Tagged Questions

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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0
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1answer
21 views

Does the medium matter when calculating Optical Period?

If I consider the optical period $T$ for a given wavelength $\lambda$ in vacuum, I can calculate it as: $$ T = \frac{1}{f} = \frac{\lambda}{c} $$ However, in a particular material/medium, I am not ...
0
votes
2answers
25 views

Using Coulomb's law to solve for a continuous charge distribution [on hold]

I have been trying to evaluate this specific integral and have been completely stuck. $\frac{1}{4\pi \epsilon_0} \int^L_0 \frac{2\lambda z}{(x^2+z^2)^\frac{3}{2}}dx$ I am a freshman and have been ...
0
votes
1answer
24 views

Using the force law to obtain total energy of an electron as a function of its radius

I am working on a problem which starts saying determine the total energy of a hydrogen atom with an electron moving with momentum $p$ at a radius $r$. For that part I got: $E = \frac{p^2}{2m_e} - ...
0
votes
0answers
12 views

Magnitude of magnetic field due to current in a semicircular wire at a specific angle θ? [on hold]

My task is to find the magnitude of magnetic field due to a current going in a semicircular wire at an angle θ. Now, using Biot-Savart law, I have the current equation at dB = (μ0 I/4piR) ...
1
vote
1answer
16 views

How to model the impedance of a coil with a dielectric slab on the top

I have sketched what I meant in the image below. Were there no dielectric slab, the coil could have been simply modeled as an inductor. But I am not sure what the impedance can be if there is a ...
0
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0answers
16 views

Driving force in a homopolar motor

I can't really figure out the driving force in the following homopolar motor: http://www.youtube.com/watch?v=w2f6RD1hT6Q I am not convinced by the usual argument involving the Laplace force, so ...
0
votes
1answer
29 views

Influence of applied voltage to an electron of a metal

I would like to ask what would happen to the potential well of an electron being trapped in a metal? If I apply a voltage trying to accelerate the electron out of the potential well. Would It make the ...
0
votes
1answer
20 views

Mimic Magnetic Repulsive Resistance through a spring

I have two identical magnets and I push them towards each other so that they are repelling. Is it possible to accurately mimic this repulsion effect (magnetic energy) just using a metal spring ...
1
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2answers
86 views

Magnetic force as a relativistic effect?

There is something I am confused about when it comes to the force between two parallel wires carrying current, specifically why when they carry current in the same direction the wires are always ...
0
votes
2answers
81 views

Why aren't Faraday's law of induction and Maxwell-Ampere's law (without sources) symmetric?

I was wondering why Faraday's law of induction and Maxwell-Ampere's law (without sources) are not totally symmetric in the sense that Maxwell-Ampere's law has a $\epsilon_0 \mu_0$ term on the right ...
1
vote
1answer
213 views

What causes the light spectrum to appear on oil surface? [duplicate]

I have observed that when petrol or oil leaks, the light spectrum appear on it. What causes light to split into its constituent colours?
1
vote
0answers
22 views

What is the rate at which the field of an electron spreads? [duplicate]

There are 2 steps in the creation of a force field: (1) first the object comes into existence (2) then the field appears. In the case when a photon splits into an electron and an antielectron, these 2 ...
0
votes
1answer
51 views

Magnetic levitation frog example

I'm trying to figure out the strength of the magnetic field needed to levitate the frog, assuming it's spherical, has mass $5$kg and diameter $20$cm. I also assume $\vec{B}$ is uniform. Since it's ...
4
votes
1answer
53 views

Distribution of current of a rotating cone

If I have a hollow cone (surface with no bottom cover ) as the one in the picture. The cone has surface charged density $\sigma$. It rotates around the symmetry axis with an angular velocity ...
0
votes
1answer
34 views

Right hand solenoid example

I am just studying for my physics final exam. I came across this example and am not satisfied with the answer. My answer is that Z is South and the current would flow from Y to X because of the ...
3
votes
3answers
168 views

How is a photon measured?

If photons transmit the electromagnetic force, which is observable: the photon or the electron? Do we ever directly measure a photon, or do we only measure it's effect on electrons. For example ...
0
votes
1answer
18 views

How do I determine the electric charge on two (initially discharged) spheres after an impact with a charged sphere?

