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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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Wave like properties of particles

If we consider a particle such as an electron to have wave like properties too, what kind of wave it is? What is exactly meant by its wavelength?
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Electromagnetic field tensor and antisymmetry

Why does the inner product between the four force (caused by the electromagnetic field tensor) and the four velocity equaling zero imply that the electromagnetic field tensor is antisymmetric? This ...
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Is it possible for electromagnetic waves in the Extreme Low Frequency range to carry HQ video and audio signals? [duplicate]

I had just finished this interesting read on a us navy project and was wondering how Brain computer interfaces are touting in the near future to pick up thoughts that might need the same bit rates as ...
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Electric field on dipole with different directions

Griffith's E&M problem 4.5 page 165 In the figure p1 and p2 are perfect dipoles a disantce r apart. What is the torque on p1 due to p2.? Wjat is the torque on p2 due to p1? well i calculated ...
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The rise of BCI

If EEG and MEG are measuring different aspects of neural oscillations, why can't we measure the combined signals as a whole, when we are doing the same for higher frequencies? Why did the us navy ...
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How can you block ELF in the range of 2hz - 20hz?

If our brains are emmiting em waves(https://engineering.mit.edu/engage/ask-an-engineer/can-brain-waves-interfere-with-radio-waves/), how can we effectively shield our brains from emitting such waves?
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1answer
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How do ion thrusters create a force against the spacecraft?

I recently saw an old thread, How do reaction engines create a force against the rocket?, get bumped up, and it asks a good question: in a chemical rocket, the fact that the rocket exhaust gets ...
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2answers
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How can be $\frac{1}{4\pi R^2}\int_{S}V_{ext}(R)da= V_{ext}(0)$ physically explained?

I was working out problem 3.1 (4th edition) of Introduction to Electrodynamics by Griffiths, which asks you for the average potential over a spherical surface due to a charge located inside the sphere ...
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1answer
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Electric quadrupole radiation and supernova core collapse

I have a question when I'm learning the electric quadrupole radiation in E&M. In the radiation zone, the vector potential $\vec{A}(\vec{r},t)$ generated by the current distribution $\vec{J}(\vec{r}...
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2answers
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How can non-polarised light exist? [duplicate]

Typically unpolarised light is shown like this: Basically, from what I've seen, unpolarised light is just a lot of overlapping individual light waves. However, if they truly overlapped, would their ...
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Why -20dB is taken as the standard rejection ratio for optical mode rejection in multimode waveguide?

In case of multimode waveguide to demonstrate perfect mode rejection, I saw researchers use boundary of -20dB. If mode power at output is less then -20dB, it is considered as fully rejected/filter out....
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The relation between the electric field and magnetic field?

I was wondering under what condition the relationship between the electric and the magnetic field holds: $\overrightarrow{B} =−\frac{\overrightarrow{𝑘}}{𝜔}×\overrightarrow{𝐸_0}𝑒^{𝑖(𝑘⃗ ⋅𝑥⃗ −𝜔𝑡)...
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Relativistic effect in a current carrying wire

In many places I read that in the rest frame of a current carrying wire (electrons in motion), the spacing between the positive ions is length contracted and equal to the length contracted spacing ...
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1answer
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Optical trapping: factor 1/2 in definition of gradient force and potential energy?

If one consider the original deduction of the gradient forces that is applied to the trapped particle we can find the following: $$F_{grad}=\frac{1}{4} \alpha \nabla E_0^2(r),$$ where $E_0(r)$ is an ...
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1answer
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Electrodynamics in 2+1 dimensions

In 2 spatial + 1 time dimensions, this answer tells us what $F$ is in terms of differential forms. I wanted to know how do we determine $A$ in terms of differential forms? (Here $A$ is the vector ...
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Questions about plane electromagnetic wave

A wavefront is defined to be the locus of all points on which the field oscillations are in phase, in the plane electromagnetic wavefront, it is said that all points on a plane perpendicular to the ...
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1answer
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Explaining receiver antennas without using Reciprocity

I am trying to understand the physical behavior receiver antennas, but I am unsatisfied with all the explanations I have seen so far in my textbook (Stuzman & Thiele, Antenna Theory and Design) or ...
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How the Magnetic field varies here? [on hold]

