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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552 views

Direction of Current, according to Lenz's law, from induced EMF

My textbook (NCERT Physics Class 12th) has the following figure (Pg 212). The Arm PQ is moved to the left side thus decreasing the area of the rectangular loop. This results in a decrease in ...
2
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2answers
479 views

Why do old transformers make sounds, but new ones don't?

What I think is that maybe insulation is gradually removed between plates, and eddy currents start passing from one plate to another and this produces sound. Am I right?
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3answers
298 views

Do mobile phones produce radiation harmful to humans? [closed]

What are the health effects of mobile phone radiation (if any)? Does using a mobile phone without an SIM card change the answer?
2
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2answers
89 views

Electromagnetism in astrophysics

What is the most important electromagnetic phenomena involved in astrophysics?, I am working in a presentation of magnetars, pulsars and magnetic field of planets and I would like to know if there are ...
2
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2answers
84 views

Copper coils for nMRI

Given the lack of financial efficacy in the medical industry, I was wondering if nuclear magnetic resonance imaging would be possible with a copper/silver coil if the use is short 10 second imaging ...
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1answer
38 views

Does zero free current entail zero $\vec H$?

There are two kinds of magnetic fields (different authors give them different names), $\vec B $ and $\vec H$ which are related by the equation $$ \vec B = \mu_o (\vec H + \vec M)$$ where $\vec M$ is ...
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2answers
54 views

Does the power weakening of an electromagnetic transmission over distance depends on the beam's width?

Does the power weakening of an electromagnetic transmission over distance depends on the beam's width? If I emit an omnidirectional electromagnetic transmission, its power weakens in the ratio of ...
0
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1answer
19 views

Why does material change electrical property based on frequency of input electrical signal?

I was investigating whether dry soil would conduct electricity. What I found was that at 60 Hz frequency (i.e. power line fell to the ground), dry soil is a very good conductor. But if you increase ...
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0answers
19 views

Mixing liquids when applying a voltage

I'm thinking in the following experiment: A) Two (non mixable (like water and oil)) liquids are going to lie in a container, in the extremes of the container there are two electrodes, those ...
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0answers
73 views

How to prove that the net magnetic force in an arbitrary current loop is zero [closed]

I believe that using Stokes' theorem makes this a laughing matter, because $$\mathbf{F}=\oint_C i d\mathbf{r} \times \mathbf{B}$$ Since the magnetic field is constant the $\nabla$ operator would ...
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2answers
184 views

Challenging Magnetostatics Problem - the “blind spot” of a magnetic dipole

I'm reviewing for an electromag exam and I stumbled upon a problem that's really hard to figure out. Here it is: A small magnetic dipole with moment $\vec m = m_o \hat z$ is in a region with uniform ...
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2answers
65 views

Is there any theoretical reason behind multiplying by $N$ when using Faraday's law of induction?

It is a well known fact that whenever we want to calculate the emf in a solenoid we usually multiply the changing flux for one loop times $N$, which is the number of turns in the solenoid. But why is ...
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2answers
97 views

Momentum conservation in an electromagnetic system?

Suppose that I have two charged particles in the configuration below. Let us assume the following: We apply a constant force $f$ to the the bottom particle so that it has a constant acceleration ...
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1answer
41 views

why an immersion lens is called so in Ion Optics?

In Ion Optics, an immersion lens is composed of two planar apertured electrodes. My question is why it is called an 'Immersion" lens? I know that an Einzel lense has three or more electrodes. But ...
1
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1answer
45 views

Can I tear apart a pipe with magnetostriction? And how big field should I use?

As I know, with magnetostriction we can "stretch" its unit cell. The stronger is the field, the more it expands the cell. Using alternating current in coils that induce magnetic field we can stretch ...
2
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0answers
124 views

Stress Energy Tensor of EM Field

Stress energy tensor for electromagnetic field is given by $$T^{\mu\nu}=\frac1{4\pi}(F^{\mu\alpha}F^{\nu}{}_\alpha-\frac14 g^{\mu\nu} F_{\alpha\beta}F^{\alpha\beta}).$$ My textbook (unpublished ...
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1answer
68 views

Proof of equivalence of two different covariant notations of Maxwell's equations [closed]

My homework assignment is Prove that $$\partial_\beta F_{\gamma \delta} + \partial_\gamma F_{\delta \beta} + \partial_\delta F_{\beta \gamma} = 0$$ with the electromagnetic tensor ...
4
votes
2answers
76 views

Charge orbiting gravitational body [duplicate]

I am currently rather uneducated on the subject, but I was thinking of a general relativity thought experiment. Say I take a charge from infinity and give it velocity to orbit a planet in a circle. ...
0
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0answers
71 views

Why is the direction of $I$ or $F$ different in case of Motors and Generators?

