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31 views

Surface electric current flows

All the models of electric current flow I have seen used in the texts I have seen are models where the current flows in a curve - let us call it $\gamma$ -, and for which, for example, the magnetic ...
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0answers
19 views

Voltage drop along electron beam

A focused electron beam represents a current and unless the charges (electrons) meet no resistance to their movement there should be a voltage drop along the length of the beam. So, assuming the beam ...
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0answers
47 views

Do gravitational waves produce real accelerations?

Do gravitational waves produce real accelerations? For example, if I have an electron and a gravitational wave passes by, will the electron emit electromagnetic waves according for instance to Larmor ...
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2answers
93 views

Non-standard representation of the free electromagnetic plane wave

The usual representation of a free electromagnetic wave in vacuum looks like this: The blue parts are the local electric field, while the green parts are the local magnetic field. The circularly ...
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1answer
40 views

Does an electromagnet use more energy when it is repelling another magnet?

Kind of a strange question. Is the amount of energy used to power an electromagnet directly proportional to the amount of work an electromagnet does? That is, if I were to power an electromagnet ...
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2answers
184 views

Relating Poyntings theorem to Lenz and Faraday's law?

In system's similar to a motor, where the armature begins to accelerate simultaneously there is induced $-\epsilon$ to reduce the applied current(hence the applied power $P(t)$ is also reduced), or ...
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0answers
34 views

One from Landau's Minimum (Macroscopic Electrodynamics)

A dielectric sphere with the electric and magnetic susceptibilities ε1 and µ1 is rotating with angular frequency ω in a constant electric field E~ in a medium, characterized by the parameters ε2 and ...
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0answers
20 views

Electric field produced by polarized medium at the surface of a conductor

Suppose we have some conducting sphere of radius $R$ and charge $Q$ submerged half way into a medium with permittivity $\epsilon_1$. I have deduced that the electric field produced by the sphere will ...
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1answer
32 views

How do I show for an ideal transformer $M^2=L_1L_2$?

I've been stuck on this problem for about an hour. In an ideal transformer, the same flux passes through all turns of the primary and of the secondary . Show that in this case $M^2=L_1L_2$, where $...
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2answers
104 views

Stored energy and momentum in a magnetic field?

How is it possible for a magnetic field to store energy and momentum? And can deliver it to another object(charge/dipole) or field? I can't depict it. I know it occurs but how? For example, the ...
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1answer
118 views

Motion of Particle due to Lorentz Force

So my professor gave us the following question: A particle with electric charge $Q$ and mass $M$ is initially traveling with velocity $v_0$ in the $x$ direction at time $t= 0$. There is a constant ...
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1answer
197 views

Why does the energy of the mechanical wave depend on frequency but the EM wave does not?

Why does the energy of the mechanical wave depend on frequency but the EM wave does not? Are there any implications?
3
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1answer
76 views

Is this a valid Gauge fixing condition?

I've been given the following gauge fixing condition: $$A_\mu A^\mu = 0 $$ And I've been asked to show if it is a valid gauge fixing condition or not. I believe that it isn't because I've already ...
3
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2answers
141 views

Does light interact with electric fields?

We know that light is an electromagnetic wave and it does interact with charges. It contains magnetic field and electric field oscillating perpendicularly but when we apply an electric or magnetic ...
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2answers
74 views

Negatively charged ball orbiting positively charged ball in space?

Would it be possible to make something like two metal balls, one positively charged and one negatively charged, in space, where the negatively charged ball would orbit the positively charged one like ...
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1answer
117 views

Force on a magnetic dipole due to a magnetic field

Imagine a solenoid which has current $i$ and is producing a magnetic field $B$ which equals $$B=\mu N i$$ Now, imagine we put a small cylindrical magnet at the end of the solenoid. Then because at ...
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0answers
64 views

induced current in loop

I'm re-asking this question that was posted by @zhzh and subsequently closed. If you have a loop that is spun clockwise as a magnet is moved near it, is a current induced in that loop? If the ...
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5answers
300 views

Applying Newton's 3rd law in electromagnetic systems

From this diagram: What is the action force here, and what is the reaction force? From two references, the wire and the magnet? *Assume the magnet being an electromagnet(air core, basically a ...
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0answers
33 views

What does the density of points (tail point of the vectors) represent in the geometrical representation of a vector field? [closed]

While trying to understand the divergence of a vector through the geometrical representation of the vector field, I found that pictures can be misleading. Even a vectors field which looks to be ...
3
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0answers
113 views

Classical electrodynamics as an $\mathrm{U}(1)$ gauge theory

Preface: I haven't studied QED or any other QFT formally, only by occasionally flipping through books, and having a working knowledge of the mathematics of gauge theories (principal bundles, etc.). ...
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1answer
53 views

How to understand the continuity equation $i\omega \rho=\nabla\cdot\mathbf{J}$?

