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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76 views

Maxwell and Bianchi equations

We frequently see: A certain action and then we are asked to solve for Bianchi identity and Maxwell equation. I have often solved for them but I never knew what is the difference between the two? In ...
2
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2answers
62 views

How magnetic field formed around Earth?

What are the factors that help the Earth to form a magnetic field around itself and why it is spread throughout from South to North?
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2answers
126 views

Velocity of an Electron as it Passes through a Uniformly Charged Ring

I've been presented with a problem in which an electron is placed a certain distance x from the center of a positively charged ring and allowed to move freely. The ring has a known charge density λ. I ...
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0answers
15 views

Software for drawing the shape of magnetic fields for different shaped magnets [duplicate]

I've been looking for some time on the Internet, for software or some system in order to draw magnetic fields based on the geometry of a magnet. Is there a specific software for this? I hope this ...
0
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1answer
84 views

How can you use the Delta Function to find the Electric field inside of a Hydrogen Atom?

This is a problem from my homework, but I'm lost conceptually on how to do it. I'm not looking for someone to do the work for me, I'm looking for some guidance on how I can set it up/understand it ...
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1answer
111 views

Is the Biot Savart Law applicable only for continuous currents?

There is a widely known formula for the magnetic field due to a moving charged particle. $$\frac{\mu_0}{4\pi} q \frac{\vec{v}\times\vec{r}}{r^3} $$ The usual derivation is as follows. $$ dB = ...
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1answer
33 views

Potential on a uniform

My doubt is in the part where he sets $V=V_s - \int{E dr}$. My idea is that he is summing two potentials, from the surface, and the one he's trying to calculate with the integral. Is that right? ...
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2answers
205 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 ...
3
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1answer
166 views

Is it possible to eliminate Van der Waals interactions?

I came to know that the friction force actually depends on the surface contact area due to weak interactions (adhesion due to Van der Waals forces) between the atoms of both materials increasing in ...
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1answer
381 views

What is the difference between an electric field and gravitational field? [duplicate]

Since the electrostatic field and the Newtonian gravitational field share a similar form: proportional to $$ \frac{1}{r^2} $$ Is there any qualitative difference between motions under the ...
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0answers
41 views

Where does the North Pole of a magnet originate?

My teachers have told me that magnetism occurs when spin, or magnetic moments of each atom in the substance align up, which creates an overall force. This would mean each atom is acting like a ...
2
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1answer
138 views

Vector Potential Oscillating E Field of the “Null” Field of a Hertzian Dipole?

The vector potential of a Hertzian dipole falls off spherically as $1/r$. The polar axis of the dipole is a "Null" field -- meaning no electric and magnetic field. The absence of magnetic field is ...
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1answer
24 views

Damping of motion of a conductor [closed]

The motion of a conductor is damped when allowed to oscillate between magnetic pole pieces. Two questions:- How is the damping force produced by eddy current? Why eddy current is undesirable in ...
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1answer
1k views

Calculating capacitance of integral PCB components

I posted this question originally with Electrical Engineering Stack Exchange but nobody seems to know the answer. So I thought a physicist might find the question more approachable. Formulas for ...
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4answers
343 views

Is this a Lorentz-scalar? How do I tell?

I'm struggling to identify whether a scalar is a Lorentz-scalar. E.g: $$\partial_i A^i \quad i \in {1,2,3}.$$ How do I determine if this is a Lorentz-scalar or not? If got the same problem with ...
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1answer
56 views

Vector calculus trouble in Hamilton's equations / particle in E.M.field [closed]

As part of applying Hamilton's equations to a particle in an electromagnetic field, one step is to take $\dot{\mathbf{p}} = - \dfrac{\partial H}{\partial \mathbf{r}} = -\nabla H = - \nabla ...
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0answers
93 views

Wave equation - Cases where separation of variables doesn't work

Separation of variables combined with the Fourier's theorem is the most common technique of solving D'Alembert's wave equation: $$ \Delta\Phi-\frac{1}{c_0^2}\frac{\partial^2 \Phi}{\partial t^2}=0 $$ ...
0
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1answer
49 views

What is a measure of how easy it is to magnetize a material?

