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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1answer
43 views
Reason behind cohesive and adhesive force
What is the real cause behind the cohesive and adhesive forces?
0
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0answers
129 views
What is the current induced in a straight wire when a magnetic field is turned from on to off?
Consider a straight wire which runs along the y-axis. A B-field exists which is uniform and only in the x-direction:
$$\vec{B}=(B(t),0,0)$$
The B-field at time $t_0$ is 'switched off' in ...
1
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1answer
32 views
Summing up magnetic fields
In the case of a complex shape filiform distribution of current, are we allowed to determine the magnetic field created by sections of the distribution and then summing them up, like we do with a ...
1
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1answer
71 views
Should the electric potential of a positively charged sphere be negative?
Because the indefinite integral of the electric field results in a negative value? (As the function is proportional to $r^{-2}$?
I've got to be missing something... Help please!! Thanks!
Also, I ...
5
votes
0answers
112 views
Local explanation of the Aharonov-Bohm effect in terms of force fields
Here is an interesting paper for the Physics SE community: On the role of potentials in the Aharonov-Bohm effect, Lev Vaidman, published in PHYSICAL REVIEW A 86, 040101(R) (2012).
You should check it ...
1
vote
2answers
102 views
Potential Inside Conducting Cube
A cubical box with sides of length L consists of six metal plates. Five sides of the box { the plates at $x=0, x=L, y=0, y=L, z=0$ - are grounded. The top of the box (at z = L) is made
of a separate ...
1
vote
5answers
88 views
Is there a more scientific term for “obstruction of EM waves”?
When EM waves pass through things like rain and hail, they can be "obstructed" and bounced back or absorbed, rather than passing through.
I'm conducting an experiment on this effect, and wondered if ...
0
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2answers
359 views
What is the force between two perpendicular wire? [closed]
What is the force between two perpendicular wire carrying current, one to the north and one to the east?
2
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1answer
417 views
Magnetic Fields created by Current-Carrying Wires
The drawing shows two perpendicular, long, straight wires, both of which lie in the plane of the paper. The current in each of the wires is I = 5.6 A. What is the force and torque acting on each wire ...
0
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0answers
45 views
Should a 1D Guassian wave oscillate?
I wrote a few lines that numerically solve Maxwell's equations.
The result is a moving wave that looks like a single pulse.
This looks strange to me because I expect waves to move in oscillator ...
0
votes
0answers
73 views
Superconductors for sale? [closed]
Does anyone know where I can purchase superconductors for a school presentation? I don't need a whole kit, just the disk itself. Thanks.
2
votes
1answer
106 views
Aharonov-Casher effect for charged particles
All the explanations of the Aharonov-Casher effect seem to imply it only "works" for neutral particles with a magnetic moment. This seems to stem from the duality of the A-C effect with the more known ...
0
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0answers
43 views
Object accelerated by electromagnet
If I have an electromagnet (powered by a battery) and a metallic object, the
power generated will depend on the voltage of the battery and the resistance of the coil.
If I accelerate the object, will ...
0
votes
0answers
35 views
Charges and Fields [closed]
Three charged particles lie along the x axis of a Cartesian coordinate system. q1 is located at x = 0,
q2 lies at x and q3 lies at x = r. All three charges are positive, but only q2 is free to move. ...
1
vote
1answer
50 views
Regarding Electromagnetic Plane and Maxwell equations
I asked this on the math.stackechange but I was told that it might be a good idea to ask here too since my problem is physics/math! Here is the question:
Hello everybody I am kind of struggling with ...
0
votes
1answer
105 views
Are normal light waves more dangerous than WiFi radiation? [closed]
I'm getting sort of tired of all the "wireless hysteria", so I thought this might be a nice comment.
To the extent of my knowledge, WiFi uses microwaves, which have a lower frequency and hence carry ...
1
vote
1answer
57 views
Scale-down modeling for radar cross-section measurement
Scale-down models for targets are used for radar cross-section measurements of huge-size objects. Is there any upper limit to the scaling factor on downscale modeling as a $100~\text{m}$ length of ...
0
votes
0answers
67 views
Minimum energy magnetic field has zero current?
