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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Derivatives involving four vectors [closed]

The Schrödinger lagrangian for complex fields is $$L=\frac{1}{2m}(D_i \psi)^* Di \psi - \frac{i}{2} \left[\psi ^* D_0 \psi - (D_o \psi)^* \right] - \frac{1}{4}F^{\mu \nu}F_{\mu \nu}$$ Where ...
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29 views

Electric field of a non-uniformly charged quarter circle

I was studying for my Electromagnetism test when I stumbled upon this problem and haven't been quite able to do it correctly, mostly because I can't find any symmetry for me to calculate the Electric ...
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26 views

Ampere right hand screw rule or Maxwell screw rule is the more correct name of the right hand grip rule?

Wikipedia states ampere right hand screw rule Some textbooks state maxwell screw rule Which one is the more correct name in representing the (colloquial) Right hand grip rule?
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38 views

Coulomb gauge and vector identites

consider a coulomb gauge and the following volume integration: $$\int d^3x{\dot{A}.\nabla A}$$ How can we show that this is zero in coulomb gauge? (A is a vector potential) this is my attempt at ...
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53 views

if the voltage is the driving force of the current, how can it lead or lag?

Correct me if I'm wrong on this, but up till now I've assumed that the voltage across a resistor could be seen as both the energy lost by a current passing through it and the energy needed for a ...
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38 views

Wave reflection

I have a simple question on wave reflection. I know that if I have a progressive monochromatic EM wave and a mirror, the reflected wave will be opposite in phase on the mirror to assure a total E ...
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1answer
39 views

Why don't stars re-emit the photons they absorb, thus restoring a continuous emission spectrum?

If you shine white light through a gas, electrons can absorb sufficiently energetic photons to reach higher excited states. This produces gaps in the spectrum and it's how Helium was discovered. So ...
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32 views

Moving point charge --> using Maxwell induction integral to find B-field

I'm trying to wrap my head around this problem. A point charge is moving with a uniform speed v. The question requires that I show how the Maxwell induction integral can be used to find the magnetic ...
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36 views

Newtons corpuscular theory

I am learning about the history and evolution of certain physics theories, one being Newtons corpuscular theory. I am reading that Newton predicted the corpuscles, which make up light would travel ...
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24 views

Is the permeability of plasma equal to the permeability of free space?

I learned that in Maxwell equations for plasma, the permeability is equal to the permeability of free space so that means the relative permeability is equal to the unit tensor. Also we have always ...
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73 views

When exactly do we substitute $\epsilon_0 \rightarrow \epsilon$ and $\mu_o \rightarrow \mu $?

If everything is embedded into vacuum, why aren't Maxwells Equations always with $\mu_o$ and $\epsilon_o$? When exactly do we have to make the substitution $\epsilon_0 \rightarrow \epsilon$ and ...
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21 views

Which formulas would tell me the gradient of an electromagnetic field at an arbitrary distance from a pole? [duplicate]

I'm a newbie to physics and was wondering where I can read about electromagnetic gradients. From what I understand (and my intuition) electromagnetic fields create force gradients around its poles. ...
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2answers
107 views

Batteries Do Not Supply Electrons?

I've read that a battery does not supply electrons, it establishes the electric field that exerts force on the electrons in the wires. This makes total sense to me for AC, but not for DC. The way I ...
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38 views

Deflection of cathode rays in a uniform magnetic field

A beam of cathode rays moves from left to right in a plane of the paper and it enters into a uniform magnetic field acting perpendicular to the plane of the paper and inwards . Now the cathode rays ...
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2answers
21 views

Link between energy per unit frequency and derivative of the energy in regard of frequency

I take a simple example to illustrate my question. I consider $\frac{du}{d \omega}$. It physically represents the volumic energy per unit interval of frequencies (imagine a system with ...
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1answer
33 views

why is the vertical component of the phasor an imaginary number?

I was learning about the phasor representation of AC circuits, and on adding two out of phase phasors they were each resolved in to a horizontal and vertical component to facilitate the addition. What ...
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2answers
55 views

Are AC waves actually a sin function or just appear in their alternating nature as sinusoidal waves?

