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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3
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2answers
123 views

Proton and electron joining

If I have a proton and an electron at rest at some distance apart. Will they form an hydrogen atom when released or they will join together? My intuition says it will form H atom. But I cannot ...
-1
votes
0answers
33 views

Maxwells equations at a point

Why is it that whenever a problem is posed for maxwells equations -say the electric field at some area- that it is only requested for a fixed/given point? It would seem like you would want the field ...
1
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0answers
28 views

Self propagating EM waves mechanism of propagation

Electric fields generate magnetic fields and visa/versa,and this is attributed to be how a light ray self propagates. Can anyone further clarify how exactly light rays self propagate? If the mechanism ...
4
votes
1answer
97 views

Lagrangian density for Lorentz force of continuous charge distribution in external field?

It's frequently an exercise to derive the Lorentz force law for a particle with charge $q$ in an external electromagnetic field given by the following Lagrangian: $$L = -mc^2\sqrt{1-\frac{\dot{r}^2}{...
1
vote
1answer
76 views

Magnetic monopoles in field theory

In standard QED, we couple the electron to electromagnetism by replacing $$\partial_\mu \to \partial_\mu + i e A_\mu.$$ Upon taking the classical limit, we find that this gives electrons an electric ...
1
vote
1answer
63 views

Force on a current carrying wire of arbitrary shape in a magnetic field

To obtain the net force by an external magnetic field on a current carrying wire, we divide the wire into small sections of infinitesimal lengths. The force due to the magnetic field on a given ...
33
votes
4answers
3k views

What is the explanation of the non-existence of magnetic monopoles? [closed]

Electric monopoles do exist, but why cannot magnetic monopoles exist? I know that magnets have two different poles, and when we cut them, both pieces will have again two different poles. But why does ...
1
vote
1answer
26 views

'Printing' magnetic polarity into a material

There is a company out there that 'prints' magnetic pixels, a.k.a. maxels, into a material. In doing so, they're able to choose the distance of effect of magnetic fields they create, independently of ...
1
vote
0answers
30 views

Can accelerated charged particle gain energy from charged medium? [closed]

Suppose a charged particle is moving in a medium. If it is accelerated, it is well known that it radiates energy (even in the vacuum). I was wondering if the charged particle would accelerate through ...
3
votes
3answers
81 views

when current passes through a wire , does it only travel through the outer surface?

In electrostatics we are taught that the field inside a conductor is always zero . So when current passes through a wire , the electricity only passes through the surface of the conductor . In ...
1
vote
1answer
32 views

differences of Aurora Borealis (North) vs. Aurora Australis (South)

I was wondering about the differences between the Northern vs. Southern lights: will one pole collect the positive charges, and the other the negatives? I know that the solar wind is composed mainly ...
2
votes
0answers
32 views

How can neutral conductive fluids create a magnetic field? [duplicate]

It's a standard textbook answer that currents of molten iron inside the Earth create the Earth's magnetic field. Matter, on the supermolecular scale, is electrically neutral. So any magnetic field ...
0
votes
0answers
23 views

How does the velocity of a magnet falling through a solenoid affect the induced voltage?

Given Faraday's Law, $\mathcal{E}=-N \frac{\partial \phi}{\partial t}$, we can induce an EMF by dropping a magnet through a coil of wire. How would the velocity of this magnet relate to the induced ...
0
votes
0answers
41 views

Deriving an equation in Maxwell's “a treatise on electricity and magnetism” [duplicate]

I am reading Maxwell's "a treatise on electricity and magnetism" and I need a derivation of formula 16 $\left(M=\iint\dfrac{\cos\varepsilon}{r}\mathrm ds~\mathrm ds'\right)$ (in the page below) using ...
0
votes
0answers
14 views

How to find the polarization (phase) of the virtual source if image theory is used

Consider a radiating element (actual source) is placed above a ground plane, and if we apply image theory to determine the E-filed, the virtual source will be placed below the ground plane and ground ...
0
votes
0answers
34 views

Magnetic field inside a solenoid

I'm trying to understand how to calculate the magnetic field inside a solenoid. However I don't understand why we choose to take a rectangular loop with one side inside the solenoid and we don't take ...
0
votes
1answer
35 views

Why does the primary coil get very warm with DC supply but not with ac supply?

It's about an investigation carried out on the transformer effect . The primary coil of 20 turns and the secondary coil of 50 turns is wound around a laminated iron core.
0
votes
1answer
46 views

Birefringent filter, optical path length difference?