Following section 9.2 of Bellan's "Fundamentals of Plasma Physics" suppose we have some domain (I assume simply connected) $D$ with a magnetic field $\mathbf{B}$ inside, but no electrostatic field. ...
1
vote
0answers
46 views
Where Is the mechanical energy flow in Poynting's theorem?
Poynting's theorem is usually interpreted as the statement of the conservation of energy, where the change in mechanical and electromagnetic energy within the volume equals the flow of electromagnetic ...
10
votes
3answers
263 views
Can the path of a charged particle under the influence of a magnetic field be considered piecewise linear?
Ordinarily we consider the path of a charged particle under the influence of a magnetic field to be curved. However, in order for the trajectory of the particle to change, it must emit a photon. ...
1
vote
1answer
91 views
Faraday's Law and magnetic monopoles
The magnetic monopoles does not exist which can be shown by $ \int {\vec{B} \cdot d\vec{A}} = 0 $.
But in Faraday's Law of electromagnetic induction, we clearly show the EMF induced is the time rate ...
1
vote
1answer
152 views
why define relative permittivity and relative permeability?
I am reading something about electomagnetic field and the first introduce the free space permittivity and permeability for the electric field and magnetic field. And later when discussing the field in ...
0
votes
1answer
151 views
Conservation, Maxwell tensor
Can someone please explain to me how the conservation of the energy momentum tensor $$\nabla_\beta T^{\alpha \beta}=0$$
imply the conservation of the Maxwell tensor $$\nabla_\beta F^{\alpha ...
4
votes
0answers
92 views
Is it correct to sum over either index of the metric the same way?
I don't know if the following is correct, i want to compute the following derivative
$$\frac{\partial }{\partial (\partial_{\mu}A_{\nu})}\left(\partial^{\alpha}A^{\beta}\partial_{\alpha}A_{\beta} ...
1
vote
0answers
73 views
Einstein +Maxwell 's tensor
Why is it true that we can deduce that Einstein's GR equations coupled with Maxwell's EM equations may be written in the form $$R_{ij}=C(F_{ik}F_j^{\,\,k}-{1\over 4}g_{ij}F_{mn}F^{mn})$$
without ...
0
votes
1answer
49 views
Ferromagnetism and electrical conductivity
Are ferromagnetic materials good conductors of electricity? If yes, please explain with proper examples.
3
votes
2answers
154 views
Influence of charged particle's own electric field on itself
I read this in my textbook: A charged particle or object is not affected by its own electric field.
Since I find this completely unintuitive and my mind is yelling "wrong! wrong! how could a particle ...
0
votes
0answers
49 views
Calculating attracting magnets?
I have two identical bar magnets...
Each bar magnet can lift an object with 10lb of force.
When they attract each other, I believe that each magnet applies a force of 10lb to attract the other.
That ...
2
votes
2answers
130 views
Should I use Coulombs law when magnets attract/repel?
When magnets attract to each other or repel.
Should I use Coulombs law? If not, why not?
Some would say that I shouldn't because: "Coulomb's law deals with static charges and force due to them. ...
2
votes
1answer
61 views
Absorption cross section for direct dissociation and predissociation - Photoelectric absorption
Why the absorption cross section in direct dissociation process is wide and structureless while the absorption cross section in the predissociation process is structured and containing lines which are ...
0
votes
0answers
79 views
Parallel straight wires [closed]
Two parallel straight wires each have current in the same direction. If I use the force law for a current in a magnetic field, do the wires attract or repel?
Thank you
0
votes
2answers
292 views
Induced emf in AC generator
The induced emf in a coil in AC generator is given as:
E = NABwsin@
Now, when the angle between the normal of plane and magnetic field is zero degrees, the induced emf is zero i.e.
E = NABwsin0 = 0
...
0
votes
0answers
36 views
State equations in magnetic systems
Assume there is an isotropic magnetic material in a homogeneous field $\vec{H}$ inside of a long coil. The work done to the material is given by $\delta A = H dM$, where $M$ is the magnetisation of ...
0
votes
2answers
235 views
conservation of energy in Lenz's law
When we bring opposite poles of two magnets together, they attract each other (or vice versa). Now, we can say that the kinetic energy gained by the magnets is due to the attractive force.
