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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Where is the amplitude of electromagnetic waves in the equation of energy of e/m waves? [duplicate]

Does the amplitude of the photon oscillations always stay constant and if it is not - what are the physical differences between the photon with higher amplitude in comparison to the one with the less ...
2
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1answer
135 views

Could we really charge metal plates using microwaves?

While skimming through Dielectric heating, I read that they use microwaves to charge the plates. How do they do that?
4
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1answer
141 views

Non reciprocal light propagation

In search for some explanation in why magneto-optical materials (like the one used in the Faraday rotator and, consequently, in the "optical diode") act in such a "strange" way, I saw that this kind ...
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3answers
289 views

Induced current using a reference system bound with a moving charge

Suppose we have a charge moving at velocity $\mathbf{v}$ in the same plane of a square wire. If I sit in a reference frame where the square wire is still, since the charge is moving with velocity ...
4
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3answers
311 views

What is the missing proportionality constant in the magnetic levitation formula?

The formula for magnetic levitation is $$ B \frac{dB}{dz} = \frac{ \rho g }{\chi} $$ but as always, I have a hard time figuring the units in SI. The left hand side is $\mathrm{T^2 /m}$, while $\chi$ ...
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3answers
2k views

Right hand rule to measure direction of electric field [closed]

The diagram: a positive plate is on the left of the page and a negative plate is on the right. The velocity of the particle is going down the page. So, with a right hand rule, I would point my thumb ...
2
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1answer
93 views

Circuit Loop Law Doubt

In a circuit with a solenoid/inductor and a resistor and a battery . Books say that $\Sigma \Delta V=0$ around a closed loop . That means work done by electrostatic field per unit charge is $0$ ...
2
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3answers
494 views

What does physics study? [closed]

Wikipedia definition: Physics (from Ancient Greek: φύσις physis "nature") is a natural science that involves the study of matter[1] and its motion through spacetime, along with related concepts such ...
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1answer
316 views

How to add a potential term to the Dirac Equation?

I've read that if you have a Hamiltonian for the Dirac Equation, you can add a potential term to it simply by adjusting the momentum operator so that $p^\mu \rightarrow p^\mu-A^\mu$, where $A^\mu$ is ...
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0answers
286 views

Finding the terminal velocity of a magnet dropped in a solenoid

We have to find proportionality of the terminal velocity with the factors of the system: Plot: a small dipole(mass $m$) with dipole moment $\mu$ is dropped in a long solenoid (radius $r$, ...
2
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2answers
86 views

Gauss's Law understanding

In the case of a point charge $q$ at the origin, the flux of $\vec{E}$ through a sphere of radius r is, \begin{equation} \oint \vec{E}\cdot d\vec{a} = \int \frac{1}{4 \pi \epsilon_0 ...
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5answers
2k views

How can there be net linear momentum in a static electromagnetic field (not propagating)?

I understand from basic conservation of energy and momentum considerations, it is clear in classical electrodynamics that the fields should be able to have energy and momentum. This leads to the usual ...
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1answer
650 views

Superconducting diamagnetic sphere in uniform magnetic field

What is the size of the magnetic dipole moment $\vec m$ of a superconducting diamagnetic sphere $radius=R$ in a uniform magnetic field $\vec B_0$? Since there is no free current, we can solve for ...
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1answer
99 views

Gauss's Law - Electric Field outside a Shell?

I'm somewhat confused as to why the electric field outside a spherical shell is $\frac{Q}{4\pi r^{2}\epsilon_{0}}$ Going through the work: $$ Q = 4 \pi r^{2} \sigma ; \frac{4\pi ...
3
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1answer
295 views

Electromagnetic black hole?

