scenario,...
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4answers
2k views

Do Electromagnetic Waves really propagate through continuous Induction?

I've often seen it said that in an Electromagnetic Wave the changing Electric Field component creates the Magnetic Field Component and the changing Magnetic Field Component in turn creates an Electric ...
4
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1answer
58 views

Why do these calculations of EM fields for a magnet and wire loop seem inconsistent?

Suppose you have a conducting circular wire loop and a magnet moving towards each other. They move along the $z$ direction with nonrelativistic constant speed $v$. Let the $B$ field of the magnet in ...
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1answer
23 views

Short Circuit Fires

I understand that if there’s low resistance, Ohm’s law shows that the current drawn will be very high. However, isn’t resistance what causes heat in a wire? I get there’s a lot of current going ...
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3answers
36 views

Open Circuits vs. Charging a Capacator

When you attach a capacitor to a battery via two wires, charge transfer occurs from one of the plates to the other. However, in an open circuit, there is infinite resistance in the dead-ended wire, ...
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2answers
2k views

How can I tell which end of a wire will have a higher potential?

I have the following setup: A <---- wire ----> B $V_b - V_a = \Delta v = \text{a positive value}$ I have two questions: Which end of the wire has a ...