Similarly, ...
2
votes
2answers
64 views
Magnetic properties of matter
When a dielectric is placed in an electric field,it gets polarized. The electric field in a polarized material is less than the applied field. Now my query is, when a paramagnetic substance is kept ...
1
vote
1answer
82 views
Thermodynamics and electromagnetic fields
The energy density in an electromagnetic field is given by: $u = (1/2) <\vec{E}^2 + \vec{B}^2>$ where $<,>$ denotes the average over time. In a cavity it holds that $u= u(T) = ...
0
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0answers
48 views
Faraday's Pail Experiment [closed]
I have a question regarding Faraday's Pail Experiment. If we place a negative charge -Q at the centre (ie inside the pail), what will be the magnitude and sign of the induced charges that appear on ...
-2
votes
1answer
67 views
Electric potential energy and speed
If we have electric field and we put electron there , the electron will move in the opposite direction as the electric field.
My question is electron in that direction will speed up or slows down ? ...
0
votes
1answer
32 views
What role has our Moon played in creating a persistent geomagnetic field?
The question comes from a comment by Mark Rovetta on my earlier question about the Earth's core going cold.
4
votes
2answers
243 views
Einstein Field Equations and Electromagnetic Stress-Energy Tensor
My question is: if we write Einstein field equations in this form:
$$R_{\mu\nu} - \dfrac{1}{2}g_{\mu\nu}R=8\pi \dfrac{G}{c^4}T_{\mu\nu}$$
Then the left hand side is one statement about the geometry ...
1
vote
1answer
104 views
Does a photon have a north and south pole?
A photon has an oscillating magnetic and electric field.
Is the magnetic field a dipole?
1
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1answer
75 views
Can electromagnetic momentum be introduced at pre-university level as for electromagnetic energy?
Electromagnetic energy is introduced at pre-university level, starting with static electric energy followed by static magnetic energy. But the introduction of electromagnetic momentum usually has to ...
0
votes
1answer
92 views
a metal bar on a pair of conducting rails that carries a current
This is a homework question, and I solved it already, but something bugs me. So the problem is stated as following:
A metal bar of mass M sits on a pair of long horizontal conducting rails separated ...
4
votes
0answers
75 views
Is the search for a Simple-group-based Electro-Weak theory over?
Just wondering:
We know that, in its current form of the $SU(2)_L\times U(1)$, the electroweak theroy rides a wave of huge success. However, is it not possible that the correct simple group ...
1
vote
1answer
165 views
How many fundamental fields / constraints are in Maxwell's Equations?
I've seen electromagnetics formulated in the potentials ($\bf A$, $\varphi$) (and their magnetic counterparts) in the Lorenz gauge for a long time. Justifications for using these include that they ...
2
votes
3answers
257 views
Maxwell Stress Tensor in the absence of a magnetic field
I'm having some trouble calculating the stress tensor in the case of a static electric field without a magnetic field. Following the derivation on Wikipedia,
Start with Lorentz force:
$$\mathbf{F} = ...
1
vote
2answers
90 views
Radiated power from a 'source volume' outside which charge & current are zero (i.e. derive radiated power from Jefimenko's equations)
In classical electrodynamics, what is the radiated power from a generalized source (consisting of charge density $\rho$ and current density $\vec{J}$) in vacuum?
Let us define $V_s$ to be the ...
1
vote
1answer
108 views
magnetic scalar potential far above a magnetic film
The situation I am looking at is a magneto-static problem of a finite magnetic film with magnetization $\bf{M}$. I would like to find the the magnetic field far above the plate. My expectation is that ...
0
votes
1answer
54 views
what is the difference between constant and changing magnetic and electric fields? How do they occur? How do they form an electromagnetic wave?
what is the difference between constant and changing magnetic and electric fields? How do they occur?
How do they form an electromagnetic wave?
2
votes
2answers
280 views
Greens function in EM with boundary conditions confusion
So I thought I was understanding Green's functions, but now I am unsure. I'll start by explaining (briefly) what I think I know then ask the question.
Background
Greens are a way of solving ...
