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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4answers
556 views

Induced EMF of a rectangular loop should be zero?

Considering the shape of a rectangular loop in a changing magnetic field: The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. ...
0
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1answer
39 views

Does the Magnetic Flux change (at a point) If the wire which causes it was cutted into a half?

If the electric charges move through a wire Does the generated Magnetic flux at any certain point chanage if this wire was cut into a half
0
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1answer
118 views

What is the connection between linear, circular and elliptical polarization versus perpendicular and parallel polarization?

In the EM course, we suddenly went from linear, circular and elliptical polarization to perpendicular and parallel polarization. What is the connection between these concepts and when it is ...
1
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0answers
156 views

Electric field due to rotating charged sphere

Consider a spherically symmetric charged object (charge $Q$) rotating about its axis. From Gauss's law we know that all that matters for the electric field $\mathbf{E}$ is the charge $Q$ enclosed ...
0
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0answers
50 views

Work done by the Lorentz force?

Im trying to calculate the work done on a single wire placed inside a magnetic field. Now for the force I've used the Lorentz force formula to calculate it, for the distance should I used the width of ...
1
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2answers
492 views

Deriving Maxwell's Equations from Electromagnetic Tensor

Given $ F_{\mu\nu} = \partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu} $ It is obvious that the diagonals are zero, as $ F_{ii} =\partial_{i}A_{i} - \partial_{i}A_{i} = 0 $ And, ...
2
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3answers
183 views

How is energy conserved in magnet?

When an object (i.e.magnetic) is attracted towards the magnet, work is done so that the energy is used but from where this energy comes from?
0
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1answer
46 views

Given $\tilde {E}$, which formula should I use to find $\tilde {H}$

Given $\tilde {E}$, there exist two formulas in my book (Cheng) to compute $\tilde {H}$: Maxwell's formula: $\nabla \times \tilde {E} = -j\omega\mu \tilde {H}$ Plane wave formula: $\tilde {H} = ...
1
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2answers
190 views

Force on a loop with induced current

Consider an infinitely long ideal solenoid with current $I$, radius $a$, turns per unit length $n$. Put a closed conducting loop around it (radius $b > a$), on a common axis through their centers. ...
0
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1answer
350 views

Why do Magnetic North and South Poles never exist by themselves? [duplicate]

As the title suggest, whilst I was reading I saw written that a 'Magnetic North or South Pole has never been found by itself'. And I was wondering why this was?
13
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4answers
805 views

Coulomb's Law in the presence of a strong gravitational field

I was under the impression that the $1/r^2$ falloff of various forces were because of the way the area of a expanding sphere scales. But that strict $1/r^2$ falloff would only be globally true in a ...
0
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1answer
76 views

Combination of Maxwell equations and other form of Maxwell equations

In reference to this paper on arXiv, page three, we have the following: We know that the Bianchi Identites are $\partial_{[\alpha F_\beta\gamma]} = 0$ and are equivalent to $$\nabla \cdot B =0 $$ ...
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3answers
198 views

Why are electrons attracted to a magnetic field?

In a cathode ray tube, the deflecting system can consist of either magnetic or electrically charged plates. I understand the fact that electrons are attracted or repelled by the electrically charged ...
0
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2answers
282 views

Domains of electromagnets

I know that magnetism is due to the alignment of domains of a magnetic substance. Due to this reason, an iron bar may be magnetized by stroking it with a permanent magnet such that its domains are ...
1
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4answers
6k views

What is the difference between electromagnet and solenoid?

What is the difference between electromagnet and solenoid? Both these terms seem as the same thing to me. The only difference that I can find seems to be that an electromagnet contains a soft iron ...
1
vote
1answer
160 views

Find magnetic field in a solenoid for different regions?

