Questions tagged [electromagnetism]
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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What are the physics behinde reflection and refraction of electromagnetic wave at a dialectric surface?
I have understood the most of the equations that lead to the Fresnel-Equations from electromagnetic waves and Maxwell equations. But not enough to understand what is happening. So I don't ask for an ...
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Conflict between the the integral and differential forms of Maxwell's equations
Recently I made a question, you can click in this link. I am having issues with the divergence of the magetic field in the orbital cylindrical frame, in which the divergence of the magnetic field is ...
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Lorentz transformation of classical point charge current and density
For a classical point charge, the electromagnetic charge and current densities can be given, respectively, as $$\rho=q \delta^3(\mathbf{r}-\mathbf r_0(t)) \tag{1} $$and $$\mathbf J = q \mathbf v \...
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Why should we expect motional emf to obey the same general relation as Faraday's Law?
There are many posts on this forum asking whether motional emf is actually an instance of Faraday's Law -- because, confusingly, it is often taught as though it is, with no qualification. The best ...
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How can there be uncertainty reflecting the polarization of a photon?
I am aware that many quantum entanglement experiments are based on uncertainties related to the polarization of a single photon, which can be measured in the horizontal/vertical, clockwise/...
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Time-scale of demagnetization of permanent magnets due to external field
I wonder what the time scale to demagnetize a fully magnetized permanent magnet with an external magnetic field is. The temperature is assumed to be constant.
I found a reference 1 for specific cases, ...
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Current Recycling in Electromagnets
Has anyone figured out a way to recycle specifically the energy used to power electromagnets? Since for many applications, the Bfields required are extremely high magnitudes, the currents must match ...
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What is 90 or 180 "degree" RF pulse in MRI?
I am studying MRI, specifically spin echoes.
Now I kind of understand how magnetic resonance imaging works, but what I'm stuck is the definition of "degree" in MRI. What I thought was that ...
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When do we derive $E=-(\partial V/\partial S)$? [closed]
As the question says when do I need to derive the equation for electric field $E$ where V=potential and S=distance? I know some equations just have the change in V over the change in distance but some ...
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Can you charge a capacitor with only voltage (without current)? If No, then how does a capacitor correct power factor?
Let me explain you why I am asking this question.
The other day I was studying about power factor correction of a (step up or any) transformer. It said that on the output side of transformer's ...
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Confusion regarding magnetic length
Consider the geometric length of a magnet to be $L$ and magnetic length to be $L'$.
Some sources claim that $L'$ = $(0.84)L$.
One such claim can be found here:-
I cannot find any good reason to be ...
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Is third photoelectric effect law wrong? [closed]
1.Ionization energy equals minimal energy electron needs to overcome atom’s Coulombs and gravitational fields.
Ionization energy depends on which state atom currently is. For ground state ionization ...
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Energy increase due to the Zeeman effect
The Zeeman effect is the change in energy of a system with a permanent magnetic moment in the presence of an external magnetic field. Consider electrons for example. In general, in $B \neq 0$ ...
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Verification of Stoke’s theorem [migrated]
Consider the vector field $$\vec{F}=-\frac{y}{x^2+y^2}\hat{i}+\frac{x}{x^2+y^2}\hat{j}$$. Calculating the curl of the vector field I am getting zero. But when I am calculating the line integral along ...
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How do scientists focus high energy electromagnetic waves onto a target?
For visible light, we are able to use mirrors to focus on what we want.
However, gamma rays' wavelengths are too short and can't see solid objects.
So how do scientists focus high-energy ...
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Question about Drude's model of conduction
Why is it proper to assume the electronic velocity after collision is zero in Drude's model?
(Where are momentum and energy conservation?)
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Why the divernce of this magnetic field is not zero?
I am working on a project on which I need to calculate the geomagnetic field in different coordinates. When I use the conventional form of the dipole field in spherical coordinates:
$$\vec{B}_{r,\phi}=...
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An infinitely spinning magnet is placed at the center of a sphere filled with mercury. Will a magnetic field be generated along the system’s $z$-axis?
The magnet’s field vector is perpendicular to its spin axis, its angular velocity is as high as it can be without destroying itself or the overall system (unless a lower velocity would produce ‘better ...
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Question about electrons in electric fields [duplicate]
I've been studying chemistry (I know this is a physics page) and I've read that electrons are supposed to lose energy because they are moving in an electric field (I know this is not actually ...
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Why atoms are not being ionized during stimulated emission?
Probably simply question, however, I need to know for sure.
To ionize atom at some state, as understand, it must receive energy of that state, so for hydrogen atom, being in ground state it $13.6 eV$. ...
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Finding motion/position of a particle travelling through static magnetic field of a bar magnet
I am trying to figure out an expression for the position of a charged particle travelling downwards in the z-direction at some constant velocity through the static magnetic field generated by a bar ...
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Circular magnetic field around current carrying wire [duplicate]
We know that a current carrying wire produce a magnetic field around it.But my question is why those magnetic lines are circles.
How it can be like that?
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Poincaré inverse square law unification
A while back, I came across a claim that was something like this: Poincaré said that no two fundamental force laws could have the same mathematical form such as $1/R^2$; that no two fundamental force ...
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Magnetic field strength beyond wire [closed]
If there is a point beyond a semi-infinite wire at a perpendicular distance r from it then what will be the magnetic field strength at the given point and what will be its direction?
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How do you determine the electromotive force when the negative plate of the capacitor is connected to the positive terminal?
$\newcommand{\d}{\mathrm{d}}$We have a square wave source with period $T$ and voltage $V_0$ connected to a circuit which is has a resistor of resistance $R$ in series with an uncharged capacitor of ...