Consider the following diagram: In the first diagram, the wire is carrying current into the screen. Motion of the wire is downwards (Left Hand Rule) In the second diagram, the wire is moved ...
3
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1answer
293 views

Hamiltonian formulation of single particle in an electromagnetic field

Consider a charged particle in a static electromagnetic field. Suppose that the domain is simply connected so that the second law of Newton's dynamics reads: $$ ...
1
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1answer
139 views

Euler-Lagrange equations of a current-loop pendulum in a magnetic field

I am reading "Nonlinear Electromechanics", by Dmitry Skubov and Kamil S. Khodzhaev, Springer 2008. Here is the relevant and freely available chapter. Essentially, a loop of area $S$, mass $m$, moment ...
1
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1answer
95 views

General solution to the wave equation of electromagnetic field

Suppose that we have the four-vector potential of the electromagnetic field, $$A^i$$ The wave equation is given by $$\Box A^i=\frac {\partial^2}{\partial x^k \partial x_k } A^i= \left(\frac {1}{c^2} ...
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0answers
19 views

If there is a violation in magnetic interaction b/w two non parellel current carrying wires

when two current carrying wires are put near, there is a magnetic interaction b/w the wires. if they are kept parellel, they'll follow newton's 3rd law. i want to ask that if they are not kept ...
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1answer
71 views

Calculation mistake some place in finding stress-energy tensor

If the Lagrangian in Maxwell's theory is $$L= R- \frac{1}{4}F_{\mu\nu}F^{\mu\nu}$$ I want to find $T_{\mu\nu} $ The procedure is that I vary the action: $$\delta S = -1/2 \int{d^4x ...
2
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3answers
629 views

How does a single charge produce magnetic field?

I have studied in Introduction to electrodynamics (Griffiths) that magnetic field is actually due to effects of relativity unequal Lorentz contraction of the positive charge and negative lines, a ...
3
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2answers
480 views

Landau level degeneracy in symmetry gauge, finite system

As we know, Landau level degeneracy in a finite rectangular system is $\Phi/\Phi_0$, where $\Phi=BS$ is the total magnetic flux and $\Phi_0=h/q$ is the flux quanta. This can be easily derived using ...
2
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1answer
492 views

Can a force stop a Photon since Photons have momentum and What does momentum mean when talking about massless particles?

Momentum measures how hard it is to stop an object. While Photons are massless they still have relativistic mass and energy. My question is can something stop photons other than being absorbed by ...
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5answers
5k views

What is canonical momentum?

What does the canonical momentum $\textbf{p}=m\textbf{v}+e\textbf{A}$ mean? Is it just momentum accounting for electromagnetic effects?
4
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4answers
844 views

Magnetostatic energy density — derivation without introducing inductance?

I was looking for a derivation of the expression for the energy density at any point in a static magnetic field. I do know that it is $$u_B=\dfrac {1}{2 \mu_0}\left|\mathbf{B}\right|^2,$$ I was just ...
12
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4answers
1k views

Fundamental invariants of the electromagnetic field

It is a standard exercise in relativistic electrodynamics to show that the electromagnetic field tensor $F_{\mu\nu}$, whose components equal the electric $E^i=cF^{i0}$ and magnetic ...
6
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4answers
9k views

Derivation of self-inductance of a long wire

Currently I am stuck, trying to derive the self-inductance of a long wire. According to literature it should be $$L=\frac{\mu_r\mu_0l}{8\pi}$$ and in literature its derived by looking at the energy ...
0
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1answer
100 views

Gravitational force and Electromagnetic force?

I found this interesting note in one of my textbooks, The enormous strength of the electromagnetic force compared to gravity is evident in our daily life. When we hold a book in our hand, we are ...
2
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2answers
85 views

What is the link between observation wavelength and spatial resolution of an instrument? [closed]

It seems that when capturing and emitting EM waves matter proceeds differently depending on the wavelenght. According to an answer from another question, EM energy is captured following three modes: ...
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5answers
85 views

Is anything coloured “infra-red”?

