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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9 views

Lorentzian nature of reflected microwave power in Electron Spin Resonance

Reflected microwave power has Lorentzian shape when plotted against frequency in ESR. How to mathematically prove this?
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1answer
40 views

Derivation of Biot Savart law for a curve

I'm ok with the following expression for Biot-Savart: $$ {\mathbf B}({\mathbf r}) = \frac{{\mu _0}}{{4\pi }}\int\frac{{{\mathbf J}({\mathbf r'}) \times {(\mathbf{r-r'})}}}{{|\mathbf{r-r'}|^3 }}dV'. $$ ...
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17 views

Amper`s Law and a flat [closed]

Here the question is written in photo.I think that this E should be integrated then should be written in the equation. I can`t find any easier way. Let me know if you see any.
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1answer
30 views

Amper`s Law on a ring [closed]

In this question i am having a bit trouble with understanding? And answer does not match totally. My question is that firstly there is no closed loop why is Amper`s Law holding here and second ...
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0answers
32 views

Why do metal filings line up in parallel (non-curved) lines on a knife blade?

My girlfriend works at a knife factory and was grinding a new knife model today made of a very hard, magnetic steel. She noticed that the metal filings were lining up on the knife blade in ...
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2answers
220 views

Is the photon truly not absorbed in Raman scattering?

In reading about Raman Scattering, I was thinking while reading it "okay, incident photo absorbed by molecule, molecule goes to higher energy vibrational state, molecule re-emits photon with either ...
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1answer
31 views

What's the difference between electromagnets and induced magnetism?

I am studying over the basic introductory concepts of magnetism, and I cannot comprehend definition of the following words below: Electromagnets- objects that are magnetic only when electric current ...
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1answer
51 views

Coulomb law and photons

When we consider process like $e^- e^- \to e^- e^-$ in QED, we see that from exchanges of one photon (tree-level diagrams) one can obtain Coulomb's law, while loop-diagrams give quantum corrections ...
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7answers
251 views

Is the electric field of a volume charge distribution well defined?

Consider a volume charge distribution $\rho({\bf r'})$. The electric field at ${\bf r}$ is $${\bf E}({\bf r})=\iiint\frac{1}{4\pi\epsilon_0}\frac{\rho({\bf r'})}{R^2}\hat{\bf R}\, \mathrm d^3{\bf ...
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35 views

What is the difference between length and velocity gauge when it comes to a dipole approximation?

Lets say we have plane wave with $\vec E$ perpendicular to $\vec k$. The dipole term will come from $\vec A\cdot \vec p$. Is the electric field longitudinal in the length gauge for the dipole ...
2
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1answer
173 views

The strength and control over the strength of electromagnets, correlation between the strength and weight of the magnets

I am currently working on a project that aims to use the attraction & repulsion between two electromagnets as an alternative to weights, from a physiotherapy and rehabilitation perspective to help ...
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2answers
25 views

gradient strength units in MRI

This may not be the appropriate forum for this but seemed to be the closest. I am trying to understand some concepts around MRI physics and it is common to use external magnetic fields created using ...
3
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1answer
90 views

How to solve “EM wave equation” for the field of uniformly moving charge?

Is it possible to show that the field of a uniformly moving charge, which is according to Biot-Savart law is given by... $${\bf E}({\bf r},t)=kq\left(\frac{1-v^2/c^2}{(1-v^2 \sin^2 ...
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2answers
51 views

Can a grounded spherical shell have a charge?

Can a grounded spherical shell lying inside another bigger non-grounded shell have charge on it? If yes then why? Won't the charge on the grounded conductor flow to the ground?
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0answers
25 views

How to solve the following wave equation? [closed]

We have paraxial beam propagating in a uniaxial anisotropic media. When such is the case wave can be divided into two orthogonal parts: Ordinary wave beam Extraordinary wave beam. The ordinary ...
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1answer
21 views

What would be the suggested material for blocking EM waves of a 600V 160kW 5kHz coil?