Suppose that $\mathbf{J}$ is alternating current and its intrinsic frequency is $\omega$, i.e. $\mathbf{J}(\mathbf{r}',t)=\mathbf{J}(\mathbf{r}')e^{-i\omega t}$ , and $\rho$ is charge density. Then my ...
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1answer
119 views

Why are agglomerated gold nanoparticles black?

I put a drop of aqueous red colloidal gold on a glass. After the drop dried I saw that remaining nanoparticles were black instead of red. Why black? Does it mean that plasmon resonance is so strong ...
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1answer
171 views

Conservation of momentum in electrodynamics and force on the northern hemisphere of a uniformly charged sphere

In chapter eight of his book "Introduction to Electrodynamics", Griffiths gives the following example: "Calculate the net force on the 'northern' hemisphere of a uniformly charged solid sphere of ...
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0answers
25 views

magnetostatics and time varying fields

Mathematical statement for magnetostatics is $\nabla\cdot \mathbf{J}=0$ which accounts for no piling of charge and hence no time varying field term in the 4th Maxwell's equation.Are there any possible ...
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1answer
75 views

emf of a moving wire in a variable magnetic field

Lets, say that Magnetic filed is either (case 1) $B=B_0$ or (case 2) $B=B_0\sin{\frac{\pi x}{L}}$in $z$ direction. We have a wire with a length of $L$ along $y$ axis which is moving in $x$ direction ...
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0answers
28 views

Capacitor connected across two isolated sources [duplicate]

Lets say there are two isolated sources of positive charges $q_1$ and $q_2$ $(q_1>q_2)$ located sufficiently far apart. If the two are connected to the two plates of a capacitor, then what is the ...
4
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0answers
137 views

Why is there no induced current in this loop? [closed]

This problem comes from Barron's SAT Subject Test Physics book. A loop of wire and a bar magnet are moving relative to one another. Which motion in the diagrams shown below will NOT induce a ...
3
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1answer
138 views

Are the conducting electrons in a metal counted as free or bound charges?

Caused by another question on the propagation of EM waves in a metal, I got a bit confused on the notion of free and bound charges in a metal (sometimes called induced or external). Are the free ...
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2answers
54 views

Torque on a disk with charge

Imagine you have a disk with some number of charged metallic spheres spaced equally around a disk at some distance from the center of the disk and those spheres are mechanically attached to the disk. ...
0
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1answer
36 views

Induced EMF, by moving the magnet is the same?

I know that when a wire is moving in a magnetic field, there is an induced EMF due to the change in area, therefore, a change in magnetic flux. However, what if things we're switched? Fixing the ...
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1answer
58 views

Why does work done for polarization per unit volume contain $1\over 2$as in $\frac{1}{2} \mathbf E\cdot \mathbf P?$

Work done per unit volume to change the polarization from $0$ to $\bf P$ is given as the integral of $\mathbf E\cdot d\mathbf P'$ as $$u_p= \int_0^{\bf P}\mathbf E\cdot d\mathbf P' \;$$ integrating ...
7
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1answer
277 views

Motion in a time-dependent uniform magnetic field

Suppose you have an infinite solenoid generating an uniform magnetic field inside. The field is oriented along the solenoid axis : unit vector $\vec{\bf n}$. The field intensity varies linearly with ...
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0answers
48 views

Why is the enclosed area of a voltage-charge Lissajous figure to the dissipated energy per cycle?

