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
100 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
53 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 ...
0
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1answer
257 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 ...
1
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1answer
56 views

What is the sum of the electrons' magnetic moments in a wire?

The electron has a magnetic moment. This magnetic moment will be influenced in a electric field; the magnetic moments will be more or less aligned. During the movement of an electron in a wire under ...
2
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2answers
70 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 ...
0
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2answers
991 views

Potential energy from opposing magnets repelling each other with a gap of 1 mm

I have two powerful rare earth magnets, that are separated by a distance of 1 mm. I applied energy to bring them closer to each other, hence increasing the potential energy. Now, when one of the ...
0
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1answer
31 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 ...
1
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0answers
64 views

Faraday's paradox

I have studied that Faraday's law of induction and motional emf are 2 different lines of thinking but are essentially same. But then , how can Faraday's paradox be explained by Faraday's law of ...
3
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1answer
64 views

Earnshaw Theorem for ionic solids

Is a single molecule of sodium chloride (say) or a cluster of molecules of NaCl unstable, although macroscopically NaCl is in fact, stable? How can I reason this based on Earnshaw's theorem?
1
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1answer
58 views

How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?

The only difference that I know of between Kitchen Microwaves and Wifi signals is how much power is pumped through them. Why is it that Wifi signals, being 1000 times weaker can travel so much ...
1
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1answer
310 views

Force from solenoid

I'd like to approximate the force from a solenoid, or at the very least find a formula which is proportional to the force so that I can experimentally find the constant for my particular case. ...
0
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2answers
120 views

Why is emf equal to PD when circuit is “open”?

I understand that the set up of a battery consists of very good, but not ideal conductor, and therefore, some internal resistance exists. Also, I get that emf is the PD that would exist if the ...
1
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1answer
60 views

Biot-Savart and Amperes law inconsistentcy

Imagine the situation below where a steady current $I$ goes through the line. The radius of the semicircle is $r$. How do I compute the magnetic flux in the point $P$, marked by a black dot? ...
0
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2answers
37 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 ...
1
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4answers
162 views

How any material can provide electrostatic and magnetic shielding

Several metallic objects like Iron, Copper etc can provide electrostatic shielding (one should remain inside the car during thunderstorm) and several superconductors like HTS (High Temperature ...
0
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1answer
17 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 ...
2
votes
3answers
200 views

Why is the inner product between divergence-free current$\vec{J}$, (satisfied $\nabla\cdot\vec{J}=0$) and a gradient field$\nabla \varphi$ zero?

I read a statement saying that the inner product between divergence-free current and a gradient field is zero. Divergence-free surface current is $\nabla\cdot\vec{J}=0$, and $\vec{J}$ could be ...
0
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0answers
24 views

Does there exist purely lossy medium?

We know that the complex propagation constant for plane wave is given by $$\gamma = \alpha + j\beta $$ where $\alpha$ is the attenuation factor, and $\beta$ is the wave number and appears in the ...
1
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3answers
77 views

What is the purpose of the factor $N/L$ in the magnetic field of a solenoid?

I am supposed to find the expression of the magnetic field on the axis of a solenoid of radius $a$, length $L$, and with $N$ the number of turns. So in order to find $dB$ (to integrate over dx), I ...
1
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0answers
18 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 ...
1
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0answers
54 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 ...
1
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0answers
79 views

Remote EEG Detection / Reading

I recently noted that Phillips, Accenture, and Emotive grouped together to create a proof of concept that would assist ALS patients to gain greater control of their lives through brain, voice, and eye ...
3
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2answers
132 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 ...
0
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2answers
51 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 ...
1
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2answers
73 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 ...
1
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2answers
74 views

Does a magnetic field sustain a current in a loop?

A particular current produces a constant particular magnetic field. If the fact holds under scrutiny, why doesn't a magnet held within a loop sustain a particular current? Are the electron spins ...
1
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1answer
908 views

Proving Ampere's Circuital Law

How to prove Ampere's Circuital Law in case of any conductor. My text gives the proof of only the special case when the conductor is long and straight. I am trying to prove it, but haven't been ...
1
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1answer
140 views

Symmetries of a Uniform Magnetic Field

Simple question. A system with a uniform electric field everywhere in space has translational invariance in the directions perpendicular to the electric field but no translational invariance parallel ...
4
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2answers
271 views

How does a magnetic field cause objects to move?

Let me start off by saying that I never got a degree in any field but I do read and study physics (both classical and quantum); although I personally can't solve quantum physics equations I do have a ...
3
votes
2answers
101 views

Looking for lists of student difficulties with electromagnetism, circuits (DC and AC)

I'm trying to help with an Instructor's Guide for an introductory calc-based physics text, and it would really help if I could locate some lists of typical student difficulties with topics like ... ...
0
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1answer
130 views

Physical representation of magnetic vector potential

In electrostatics, for scalar potential $V$, we can represent the equipotential surface as a perpendicular surface of the direction of derivative. such as $$ {\bf E}~=~-\nabla V \tag{1} $$ & the ...
1
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1answer
26 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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2answers
57 views

Uniformity in a solenoid

I know the magnetic field strength increases as the number of turns in the solenoid increases. However, I've learnt the field inside the solenoid is usually nearly uniform. So, does the number of ...
1
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1answer
36 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
99 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 ...
5
votes
3answers
257 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 ...
2
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1answer
65 views

Can a single technology be used to observe light at any wavelength?

Radio waves are observed using induction in antenna. Visible light is observed exploiting electronic transition in matter. Is it possible to do the opposite? If not why?
0
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1answer
48 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
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2answers
65 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. ...
1
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0answers
55 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
238 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: $$ ...
0
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0answers
12 views

Reaction force due to the curvature and gradient drift

We know that a charged particle will have a drift velocity in both a curved magnetic field and when there is a transverse spatial gradient in the magnitude of the magnetic field. This drift velocity ...
0
votes
1answer
47 views

Magnetic flux of a loop due to another loop

The scenario is the following, I am given 2 loops with the same radius, r, a distance of d, and same current of I. In the left loop the current goes counter clockwise, in the right loop the current is ...
1
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1answer
84 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
45 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} ...
1
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0answers
14 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 ...
0
votes
1answer
55 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
367 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
84 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
votes
1answer
357 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 ...