This is the first part of a problem on my book: Two identical conducting spheres, initially discharged, of mass $ m = 0.5 \ kg $ come in contact, in consecutive moments, with another sphere, ...
0
votes
1answer
61 views

Derivation of magnetic vector potential [on hold]

While solving the equations for magnetic vector potential due to a infinitesimally small line current, I got a result as follows $$\mathbf{A}(r) = \frac{e^{j\beta r}}{4\pi r}\int_v \mu\mathbf ...
2
votes
2answers
56 views

Electric and Magnetic field's phase difference shift in linearly polarized electromagnetic waves

I am a high school student and we currently studying the electromagnetic theory. In my textbook i read that the oscillating electric the magnetic fields have phase difference equal to π/2 rad near the ...
1
vote
0answers
39 views

How to solve the following set of equations (magnetohydrodynamics with anomaly term)?

Let's have system of equations: $$ \tag 1 [\nabla \times \mathbf E^{(1)}(\mathbf r , t) ] = -\frac{\partial \mathbf B^{(1)}(\mathbf r , t)}{\partial t} , $$ $$ \tag 2 [\nabla \times \mathbf ...
1
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0answers
44 views

Online physics lecture [on hold]

I'm looking for online courses "video" for Classical electrodynamics and nuclear physics for undergraduate students .. do you know where I can find it ?
0
votes
1answer
50 views

Does alternating current come from DC?

I wonder whether alternating current is produced from DC current or whether AC and DC are entirely different concepts. Is there any relationship between AC and DC?
0
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3answers
72 views

Why in a solenoid, do the magnetic field lines resemble that of a bar magnet?

Each loop in the solenoid will have its own magnetic field wrapped around it. In that case it won't resemble the magnetic lines formed by a bar magnet...; but we see that it is the same. Why is that?
0
votes
2answers
72 views

why do magnetic field lines leave the north pole and enter the south pole? [duplicate]

suppose we have a bar magnet |S_______________N| ... we can imagine it to be |S-N-S-N-S-N-S-N| So, why do we see the lines leave the north pole and enter the south pole externally? What ...
0
votes
0answers
24 views

How derivate paradox of dicressiang the charge in Liénard–Wiechert potential? [closed]

My professor discuss the Liénard–Wiechert. and give the formula for the moving point charge. there was a paradox by which it charge density is replace by the charge with paradox times of decreasing ...
0
votes
1answer
41 views

constant $c$ in Lorentz force

I met some of examples of force in studying classical dynamics, and Lorentz force was represented as $$ \mathbf{F}=\frac{q}{c} \mathbf{\dot{r}} \times \mathbf{B} $$ I've searched for meaning of $c$, ...
2
votes
2answers
81 views

Why does light fade from a room?

So basically in my toilet I have this really bright ceiling light, but when I turn it off it doesn't just instantly plunge me into darkness, but rather sort of fades out over a few seconds. It looks ...
0
votes
0answers
21 views

Electrolysis of Paramagnetic Ferric Chloride in Water

I'm trying to settle a bit of a debate between a friend and myself over the cause of an effect observed when dissolving Ferric Chloride, Iron(III) Chloride (FeCl3), in water and supplying an ...
0
votes
1answer
33 views

Alternating Current/Electromototive Force equaling 0?

All alternating current diagrams are drawn as sinusoidal waves that intercept with x axis. Given our usual sine wave that equals to 0 at 180° it would mean the alternating current would equal to 0. If ...
0
votes
1answer
35 views

Deriving electric field

Can anyone explain where the following expression for the electric field vector comes from? $$ \mathbf E(\mathbf r,t) = -\nabla \phi(\mathbf r,t) - \frac{\partial}{\partial t}\mathbf A(\mathbf r,t) ...
1
vote
1answer
24 views

a charged particle path [closed]

A particle of mass $m$ and charge $e$ enters a homogeneous and stationary electric field $E$ with velocity $v_0$ perpendicular to the direction of the field. Calculate the particle's path? What does ...
1
vote
1answer
16 views

Can iron be magnetized Via electric current in aqueous solution?

