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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Radiation interactions: how is the transition from the “electric” regimen to “particle-like” regimen?

When we study the interaction of the electromagnetic radiation with free electrons we can find two different approaches in the literature: for low frequency (RF, light...) a classical view is used and ...
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1answer
157 views

Show that charge conservation $\partial_\mu J^\mu = 0$ implies global U(1) invariance?

The $U(1)$ global gauge symmetry of electromagnetism implies - via Noethers theorem - that electric charge is conserved. Actually, it implies a continuity equation: $$ \psi \rightarrow ...
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439 views

Electrolytes and electric field

Let me assume, that I have an arbitrary electric field. Is there any way to determine what happens to this electric field if it is appolied to a charge in let me say water with natrium chloride in it? ...
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2answers
1k views

Does a single electron moving at velocity $v$ have an associated magnetic field, ignoring intrinsic spin?

I have seen explanations of the magnetic field due to an electric current as being due to a Lorentz contraction of the moving electric charges. Would this explanation work for a single electron. There ...
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320 views

Does the spin precession change sign when the angular momentum does?

Say you have a charged particle moving circularly in an electromagnetic field. Basic quantum mechanics tell us that its spin will precess with a certain frequency. If the same particle were traveling ...
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124 views

I'm reading this in a senior year physics text book, need to know if this is true, and if so then how? :

"Even if a magnet is broken into atoms, each atom shall be a complete magnet. If the atom is further broken into electrons, protons, neutrons, etc. even then each particle shall behave like a complete ...
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120 views

How to derive magnetic moment for integrated circuit level?

Imagine we have a circuit on the xy-plane, with a random geometry (it just need to be closed). I want to calculate the magnetic moment of this setup: $$\vec{m} = \frac{1}{2} \int_V d^3 x' \vec{x}' ...
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1k views

Question regarding inverse relation of resistance with area of cross section

It is said that resistance is inversely proportional to area of cross section. But greater area will have greater electric flux, and greater electric flux will have greater magnetic flux, and greater ...
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5k views

From where do the permanent magnets get energy from? [duplicate]

I have a doubt about permanent magnets. If a magnet is permanent it can attract some materials permanently. Attracting something involves energy. If a permanent magnet can do this forever, from where ...
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1k views

Magnetic force on a magnetic dipole

Can someone please help me understand and answer the question of "why are the forces on a magnetic dipole different in a uniform magnetic field and non-uniform magnetic field?" I know in a uniform ...
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1answer
137 views

Behavior of the electric field on boundary surfaces

Consider this picture. Integrating over this infinitesimal box gives the following equivalencies: $$\int_{\Delta V} d^3r~{\rm div} \vec{E}(\vec{r}) = \int_{S(\Delta V)} d\vec{f} \cdot ...
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325 views

iPhone compass not being affected by current

As you probably know, electrical current and magnetic fields are close friends. In an iPhone there is a compass, however there are also lots of cables and in all of them, or at least in most of them ...
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801 views

inductance value of a single turn thin circular coil

the question may seem simple but I haven't found any fitting formula yet. The problem is the following: consider a single-turn, circular coil made of reasonably thin wire (diameter of the wire much ...
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2answers
471 views

EM Fields in a Rotating Frame of Reference

I'm struggling on my approach to the problem of figuring out E and B fields in a non-relativistic way for a rotating frame of reference in the x-y plane around the z-axis. I am attempting to do this ...
2
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1answer
355 views

Induced Current, Dipole Falling Under Gravity Towards Circular Loop [closed]

Suppose a magnetic dipole $\mathbf{m} = m \hat{z}$ is falling towards a circular loop of radius $b$ under gravity. Assuming the dipole always stays along the $z$-axis of the loop, determine the ...
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1answer
125 views

Field Tensor and classical limits

I would be very grateful if someone would kindly explain this generalization of the Lorentz force law to the special relativity domain. Please bear with me. Classically, the Lorentz force law is ...
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1answer
48 views

Minimum atomic clearance permitting motion

Suppose you were to build the piston and cylinder in a car engine atom-by-atom. Let's just say carbon, since you can make a lot of different shapes due to it's high valence. So assuming you make the ...
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1answer
43 views

Does an Ising lattice that returns to equilibrium create a current by induction?