I mean do we use the graph of a sin wave just as a representation of how the current varies, or are the actual instantaneous values of current really an output of a sin function of the angle the ...
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1answer
25 views

How would the Compton Effect be if an electron absorbs 100% of the energy of the incident photon? [duplicate]

How can I prove that the relativistic energy and the impulse cannot be conserved if an electron, which is essentially free, absorbs all the energy of the incident photon?
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How does the electromagnetic field of an electron and a rotating ball of charge behave in a co-rotating reference frame?

First time poster, hope I'm not breaking any rules. Basically I'm curious about how far the classical analogy of an electron as a rotating ball of charge can be stretched. The situation I'm ...
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39 views

How is it possible to have an electromagnetic wave inside a superconductor if E = 0 and B = 0 inside it?

I am studying "Wave guides" from the book "Introduction to electrodynamics" by Griffiths. We study the propagation of electromagnetic field inside a superconductor cylinder. We can prove that E = 0 ...
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12 views

What is the link between a magnet's position and alternating current?

Was wondering what the link was between a spinning magnet's position and the polarity of the AC current at the time. I know that if, instantaneously, a bar magnet was horizontal, current is 0A, if ...
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15 views

Magnetic Fields and their strength

1) Does a larger magnetic field make a magnet stronger or extend the range (space) in which it can influence objects? 2) What makes a magnet 'stronger' (have a larger attractive pull)?
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27 views

Jackson Ionosphere Question

I am looking at Jackson 7.14 (3rd ed). We are given $\epsilon=\epsilon(z)$ and the fields are $F(z)\exp(i(kx-\omega t))$. My first step was to find a wave equation, which I get to be ...
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16 views

Commutation of photonic crystal master equations

In studies of photonic crystal's, there are two "master equations" for the electric and magnetic fields: ...
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2answers
39 views

Movement of charges in presence of electric potential difference

This might appear to be a bad question to some but i can't figure out the answer. While reading about an electric circuit, i found this on the web: Over the course of time, one could think of ...
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423 views

Why are electrons treated classically in cyclotron measurements?

As I understand , systems having large angular momenta relative to the planck constant (limit of large quantum numbers, e.g. $J/\hbar \to \infty$), can be treated as classical systems. Now in the case ...
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Why is the longitudinal term removed to form the Lorentz force equation?

Beginning with the expression for interaction energy, and deriving the potentials form for the force on a test charge moving at velocity $\vec v$ yields: $$\vec F = q(-\nabla\phi - {D\vec A \over ...
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What is the importance of vector potential not being unique?

For a magnetic field we can have different solutions of its vector potential. What is the physical aspect of this fact? I mean, why the nature allows us not to have an unique vector potential of a ...
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1answer
23 views

Deriving Electromagnetism energy-stress tensor in GR [closed]

Please find the mistake in the following calculations. We have $L=-F^{\mu\nu}F_{\mu\nu}$, and try to derive the energy-stress tensor using $\delta(-g)^{1/2}=\frac{1}{2}(-g)^{1/2}g^{\mu\nu}\delta ...
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57 views

Are Coulomb's Law and Lorentz force always true for all phenomenon in physics?

I saw two equation about charge: Coulomb's law $$\mathbf F=k \frac{q_1q_2}{ r_{12}^{2} }\, \mathbf{\hat r} $$ and magnetic Force $$ \mathbf F=q \, \mathbf v\times \mathbf B$$ My question ...
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23 views

Thermal imaging

If we have thermal imaging camera and the sound is wave is produced and propagates in hot air can we see the electromagnetic emission of molecules producing pressure packets of sound wave?
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Electromagnetism and atomic space [duplicate]

Since there is much space in the atoms and between them but eletro-magnetism exists, would it be possible to open that space with an electromagnetic field allowing say a person to pass through a wall ...
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May have solution to magnetic perpetual motion [duplicate]

The theory behind my design is if you can rotate the magnets as they pass a stationary magnet you can generate perpetual motion using magnatism as the only energy source. the design: if you ...
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39 views

Can a circuit produce a magnetic force on itself?