So I was thinking about something for the past while Consider a large spherical foam-ball with homogeneous density. Where a foam ball is defined as an object that can absorb matter with 0 friction ...
2
votes
1answer
847 views

Magnetic force on a magnetic dipole

Can someone please help me understand and answer the question of "why are the forces on a magnetic dipole different in a uniform magnetic field and non-uniform magnetic field?" I know in a uniform ...
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3answers
404 views

Magnetic B Field of Point Charge Not at Constant Velocity

I'm working on an N-body simulator for charged particles. I know that moving charged particles generate a magnetic field, and another moving charged particle could be effected by this magnetic field. ...
3
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1answer
155 views

Magnetic field due to a conducting sphere in unpolarized light

From Jackson, problem 10.3: A solid uniform sphere of radius $R$ and conductivity $\sigma$ acts as a scatterer of a plane-wave beam of unpolarized radiation of frequency $\omega$, with $\omega R ...
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2answers
155 views

Tensor fields on space-time

I'm reading landau electrodynamics . For the investigation of the dynamics of a charged particle in an electromagnetic field , the author introduces a four vector potential field on spacetime . The ...
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0answers
114 views

How sharp is the direction of magnetic field of a cone magnet?

I heard recently that cone magnets can be used to focus and direct the field sharply at one particular point in a particular direction. I jus want to kno how accurate and sharp this field is? And if ...
2
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3answers
704 views

continuity of the electric potential due to a surface charge

The Electric potential due to a charge distribution on a surface is : $\Phi \left ( x \right )=\int \frac{\sigma \left ( {x^{}}' \right )dx{}'}{\left \| x-x{}' \right \|}da$ I want to show that it's ...
2
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1answer
262 views

Transformators (primary and secondary windings)

Consider the following problem: Robby wants to put a light in the shed so he puts a cable between his house and the shed. In the cable there are 2 wires with a combined resistance of $0,92 \space ...
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3answers
201 views

Violation of Newton's 3rd law

I'm just expressing my guess. Let two particles A and B experiences forces $F_1$ and -$F_2$ between them and let guess also there are two observer, one is stationary and other is moving with ...
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2answers
788 views

Eddy currents: Explanation of the force slowing down a moving metal plate

Suppose a solid metal plate moves trough a homogenous magnetic field $\vec{B}$ as shown in the following image. You will see that the plate will slow down (as long as not the entire plate is in the ...
3
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2answers
69 views

what sort of set up would i need to create an electro magnetic chair with no legs

I am a furniture design student therefore please keep it simple. a system strong enough to hold the avergae male of say 90 kg is this possible even....
0
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1answer
76 views

Doubt on the analysis of $RC$ circuit

I'm studying $RC$ circuits and I'm in doubt on how to deal with one kind of situation. Well, first when we have simply a circuit with just one resistor and one capacitor in series, or a circuit that ...
2
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1answer
149 views

Magnetic Force on a Loop and number of turns

Suppose we have a rectangular current-carrying loop with current $i$, then we know that the magnetic force on each side can be found as: $$F=iL\times B$$ Where $L$ is the vector in the direction of ...
3
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1answer
79 views

What is a $TEM_{900}$ cavity radiation mode?

I am reading a paper by Serge Haroche stating the cavity they use sustains a Gaussian mode of the e.m. field called $TEM_{900}$. I understand what Gaussian means. I found this explaining what TEM ...
2
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2answers
67 views

What really is the Magnetic Force on a wire?

I have a doubt regarding the significance of a force on a wire. Well, first of all, I know that if I have a particle and if there are several forces acting over it, then we can compute one total force ...
0
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1answer
608 views

Doubt in problem regarding resistance a of cylinder

I've been working on some exercises and I'm in doubt if my procedure with this one is correct. We have a hollow cylinder with internal radius $r_a$, external radius $r_b$, resistivity $\rho$ and ...
2
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1answer
9k views

What is the difference between a Rodin coil and a Rodin starship?

I've seen various designs for Rodin coil and a 'Rodin starship'? Are these just regular electromagnets? Or something different? How do they differ from regular electromagnets?
5
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3answers
408 views

Negative Mass and gravitation

Since Newtonian gravity is analogous to electrostatics shouldn't there be something called negative mass? Also, a moving charge generates electric field, but why doesn't a moving mass generate some ...
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2answers
171 views

Free spin (Curie) Paramagnetism

I'm working through a derivation for Curie paramagnetism and hope someone could help clarify a couple of steps. The way that makes sense to me (although now I have seen the wikipedia derivation below ...
5
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2answers
261 views

Why isn't data lost when sent over large distances?

