Tagged Questions

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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37 views

Why doesn't an electromagnetic wave violate conservation of energy?

I'm starting to study electromagnetic waves and as i understand, an electromagnetic wave projects a varying electric field. This electric field can in turn give forces of repulsion/attractions to the ...
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0answers
12 views

Why does freshwater attenuates signal less than seawater?

One of the most interesting problem in underwater communication is how deep the device (i.e. submarine) could go to still be able to communicate with on-shore data center. The attenuation in a medium ...
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0answers
33 views

Is there an equivalent force to magnetism for gravity? [duplicate]

In electrodynamics, one way to explain the origin of the magnetic field is to analyze how a moving particle perceives a static electrical field under the effects of special relativity. Due to the ...
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0answers
27 views

Larmor Precession of a macroscopic number of electrons

I know that there are some similiar questions out there, but I'm still quite puzzled by the following problem. Say i have a box full of interacting electrons ( I'm not sure if it would change anything ...
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0answers
9 views

Tunable ferroelectric material

Is there any tunable ferroelectric (whose permittivity changes by applying electric field) material for visible or near IR applications?
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2answers
39 views

What does it mean to say “a paramagnetic material is attracted to an external magnetic field?”

I'm just having a hard time wrapping my head around what actually goes on when a paramagnetic material is exposed to an external magnetic field. I understand that the individual dipoles line up so ...
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0answers
23 views

Is there an analytical expression for the conductivity of the surface of topological insulators?

I have a question about the conductivity on the surface of Topological Insulators (TI): Is it accurate to model the conductivity by the Drude model (I read a paper that modeled the conductivity with ...
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3answers
109 views

Rigorously prove that electric field is zero in a perfect conductor

I have ran into a problem while trying to prove that the electric field is zero in a perfect conductor My argument went something like this: We know that: $$\vec J = \sigma \vec E$$ In a ...
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0answers
31 views

Is the Earth can be protected by a loop of supraconductor in space? [duplicate]

If there is a problem from the Sun or from the magnetic field of the Earth, is it possible to build a loop of gold around the equator and create an artificial field with electricity ? How much energy ...
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4answers
65 views

How do radio waves reach receivers without being canceled out by interference?

When I think of waves traveling through a medium, I tend to think of the double slit experiment or waves in a pond. In those cases, waves are canceled out by destructive or constructive ...
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0answers
29 views

Ohm's Law for Accelerated Conductors

My questions are about the Ohm's law for a moving conductor in a stationary magnetic field. As we know this law is stated based on following relation with respect to the fixed frame of reference ...
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0answers
19 views

Magnetic,Uniform Magnetic field [on hold]

If a circular current carrying loop ,carrying current $I$, is placed in a uniform magnetic field $B$,calculate the force of expansion on the loop. Radius is $R$. Is the answer $IRB~~$ or $~~2 IRB$?
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17 views

If an common metal is induced with a static dipole moment, won't the the positive side be weaker?

To my knowledge, common metals with metallic bonding is formed through free electrons within its lattice structure. If say we are to shift the free roaming electrons to one side and therefore minutely ...
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0answers
19 views

Induced EMF of a fixed wire?

I've studied many examples where there is a "moving" conductor passing a magnetic field or in a magnetic field, or a moving magnetic passing a solenoid or conductor, and the induced $\epsilon$ is ...
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1answer
48 views

Moving frames - a blow on “virtual particles”?

There is a polemic about whether virtual particles EXIST, or are only a tool for making our calculi easier. Here is a doubt about their existence, and on the other hand, an argument in support of ...
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1answer
87 views

What is the Poynting vector of an electron?

On chapter 27 of "The Feynman Lectures on Physics Volume II: Mainly Electromagnetism and Matter", Richard Feynman says: Finally, in order to really convince you that this theory is obviously nuts, ...
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2answers
52 views

Why does $\hat n \times (\vec E_1 - \vec E_2) =0 $ imply that the tangential electric field components are equal?