In 'The Light Fantastic' by Kenyon, I.R. (p424), it is said that for a birefringent material inclined at Brewster's angle and who's optical axis lies in the plane of the plate, we have an optical path ...
1
vote
1answer
29 views

Verify Faraday's law of induction [closed]

I need to design a procedure for an experiment to verify Faraday's law of induction. I'm given: A Helmholtz coil of radius 15 cm Three plastic spheres of radii 1 cm, 1.5 cm, and 2 cm. A spool of ...
5
votes
3answers
107 views

Energy of an accelerated charge?

Thought experiment: Let's take a positively charged particle and place it in a vacuum with no electric field. It just sits there. Now, we instantaneously introduce an electric field. The moment ...
-4
votes
0answers
50 views

Books about electomagnetism and quantum mechanics [duplicate]

suggest me some books about electromagnetism. anchor tags:everything about ac,dc circuits,magnetic circuits,semiconductors etc things i need to know through my college years(i am currently doing my ...
0
votes
0answers
33 views

Calculation of Phase Delay of a Curved Planar Dielectric Waveguide (i.e., bend)

I am working on a very phase sensitive planar dielectric waveguide device and I need to calculate the phase delay induced by a bend of some radius, $r$ and of $\phi$ radians for a give planar ...
1
vote
0answers
31 views

Effect of shielding on a miniature Helmholtz coil

So a bit of background... I'm a university graduate student with primarily digital signal processing and embedded experience. However I've managed to get myself involved with a project that requires ...
0
votes
1answer
27 views

Does the sign of imaginary part of complex permittivity have any physical meaning?

I have noticed some papers having written complex permittivity as $e' + je'' $ and others as $e' - je''$. The data in literature does not specify the sign. What should I use and does the sign of $e''$ ...
0
votes
2answers
53 views

Electric field around a moving charge

A moving charge has magnetic field around it. Is it just increased electric field in relativistic frame? If yes, does a moving charge has more electric field around it?
0
votes
1answer
86 views

Wave operator applied to electromagnetic field tensor

I'm trying to understand an argument in "An introduction to general relativity" by Hughston and Todd (p37). Let $F_{ab}$ be the electromagnetic field tensor, I'm trying to show: $$\Box F_{ab} = -4 \...
0
votes
0answers
20 views

Parallel wires in a changing non-uniform magnetic field?

Given this diagram: Where four copper wires are parallel to one another, connected to an exterior circuit(not shown) placed in a non-uniform magnetic field $B$ that is changing over time. Due to $...
1
vote
0answers
35 views

Amplifying an electromagnet's generated field

Given an electromagnet like this: which is essentially an iron casing with coils inside: Is it right that the centre disc would have one polarity whilst the outer housing another? To boost the ...
0
votes
0answers
8 views

Rotating magnetic field dencity

How to calculate the magnetic field density (Tesla) in a rotating magnetic field produced by 3 sets of coils at 120 degree angles.
0
votes
1answer
40 views

What happens when a magnet is dropped through a solenoid with a current running through it?

I know when a magnet is dropped through a copper tube or a copper solenoid it induces a current in the copper and the magnet actually slows down as it falls through the tube, but if I did this with a ...
1
vote
1answer
49 views

Are neutrinos affected by electromagnetic forces?

I know they don't interact with matter nor any electromagnetic forces, but I know they have mass, very little mass but they have. When something has mass it must be attracted by gravity and other ...
0
votes
2answers
36 views

Why does electromagnetic induction creates both voltage and current but static electricity only creates voltage and no current?

Why is it that electrons do not move the same way in triboelectric as in magnetic induction when both creates voltage? Triboelectricity can even create tens of thousands of volts like Van De Graf ...
0
votes
0answers
48 views

Sum rule and dense limit

Is the finite size of lattice constant important for derivation of sum rules, for example, the sum rule for diagonal conductivity $\sigma_{xx}$: $$ \int \limits_{-\infty}^{\infty} d\omega\sigma_{xx}(\...
1
vote
0answers
74 views

World's Largest Radio Telesope : How accurately does it need to be figured?