Well I think that it is kind of obvious, because the magnetic force does not specify if the fiel is produced or not by the circuit that feels it, but I had never heard about "autoinduced forces" or ...
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Clear, coherent electromagnetism textbook [duplicate]

I have recently finished a year-long introductory undergraduate course on electromagnetism, but I did not get a very good sense of how everything fits together conceptually. Are there any good ...
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1answer
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How exactly does Magnetic Hysteresis lead to Power Loss in Transformers?

In reference to the Power losses in a Transformer, the author of Physics Textbook writes: As a result of magnetic hysteresis, the magnetic energy stored in the magnetic field as the magnitude of ...
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Why is there a minus in the Gauge Field Lagrangian kinetic term? [duplicate]

For vector Gauge fields we usually write the kinetic term: $$ \mathcal{L} ~=~ - \frac{1}{4} F_{\mu \nu} F^{\mu \nu}$$ while for matter fields e.g. for a real scalar: $$ \mathcal{L} ~=~ \frac{1}{2} ...
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Half wave antenna

I need to calculate the magnetic field intensity at a distance $x$ and angle $\theta$ for a half wave dipole antenna. I know the length of the antenna is $$ l = 333.33mm$$ and the magnetic field ...
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1answer
25 views

About field lines and continuity of magnetic and electric fields

In my laboratory class we were doing an experiment with grass seeds and iron filings to visualize the electric and magnetic field lines. So, we were discussing why they appear, because we know that ...
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can you explain the Lenz's Law **with** conservation of energy?

can you explain the Lenz's Law with conservation of energy?? there is another question in this website which explains it without conservation of energy but my emphasis is on explaining it, with ...
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36 views

Calculating instantaneous field strength for known signal source

I was referred here from electronics stackexchange. Suppose I have a time varying signal source V = Vm cos(wt). This source is fed to a half wave dipole antenna which in turn radiates electromagnetic ...
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42 views

Magnetic field of multiple loop turns?

Given this equation to calculate the magnitude of the magnetic field: $B = \mu_0 \frac{N }{l} I .$ Where $\frac{N }{l}$ is the number of turns per unit length. $I$ is the current, $\mu_0$ ...
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24 views

Conservation of Energy in induction due to variable current

An infinite wire with current is kept next to a square loop. Now, I change the current at a constant rate in the long wire. This would induce a current in the loop, the direction being determined by ...
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When do we have a coherent magnetization rotation? (single-domain particles)

So I am studying micromagnetics + spintronics and I am little bit confused with certain concepts regarding the physics of magnetic domains (especially single domain particles): What I already knew: ...
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26 views

Value of $E_x$ through an $X-\textrm{boost}$

Suppose I'm standing at the (0,0,0) point of a coordinate system and I see an electric field where $E_x=f(t,x,y,z)=\sin(x\cdot y\cdot z\cdot t)$ and at time $t_0=0$ I start moving on the x-axis at ...
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18 views

can we actually substitute electrodes in linac to electromagnetic coil?

I did some readings and researches for linear particle accelerator as I want to create one for my school science project. But I realised that most linac uses electrically isolated cylindrical ...
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Electromagnetic Tensor under Lorentz transformation, $F'(\Lambda x)= \Lambda F(x) \Lambda^T$?)

Suppose we have a configuration of electric and magnetic fields $E(x1,x2,x3,x4)$ and $B(x1,x2,x3,x4)$, they depend of position and time for a fixed coordinate system. And I want to know how someone in ...
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Hertzian Dipole Antenna

Hey guys have a physics question. I'm trying to find the current that is necessary to radiate a total power of $$60mW$$ Current is constant over dl and the length is equal to $$l=0.02*\lambda$$ I'm ...
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1answer
31 views

Shape of Eddy Currents

I've always had this question and didn't know how to answer it - Why do eddy currents have the shape they do? Why are they circular eddies?
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68 views

What is the purpose of the Maxwell Stress Tensor?

In the calculation of the forces acting on a charge/current distribution, one arrives at the Maxwell stress tensor: $$\sigma_{ij}=\epsilon_0 E_iE_j + \frac{1}{\mu_0} B_iB_j ...