On page 8: http://local.eleceng.uct.ac.za/courses/EEE3055F/lecture_notes/2011_old/eee3055f_Ch4_2up.pdfele I don't understand why $E_{t1} = E_{t2}$ is equivalent to $\hat n \times (\vec E_1 - \vec ...
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0answers
21 views

Standing Wave ratio vs frequency in power transmission lines? [migrated]

What is the relationship between Voltage Standing Wave Ratio (VSWR) and frequency in context of transmission power lines?
6
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2answers
71 views

Why the extra term $\frac{1}{2}(\partial_{\rho}A^{\rho})^2$ in the photon Lagrangian?

In my quantum field theory class we have been told to use this Lagrangian for the photon field $$\mathcal{L}=-\frac{1}{4}F_{\alpha\beta}F^{\alpha\beta} -\frac{1}{2}(\partial_{\rho}A^{\rho})^2.$$ but ...
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0answers
21 views

Question on electromagnetic field tensor

if few particles are being accelerated in electromagnetic field and the field tensor is $F^{\mu \nu}$, and also if the acceleration of the particles are given (or known), then how to prove that we ...
1
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1answer
41 views

Forces on moving charges

I want to know what are the total forces on a moving charge due to another charge. I heard that moving charges is the cause of all magnetism.So what I think is that if we have a charge in motion ...
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1answer
47 views

Is the overall force of a test charge in a electromagnetic field always the same?

I had a conversation with my physics teacher today about electric fields and he told me something I couldn't believe and because I found no information elsewhere I am going to ask you guys. Imagine a ...
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0answers
31 views
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42 views

Work Done Against Magnetic Force [closed]

I have been trying this question and I am failing to understand how to work out the work done against the magnetic force or indeed what causes the magnetic force in the following situation. A long ...
0
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1answer
23 views

Has a “phase” change ever been considered between RF radiation and photons [closed]

Where photons have properties such as spin and line of sight propagation, radio frequencies do not to the same degree, Photoelectric effects are attributed specifically to photons but not to radio ...
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2answers
45 views

Why can't unequal current sources be connected in series?

Imagine two power sources, PS-1 that can supply $5A$ at $100V$,and PS-2 can supply $10A$ at $40V$, due to conservation of charge, and Kirchoff's current law(KCL) this is incorrect, and would lead to ...
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2answers
116 views

How does the Lorentz force work?

How does the Lorentz force work? In the context of a homopolar motor. I am a year 13 student so please keep your answers simple! I am having trouble understanding the Lorentz force; does the magnetic ...
2
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2answers
447 views

Why do old transformers make sounds, but new ones don't?

What I think is that maybe insulation is gradually removed between plates, and eddy currents start passing from one plate to another and this produces sound. Am I right?
2
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1answer
32 views

Can a Coronal Mass Ejection (CME) cause a blackout on Earth and why?

Can a Coronal Mass Ejection (CME) cause a blackout on Earth and why is it so what's the relation between electro magnetic radiations and electrical and electronic appliance.what exactly does it do to ...
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1answer
31 views

Does zero free current entail zero $\vec H$?

There are two kinds of magnetic fields (different authors give them different names), $\vec B $ and $\vec H$ which are related by the equation $$ \vec B = \mu_o (\vec H + \vec M)$$ where $\vec M$ is ...
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3answers
98 views

Do mobile phones produce radiation harmful to humans? [closed]

What are the health effects of mobile phone radiation (if any)? Does using a mobile phone without an SIM card change the answer?
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1answer
55 views

How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?

The only difference that I know of between Kitchen Microwaves and Wifi signals is how much power is pumped through them. Why is it that Wifi signals, being 1000 times weaker can travel so much ...
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0answers
62 views

Faraday's paradox

I have studied that Faraday's law of induction and motional emf are 2 different lines of thinking but are essentially same. But then , how can Faraday's paradox be explained by Faraday's law of ...
1
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1answer
52 views

Biot-Savart and Amperes law inconsistentcy

Imagine the situation below where a steady current $I$ goes through the line. The radius of the semicircle is $r$. How do I compute the magnetic flux in the point $P$, marked by a black dot? ...
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2answers
35 views

Does the power weakening of an electromagnetic transmission over distance depends on the beam's width?