A uniform beam of positively charged particles is moving with a constant velocity parallel to another beam of negatively charged particles moving with the same velocity but in opposite direction ...
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2answers
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inverse square law and far-field radiation

This question is related to Feynman on inverse square law of EM radiation. It is basicly the same question except that I don't see that the question was ever answered, and I hope someone will answer ...
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Calculate the electric field and potential given a potential difference [on hold]

I have been asked to calculate the electric field and potential laws inside this coaxial cable: My first try was to apply the Gauss' Law between the two conductors, so if the battery charges the ...
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3answers
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Is magnetic force relativistic effect or intrinsic property?

In this video, they explain that magnetic force is an effect of electric force due to special relativity. https://www.youtube.com/watch?v=1TKSfAkWWN0 But then, the same authors explain in this video ...
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2answers
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Is it possible to induce left-handed electromagnetic wave from positrons? [duplicate]

All observations of electromagnetic induction we have, are from the flow of electrons. The accelerated electrons in the antenna rod induce an EM wave with a right handed system of the electric field ...
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If light is an electromagnetic wave then can we create light? [on hold]

Can we light up a dark room by changing electric and magnetic fields in that room to create light?
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Quantising electromagnetic field in a box. Why can't I use the easy cartesian modes?

In this paper, the electric field eigenmodes of a box with perfect metallic walls are derived. They are (eqn. 2.39) But the author does not use these modes to construct the Electric field, and then ...
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Why do EM waves not require a medium to travel trough? [duplicate]

Why do EM waves not require a medium to travel trough? I'm not able the understand the topic, unless i understand why they don't require a medium.
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1answer
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Force on current carrying wire in magnetic field

I was doing an experiment on the force on a current carrying wire in a magnetic field. I used an electric weight to register the magnetic force. When I used the force, current, and length of the wire ...
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What is the chronological order of math classes need for college physics starting from calculus? [closed]

Can someone list all of the mathematics needed to reach the highest of the high in physics?
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On the applicability of Coulomb's law and the Biot-Savart law

Jefimenko's equations are $$\textbf{E}(\textbf{r}, t_r) = \frac{1}{4\pi\epsilon_0}\int \left[\rho\left(\textbf{r}', t_r\right)\frac{\textbf{r} - \textbf{r}'}{\left|\textbf{r} - \textbf{r}'\right|^3} + ...
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1answer
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Oscilloscope measurement on primary coil changes secondary coil voltage

So I was measuring the induced voltage in a solenoid (air-core) with the primary solenoid being on the outside (parameters:$N_p=100$, $l_p=12.6cm$, $d_p=5.1 cm$, $N_s=50$, $l_s=6.3 cm$ and $d_s=3.1 cm$...
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1answer
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energy storage and flux produced by an inductor

The unit of magnetic flux is the Weber and is proportional to L x I or Henrys X Amps. Flux is also proportional to J/I or energy per Amp. Energy stored in an inductor is given as 1/2 LxIxI. When I ...
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HARD EMI question, IV been at it for about a week but still not getting it [closed]

So I'm a high-school senior and this is in my study package I believe I need to use magnetic flux change = integral E.dl. But no idea how to apply it
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“flowing” speed of an electric field?

Consider a capacitor made of two infinite plates which are at a very large distance $d$ from each other. We place a positive charge on a plate and a negative one on the other one. Two electrical ...
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1answer
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Are all solutions of Maxwell's equation related by a gauge transformation?

Consider Maxwell's equation (without source): $$ \partial_\mu F^{\mu \nu} = 0 \implies \partial_\mu \partial^\mu A^\nu = \partial_\mu \partial^\nu A^\mu.$$ Can we find a pair of classical field ...
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1answer
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Calculating Noether Current for Electromagnetic Current interacting with a Dirac Fermion

I'm trying to confirm that the conserved current of the Lagrangian $$ {L} = -\frac{1}{4}F_{\mu\nu}F^{\mu\nu} -j^{\mu}A_{\mu}+\bar\psi(i\gamma^{\mu}\partial_{\mu}-m)\psi $$ associated with the ...
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1answer
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Why does the charge on a charged metal sphere decrease when different sites on the sphere are measured?