Is there a measure (i.e. is there a word for..) how easy it is to magnetize a particular material? From some searching, I have found that 'coercivity' is a measure of how difficult it is to ...
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2answers
45 views

Does observing an EM wave take energy from it?

Here is the question that got be started on this: Suppose 1,000 people were sitting in a room with their laptops out and a WiFi transmitter in the front of the room. If everyone was connected to the ...
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1answer
43 views

Is it possible to overcome the strong radiational force?

If the electro magnetic force is 317 times weaker then the strong force then if 317 electromagnetic fields overlapped an area where the strong force is than couldn't it "turn off" the strong force?
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0answers
11 views

Why electron acts like massless when it's moving through graphene sheet? [duplicate]

It has been observed that when electron moves through graphene sheet,it shows a relativistic motion.why does that occurs?
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0answers
90 views

Magnetic ball Inside a rolling hollow copper sphere?

Just curious with regards to the production of electricity. What would happen if you put a magnetic ball inside a hollow copper sphere and rolled it down a hill? Where would current be generated ...
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0answers
27 views

What is the minimum size of a steel bar to 'absorb' the magnetism of one of the poles of a magnet?

I have two $51~\text{mm} \times 41~\text{mm} \times 17~\text{mm}$ Neodymium magnets ($N35$) which I want to connect with a bar of mild steel to form one horseshoe-magnet. The poles are on the $51 ...
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2answers
945 views

How is electrical energy transmitted?

My original question was in an effort to understand the electrical analogy to Markov chains, which is explained in Snell's article. There are some neat parallels that involve taking a Markov chain ...
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1answer
76 views

Work out components $F^{01}$ and $F^{ij}$ of the antisymmetric tensor $F^{\mu\nu}$ under the Lorentz Transform [closed]

Work out explicitly how the components $F^{0i}$ and $F^{ij}$ of the antysymmetric tensor $F^{\mu\nu}$ introduced in chapter I.6 transform under a Lorentz transformation This problem is from Zee, ...
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2answers
296 views

What is the pressure of a charged gas?

I have $N$ ${\rm H}^+$ gas molecules in a sealed, electrically insulated container. What equation can I use to accurately calculate the pressure of the gas? It seems to me that $PV = nRT$ will not ...
2
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1answer
48 views

Is a flexible space tower using coaxial superconducting rings possible?

Suppose we stack coaxially, vertically a large number of rings, made of some high temperature superconducting material, and start a current through each of the rings in alternate directions (e.g. the ...
2
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1answer
130 views

Two rings carrying opposite current directions

Suppose I placed a conducting ring on the ground with clockwise current, and then above it a second similar ring carrying CCW current. Wouldn't the repulsion between the two rings get the top ring to ...
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0answers
73 views

Transverse electromagnetic (TEM) mode waveguide

Suppose we have a parallel plate waveguide operating solely in the TEM mode, composed of perfect conductors. I understand that the electric field must terminate perpendicularly on the plates, where a ...
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3answers
2k views

Maxwell's Stress Tensor

What really is the Maxwell Stress Tensor? I understand that it's derived from $$\mathbf {F} = \int _V ( \mathbf E + \mathbf v \times \mathbf B )\rho \ d \tau$$ Griffiths describes this as "total EM ...
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1answer
1k views

What is the difference between the magnetic field of a permanent magnet, and that of an electromagnet?

What is the difference between the magnetic field of a permanent magnet, and that of an electromagnet? In the diagram below, you can see that the magnetic field of a wire which carries a DC ...
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1answer
94 views

Rutherford scattering in Quantum Mechanics terms?

Rutherford scattering stands as the first evidence of atoms having the positive charge concentrated in a small nucleus. While the classical description of the phenomenon is satisfactory, it puzzles ...
3
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1answer
151 views

How does a “permanent” magnet with switch work?