Does the power weakening of an electromagnetic transmission over distance depends on the beam's width? If I emit an omnidirectional electromagnetic transmission, its power weakens in the ratio of ...
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0answers
23 views

Does there exist purely lossy medium?

We know that the complex propagation constant for plane wave is given by $$\gamma = \alpha + j\beta $$ where $\alpha$ is the attenuation factor, and $\beta$ is the wave number and appears in the ...
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1answer
17 views

Why does material change electrical property based on frequency of input electrical signal?

I was investigating whether dry soil would conduct electricity. What I found was that at 60 Hz frequency (i.e. power line fell to the ground), dry soil is a very good conductor. But if you increase ...
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0answers
18 views

Mixing liquids when applying a voltage

I'm thinking in the following experiment: A) Two (non mixable (like water and oil)) liquids are going to lie in a container, in the extremes of the container there are two electrodes, those ...
2
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2answers
49 views

Electromagnetism in astrophysics

What is the most important electromagnetic phenomena involved in astrophysics?, I am working in a presentation of magnetars, pulsars and magnetic field of planets and I would like to know if there are ...
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2answers
70 views

Momentum conservation in an electromagnetic system?

Suppose that I have two charged particles in the configuration below. Let us assume the following: We apply a constant force $f$ to the the bottom particle so that it has a constant acceleration ...
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2answers
50 views

Is there any theoretical reason behind multiplying by $N$ when using Faraday's law of induction?

It is a well known fact that whenever we want to calculate the emf in a solenoid we usually multiply the changing flux for one loop times $N$, which is the number of turns in the solenoid. But why is ...
3
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2answers
129 views

Challenging Magnetostatics Problem - the “blind spot” of a magnetic dipole

I'm reviewing for an electromag exam and I stumbled upon a problem that's really hard to figure out. Here it is: A small magnetic dipole with moment $\vec m = m_o \hat z$ is in a region with uniform ...
1
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1answer
35 views

Can I tear apart a pipe with magnetostriction? And how big field should I use?

As I know, with magnetostriction we can "stretch" its unit cell. The stronger is the field, the more it expands the cell. Using alternating current in coils that induce magnetic field we can stretch ...
2
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0answers
97 views

Stress Energy Tensor of EM Field

Stress energy tensor for electromagnetic field is given by $$T^{\mu\nu}=\frac1{4\pi}(F^{\mu\alpha}F^{\nu}{}_\alpha-\frac14 g^{\mu\nu} F_{\alpha\beta}F^{\alpha\beta}).$$ My textbook (unpublished ...
2
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1answer
65 views

Can a single technology be used to observe light at any wavelength?

Radio waves are observed using induction in antenna. Visible light is observed exploiting electronic transition in matter. Is it possible to do the opposite? If not why?
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0answers
19 views

polarization of a transverse wave travelling in ionosphere with polarization direction perpendicular to earths magnetic field

Assume a transverse electromagnetic wave entering ionosphere such that its Electric field of wave is perpendicular to earths magnetic field. Now, i read that as it will enter plasma, the wave will ...
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0answers
51 views

Why is the direction of $I$ or $F$ different in case of Motors and Generators?

Consider the following diagram: In the first diagram, the wire is carrying current into the screen. Motion of the wire is downwards (Left Hand Rule) In the second diagram, the wire is moved ...
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0answers
12 views

Reaction force due to the curvature and gradient drift

We know that a charged particle will have a drift velocity in both a curved magnetic field and when there is a transverse spatial gradient in the magnitude of the magnetic field. This drift velocity ...
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1answer
48 views

Proof of equivalence of two different covariant notations of Maxwell's equations [closed]

My homework assignment is Prove that $$\partial_\beta F_{\gamma \delta} + \partial_\gamma F_{\delta \beta} + \partial_\delta F_{\beta \gamma} = 0$$ with the electromagnetic tensor ...
1
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1answer
43 views

General solution to the wave equation of electromagnetic field

Suppose that we have the four-vector potential of the electromagnetic field, $$A^i$$ The wave equation is given by $$\Box A^i=\frac {\partial^2}{\partial x^k \partial x_k } A^i= \left(\frac {1}{c^2} ...