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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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### I know that if $σ$ is the conductivity of the wire, then, by Ohm’s law in its general form we have: $\vec{J}=\sigma \vec{f}$

I know that if $σ$ is the conductivity of the wire, then, by Ohm’s law in its general form we have: $\vec{J}=\sigma \vec{f}$ that $\vec{f}=\vec{f}_{em}+\vec{f}_{app}$ where where $\vec{f}_{em}$ is the ...
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### Could a disturbance to a magnetic field be detected near the source?

(Forgive my crude MS paint drawing) Imagine we have a large magnetic field coming from a powerful source. At the edge of the field where it is weak (A), we introduce a disturbance using another ...
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### Relation between AC Current and electromagnetic wave in transmission lines?

I m so confused we started by learning about electromagnetic waves in the vacuum , then we went to transmission lines and there the professor mentioned the em waves are traveling through them. I ...
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### What distance do I use when examining the effect of an electric field on a point outside of a nonconducting sphere with an arbitrary volume charge?

It's easy enough for me to do this for a ring (as I interpret the field lines as "exiting from the centre of the ring") or a point (simply the distance from the point to the other point), but I just ...
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### Nature of induced magnetic field lines in cylindrical capacitor

What is nature of the induced magnetic field lines inside a cylindrical capacitor which has a time variant potential gradient across it? For a parallel plate capacitor, its straightforward: the ...
56 views

### Conceptually understanding RL circuits

I'm struggling to conceptually understand the current-time profile of an RL circuit. Specifically, what causes the rate of change of current, $\frac{\partial i}{\partial t}$, to start off high when ...
19 views

### Collective dipole oscillation and specular reflection

Let's say I shine light upon a surface and observe a specular reflection. If I look with a electron microscope, I see tiny particles much smaller than the wavelength of light (~10 nm) and also closely ...
55 views

### Could you transmit electricity wirelessly?

Since you can wiggle an electron to get a radio wave And if you 'wiggle' it fast enough you get higher and higher frequency right? So couldn't you just broadcast an electromagnetic beam or whatever ...
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### Screened magnetic interactions

I hear all the time people saying magnetic interactions are not screened. I do not understand what it means. Is it related to the nonexistence of magnetic monopole? I assume you can always find some ...
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### How to find the mixed tensor, contravariant tensor and tensor trace of $F$

I have a question in particle physics that ask me to find the mixed tensor, contravariant tensor and tensor trace of $F$: Our professor didn't teach us that much about the math of tensor, which makes ...
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### Why is flux through a closed surface not infinite? [duplicate]

The Electric field lines coming out of a charge are infinite, So the lines passing through a closed surface should be infinite , right?
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### How to do colour map in MRI?

For this question, I assume if we do a colour map, it is based on the magnitude of the magnetic strength? After that how do we know where each coordinates belong to? Like for example at p= 0 degrees,...
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### Is it possible to measure the amplitude of a magnetic field independent of the orientation?

The strength of a magnetic field can be measured as strength in a specific direction. We can calculate the strength of a magnetic field along the field lines from measuring the strength in x,y,z ...
1k views

### Can a magnet bend light? [duplicate]

As light is electromagnetic radiation. Then why I don't see any magnet bending light wave? Or why light doesn't diffract whenever it passes by a live wire?
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### Metric (in)dependence of the electromagnetic field strength

In GR, the vector potential is defined as $A^\mu$ which is a contravariant vector. Then lowering the indices requires the metric $A_\mu =g_{\mu\nu} A^\nu$, using this vector one defines the field ...
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### Pull force of neodymium magnets - is there a way to be more realistic?

I have a mirror attached to a 360 degrees swivel/tilt base. The mirror with the base weights 1 lbs. I glued a N52 round magnet on the base, and another magnet on the wall. Each magnet measures: 1.25 ...
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### What was the real need of the operators of divergence and curl?

As I'm advancing my study in Electromagnetism I'm getting introduced to more mathematical operators which are exclusively used in Electromagnetism and Fluid Dynamics only. Let me try to explain ...
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### Interpretation of real and imaginary parts of the Poynting vector in a vacuum

It is known that the flux of the Poynting Vector through a certain surface represents the total electromagnetic power flowing through it. Then, its real part is the active power, while its imaginary ...
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### Velocity of particles in electrolysis

In an electrolysis (H20) water is split up and the protons start to move towards the anode. So one would be able to calculate the acceleration of the proton from the electric field in that area. ...
1k views

### In a double slit experiment, does each and every photon leave a dot on the screen in the bright area?

I have read this question: Why does the photon strike at one or another place on the tape? where PhysicsDave says: All photons passing thru the slits leave a dot on the screen, this is true for ...
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### How is the Time Derivative of the the Electric Field Equal to the Current Density in Gaussian Units?

The microscopic form of Ampere's law with the Maxwell addition in Gaussian units states, \nabla \times \vec{B} = \frac{1}{c} \left ( 4 \pi \vec{J} + \frac{\partial \vec{E}}{\partial ...
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### Can a moving current carrying wire produce electric field?

As we know, a current carrying wire can create a magnetic field which is perpendicular to the direction of current (From relativity, we can understand why magnetic fields are produced ) and the net ...
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### Stumped on understanding a Feynman lecture about force from wire on magnet

I must (sheepishly) admit that I'm stumped on a beginning page of The Feynman Lectures Volume 2. I have included a picture from the page. [Let me know if I'm breaking copyright, or if I can include ...
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### Two solenoids of co-axis in relativity

Lets think about two solenoids sharing their axis. Their radius is almost same, so you can ignore the difference of the area of the cross-section. Two solenoid has same magnitude of current, but ...
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### Deriving the speed of light from Maxwell's equations?

Relationship between speed of light and EM force? Can it be said that Maxwell used measurements of the "strength of electric force and strength of magnetic force", to derive the value for the speed ...
100 views

### Does the magnetic part of the Lorentz force do work?

The magnetic part of the Lorentz force is $$\vec F_L=q\left(\vec v\times\vec B\right)$$ As this force is always perpendicular to the direction of the movement, we learned that no work is done by it. ...
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### What is the strength of gravity compared to electromagnetism?

I would like to know the strength of gravity as compared to electromagnetism. Im not looking for their classical constant values. It can be compared by saying i.e. gravity is a million billion times ...
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### Calculate Force on Permanent Magnet from a Coil

I was wondering how I could calculate the force on a permanent neodymium magnet. For this setup, I have a magnet on a scale with a coil directly above it. The magnet and the coil repel each other when ...
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### Does a relaxing electron really accelerate?

This is not a duplicate. I am not asking about quantum leaps or quantum jumps or whether the transition is instantaneous (yes I asked that question before here Do electrons really perform ...
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### Difference between Motional EMF and Hall EMF

Is there any difference btw motional emf and hall emf ?
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### What is the nature of the multipoles used to shape/re-direct an e-beam?

Imagine having an electron beam, just like in an electron microscope. Now imagine that we have some multipoles prepared in the beams path to influence the shape/path of the beam. My question is: ...
The current density is defined as: $$\textbf{J} (\textbf{r},t) = \rho(\textbf{r},t) \cdot \textbf{v}(\textbf{r},t)$$ where $\rho (\textbf{r},t)$ and $\textbf{v}(\textbf{r},t)$ is the charge density ...