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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15 views

Confusion about magnetic vector potential

I'm confused about two things : the illustration and the "vector" in magnetic field . I didn't understand why the potential created by magnetic field got vector . I know about Helmholtz theorem , but ...
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How do the formula of electrostatic energy change from vacuum to with matter?

I know that, in vacuum, the electrostic energy is: $$ U_E = \frac12 \int \rho(\mathbf r)\varphi(\mathbf r) d^3\mathbf{r} $$ But I don't know how to pass to the matter version? The formula would be ...
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Why is magnetic attraction field lines when seen with ferrocell reverse that with iron filings?

Magnetic attraction seen with ferrocell. The magnetic field pattern is the same as that for repulsion when viewed with iron filings. Why?
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Current density of a system of charges in QM

Consider a quantum mechanical system of charges, e.g. a molecule, in its stationary state (thus it is a bound state). Is it possible to write down a formula for a current density of this system ...
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Teaching Electromagetism to Life-Science Students

I am going to be teaching an introductory university class on E&M and Modern Physics (from Coulomb's Law through Maxwell's Equations and optics, followed by a brief mention of quantum and ...
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What is Cerenkov radiation? [closed]

What is Cerenkov radiation? Why is electron faster than speed of light in water? `[![ Blockquote ]1]1`
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Motion of charged particle in uniform magnetic field and a radially symmetric electric field

This question posted by me on MSE talks about a physics problem. This is what it was: (I hope someone can help me with this) Consider a region of 2-dimensional space with a uniform magnetic field of ...
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Neat expression for the dipole potential in Fourier space?

In textbook electromagnetism we are used to seeing neat, coordinate-free, expressions for the scalar potential from an electric dipole (using Gaussian units) $$\phi(\mathbf{r}) = \frac{\mathbf{p} \...
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Could sufficiently powerful Diamagnetism be used to repel a light macro-sized object above the ground?

Take, for example, a futuristic-looking square "lamp" (effectively a metal cube that emits light). Overall we assume the lamp can be made to have a mass of around ~20 grams as this is ...
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Why does a floating capacitor act differently than a grounded capacitor?

I'm having difficulty understanding why a floating capacitor will store less charge than a grounded capacitor. Imagine you have two parallel plates and a low DC voltage source like 5V, with the ...
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Why does the DC motor stop when I put my finger in?

When I insert my finger into a moving motor with a wing, when my finger collides with the wing of the dc motor, the motor stops, and when I remove my finger, the motor rotates again. What is this ...
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Estimating the magnetic field outside a tesla coil

Recently I was studying about coils and a question raised to me: How can I estimate the outside magnetic field of a tesla coil, take a look in the scheme above. In this sense, I would like to know ...
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Proving a general formula for the boost transformation of the electromagnetic field

In inertial frame $\mathcal{O}$, a region of space-time is filled with constant electric field $\vec{E}$ and magnetic field $\vec{B}$. Another inertial frame $\mathcal{O}'$ has 3-veclocity $\vec{V}$ ...
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Solving Maxwell equations on curved spacetime

I have difficulties to understand how to solve the Maxwell equations on curved spacetime. I want to solve the equations in the weak regime $g_{\mu\nu}=\eta_{\mu\nu}+h{\mu\nu},~ h_{\mu\nu}\ll 1$ ...
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How does a plasma react to a non-uniform, oscillating magnetic field?

The ponderomotive force tells me that if I apply some non-uniform oscillating acceleration field $\ddot{x} = g(x) cos (\omega t)$ where $g(x)$ is the strength of the field as a function of location ...
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Using an electric/magnetic field to cool particles

I was just thinking that temperature and the motion of atoms are connected. Hence, I was wondering why we can’t just use a magnetic or electric field to hold atoms more or less in place, lowering ...
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Induced field on a solenoid - dependance on position [closed]

Consider a cilindric solenoid of lenght L, with N spires of radius a. Calculate the induced field in a point P, in function of the angles $\theta_1 $ and $\theta_2$. The magnetic field for a solenoid ...
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A non-conservative electric field inside a conducting wire within a magnetic field?

We consider a region of space confined within a box, in which there is a time varying magnetic field $\mathbf{B}$ as well as a perfectly conducting loop of wire that passes into and out of the box to ...
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Falling EM system contradicts equivalence principle?

The following is an improved version of my previous post Falling electric dipole contradicts equivalence principle? Consider the following system comprising a particle on the left with charge $+q$ ...
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132 views
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Radiation field of a scattering experiment moves faster than light

Consider an electron with 4-momentum $p$ which receives a sudden "kick" at $t=0$. After the collision, it has momentum $p^\prime$. Now I am interested in the radiation, that the particle is emitting. ...
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Relative permittivity/Delectric constant

What Is the physical meaning of the dielectric constant ? (I don't want the mathematical expression) Thanks In advance.
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Explain the reason behind electromagnetic induction of closed loop wires

Don't mistake me for asking why Faraday's law of induction works fundamentally. (I know there exists duplicates if that was my question). Firstly, I can explain why when a straight infinite wire moved ...
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1answer
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if an oscilating LC circuit without antenna could produce electromagnetic waves since there is changing electromagnetic field?

Just shown as the above picture, if the right part removed, would electromagnetic waves still be produced near the inductor and what the changing electromanetic field are in this situation? I used to ...
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Does electromagnet work without heat of wire?

If we isolate wire with thin wire and assuming that no heat rejected by wire then does electromagnet work. If it work then it violate conservation of energy .
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Missing equations in Maxwells Equations

We have Maxwell's Equations (ignoring permittivity and permeability of free space) $$ \nabla\cdot E=\rho\;;\;\nabla\times E=-\frac{\partial B}{\partial t} $$ $$ \nabla\cdot B=0\;;\;\nabla\times B=\...
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Plastic permittivity compared to vacuum permittivity?