A steady current I flows along an infinitely long hollow cylindrical conductor of radius R. This cylinder is placed coaxially inside an infinite solenoid of radius 2R. The solenoid has n turns per ...
1
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0answers
58 views

$G_{\mu\nu}$'s field equations in electromagnetic duality

Given $$G^{\mu\nu} = -2 \frac{\partial L}{\partial F_{\mu\nu}}$$ it is written in http://arxiv.org/abs/hep-th/9506035 that the field equations are (eq 1.8) $$\partial_{[\alpha} \star G_{\beta ...
2
votes
3answers
545 views

Magnetic monopole and vector potential

Does anyone know how to prove (in a simple way if possible) that it is impossible to define a single-valued globally defined magnetic vector potential $\vec{A}$ on the manifold ...
0
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1answer
40 views

Formulas which describe Electric power loss accurate

What formulas describe the real power loss in conductors the best? I heard that there are different effects, like skin effect etc. I want to calculate the % the power drops per meter distance. ...
0
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1answer
494 views

Alternative air transportation and magneto-aerodynamics

Firstly, I am not a big fan of UFOs, however, a flying saucer makes me think if is there another, comparatively agile way to travel through the air without jets, turbines or propellers (balloons are ...
0
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1answer
44 views

When mutual inductance are occurring between two coils, why the two coils have the same mutual inductance?

I learned that $$\epsilon_1 = -M_{12}\frac{di_2}{dt}$$ $$\epsilon_2 = -M_{21}\frac{di_1}{dt}$$ And the book tells us directly that $M_{12} = M_{21}$ without a reason. Is there a mathematical proof ...
43
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1answer
53k views

How does this “simple” electric train work?

In this YouTube video, a dry cell battery, a wound copper wire and a few magnets (see image below) are being used to create what can be described as "train". It looks fascinating but how does this ...
0
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3answers
325 views

Can you stop an electron in vacuum?

If we shoot an electron in vacuum tube, then stop it with electromagnetic field, and switch off the field, what will happen with electron? Will it continue its movement? If there is a gravitational ...
0
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1answer
211 views

The Electric field & Coulomb's law [closed]

A point charge is moving in an electric field at right angles to the lines of forces .Does any force act on it ? Please explain why.
1
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1answer
48 views

Can two transformers used to make tesla coils?

Tesla coils seem to perform the same as a transformers: they use two coils and use magnetic field to transfer the energy. Doesn't using two transformers produce the same result? It doesn't seem to be ...
0
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0answers
43 views

What happens to a superconductor when a magnetic fields strength increases?

I'm curious about what happens when a HTS is locked in place above a magnet via the Meissner Effect and say another magnet is added. Would the superconductors height fluctuate? Or would the extra ...
1
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1answer
186 views

Can the physical properties of the EM field be described directly from the 4-gauge potential?

I'm trying to make an argument that classically, the EM field is considered a more 'real' physical quantity than the potentials, and am tempted to say that the fact that the field carries energy & ...
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2answers
121 views

Magnetic Hysteresis

I'm a bit confused about when magnetic hysteresis occurs in ferromagnetic materials. Does it only occur with a rapidly changing external magnetic field or with a slowly changing field? Does the speed ...
1
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2answers
275 views

Is four-current a vector or a vector density?

According to MTW, $$F^{\alpha\beta}{}_{;\beta} = 4\pi J^\alpha$$ and we can infer that the four-current must be an ordinary vector field because the left side is tensorial. But Wikipedia says that ...
4
votes
1answer
194 views

What is the refractive index of a gas-like system solely consisting of protons?

I know that the permittivity of a medium is somewhat dependent on the density of electrons orbiting protons. What if we have a gaseous system with no electrons consisted of protons like H+ or He++? ...
0
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0answers
109 views

In the Pound-Rebka experiment, does light lose energy?

In the Pound–Rebka experiment the redshift / blueshift of photons is measured in small distances. This experiment one explain by the influence of gravitational field on the photon: "When the photon ...
-1
votes
2answers
433 views

Reflection coefficient as a function of frequency

I am trying to relate the equation for reflection coefficient in oblique mediums to the frequency but can't figure out how the frequency affects the reflection of light. $n_1$= intrinsic impedance of ...
2
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0answers
128 views

Electric and magnetic field in a black hole

I have many questions about this topic: Does the electric field of a charged black hole look like this? I mean how can it have an electric field if nothing can escape from a black hole, and what is ...
0
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3answers
752 views

Why is signal bad in a tunnel?