All our day-to-day objects are things we can see, and thus we can conclude that their colour lies in the visual spectrum. Is there matter that does not have a colour within the visual spectrum? What ...
3
votes
3answers
883 views

Why the magnetic flux is not zero?

If $\vec{\mathbf B}=B\vec{\mathbf a}_z$, compute the magnetic flux passing through a hemisphere of radius $R$ centered at the origin and bounded by the plane $z=0$. Solution The hemisphere and ...
4
votes
2answers
516 views

Deriving Heaviside-Feynman formula for the electric field of an arbitrarily moving charge from Lienard-Wiechert potential

I've been trying to derive this (which Feynman warns takes a lot of work) for a couple of days now, without success. My current best derivation which however doesn't give the right answer is: First, ...
0
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3answers
107 views

What is the experimental evidence that light is an electromagnetic wave? [closed]

Do we have any experimental evidence to confirm this? How can maxwells equation confirm light as an EM wave simply based on a similarity in speed? Just curious!
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0answers
273 views

Why do we apply Fleming's *left* hand rule in case of DC motors but use Fleming's *right* hand rule in case of generators?

Why do we apply Fleming's left hand rule in case of DC motors but use Fleming's right hand rule in case of generators to find the direction of induced current?
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0answers
30 views

BH-demagnetazation curve of NdFeB, N50

Is there a trusted literature source (material handbook) where someone can find the BH-demagnetazation curve of the Neodymium Iron Boron (NdFeB) permanent magnet, grade N50? Why there exists ...
0
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1answer
93 views

Can Gauss' and Ampere's Laws be written in terms of the divergence of an energy four-vector?

In the first 20 minutes of this video, Susskind derives the continuity equation for charge conservation: $$\dot{\rho}+\nabla\cdot\vec{J}=0$$ (Where $\vec{J}=\frac{\partial\dot{q}^m}{\partial A^m} ...
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3answers
1k views

Relation between complex dielectric constant and resistance

I am having a real hard time understanding how I reach the expression that relates resistance and $\varepsilon''$ as the complex part of the dielectric constant. I was trying to follow a clear and ...
0
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0answers
42 views

Electric Field Inside Dielectric Filled Cavity

I want to determine the electric and magnetic fields inside the following cavity: perfect conducting walls at $z = 0, z=d, x=0, x=b, y=t, -t^\prime$, with a dielectric $\epsilon$ filling ...
10
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3answers
643 views

Where's the energy in a boosted capacitor?

Suppose I look at a parallel plate capacitor in its rest frame and calculate the electrostatic energy, $E$. Next, I look at the same capacitor in a primed frame boosted in the direction perpendicular ...
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1answer
38 views

Tranverse electromagnetic waves

TEM waves do not exist in waveguide. Is this the correct explanation- Both curl and divergence of TEM are zero inside the waveguide and because of the boundary conditions (electric field zero at every ...
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0answers
20 views

Assumption of Positive Dielectric Value in Derivation of Dipole Field Around a Spherical Dielectric? Implications for Plasmonics

I am going through the solutions to Laplace's equation in spherical coordinates. In particular the boundary value problem of a dielectric sphere of radius $a$ and permittivity $\epsilon$ which has a ...
2
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1answer
50 views

momentum and energy of the rods

I have two electromagnets and I attach them to two rods(same mass) such that, first one will be attached to centre of first rod and the second one will be to the end of second rod. Now I bring them ...
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0answers
43 views

Reflectivity of gases

According to the principle of the conservation of energy, the following is true for an object: emissivity+reflectivity+transmittance=1; where emissivity=absorbtivity according to Kirchoff. When ...
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2answers
74 views

Divergence of Magnetic Field in an Electromagnetic Wave

In many text books of physics (especially those coping with Electromagnetic Theory), we see that an electromagnetic wave (EMW) is drawn as an electric field varying with a sinusoidal form and exactly ...
1
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1answer
85 views

Physical meaning of magnetic vector potential [duplicate]

Can anyone give me an intuition of why the magnetic vector potential $A$ is sometimes interpreted as the electromagnetic momentum ? I don't know analytical mechanics, just classical electromagnetism ...
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0answers
82 views

A-level electromagnetism experiments? [closed]

Does anybody know any A-level experiments investigating electromagnetism that can produce good data relatively simply?