Consider you have an Ising lattice with a dominant up component out of thermal equilibrium, that's your initial state. The down spins want to flip up and align with the ups, and they'll do so until a ...
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596 views

Reason behind cohesive and adhesive force

What is the real cause behind the cohesive and adhesive forces?
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336 views

Should the electric potential of a positively charged sphere be negative?

Because the indefinite integral of the electric field results in a negative value? (As the function is proportional to $r^{-2}$? I've got to be missing something... Help please!! Thanks! Also, I ...
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2answers
185 views

Radiated power from a 'source volume' outside which charge & current are zero (i.e. derive radiated power from Jefimenko's equations)

In classical electrodynamics, what is the radiated power from a generalized source (consisting of charge density $\rho$ and current density $\vec{J}$) in vacuum? Let us define $V_s$ to be the ...
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1answer
2k views

Length of wire required for solenoid to produce desired magnetic field

In a question: To construct a solenoid, you wrap insulated wire uniformly around a plastic tube 12cm in diameter and 50cm in length. You would like a 2.2 A current to produce a 2.6 kG magnetic ...
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2answers
796 views

Irradiance (or intensity) of an electromagnetic wave

Is the irradiance (or intensity) of an electromagnetic wave given by \begin{equation} I = \left<\|\mathbf{S}\|\right>_{T} \end{equation} or by \begin{equation} I = ...
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2answers
1k views

Basic magnet through a copper coil experiment - What if poles were rotated 90 degress

I was wondering what voltage or current, if any, would be produced if the basic magnet through a copper coil experiment had the poles rotated 90 degrees so north and south faced the top/bottom of the ...
2
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1answer
326 views

Transformators (primary and secondary windings)

Consider the following problem: Robby wants to put a light in the shed so he puts a cable between his house and the shed. In the cable there are 2 wires with a combined resistance of $0,92 \space ...
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1answer
94 views

Rosenberg-Coleman effect

I know only that it states some variations of the Sun's magnetic fields. What is the Rosenberg-Coleman effect specifically?
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849 views

D'Alembert operator and special relativity

We are currently covering special relativity in the theoretical physics lectures where we defined: $$ \mathrm ds^2 := \mathrm dt^2 - \mathrm dx^2 - \mathrm dy^2 - \mathrm dz^2 $$ In Road to Reality, ...
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168 views

What happens when a ferromagnetic object encounters a field too strong for it?

Just thinking aloud ... It is possible for a star to grow so large it collapses under it's own gravity. Along a parallel path (so to speak) when a conductor carries a current too large for it to ...
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2answers
236 views

Power due to dipole radiation and time reversal symmetry in classical E&M

The dipole formula for the power loss emitted by a time varying electric dipole is (in natural units) $P = \frac{\dot d_i^2}{6 \pi}$. This is clearly even under time reversal symmetry $T$, but a ...
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1answer
143 views

Could we really charge metal plates using microwaves?

While skimming through Dielectric heating, I read that they use microwaves to charge the plates. How do they do that?
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1answer
981 views

Proof of equality of the integral and differential form of Maxwell's equation

Just curious, can anyone show how the integral and differential form of Maxwell's equation is equivalent? (While it is conceptually obvious, I am thinking rigorous mathematical proof may be useful in ...
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1answer
141 views

Crystal magnetic response only skin deep?

The Hamiltonian for a single electron in a magnetic field reads $$H=\left(\frac{{\bf p}^{2}}{2m_{e}}+q_{e}\phi\right)+\mu_{B}\left({\bf \hat{L}}+g{\bf \hat{S}}\right)\cdot{\bf ...
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1answer
189 views

How does one prove:$\nabla(\vec{\mu_m}\cdot\vec{B})\cdot\vec{dr}=0$?