We have several magnetic stands on my university, see here. This stand has a button which makes you able to turn the magnet off and on. It is quite hard to press the button, it seems like something ...
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1answer
255 views

Repulsive Force between two Electromagnets

This question might seem a bit too basic(or advanced!), but it is the one to which I'm really desperate to find a simple enough explanation and solution. Ok, So here's the problem, I have two ...
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3answers
278 views

Why the induced field is ignored in Faraday's law?

Suppose we have a conducting ring in a constant magnetic field $\vec{B}$. Suppose that the ring is being deformed. We know from Faraday's law that such an action will induce a current in the loop. ...
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2answers
86 views

Compass aboard the International Space Station

If an astronaut brought a compass aboard the ISS and watched it for the duration of an orbit, what would it do?
3
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2answers
105 views

Magnetic Flux Quantization

I remember a few years back in my introductory AC/DC circuits class reading a little bit in the back of the book about magnetism. By this point, I was well aware that magnetic fields are commonly ...
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2answers
81 views

What are the most efficient ways to transmit force through air? [closed]

I am taking part in a robotics competition, where the challenge is to create a pair of robots which successfully navigate a series of obstacles. However, the rules state that of the two robots, only ...
0
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1answer
54 views

Why is it valid to say $\rho dx = \sigma$ in electromagnetism?

Some problems, in electromagnetism are solved by converting a given volumetric density into a surface density. Or a surface density into a linear density. For example, and this is a procedure done by ...
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1answer
331 views

Work functions of ceramics/insulators

Is anyone aware of or know of a good source or means of estimating the work function of a ceramic material? Typically, work functions are given for pure elemental metals, rather than for compounds, ...
2
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1answer
67 views

EMF produced when a square wire is pulled from the corners [closed]

The shown diagram is of a square conducting frame being pulled from the corners. The velocity of A and B is given as $u$. Each side has length $a$. I was trying to find the induced emf at the instant ...
10
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2answers
6k views

How do superconducting materials float in magnetic field?

The movie Avatar got me interested in the subject, but so far I only found sophisticated articles loaded with unfamiliar words. Is there a simple way to explain how magnetic field affects ...
37
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4answers
5k views

How wrong are the classical Maxwell's equations (as compared to QED)?

Now, I don't really mean to say that Maxwell's equations are wrong. I know Maxwell's equations are very accurate when it comes to predicting physical phenomena, but going through high school and now ...
5
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2answers
111 views

Is Quantum Coulomb still singular?

A single free charge (e.g. electron) $q$ fixed at the coordinate origin has the well-known Coulomb/electric potential $$\phi(\vec r) = \frac q{4\pi\epsilon_0}\frac 1r \tag{A}$$ where $r=|\vec r|$ of ...
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2answers
148 views

What is the divergence of a uniform field?

I was recently studying Maxwell's equations. Got a bit confused about divergence. Would someone please tell me if the divergence of a uniform electric field = $0$, & why it is so?
3
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1answer
71 views

Effects of magnets on memory cards

I have always believed in keeping magnets and data storage devices far away from each other. Friends say I'm excessively cautious about it, but I seem to have a lot less data loss than they do! As ...
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1answer
9k views

Magnetic field inside and outside cylinder with varying current density [closed]

I am reading through Introduction to Electrodynamics by David J. Griffiths and came across the following problem: A steady current $I$ flows down a long cylindrical wire of radius $a$. Find the ...
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1answer
3k views

Was Nikola Tesla right about his ether theory?

There are a lot of articles on the net that mention Nikola Tesla's ether (aether) theories, such as this quote: Only the existence of a field of force can account for the motions of the bodies ...
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4answers
1k views

Why is it so hard to make objects invisible in visible light?

We can make objects invisible in the realm of radio and infrared wavelength. We can, for example, hide a plane radar and heat signature from being detected. However, it seem, even after years of ...
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1answer
38 views

How to Vary the wavelength of UV CFL? [closed]

I have a $12$ $V$ $DC$ operated UV $[CFL]$(http://en.wikipedia.org/wiki/Compact_fluorescent_lamp) with $365$ $nm$ wavelength. I need to vary this wavelength in the $250-300-350-400-450-500$ $nm$. ...