I am building up a reactor for heating material inside, but I need its EM waves not to interfere with the rest of my near circuitry. Its working conditions are: 600 VAC 300 A 160kW 5kHz What ...
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1answer
57 views

Effect of magnetic field on stationary charge

Consider a long infinite current carrying wire in N-S direction. Consider a charge +q at a distance of r from wire. Clearly, Lorentz force on charge is 0. But, their is a relative motion between ...
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4answers
555 views

Dipole in a spherical cavity in an infinite dielectric

In this example the assumption is made that inside a spherical cavity (with an infinite dielectric) with a dipole placed at its centre, the potential takes the following form: $$\phi_{in}= \frac{p ...
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0answers
18 views

Magnetic field in the centre of a square loop [closed]

I've a problem with the Biot-Savart's law. In particular i'd like to know if I could get the magnetic field in the centre of a square loop without integrating. Could you help me? Thankyou so much!
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1answer
673 views

Homopolar motor and Lorentz force

My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a ...
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0answers
29 views

Electron travel speed in spinning object [closed]

My question is in the hypothetical circumstance that you could spin a disk or something close to the speed of light how would that effect the travel of electrons through the material. Picture a circle ...
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2answers
100 views

What is the inconsistency between Maxwell's electrodynamics and newtonian mechanics?

As far as I understand, when a modification of a theory is made it is because some observation required this modifcation. Quantum Mechanics is a nice example of that: observations of microscopic ...
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0answers
33 views

Amplitude of unpolarized EM wave after a polarizer

The amplitude of a linearly polarized wave after a polarizer is $$E_1 = E_0\cos(\theta)$$ and the intensity is $$I_1 = I_0\cos^2(\theta)$$ Now for unpolarized light, the time averaged amplitude is ...
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1answer
308 views

What do I see if I move quickly past a charge surrounded by iron filings?

This might be a straightforward exercise, in which case I apologize. Suppose I surround a charge by iron filings initially oriented in some fixed direction, and I then move past the charge at an ...
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1answer
133 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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2answers
864 views

Do intergalactic magnetic fields imply an Open Universe?

According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty ...
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12 views

Derivation of Poynting theorem in matter

In most textbooks I have read they derive the Poynting theorem using the Maxwell's Equation in vacuum and the fact that the force density $f=\pmb{E} \cdot \pmb{J}$. Then they just generalize it ...
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2answers
82 views

What does the Poynting flux represent?

I know that the Poynting flux is the cross product between $\vec E$ fields and $\vec B$ fields, but I'm wondering, are the field lines of the Poynting flux considered as electromagnetic energy? Let's ...
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2answers
38 views

An experiment related to Faraday's laws

This question is out of an Indian competition. I do not know calculus yet. I have some idea about how magnetic fields and electric fields work together but i don't know how to go about thinking ...
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1answer
160 views

Find magnetic field in a solenoid for different regions?

A steady current I flows along an infinitely long hollow cylindrical conductor of radius R. This cylinder is placed coaxially inside an infinite solenoid of radius 2R. The solenoid has n turns per ...
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1answer
22 views

Are deflection of compass and speed of electron relative to each other?

If one holds an electron in one hand and a compass in the other hand, and the starts running at the speed of light holding those, then will there be any deflection of the compass ?
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34 views

Calculating magnetic field strength in gap of tapered toroidal electromagnet

Consider a toroidal electromagnet filled with a magnetic material of large permeability $\mu$. The torus contains a small vacuum gap of length $h$. Over most of its length the torus has a circular ...
3
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1answer
170 views

Hidden momentum

I'm trying to learn about hidden momentum. After reading what I could find with a google search, I understand that it is equal to the momentum carried by radiation, calculated with the Poynting ...
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1answer
186 views

Can the physical properties of the EM field be described directly from the 4-gauge potential?

