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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3answers
113 views

Why can the divergence of vector potential be anything?

Purcell in his book was deriving the vector potential $\bf A$ using $\text{curl}\;(\text{curl}\; \mathbf A)= \mu_0 \mathbf J\; .$ After some algebra, he came to this: $$-\frac{\partial^2 A_x}{\...
2
votes
2answers
62 views

A general complex electric field

When dealing with a plane wave solution to the electric field such as $$\vec{E}(r,t)=E_{0}\cos(kz-\omega t+\phi)$$ we usually introduce a complex electric field $\tilde{E}(r,t)$ such that $\vec{E}(r,t)...
0
votes
1answer
556 views

Why do we always consider the area of inner solenoid while calculating the mutual inductance of two coaxial solenoids?

While we know that flux linked to an area is the dot product of the area vector of the loop(and not the actual area encountering the field lines) and the magnetic field linked to it, why then do we ...
-1
votes
1answer
19 views

Armature of electric bell [closed]

I want to know that why is the armature of the electric bell made up of soft iron ? I googled this out and everywhere it was written that it is because of its propert to aquire electromagnetism but I ...
2
votes
1answer
296 views

What kinds of behavioural anomalies can a zero-field-cooled (ZFC) / field-cooled (FC) split indicate?

If a material shows a spiltting in the ZFC and FC curves, is it necessarily superparamagnetic, or could there be any other reason for the irreversibility? I have heard spin glasses also show ZFC-FC ...
1
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1answer
201 views

Why we unified electromagnetic and weak force?

We have unified electromagnetic and weak force into one single force called Electroweak force. I mean we can use these different forces within their respective domains like weak interaction for short ...
0
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0answers
43 views

Defining electromagnetic stress tensor for non-linear media

In textbooks, the electromagnetic stress tensor (in vacuum also called Maxwell stress tensor) is usually derived for linear media, implying that $$ \vec D = \epsilon_0 \epsilon_r \vec E$$ My question ...
-1
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0answers
22 views

What is the physical significance of Curie-Weiss temperature?

From the Curie-Weiss law, the CW temperature is negative for antiferromagnet (afm). what is the significance of the negative value of CW temperature of a afm?
2
votes
1answer
64 views

Why does the divergence of the Ponyting vector have energy flux density?

The poynting vector is defined as $\vec{S}=\mu_{0}^{-1}\vec{E}\times \vec{B}$ Taking the divergence of the poynting vector, one arrives at $\vec{\nabla} \cdot \vec{S}=-\frac{\partial u}{\...
0
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1answer
40 views

Why metal sphere (conductor) is used in Van de graff as the output terminal, not any other material , specifically any insulator?

Why metal sphere (conductor) is used in Van De Graff as the output terminal, not any other material, specifically any insulator?
0
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0answers
23 views

Intrinsic and normal B-H curves in magnets

In a magnet, what is the difference between the intrinsic $B-H$ demagnetization curve going from $B_r$ to $H_C$ (which has a knee) and the normal $B-H$ curve going from $B_r$ to $H_{ci}$ (which is ...
0
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1answer
5k views

How can a magnet have a greater magnetic field strength than a bigger, heavier and more powerful magnet made of the exact same material?

Here are some informations I found on the magnetic field strength (also called "magnetic flux density" or "magnetic induction", commonly denoted as B and expressed in tesla or in gauss) of an average ...
-1
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1answer
40 views

what is the physical significance of dielectric constant and loss tangent?

I want to know that the significance of dielectric constant and loss tangent behaviour. How it characterises the materials. and possible relation between the dielectric constant and polarisation ?
0
votes
1answer
47 views

Would moving quarks constitute a current?

I know that it would take an almost infinite amount of energy to isolate quarks, but let's say we've gotten past that and we have isolated quarks and put them in motion. Would that be a current since ...
0
votes
1answer
34 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
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0answers
24 views

Double Fraunhofer , Fourier transform to the image plane?

Consider the diagram below: I know that from Franhofer diffraction we have: $$u_f(x_f)\propto \int u_o(x_o) e^{-k\frac{x_f}{f}x_o} dx_o$$ Assuming small angles. I also know that $u_i(x_i)$ is ...
0
votes
1answer
780 views

Magnetic fields in transformers and back emf?

i am having some trouble with understanding the magnetic fields inside transformers and how they relate to back emf. My first query is: is the magnetic field produced inside the soft iron core ...
5
votes
2answers
252 views

What is the area in Faraday's law if we have only a piece of metal moving in a magnetic field?

If a piece of metal of length $l$ is moving with a speed $v$ in a region where there is a uniform magnetic field $B$ perpendicular to it, there will be a potential difference across its terminals ...
1
vote
2answers
214 views

Is this an example of Parity violation? [duplicate]

I always hear about parity violation in high energy physics, but what about examples in classical physics? Say we have a wire carrying current in the $+x$ direction, thus generating a magnetic field $...
1
vote
1answer
158 views

Spherical magnet inside a solenoid

When passing a bar magnet through a long solenoid why is it that the induced emf when the magnet is in the middle of the solenoid is zero? And if a spherical magnet is put inside the solenoid, will ...
2
votes
4answers
610 views

Induced EMF of a rectangular loop should be zero?

Considering the shape of a rectangular loop in a changing magnetic field: The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. ...
2
votes
1answer
42 views

Problem with magnetic field due to relative motion

We know that, moving charge produces magnetic field in the surrounding space. Consider this scenario : A charge 'q' is moving with a constant speed 'v' in the direction of positive x axis of a ...
2
votes
2answers
100 views

Why doesn't sunlight ALWAYS get split (into monochromatic) when going through the clouds?