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Screening of the electric field by a grounded conductor in the presence of external charge
If we have an incompressible (constantly $\rho=const.$) charged fluid in some parts of space, this will trivially create some electric field.
Suppose we surround each of these domains of charged fluid ...
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The medium of electromagnetic waves [closed]
I'm just wondering if anyone else has ever pondered the question: what medium do electromagnetic waves travel through? and like me remained unsatisfied with the explanation that they are 'non-...
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Magnetization Enhancement Using Various Materials
I was looking online at how inserting a material inside a solenoid can enhance/reduce the magnitude of a magnetic field in all space depending on what material is inserted. This makes sense from a ...
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Solar powered homopolar motor does not rotate [closed]
After experimenting with a battery powered homopolar motor, I thought it would be interesting to make a solar powered version. I 3d printed a frame, painted conductive ink on one side and glued ...
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Why does voltage increase as charge decreases?
Given that J/C=V, where J is energy in joules, C is charge in coulombs, and V is voltage in volts, it makes mathematical sense that as C increases, V would decrease. But from a practical standpoint, ...
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Gauge choice and observable quantities
Assume that I have the usual $U(1)$ gauge field $A_{\mu}$. We know that observable quantities are invariant under global transformations of the form $A_{\mu}\rightarrow A_{\mu}'=A_{\mu}+\partial_{\mu}\...
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Why ionic polarizability is temperature independent?
Why is ionic polarizability temperature independent? According to me, as we increase the temperature ions should start oscillating and an ionic polarizability should change. Can we see this through by ...
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Conserved quantities with charge and Killing's vectors
I'm trying to solve the following problem:
A particle with electric charge e moves with 4-velocity $U_{\alpha}$ in a spacetime with metric $g_{αβ}$ in the presence of a vector potential $A_µ$. The ...
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Hi, I have a little doubt, do electric charge create EM field or disturb or enhance it? [closed]
What i mean to ask is that, i learnt that EM waves are just disturbance in the already ever existing EM field, so when there is a charge in space, does it enhance the field in its required direction ...
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Finding the Lorentz Velocity for Relativistic Electrodynamics
I'm trying to understand something from my Special Relativity notes.
Assume here that $\vec{E}$ and $\vec{B}$ are perpendicular and $|\vec{E}|>|\vec{B}|$.
Show that there is a frame $\bar{\Sigma}$ ...
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Electromagnetic electrons [closed]
Electromagnetic waves, to my knowledge, are produced by the oscillation of forward-propagating photons in an electric field, which generates a magnetic field, due to Lenz' Law. Based on this principle,...
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How would power the baby Yoda (Grogu) Pod? [closed]
Lets go wild! Throw in your theories and ideas how you would power such a floating device? Lets have some fun wild theories!
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Hand-wavy method to ampere's law for a single coil of wire
Based on this question
Ampere's law and Biot-Savart law gives different terms for magnetic field in middle of a current running in a loop
I made the following square Amperian loop:
where I have ...
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What is the boundary from N to S called in magnetism? [closed]
I am doing some research on asymmetric magnetic fields. I have simplified my problem to an isosceles triangle mangnetized with the N side on the tip with the smallest angle and the S side opposite of ...
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Solution to Laplace Equation [closed]
Consider two dimensional Laplace equation in cylindrical coordinates
$$\frac{\partial^2 \phi}{\partial^2 r} + \frac{1}{r}\frac{\partial \phi}{\partial r} + \frac{1}{r^2}\frac{\partial^2 \phi}{\partial^...
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QFT symmetry breaking and CP violation
Since the weak interaction violates CP symmetry but electromagnetism doesnt,does it mean that the electroweak fields also violates CP symmetry?Can a field which violates some symmetry be the product ...
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$R_\xi$ gauge and degrees of freedom counting
In the standard classical Maxwell theory, we use the following arguments to claim that there are only two propagating degrees of freedom
$A_\mu$ has 4 components
$A_0$ is non-dynamical (-1)
$\...
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Radiation back reaction in classical electrodynamics with point and extended sources [closed]
I've been doing some research on the topic of radiation reaction force/self force in classical electrodynamics and although there are some discussions on the internet I would like direct answers to ...
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If I spin a magnet in a coil which has a length parallel to the magnet’s spin axis, is a magnetic field generated along the length of the coil?
This I feel is a bit of a complicated question, so I’ll quickly try to paint as clear a picture as I possibly can:
The magnet is spinning on an axis perpendicular to its poloidal axis (the line which ...
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Why do most MRI machines and fusion reactors use helium-cooled superconducting magnets instead of cheaper, higher-temperature nitrogen-cooled ones?
I am always hearing about the high (and increasing) costs of helium, as well as the difficulty of handling it....
Are there no high(ish) temperature superconductors that can create a strong enough ...
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Measuring electric potential with a flame probe
I'm reading up on an experiment to measure the electric potential at some point in space. There's a charged metal sphere that sets up an electric field in space. We want to measure the voltage at some ...
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Describing the strength of a magnetic field
There is one thing I can never get right. If I want to describe the strength of a gravitational field I can use the standard gravitational parameter ($\mu$), which does not take into account the ...
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Can high energy photons be used to accelerate particles?
So I know that particle accelerators use electromagnets to accelerate particles. However, since photons can be used to recoil particles (such as in Compton scattering), is it possible to use high ...
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Permittivity of air's dependence on frequency
It is known that for most materials, permittivity is a function of frequency, and thus the value of permittivity can have a vast variation. However, in the case of air, it is often considered $\approx$...
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Boundary Condition in Capacitors and Antenna to Free Spaces [closed]
I am reading the EM Field books by Hyatt and Griffith. Chapter Boundary condition.
in a Capacitor and in the Antenna.
I am confused between the normal vectors and the one side Field vectors. May ...