If you suspend iron particles in water and run an electric current through, the particles gather in a line along the current. Is this a form of magnetism?
3
votes
0answers
31 views

Building a Faraday cage for mobile networks that is transparent for optical wavelenghts

A friend of mine is working on an architectonic project, where she designs a Faraday cage type of open space built in parks or other public places. The initial thought is to build it from some sort of ...
0
votes
1answer
20 views

What are the advantages of Magnetocardiography?

I am reading the book Bioelectromagnetism by Malmivuo: Difference between the bioelectric and biomagnetic measurements lies in the sensitivity distributions of these methods. Diverse technical ...
1
vote
1answer
20 views

Is electricity constant in standard ECG device? [closed]

I am reading the book Bioelectromagnetism by Malmivuo et al. I am thinking if you need to use Maxwell equations in electromagnetism of ECG device. I am not sure if you need to change the current ...
1
vote
0answers
17 views

Conductivity Matrix (Symmetry Information)

I'm trying to understand the symmetry content of the conductivity matrix: one information is, presence of time-reversal symmetry causes the off-diagonal terms to vanish. When this is broken (e.g. in ...
1
vote
1answer
16 views

Direction of induced EMF(and current) and Lorentz force direction?

Is the right hand rule for the Lorentz force (given the current and magnetic field directions)and the direction of induced EMF the same?
1
vote
0answers
42 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 ...
3
votes
1answer
52 views

Super Strong Magnets

I've seen the size of electric motors get smaller over the past 10-15 years...especially those used in small remote control vehicles like helicopters. I heard that the magnets being used in the ...
1
vote
1answer
28 views

Ampère's law and superconductors

I have been reading about superconductors and emerge me a inquietude about the explanation of existence of currents inside the superconductor while their magnetic field (inside too) is zero. Based in ...
2
votes
3answers
85 views

Self-energy of electron from classical reasoning

If it takes energy to group charge together(self energy) how can it be possible for every single electrons, etc, to have exactly same amount of charge? (think of if we hold some sand in our hand, then ...
5
votes
2answers
2k views

How does Hendo hoverboards achieve the self-propelling motion? What is the MFA?

From what I understand of the idea behind Hendo hoverboards, they use four disc shaped hover-engines to generate a self-propelling motion. I also understand that it uses Lenz's law and eddy currents. ...
0
votes
2answers
43 views

Is there a relationship between the diameter of a copper wire and the bandwidth it can carry?

I'd suspect intuitively that the bandwidth should decrease as the diameter decreases, but I don't have any reasoning to back it up. Secondly, would the actual wavelengths that it can carry, also ...
1
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0answers
67 views

The classical hydrogen atom

Suppose we want to analyze a hydrogen atom using purely classical mechanics. This obviously is not exactly how things work - quantum mechanics plays a huge role and probability distributions are ...
10
votes
6answers
601 views

Recommended books for undergraduate electrodynamics

What books are recommended for an advanced undergraduate course in electrodynamics?
1
vote
1answer
71 views

How can we derive the gauge field Lagrangian?

I learned the gauge field Lagrangian is given in this form: $$\mathcal{L} = -\frac{1}{4} \mathrm{Tr}(F_{\mu \nu}F^{\mu \nu}).$$ But how one can derive this equation starting from defining the ...
1
vote
1answer
25 views

Right Hand Rule for Proton Magnetic Field

I thought that right hand rule was for find the magnetic field generated by a current, where your thumb point in direction of the current. However, I was watching something that said we can curl our ...
1
vote
1answer
44 views

How to show that the force and velocity are perpendicular and that both have constant magnitude? [closed]

The force acting on a moving charged particle in a magnetic field $\hat{B}$ is $\hat{F}=q\left(\hat{v}\times \hat{B}\right)$ where $q$ is the electric charge of the particle, and $\hat{v}$ is its ...
0
votes
1answer
53 views

Energy Imparted by a Magnetic Field

As I understand it, when you switch on an electromagnet it generates a magnetic field that pervades all space, imparting potential/kinetic energy to every object with which it interacts. Where does ...
3
votes
1answer
60 views

Reflection of two electromagnetic waves

Can Electromagnetic waves be reflected by another electromagnetic wave without having any physical (transparent or opaque) material (i.e., in free space with one wave having twice the amplitude of the ...