In Guizhou Province, China, the worlds largest radio telescope is almost complete, measuring 500 metres across. I am aware that todays largest optical telescope require very accurate figuring and ...
-4
votes
0answers
39 views

Is the magnetic field caused by the lenght contraction, as seen by us, in the frame of a fast moving charged particle? [duplicate]

If for example an electron moves very fast relative to us, then the length contraction as seen by us, causes the electric field lines to compress themselves in the direction of motion. Is this the ...
4
votes
1answer
69 views

Schrödinger-Pauli Equation Solutions

The Schrödinger-Pauli equation is the non-relativistic limit of the Dirac equation, and therefore describes spin-1/2 particles in an external electromagnetic field. It is given by: $$\left[\frac{1}{...
1
vote
0answers
18 views

Variation of spin in the rest frame of an electron in an external magnetic field

In Jackson's Classical Electrodynamics, the equation of motion for the angular momentum $\textbf{s}$ of an electron in an external magnetic field is given by (eq. 11.101) $$\left(\frac{d\textbf{s}}{dt}...
0
votes
1answer
30 views

If $T_{kl}=\epsilon_{kil}m_i$, how to show $m_i=0.5\epsilon_{ilk}T_{lk}$? [closed]

In a book I am reading (about magnetic dipole), it is given that $T_{kl}=\epsilon_{kil}m_i$. Then, it says since $T_{kl}=-T_{lk}$, it can be shown that $m_i=0.5\epsilon_{ilk}T_{lk}$. I understand that ...
0
votes
1answer
31 views

Calculating the B flux density of a flat spiral coil with N turns [closed]

Consider the following topology for a flat spiral coil (with an air gap in the middle). It's been a very long time since I have used Maxwell's Equations and now I find myself trying to figure out the $...
1
vote
0answers
22 views

Magnetic field between 4 wires with the same current

if I have 4 long wires with a current I and all of them have the same distance from each other (looked at from the front you'd have a square) shouldn't the magnetic field vanish in the middle of all 4 ...
1
vote
1answer
40 views

Rewriting Maxwell's equation in tensor form [closed]

Suppose $F_{ij}=\epsilon_{ijk}B_k $, how to prove the following: $\partial_iB_i=0$ becomes $\partial_iF_{jk}+\partial_jF_{ki}+\partial_kF_{ij}=0$ $B_iB_i$ becomes $F_{ij}F_{ij}/2$ I can see that ...
2
votes
1answer
35 views

Is it possible that propagation of acustic waves leads to emission of radiation?

Question: Consider large cloud of gas. Assume it is electrically neutral (but as always, matter is composed of smaller things which are actually charged). Is it possible that propagation of sound ...
0
votes
1answer
23 views

Induced current in a rectangular loop [closed]

The question is How much heat is generated in the resistor during the loop's entire path into and out of the magnetic field? Give your answer in J. I used Faraday's law and managed to get the ...
0
votes
1answer
43 views

Correct Definition of Angular Momentum of a Charged Particle in an Electromagnetic Field? (Classical Mechanics) [duplicate]

What is the more correct definition of angular momentum $\vec{\mathbf{M}}$ in three dimensions? (I.e. classically/Lagrangian/Hamiltonian, not necessarily quantum or relativistic) $$\vec{\mathbf{M}}...
3
votes
1answer
76 views

Can the Biot-Savart law be derived from QED?

It's important that a new theory of physics contains the equations and results of previous related ones. Maxwell theory and QED both have explanations for electromagnetic phenomena so I'm wondering if ...
1
vote
1answer
42 views

A Formula for the Phase Difference Between the Electric and Magnetic Wave Oscillations?

A) Is there a formula for the phase difference between the electric and magnetic field oscillations, in vacuum, in an electromagnetic wave emitted from an antenna, as a function of the frequency the ...
0
votes
0answers
27 views

Does a cathode ray tube emits electromagnetic waves from a steady DC electron beam?

In cathode ray tube, heated cathode emits stream of electrons but when I wrap a coil around tube a current will be inducted in that coil? after all electron beam is a steady DC current? so no emf?
-3
votes
0answers
41 views

Can someone explain me magnetic potential? [duplicate]

I don't understand why we can write the magnetic field as a potential vector A, and what exactly this potential is. Is it just a mathematical thing? And how can it help me calculate the magnetic ...
6
votes
1answer
199 views

Why is U(1) the gauge group in classical electromagnetism?

Can anyone give a physical reason that $U(1)$ is the gauge group for classical electromagnetism? I am familiar with the principal bundle formalism for Yang-Mills theory and see that since the Lie ...
2
votes
2answers
24 views

Should static electricity affect a Crookes radiometer?

I've been playing around with a Crookes radiometer for fun and I happened across something I can't figure out. I know the fundamentals of the radiometer (e.g. partial vacuum, thermodynamics, etc.), ...