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5answers
272 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 ...
1
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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
votes
0answers
81 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.). ...
1
vote
1answer
52 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 ...
4
votes
1answer
107 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 ...
0
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1answer
150 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 ...
0
votes
0answers
24 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 ...
0
votes
1answer
65 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 ...
1
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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
votes
0answers
128 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
votes
1answer
120 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 ...
0
votes
2answers
53 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
votes
1answer
32 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 ...
1
vote
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
votes
1answer
267 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 ...
0
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0answers
40 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 ...
1
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2answers
94 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 ...
0
votes
0answers
55 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. ...
0
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0answers
33 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 ...
0
votes
1answer
38 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 ...
0
votes
1answer
36 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 ...
0
votes
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
49 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 ...
12
votes
4answers
426 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 ...
-2
votes
1answer
49 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
votes
1answer
47 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 ...
0
votes
2answers
163 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
vote
1answer
81 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
votes
1answer
49 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 ...
0
votes
0answers
22 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 ...
1
vote
1answer
150 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
votes
1answer
40 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
votes
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 ...
0
votes
1answer
59 views

Induced dipole in a molecule with a permanent dipole moment?

How can there be an induced dipole in a substance with a permanent dipole moment? I thought an induced dipole can occur only in a non-polar substance. Yet, this is part of the proposed dipole-induced ...
0
votes
1answer
39 views

TM modes expression for a cylindrical cavity, Jackson's book

Consider a cylindrical cavity with circular cross section of radius R. The height of the cylinder is d; the walls are considered as being a perfect conductor. I'm facing a difficulty findind the TM ...
1
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2answers
62 views

Magnet and needle

When I place a needle near a clamped magnet and let it go, it moves towards the magnet. Since magnetic field cannot work on individual charge particles, we must conclude that the needle loses its ...
1
vote
2answers
76 views

Flow from higher potential region to lower potential region

In Thermodynamics, it is said that heat transfer takes place from higher temperature to lower temperature. Things having higher temperature have higher potential energy because of more vibratory ...
0
votes
0answers
48 views

Trapping force in optical tweezer

I am reading a paper on theory of optical tweezers in ray optics regime by A. Ashkin. For reference "click here". It is given that Gradient force $F_g$ is directed perpendicular to the light ray and ...
0
votes
1answer
207 views

How can I prove that the axial gauge is a valid Gauge fixing condition?

I am studying classical electrodynamics and I have been introduced to the concept of gauge transformations and gauge fixing conditions. Right know I am trying to prove that some conditions are valid ...
0
votes
1answer
77 views

What is the physical explanation of Fresnel's equation?

When EM radiation is travelling between the media of different refractive indices, parts of it are reflected and transmitted according to Fresnel's equations. The amount of reflectivity and ...
1
vote
1answer
153 views

Does a laser beam converge and diverge without lens?

In the case of $TEM_{00}$ mode laser produces gaussian beam. I read in wikipedia that it converges till some point called 'beam waist'and then it diverges to infinity. If there is a lens placed along ...
1
vote
0answers
39 views

Is the Dirac string continuous?

Is the Dirac string continuous? Suppose I have a point magnetic charge. Do the necessary singularities of the vector potential lie on a continuous curve in 3D space?
0
votes
1answer
47 views

Demonstration for the existence of longitudinal electrostatic oscillations

How could I demonstrate that in a linear, homogeneous and isotropic medium without losses but electrically charged, Maxwell's equation admit as solutions longitudinal electromagnetic waves, beyond ...
0
votes
1answer
38 views

Relaxation time for moving charges in a medium

This will be a short question: How could I show that the quotient between permittivity and conductivity $\left(\frac{\varepsilon}{\sigma_c}\right)$ is the relaxation time for moving charges in a ...
0
votes
1answer
66 views

What is the effect of circularly polarized light on a charged particle?

The force on an electron due to electric field is given by $F=qE$. For a circularly polarized light, $E=E_x+iE_y$. So $F=q(E_x+iE_y)$ which means particle travels in a circle. As there is magnetic ...
1
vote
1answer
54 views

Determining the phase delay between H and E fields

I want to determine the phase delay between H and E fields in a medium with losses and not electrically charged. In this medium we also have $\sigma_c\approx \varepsilon \omega$. The enunciation of ...
0
votes
0answers
49 views

Induced dipole moment of an atom

Assume that we have a neutral atom that consists of a $+Q$ particle in its center surrounded by a uniform charge density $\rho$ such that $-Q = (4/3)\pi R^3 \rho$, where $R$ is the radius of the atom. ...
0
votes
1answer
131 views

how do photons move with respect to EM (I'd like to picture wave magnitudes frame by frame) [duplicate]

(I'm aware treating photon as particle and talking about its position is not exactly, conceptually right but I think it makes sense, at least in the point of view of a beginner. Please just assume ...
1
vote
1answer
83 views

Can one make a propagating field along a waveguide only have longitudinal/transverse components?

We know that in free space, a propagating electromagnetic wave is always transverse. However, along a dielectric waveguide, the propagating wave can have longitudinal components. The exist of the ...
2
votes
4answers
88 views

the relation between frequancy and energy of EM waves

In quantum theory, $$E=\hbar w$$ In classical theory, we have the Poynting vector: $$\vec{S_\space}=\frac{1}{\mu_0}\vec{E_0}\times\vec{B_0}{\cos}^2(kr-wt)$$ given S is energy flux density (the ...