I'm trying to make an argument that classically, the EM field is considered a more 'real' physical quantity than the potentials, and am tempted to say that the fact that the field carries energy & ...
3
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1answer
37 views

Gauss law and electrostatic induction. Concentric spheres

Imagine you have hollow concentric spheres A and B with radius a and b (b>a), respectively. If A is a conductor and B has a certain density charge, I´ve been taught that B will induce some net charge ...
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1answer
29 views

How does sunlight undergo interference as shown in the video?

As far as I know, for interference to happen in a double-slit experiment, the light source should be coherent and monochromatic. If that is the case then how come sunlight undergo interference and ...
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1answer
24 views

Motion of an electric charge within an electric field with two electric charges

Assume that air resistance and gravity are negligible and the only significant force acting in the scenario is the electric force. There are two electric charges, both with an equal and positive ...
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2answers
35 views

Find the radius of the path of an electron in a magnetic field [closed]

In a question I am asked to find the radius of the circular path that an electron follows in a magnetic field. The field strength is $B=10^{-5}T$ along the positive y axis, the velocity of the ...
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1answer
42 views

Why do electric motors stop working when physically stopped (burned up)? [closed]

I found a rock lodged between the fan and the radiator on my motorcycle. Now the fan does not work when the computer prompts it to. I tried directly attaching the 12 volt battery to it as well. No ...
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1answer
21 views

How, if at all, does the behavior of a parallel plate capacitor change in the presence of a magnetic field?

I'm an AP physics student curious about magnetism and my teacher couldn't answer this question. We already learned about velocity selectors, so I know that, when the plates of the capacitor are in the ...
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3answers
72 views

Why don't E&M fields change orientation after hitting a surface?

In essentially every derivation of the Fresnel equations, the general problem of radiation hitting a surface at a certain angle is broken into two parts (out of which we hope the solution any general ...
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1answer
50 views

Questions about Magnetic field [duplicate]

I have two questions about Magnetic Field. Given a particle with mass m, charge q and uniform B field. B is going into the page, and particle is moving to the right Like this. The textbook says ...
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2answers
165 views

How can the thrust due to radiation pressure be amplified in photonic laser thruster?

The thrust is amplified due to repeated bouncing of photons between two mirrors as shown in the diagram in this: Why does repeated bouncing of photons produce amplified thrust when the answer in ...
3
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0answers
41 views

Question about the fields in a solenoid

I am very familiar with using Ampere's Law to find the B field inside a solenoid ($\mu_0nI(t)$). Then I can use Faradays to find the E field inside ($\propto\dot I$). I want to get this result ...
3
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1answer
203 views

How can Drift Velocity of an electron causes change?

Assume we have a conductor that has current flowing and its placed in a magnetic field so that it experiences the Lorentz force, and it gains Kinetic energy due to that force. Would drift velocity ...
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2answers
51 views

Is magnetic field of a uniformly moving charge constant everywhere?

As far as I understand, for the field of a uniformly moving charge, curl of $\mathbf E$ is zero everywhere. Since $\nabla \times \mathbf E = -\dfrac{\partial\mathbf B}{\partial t}$, magnetic field ...
3
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3answers
132 views

At an instant, does a system of gravitational charges exhibit equivalent behavior to a time-reversed system of like electric charges?

Question: In principle, does a system of gravitational charges exhibit equivalent behavior to a time-reversed system of like electric charges? (At a single instance in time?) Additional Notes: I am ...
2
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1answer
64 views

Why doesn't the changing field of a moving charge create EM waves? [duplicate]

EM wave phenomenon is usually described as "changing electric field creates changing magnetic field which creates a changing electric field etc." But fields of a uniformly moving (not accelerating) ...
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0answers
13 views

Easiest way to levitate a ferromagnetic material that is not a permanent magnet?

I want to levitate an iron cube that is not a polarized permanent magnet. What is the easiest way to do this?