I'm studying electromagnetism and I have found that vacuum permittivity is equal to: \begin{equation} 8.854 × 10^{-12} C m^{-1} V^{-1} \end{equation} and the relative permittivity of plastic is ...
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Why diamagnetic materials behave the way they do

I have problems to understand the reason why in diamagnetic materials, in presence of an external magnetic field, the atoms develop a magnetic moment that opposes the external field. I have read two ...
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Transmission lines: Why does a real permittivity mean you have a lossless line?

My prof said that if you have a relative permittivity which is real, the reflection coefficient is 0 (which means a lossless line). Why is this?
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If monochromatic light shines into our eyes, will that activate multiple types of cones or only one type of cone?

I have read this question: Explanation about black color, and hence color where John Rennie says: For example suppose you're looking at red light. Only the "red" cones will generate a signal and ...
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Why symmetric magnetic field in cylinder cordinates is zero in $\hat \phi$?

I just not understand the intuition and also how to prove it mathematicaly that if I have symmetric magnetic field around z axis, why it would be zero at $\hat \phi$ but none zero at $\hat \rho$?
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Does an accelerating atom emit EM wave?

Every atom contains charges and charges emit EM wave when they are accelerated. So if an atom is accelerated, the charges inside will also accelerate. Which means that the charges inside will also ...
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Electromagnetic waves: The speed of light and relative permittivity

In the chapter about transmission lines, there was the following question: Given a frequency of 1GHz and a relative permittivity of 2.25, what is the wavelength of the sinusoidal wave? In an ...
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Null point for a infinite current carrying conductor [closed]

If a current carrying conductor of infinite length is placed along the magnetic meridian and current is flowing towards north pole, then is null point possible at any point in space?
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Do stationary charges get attracted to a current carrying wire (due to change in electric charge density caused by length contraction)?

I've seen magnetic fields getting explained away by special relativity, for it accounts for the magnetism in the electron's reference frame as an electric field caused by a greater charge density due ...
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34 views

Why don't (or why do) current carrying wires attract a stationary charge placed at a distance?

I've learned that moving charges produce magnetic fields which in turn affect other charges in motion. After seeing explanations that point to special relativity, I am kind of confused. Can ALL ...
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29 views

Current induced in straight wire by a moving point charge [closed]

I have a straight wire that is grounded through a resistor. It initially has no current flowing through it. A point charge $q$ moves past the wire with velocity vector $v$ relative to the wire. How ...
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How does earth's magnetic field trap charged particles?

I am very new here and I know this question is probably very basic for this place but I thought I would ask any way. I am in year 12 physics and this question is for a research paper that makes up a ...
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3answers
132 views

Schrodinger equation for free EM field

The question come from the fact that I've seen for the first time in my life the quantization of a field, in particular of the free em field. I've study how it is possible to write the energy of the ...
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2answers
111 views

Effect of a current carrying wire on a point charge

Given a point charge $q$ near a wire carrying current as shown, what would be the effect of the magnetic field produced by the wire on this point charge $q$? I think the sign on $q$ is important so ...
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What are the mathematical prerequisites to understand Introduction To Electrodynamics by David J. Griffiths? [closed]

I am in the second year of my high school and I want to use Introduction To Electrodynamics by David J. Griffiths for physics olympiads. I know single variable calculus and I have gone through the ...
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How symmetric Halbach array can produce asymmetric magnetic field?

I don't understand. If I look at a picture of Halbach array I see it is apparently symmetric. At least both pairs of opposite sides are. Then how can it have "weak" and "strong" opposite sides?
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ElEctRIc--POTENTIAL's sign [duplicate]

COULD ANYONE PLEASE EXPLAIN TO ME HOW TO TAKE SIGN OF ELECTRIC POTENTIAL ? I can't understand it
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Regarding Faraday's law as written in Pozar's Microwave Engineering

We can clearly see in page $6$ of Pozar's Microwave Engineering Faraday's law written under this form $$\nabla \times \bar{\mathcal{E}}= \frac{\partial \bar{\mathcal{B}}}{\partial t}-\bar{\mathcal{...
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393 views

Finding equation of motion of Lagrangian density: What does the location of the indices mean?

We are given the following Lagrangian density: $$\mathcal{L}=F_{\mu \nu} A^{\mu} \mathcal{J}^{\nu}$$ where $F_{\mu \nu}$ is the electromagnetic field tensor, $ A^{\mu}$ the 4-vector of the vector ...
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Induced emf expression in Faraday's law of induction

Magnitude of induced EMF in a loop is given as $|\frac{d\phi}{dt}|$. If the loop has n turns then induced EMF is given as $|n\frac{d\phi}{dt}|$. As we can consider each turn to be a battery which is ...
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1answer
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Difference between heat emited from resistor and power on the resistor emited to heat

Consider a simple electric circuit. I just not understand what is the difference between heat emited from resistor and power that is on the resistor that emited to heat why one is $p=I^2R$ and the ...
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1answer
208 views

Eddy current brake with superconductor

There are eddy current brakes used with metal plates to stop trains. It converts kinetic energy of the metal plate to heat via eddy current on it. So what if we have a superconducting metal plate and ...
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Electric motor magnetic moment

Suppose I know that an electric motor winding produces 200 Nm torque in 0.2 T magnetic induction field. Is it possible to calculate the rotational magnetic moment produced by that motor winding just ...
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Electric field due to a general charge distribution with finite charge density

Can it be proven that for a body having finite charge density at every point Electric field is finite at every point, especially in its interior and on its outer surface?If yes please provide an ...

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