I was thinking about how information is sent, for example through the atmosphere. There are plenty of obstacles, as well diffraction, etc. Still, no information is lost. How is information sent to ...
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0answers
241 views

Easy question about magnetism?

I have to build a simple electric motor by attaching a magnet to a battery, extending the terminals of the battery (with stiff wires so they could act as supports), and placing a coil of wire on top ...
7
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3answers
4k views

What is the cause of the normal force? [duplicate]

I've been wondering, what causes the normal force to exist? In class the teacher never actually explains it, he just says "It has to be there because something has to counter gravity." While I ...
3
votes
3answers
257 views

Energy conserved… or not? Confused!

I am confused. Could someone kindly explain what's going on in this question? A particle of mass $m$ and charge $e$ moves in the $x,y-$ plane. There is a constant magnetic field $B$ that points in ...
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2answers
250 views

Ferromagnetism with mobile spins

How can electron spins in Iron at room temperature have ferromagnetic order even though they are travelling at very high speeds? One could argue that spin and motion are completely uncorrelated and ...
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2answers
5k views

Derivation of the Biot-Savart Law [duplicate]

Can someone provide a derivation of the Biot-Savart law for electromagnetic induction? To be clear, $$ d\vec{B}~=~\frac{\mu_0}{4\pi}\frac{I d\vec{\ell}\times \vec{r}}{r^3}. $$ Is there a simple way ...
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vote
1answer
245 views

Capacitance of a conducting disk

I'm reading this(PDF) derivation of the capacitance of a thin conducting disk. The surface charge density of such a disk can be shown to be: $\sigma(r) = \frac{Q}{4\pi a\sqrt{a^2 - r^2}}$ (in ...
2
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1answer
113 views

Trying to understand EM wave and photon

When electrical fields and magnetic fields couple together, it forms electromagnetic waves. And we can "quantized" it and see each "package" of it as photon. So can electrical fields and magnetic ...
2
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1answer
98 views

Falling through the ground [duplicate]

I do not know much about physics but I know that according to Newtons third law of motion when we walk we are pushing the ground down but the ground is pushing us up. What force is making the ground ...
3
votes
2answers
3k views

Electric field and electric potential of a point charge in 2D and 1D

in 3D, electric field of a piont charge is inversely proportional to the square of distance while the potential is inversely proportional to distance. We can derive it from Coulomb's law. however, I ...
0
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0answers
100 views

Is this picture of the electron dipole moment correct?

It's the electron spinning on its axis, and a magnetic moment shoots up. The direction of the North-South is opposite what I thought it should be. Why is the vector arrow pointing from North to ...
3
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1answer
3k views

Charge inside conductor

I know that the E field inside a conductor is zero. What happens if I put a source of charge inside the conductor? Say the conductor was spherical centered on the origin and there exists a charge at ...
4
votes
1answer
2k views

What are the limits of applicability of Coulomb's Law?

Coulomb's law is formally parallel to Newton's Law of Universal Gravitation, which is known to give way to General Relativity for very large masses. Does Coulomb's Law have any similar limits of ...
4
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4answers
6k views

Displacement Current - How to think of it?

What is a good way to think of the displacement current? Maxwell imagined it as being movements in the aether, small changed of electric field producing magnetic field. I don't even understand that ...
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2answers
438 views

Do black holes have charges?

Do black holes have charges? If so, how would they be measured? Also, does electricity behave the same way? Black holes affect photons, which are carriers of EM radiation, so do black holes have any ...
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2answers
315 views

Simple ohms law on a battery ? Paradox or conceptual error?

Suppose we have a regular pencil battery which supplies DC voltage $V$. Say we take copper wire and connect the ends of the battery to an $R$ ohms resistance. Then Ohm's law tells use the current in ...
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1answer
147 views

Maxwell's Equations-Relativity

How did Maxwell develop the magnetic field without relativity? Was it purely experimental? I don't see how else he would have developed any understanding for the magnetic field.