Consider this: you are driving through a tunnel listening to your AM radio and a GPS, all of the sudden AM radio has nothing but statics and your GPS says "reconnecting - please hold". You say to ...
0
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1answer
146 views

Stimulated emission direction

Place a sub-micron clump of crystal violet molecules in front of a multipixel detector. Raise the molecules to an electronically excited state with a beam of 590 nm light, illuminating from the side ...
1
vote
1answer
91 views

Electric Magnetic duality

In this paper http://arxiv.org/abs/hep-th/9705122 Section 2 We have $$S_A = \frac{1}{4g^2} \int{d^4x F_{\mu\nu}(A)F^{\mu\nu}(A)}$$ where $F_{\mu\nu}(A) = \partial_{[\mu A\nu]}$. Its Bianchi Identity ...
0
votes
1answer
694 views

Force per unit length between two long parallel wires, one of which is insulated

Suppose we have two very long parallel wires of radius $a$ and distance between their axes $d$ ($d>>a$). The first wire if is carrying charge of linear charge density $\alpha$ and is insulated ...
1
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1answer
150 views

Can someone prove that the $\tilde{E}$ and $\tilde{H}$ fields in a waveguide looks as pictured?

Hi, I'm trying to use the solution to the wave's equation in a rectangular waveguide for $\tilde{E}$ and $\tilde{H}$ to show how I can get the above picture. For example, why is the magnetic field ...
0
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2answers
60 views

Power transmission loss in a large circuit

This is a followup question to this question Consider a large circuit like the german electric power transmission network. You have a lot of of people consuming power and a lot of power stations ...
0
votes
3answers
74 views

Power transmission in a circuit

lets say you have a battery and a resistance which form a circuit. The electrons flow through the resistance. How can you visualize the flow of energy? How do you visualize the energy field? I ...
3
votes
3answers
598 views

How would you define electrostatics and magnetostatics starting from Maxwell's equations?

I'm reading Griffith's text, and he starts by defining Electrostatics as requiring the source charges don't move. I've seen a few slightly different definitions of electrostatics and magnetostatics. ...
0
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2answers
122 views

Do there exist functions $\phi$ and $A$ such that $\vec E$ satisfies the Helmholtz Theorem $\vec E = -\nabla \phi + \nabla \times \vec A$?

Helmholtz Decomposition theorem stats: "Let $\vec F$ be a vector field on a bounded domain $V$ in $\mathbb R^3$, which is twice continuously differentiable, and let $S$ be the surface that encloses ...
0
votes
1answer
120 views

Does a proton oscillating at RF produce an EM field with a -E vector

to clarify - FACT: oscillating charges of sufficient frequency produce a propagating EM field. All communications is based on electron oscillation. Question: IF a proton was oscillated at RF would ...
2
votes
1answer
57 views

Question about wavelength of EM wave in dielectric

Many websites (http://maxwells-equations.com/materials/permittivity.php) lists out the following formula as to how you can compute the wavelength of an EM wave in a dielectric $$\lambda_d = ...
2
votes
3answers
628 views

How do two electrical charged particles know to repel or attract each other?

Now per QED, electrical charges interactions are effected by photons. Suppose you are one of the two charges. How do you know to attract or repel the other charge? In other words, how do you know if ...
0
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0answers
32 views

Ground influence on 434 MHz radio signal

I have read in a paper that ground acts as reflector for radio signals in 434 MHz band. Why is that the case?
11
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3answers
882 views

Electromagnetic waves should stop while encountering a conducting shell?

I am a high school student who has just started reading elementary electromagnetism and am a completely beginner in this subject. I have read in books that EM waves are nothing but sinusoidal ...
2
votes
1answer
76 views

FDM versus fdtd for computational electromagnetics

Every article I've seen on computational electromagnetics uses the Yee fdtd algorithm, and I'm wondering if would be possible to do the same analyses using out of the box FDM with more data points, ...
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0answers
49 views

Does the total energy in an electromagnetic field depend on the acceleration of the sources?

If I accelerate a charge from rest to some velocity $\vec v$, does the total final electromagnetic energy contained in the field depend upon the acceleration of the charge?
0
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1answer
153 views

Why doesn't the electron lose or absorb energy while remaining in a selected orbit? [closed]

Postulate 2: When an electron revolves in any selected orbits, it neither emits nor absorbs energy . The energy of an electron in a particular orbit remains constant. Thus, Bohr, by postulating ...