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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209 views

What is the magnetic flux density “outside” the solenoid when AC current is passing through it?

I know that there are well defined equations explaining the magnetic flux density in the solenoid. However what about magnetic field outside the solenoid? How is the magnetic flux density related ...
4
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1answer
69 views

Specific heat of the classical ferromagnetic Heisenberg model

I have simulated the classical ferromagnetic Heisenberg model on a cubic lattice using Monte Carlo and I get a finite specific heat near zero temperature. My understanding is that from the magnon ...
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1answer
306 views

Surely force on shell can't be balanced by field momentum?

Imagine a particle with charge $q$ at rest at the origin. It is surrounded by a concentric spherical insulating shell, also at rest, with charge $Q$ and radius $R$. At time $t=0$ I apply a constant ...
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1answer
135 views

Do magnets also attract super conductors?

If a superconductor can be repelled by a magnet (or, well, locked in a certain position by the magnet) can a magnet be attracted to a superconductor. What would happen if you take a peace of metal, ...
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2answers
164 views

Question regarding Van De Graff generator Belt

I have made a VDG generator with a rubber band as the belt and a glass roller. It doesnt seem to work because I think the rubber band may be conductive. I was thinking of using other materials for ...
2
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1answer
189 views

Cylindrical capacitor in an electric circuit

I've come across a tricky question and would appreciate some hints or explanations as to why the given solution is the way it is. The question reads as follows: A coaxial cable consists of a wire ...
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1answer
56 views

Meaning of components of Maxwell's stress tensor $T_{ij}$

I am learning about Maxwell's stress tensor and what I understood is that the components, say $T_{ij}$ is something like a force parallel to the $j$th-direction acting on the surface with its normal ...
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1answer
43 views

Different voltage drops depending on the path

I was working through a problem in electromagnetic induction when I discovered that I have a fundamental ignorance of some sort that is puzzling me. Consider a circular wire, at rest, perpendicular ...
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2answers
2k views

Meaning of boundary condition for steady current density?

Although I understand the derivation of boundary condition in case of steady electric current but I did not understand, that the electric field which is in direction of $J$ current density that is ...
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4answers
437 views

Why can't the electron enter the nucleus?

Actually, there exists an attractive force between an electron and the protons inside the nucleus, but the electron cannot be attracted towards the nucleus! What force balances that attractive force? ...
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1answer
34 views

Is it possible to measure the resistivity of something using A.C. Current and a capacitor?

I've read that A.C. current "passes" through a capacitor. Is it possible to use this and measure the resistivity of the medium between the plates? Any sources or links would be appreciated.
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16 views

Comparison of electron deflection tube and Hall effect

Consider an electron deflection tube (something like this: http://www.ld-didactic.de/documents/en-US/GA/GA/5/555/555624e.pdf). Suppose you apply a magnetic field perpendicular to the screen. Then ...
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5answers
2k views

Do radio waves travel around the Earth or through it?

Whenever you hear someone illustrating/describing the transmission of radio waves they always make it seem like they'd travel perfectly around the Earth to another distant location. For example, a ...
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1answer
108 views

Experimentally finding units of physical quantity?

Say you had a new physical quantity you wanted to determine the units for. How do you go about this? For the strength of an electromagnet for example, you could carry out a simple experiment like the ...
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1answer
42 views

Does magnetic fields exert forces only on moving charges?

I am confused here a bit,i read that magnetic fields exert force only on charges moving but when we take magnet and keep it near a stationary metal (assume light in weight), the magnet attracts it? ...
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0answers
8 views

What kind of flux-pinning effect will occur if a type two superconductor is subjected to an AC electromagnet?

When a supercooled type two superconductor is subjected to a static magnetic field, the superconductor pins to the flux of the field (the mixed-state meissner effect is apparent). What happens if it ...
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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}{\...
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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)...
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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 ...
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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
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1answer
297 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 ...
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1answer
202 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 ...
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0answers
44 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 ...
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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?
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1answer
41 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?
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0answers
27 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 ...
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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 ...
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1answer
43 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 ?
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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 ...
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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.
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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
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1answer
783 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 ...
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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 ...
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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 $...
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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
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4answers
611 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
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1answer
44 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
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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 ...
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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 ...
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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
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1answer
124 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 ...
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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 ...
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1answer
237 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, ...
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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 ?
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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 ...
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0answers
42 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
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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 ...
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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 $|\...
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2answers
38 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?
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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 ...