These Lissajous figures turn up in the study of Dielectric Barrier Discharges driven by AC voltages, but I believe the concept of the "dissipated energy=area enclosed curve" is much more general. By ...
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2answers
98 views

How does a charge distribution evolve with time? (in classical electrodynamics)

It is often stated that, in classical electrodynamics, the electric and magnetic fields determine uni vocally the dynamics of a charge distribution distribution (how it evolves in time). I can more or ...
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0answers
63 views

How to calculate the equations of motion for a dipole in an electric field in three dimensions

I'm having trouble modelling a system, which I thought would be simple but has turned out to be quite complicated. The system is a charged sphere in an oscillating three dimensional electric field. ...
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0answers
37 views

Cylinder in a magnetic field with its axis parallel to the field

Consider the case of a cylinder of some permeability $\mu$ in a constant external field. Now we are familiar with finding the scalar potential and $B$ for axis of cylinder being perpendicular to the ...
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1answer
39 views

Moving the magnetic field around a fixed conductor?

In this example: and many others related to electromagnetic induction, the main component of motion would be the conductor perpendicular to a magnetic field. I know it's possible to have it the ...
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1answer
38 views

Induced EMF dependent on terminal wire connection?

Figure(a): When a conductor moving inside magnetic field, of a given length at a certain velocity the induced EMF is: $$\epsilon = vBL$$ However, what if we changed the position where the bottom ...
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0answers
12 views

Analysis of diagonal Lorentz force & Induced EMF of this orientated wire

Given this diagram: A conductor of a certain length $L$ placed in a magnetic field $B$, has the terminals connected to the top&bottom edges of the conductor. Current(even though scalar) would ...
3
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0answers
51 views

c “Propagation” in Weber Electrodynamics [closed]

The telegraph model of Weber-Gauss, relying on Weber's electrodynamics, modeled instantaneous action at a distance of the electric scalar potential (Coulomb potential) manifesting as propagation of ...
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4answers
436 views

How does the emf “know” what the magnetic flux is?

I came across an example in my book where it has the changing flux from a solenoid passing through a larger ring at the end. Here's a picture: How does the large loop (or radius $r_1$) even “know”...
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1answer
50 views

How to find the magnitude of electric and magnetic fields in an arbitary inertial frame?

In the context of special relativity, and using proper Lorentz transformations: If the electromagnetic fields have an uniform value [= constant in space and time] in an inertial frame, how will the ...
2
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1answer
49 views

Time dependent electric field: Mathematical expansion for local electric field

In many articles and books I see that local electric field is expanded as $$\vec E_0(\vec r(t)) = \vec E_0(\vec R_0) − (\vec a(t) \cdot \nabla) \vec E_0(\vec R_0) \cos(\Omega t) + \ldots $$ For ...
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2answers
182 views

Is Coulomb's law valid if the electric charge is not constant in time?

If the electric charge is constant the law of Coulomb says that: $E(r) = kQ/r^2$. The question is, if $Q = Q(t)$, can I consider that: $E(r,t) = kQ(t)/r^2$? Update: It appears that the first ...
1
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1answer
97 views

Does the direction of propagation of the natural light is perpendicular to the direction of electric and magnetic field making up natural light? [closed]

I know that the direction of propagation of the wave(light) is perpendicular to the direction of electric and magnetic field in the situation of plane waves. And I want to know the relation between ...
0
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1answer
54 views

Applying conservation of energy to a railgun problem?

When a projectile is lunched due to the Lorentz force($F_L$), how can I apply the conservation of energy that the electrical energy inputted to generate the Lorentz force & magnetic field equals ...
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0answers
23 views

Reflection and absorbtion of EM-wave in dielectric plate (calculate through dipole)

The task: there is a plate with thickness $h$. Also $\sqrt{|\epsilon|}kh \ll 1, k = \omega/c$. Find reflection and absorbtion of EM-wave in this plate. EM-wave falling normally on plate. What I did ...
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1answer
175 views

Behavior of the Electric- and Magnetic-field under time reversal and parity

The behavior of the electric- $\mathbf{E}$ and the magnetic-field $\mathbf{B}$ und time reversal and parity can be calculated in different ways. My first solution is to study the transformation ...
0
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1answer
41 views

The counterfactual possibility to explain discrete energies in Thomsons plumpudding model

In the german edition of "Tipler: Physics for scientists and engineers" there i a small text about the plum pudding model of the atom. i give a direct translation: "Thomsons plumpudding model of a ...
2
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1answer
74 views

Determination of radiation pressure

Consider the incidence of an electromagnetic wave on the plane $x=0$. We have that: $$f_x=\frac{dF}{dV}$$ $f_x$ is the volumic force density on the medium. My doubt is purely mathematical. I ...