Work done by a magnetic force(even over an infinitesimally short displacement)=0 Net Force on a current loop in an external magnetic field is given by: $$\vec{F}=\nabla(\vec{\mu_m } \cdot \vec{B})$$ ...
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477 views

why is advanced radiation absent?

the Lienard-Wiechert green functions have future and past null cones of radiation. Maxwell equations allow for a continuous range of mixtures between the retarded and advanced components, but we have ...
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2answers
993 views

Magnet and energy conservation

If we consider a steel ball falling under gravity in a cup (potential well) and being stopped at the bottom by an obstacle then energy conservation implies that the gravitational potential energy has ...
2
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2answers
702 views

Electron model under Maxwell's theory

I was not able to recall my memories, so: What is the formula that states the frequency of electrons revolving around nucleus is equal to the frequency of light (or photon) emitted (or radiated)? (I ...
2
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1answer
561 views

Strength of Magnetic Field Around a Superconductor

I recently learned that the strength of a Magnetic field around a conductor is proportional to the current flowing in it. So if we have a Mercury wire at absolute zero and pass a current through it ...
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1answer
155 views

What is the reason behind the shape of the absorption curve of electron paramagnetic resonance

In our EPR experiment, the signal looks like the "first derivative" part of the above picture. Why is this? What does the "first derivative" mean and why is it the quantity our instruments detect in ...
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2answers
410 views

A Question From Jackson Electrodynamics

I have a question regarding Jackson's Classical Electrodynamics. Consider the equation $$\varphi \left ( x\right )=\tfrac{1}{4\pi\epsilon _{0}} \int_V \frac{\varrho ( x )}{R}d^{3}x+\tfrac{1}{4\pi} ...
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1answer
189 views

Earth's magnetic field, could the poles be reversing?

Lately, I've been 'monitoring', if you will, the earths magnetic field, i've come across some strange looking activity periodically. I exchanged some emails with a guy from NASA today, Joseph Gurman, ...
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640 views

Maximize Magnetic Self-Inductance Through a Wire

You are given a long length W of copper wire. How would you arrange it to obtain the maximum self-inductance? Why? I am trying to use the equation $$L=\mu_o n^2 l A$$ I try to solve it using a ...
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2answers
4k views

Magnetic field in a cavity

We are given an infinitely long cylinder of radius $b$ with an empty cylinder (not coaxial) cut out of it, of radius $a$. The system carries a steady current (direction along the cylinders) of size ...
2
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1answer
3k views

In what position can a current-carrying loop of wire be located in a magnetic field so that it doesn't tend to rotate?

I understand that if the wire is not aligned with the magnetic field, it won't rotate, but I'm still confused on how so. Also, which direction is the current flowing?
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6k views

Properties to select suitable materials for making permanent magnets

I have read that soft iron is suitable for making a permanent magnet. Because it is required for permanent magnet to have high coercivity and high retentivity. Same text also said earlier that soft ...
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1answer
245 views

Asynchronous generator run in vacuum chamber

what will be happen if we put asynchronous generator in vacuum chamber & run it above its synchronous speed. After reaching its over synchronous speed we will cut off electrical supply. Can it run ...
2
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1answer
248 views

Question on 1st order Lagrangian Derivation in Faddeev-Jackiw Formalism

I'm looking at this reference (sorry it's a postscript file, but I can't find a pdf version on the web. This paper describes a similar procedure). The topic is the Faddeev-Jackiw treatment of ...
2
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1answer
89 views

Close electric field lines in wave guides

In a wave guide, graphics of propagation of Transversal Magnetic modes show closed field lines for the electric field. For example, for a rectangular guide: $E_x (x,y,z) = \frac {-j\beta m \pi}{a ...
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2answers
232 views

Why, intuitively, must a solution in physics be unique?

When solving Laplace's equation or Poisson's equation say, we require that the solution must be unique, which can be shown. In general, what is the physics behind seeking a unique solution? Are ...
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3answers
5k views

Do multiple permanent magnets aggregated together approach the same strength as a single magnet of the same size?

Here's an applied physics question. ;) If I buy some cube or sphere magnets like these, can I aggregate them together to create a stronger magnet (almost as strong as a single magnet)?
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203 views

Spherical magnets shooting like bullets and demagnetizing

I was playing around with a 2x chain of bucky balls and winding it around itself. Suddenly several of the little balls went shooting like bullets across the room in 3 directions and now all of the ...