In our experiment the metal sphere was charged to 3000v and when measured with a proof plane was positively charged. 2 subsequent readings (both discharged between) on different parts of the sphere ...
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1answer
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Non-gauge-invariance of EM angular momentum

In 16.7 b) of Zangwill we're asked to show that the expression $$ {\bf L}=\epsilon_o\int d^3r E_k ({\bf r}\times\nabla)A_k + \epsilon_o\int d^3r {\bf E} \times {\bf A} $$ is not gauge invariant. ...
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How to calculate the electric field using Gauss' s Law in this example? [closed]

Suppose we have an infinite cylinder (infinite in the z-direction) with radius $a$. Inside it, there is free charge density given by $\rho = \rho_o \cos (kz - \omega t)$ and current density given by $\...
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Internal energy of paramagnetic obeying m = function of H/T independent of magnetization?

I'm stumped. In this exercise, I've got a paramagnetic obeying a $m = \psi\left(\frac{H}{T}\right)$ law, where $m$ is the average magnetization (with the volume $V = 1$, so it's equivalent to the full ...
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Magnetism and magnetic field [closed]

A current carrying ring, carrying constant current 2/π A, radius is 1m and mass is 2/3 kg having 10 windings is free to rotate about its tangential vertical axis. A uniform magnetic field of 1 tesla ...
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Is a radio receiver antenna a capacitor?

I think so, but its capacitance should be very small, so how can the antenna correctly receive the signal, the low capacitance wont bother the signal?
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1answer
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Which law is true at the transformator?

According to Faradays law The changing flux will generate voltage in the secondary winding/circuit, so if the N2 = 2*N1, then the U2 =2*U1, and according to conservation of energy in that case I1 = 2*...
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1answer
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Force on a magnetic dipole in an external magnetic field

I want to find an expression for the force acting upon a magnetic dipole with dipole moment $\mathbf{m}$ if that dipole is positioned in a stationary, external magnetic field $\mathbf{B}$. The ...
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1answer
64 views

I have a pair of magnets that when I let go they attracted and broke

I let two magnets attract from a distance and when they hit they both broke a piece off of each-other. Why did the magnets damage each-other when they collided and is it possible to put the broken ...
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1answer
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Is the electric force that drives current equivalent to the Lorentz force acting on those currents?

Current flow ($I$) and the electric force responsible in moving the charges are proportional to the Lorentz force acting on those charge due to the magnetic field that the current flow produces, all ...
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1answer
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Determine electromagnetic potentials that satisfy Coulomb and Lorenz gauge condition

In our physics lecture, we did the following example of constructing potentials $\vec A$ and $V$ that supposedly satisfy both the coulomb ($\nabla \vec A=0)$ and the lorenz condition $(\nabla \vec A+ \...
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Can an electromagnet find wires under ground? [closed]

I was seeing how an electromagnet could attract a car when it's destroyed, and I was wondering, because I have to find the water tubes in my house (20/40 cm depeer), if I can put a wire inside the ...
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Wigner Rotation of static E & M fields is dizzying

I am looking for a formula to describe the set of inertial frames in which an electromagnetic field with non-parallel, non-perpendicular, and non-zero $E$ and $B$ transform to an electromagnetic field ...
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Is Coriolis force necessary for geomagnetism

I understand that the Coriolis force from the earth rotation organizes the current flow in the earth outer core into columns of rotation the axis of which parallel the earth rotation axis. That ...
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emf = velocity of conductor * length of the conducter* magnetc flux [closed]

In a sound to electricity experiment we are trying a setup similar to reverse speaker it operates in reverse manner. Above title is the formula from faraday's law of e.m.f generation . ...
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1answer
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The curious case of parallel $E$ and $B$ fields and inertial frames

In a comment to this Physics SE question, @MichaelSeifert stated, For the more general case, IIRC there's always a frame in which $\vec{E}$ and $\vec{B}$ are parallel when $\vec{E}\cdot \vec{B}\...