It is my understanding that clouds are largely made up of water, which is known to split white light into its frequency components, and that's why we see rainbows sometimes. My question is, with ...
1
vote
1answer
62 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 ...
0
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0answers
26 views

Scanning Electron Microscopy on a smaller scale

Is it possible to sufficiently focus a ~1nm beam of electrons into a sample chamber in less than 10cm of vertical space, therefore reducing the volume of the entire vacuum chamber, and subsequently ...
3
votes
1answer
123 views

Purcell's approach to Larmor's formula, assumption?

In 'Electricity and Magnetism' By E.M.Purcell, a derivation is given of Larmor formula (a version of which can be found here). I will give a brief overview here: A particle is considered at ...
1
vote
1answer
59 views

B field of an EM pulse

I am trying to find the B field associated with the following E field (to eventually find the effect it has on a charged particle): $$E(\vec{x},t)= \Re[E_0(z-ct)]e^{ik_0(z-ct)}$$ Where $E_0$ is an ...
1
vote
1answer
236 views

Electrical Resistance of Spherical, Contacting Colloids

So, I work a lot with Magneto-Rheological (MR) fluids - which are essentially electrolyte solutions with suspended, paramagnetic colloids/particles. Once a magnetic field is applied to such a fluid, ...
0
votes
1answer
46 views

Electromagnetic waves of accelerating charges

Is it true that more accelerating a charge will cause electromagnetic waves of more energy than the energy of the electromagnetic waves cause by lesser accelerated charge ?
33
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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
0answers
41 views

Mass term in Maxwell's Lagragian for Electromagnetism

In the scalar field Lagrangian the mass term is given by $$m^2 \phi^2.$$ But the equivalent term in Maxwell's Lagrangian for electromagnetism is $$m^2A_{\mu}A^{\mu}.$$ But I don't know why the ...
3
votes
2answers
381 views

Hall effect with similar positive and negative carriers?

The Hall effect includes the transverse (to the flow of current) electric field set up by the charges which accumulate on the edges, to counter the magnetic component of the Lorentz force acting on ...
0
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1answer
39 views

Amplitude of electromagnetic wave

For an EM wave propagating in +z direction, it is described by $ \underline{E} = \underline{E_0}(x, y)e^{i(kz - wt)}$ I know that the imaginary term in the expression describes phase and $|\...
0
votes
2answers
35 views

northern hemisphere compass points where in southern hemisphere?

If Northern Hemisphere compass points to magnetic north and this same compass is brought to the Southern Hemisphere, will the 'painted red north' compass needle be drawn to the magnetic south pole?
0
votes
1answer
476 views

How to calculate flux density in a toroidal CT that has an electrical line pass through the center?

I have seen the Biot–Savart equation, but I don't know how to apply it. Does the conductor length need to be considered finite and equal to the core length, or will the calculation be for a conductor ...
0
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0answers
19 views

Constant scalar magnetic potential in ferrofluid

I have read in an article that a ferrofluid with very large magnetic permeability (I gess it means that the magnetization of the fluid is the saturation magnetization) under a constant external ...
3
votes
1answer
124 views

Simple Explanation of Kondo Effect

Does anyone have a simple explanation of the Kondo Effect? (i.e. a simple physical picture + maybe equations to think of?) My current understanding is this: If we consider an electron scattering ...
7
votes
4answers
255 views

How do electromagnetic waves carry energy?

Its said that electromagnetic waves carry energy. Is this because these waves are made up of electric and magnetic fields which can cause changes to the stuff that falls with in their range? Is that ...
1
vote
0answers
30 views

Charge that moves inside a conducting sphere

I have a conducting sphere of charge Q and inside it we set a charge -q from the centre with initial velocity uo and i would like to know with what speed it reaches the surface of the sphere and what ...
1
vote
4answers
2k views

Why Change Of Magnetic Flux Induces Current In circuit?

Why change of magnetic flux with the time through a coil induces current in it? Please tell me that what happen to charge in the coil if the magnet or electromagnet wave around this? Also, why a ...
0
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0answers
15 views

Sources for the magnetic exchange integral in group d transition metals

Does anyone know where I can find values for the magnetic exchange energy, J for elements with the atomic numbers 20-30? I've found values of 0.015meV, 0.03meV and 0.02meV for iron, cobalt and Nickel ...
5
votes
1answer
113 views

What is relationship between electromagnetic mass and rest mass?

Is there a direct equation which compares rest mass $m_°$ and electromagnetic mass $m_{em}$? Nothing on web I found. $m_{em} = \frac{4 E_{em}}{3c^2}$ 4/3 problem The final solution of the ...
0
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0answers
17 views

Boundary conditions while finding electric potential inside some specific volume

Please see the figure. Continuous charge distributions are in blue color. Inside cavity there is some charge distribution. Also On surfaces S1 and S2 there are some charge distributions (not shown)....
4
votes
1answer
96 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}{...
3
votes
2answers
265 views

Magnet gyroscopic force spin

I was wondering whether a magnet exerts any measurable gyroscopic effects. I understand that magnetism is caused by alignment of spins of electrons which have angular momentum. (I realise that that ...
1
vote
1answer
201 views

How do you derive Fleming's left hand rule?

How do you derive Fleming's left hand rule? What is the theoretical explanation for the directions of the magnetic field, current and the force on the current for being oriented in that way relative ...
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0answers
32 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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2answers
7k views

Fleming's right hand and left hand rule

Why are there two rules: Fleming's left hand and right hand rules? What is the difference between the two and why can't we use just one rule? Suppose the magnetic field is from right to left and ...
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 ...
1
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1answer
108 views

Force on a magnetic dipole due to a magnetic field

Imagine a solenoid which has current $i$ and is producing a magnetic field $B$ which equals $$B=\mu N i$$ Now, imagine we put a small cylindrical magnet at the end of the solenoid. Then because at ...