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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Can unbound electrons jump energy levels?

If an unbound electron (or indeed any charged particle) is moving through free space, is there a probability that it can spontaneously change energy by emitting a photon, or does this require the ...
3
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1answer
161 views

Can you safely draw sparks from the nose of an electrified boy?

From Purcell's Electricity and Magnetism A spectacular conclusion of one of the popular exhibitions of the time was likely to be the electrification of a boy suspended by many silk threads ...
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2answers
3k views

Magnetic field in a cavity

We are given an infinitely long cylinder of radius $b$ with an empty cylinder (not coaxial) cut out of it, of radius $a$. The system carries a steady current (direction along the cylinders) of size ...
2
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2answers
107 views

Origin of field deduced from potential

Related: Tubelights+power lines pictures? I would've edited this into the above question, but I realized that there' enough to it to qualify as a new one. Plus this seems to be a confusion of ...
2
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1answer
2k views

In what position can a current-carrying loop of wire be located in a magnetic field so that it doesn't tend to rotate?

I understand that if the wire is not aligned with the magnetic field, it won't rotate, but I'm still confused on how so. Also, which direction is the current flowing?
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1answer
303 views

What is the mathematics behind artificially generated plasmas via electric fields?

The ionization degree of a plasma is given by the Saha equation, which depends on the temperature and the particle specific ionization energy. In thermal equilibrium, the relation between ionization ...
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1answer
104 views

Formula for potential for 2 coaxial tubes

Could someone remind me of the formula for potential function for a system consisting of 2 coaxial tubes maintained at a fixed potential difference and the in between medium has uniform conductivity ...
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2answers
1k views

How long will my fridge magnet stay attached to my fridge?

I have a 500 gram neodymium magnet. It is stuck to my fridge. With the constant pull of gravity trying to pull it off how long would it stay attached to fridge? For arguments sake lets say my fridge ...
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1answer
315 views

What is electric/magnetic field in a medium?

One method of introducing electric field is based on the measurement of the force acting on moving charged particle. By equating F to qE we determine the electric field E if the electric charge q is ...
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0answers
488 views

Dielectric in Parallel Plate Capacitor

Given a parallel plate capacitor of width $w$, length $l$, with a dielectric moving along the length $l$. Let the dielectric be from $x$ onwards. The capacitance will be $\frac{w \epsilon_0}{d} ...
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2answers
2k views

Properties to select suitable materials for making permanent magnets

I have read that soft iron is suitable for making a permanent magnet. Because it is required for permanent magnet to have high coercivity and high retentivity. Same text also said earlier that soft ...
2
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3answers
513 views

What does physics study? [closed]

Wikipedia definition: Physics (from Ancient Greek: φύσις physis "nature") is a natural science that involves the study of matter[1] and its motion through spacetime, along with related concepts such ...
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4answers
2k views

Some questions about car radio and cellphone antennas

1-Why the antenna of the radio of cars is located outside the car and not inside? 2-If the answer to 1 is because that cars are like Faraday cages then how come my cell phone can receive signal ...
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214 views

What is the electric field part of an EM wave? Radiation field or the induction field?

Look at this image: I wonder if the electric field is from the induction field from a vibrating electron or the radiation field? If it is from the radiation field, as I suppose, than can someone ...
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2answers
886 views

Mechanism by which electric and magnetic fields interrelate

I read that force due to electric field on some particle in one reference frame can exhibit itself as force due to magnetic field in some other reference frame and that electric and magnetic fields ...
1
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1answer
964 views

How do home-made capacitator bottles affect a tesla coil?

In the tutorial I'm following, the guy uses 6 snapple bottles to make capaciter (by covering them with foil and filling them with salt water) for a 9kV 30mA input. I have a stronger transformer (15kV ...
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1answer
343 views

Will relative velocity with respect to electrons effect current in wire and magnetic field produced by wire? [closed]

Question: consider a long, straight wire of cross-sectional area $A$ carrying a current $i$. Let there be $n$ free electrons per unit volume. An observer places himself on a trolley moving in ...
2
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1answer
608 views

Would a metal enclosure (such as a shipping container) protect its contents from the effects an electromagnetic pulse?

I was watching a program about disaster preparedness, and it was suggested that the metal enclosure of a common shipping container (of the intermodal variety) would be sufficient to protect its ...
2
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1answer
221 views

Asynchronous generator run in vacuum chamber

what will be happen if we put asynchronous generator in vacuum chamber & run it above its synchronous speed. After reaching its over synchronous speed we will cut off electrical supply. Can it run ...
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3answers
1k views

Shielding magnetic field without mu-metal

I need to shield my device from magnetic interference, including earth magnetic field (if you move device around, it might be enough to cause slight currents i guess) and magnetic field caused by ...
2
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1answer
272 views

Does inducing a current in a wire have any effect on the strength of a permanent magnet?

Does inducing a current in a wire result in any changes in the strength of a permanent magnet? Specifically, what would the results of the following controlled experiment be? You set up two ...
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2answers
804 views

Image charges, laplace equation and uniqueness theorem

Consider a well-known problem of the electric field generated by a system composed of a point charge in proximity of a large earthed conductor. It is said that the potential due to an image charge ...
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1answer
1k views

How does a speaker produce sound?

What I read is that a speaker produces sound by the movement of a coil attached to a cone which moves back and forth. So, If I try to move the coil by hand, would it produce sound? If not, why? or Why ...
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0answers
345 views

Electrical machines: electrical and mechanical properties of steels and cast iron

I would like to know a good source of information for the common metals (steels and cast iron). Information that I need are basically the mass density and the electrical conductivity or resistivity. ...
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2answers
2k views

Conservation of Energy and the Poynting Theorem

Conservation of energy in an electrical circuit can be expressed by Ampere's law $$\nabla \times \textbf{B} = \mu_o \textbf{J} + \epsilon_o \mu_o \frac {\partial \textbf{E}} {\partial t}$$ when ...
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2answers
5k views

How to determine directions of vectors of an electromagnetic wave

I did an exercise which probably is quite popular, in which you draw an electromagnetic wave and prove that it should propagate at the speed of light $1 \over \sqrt {\mu_0\epsilon_0}$ using Farday's ...
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3answers
733 views

Electromagnetic wave reflection vs. light reflection

Related: x-ray interaction with atmosphere I know that electromagnetic waves of particular frequencies reflect from the ionosphere. And the light (which from one perspective is an electromagnetic ...
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1answer
72 views

Potential at a point

What is the electric dipole moment of the charge distribution with $q$ at $(0,0,1)$, $q$ at $(0,0,-1)$ and $-2q$ at $(0,0,0)$? I would think that it is $\vec{0}$ by the definition ...
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1answer
715 views

Blandford-Znajek process: Why/how does the current flow along the magnetic field lines

Related: How would a black hole power plant work? I have put a bit of commentary enumerating my confusions in parentheses I read in Black Holes and Time Warps (Kip Thorne), that quasars can generate ...
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1answer
1k views

Given charge distribution find electric field

Given a charge distribution $\rho(\vec{r})$ where $\vec{r}$ is the position vector and that $\rho$ is a function of only $|x|$, Why is it that the corresponding electric field $E$ is necessarily of ...
2
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1answer
219 views

Question on 1st order Lagrangian Derivation in Faddeev-Jackiw Formalism

I'm looking at this reference (sorry it's a postscript file, but I can't find a pdf version on the web. This paper describes a similar procedure). The topic is the Faddeev-Jackiw treatment of ...
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3answers
2k views

Electric field at a point inside a capacitor

Let's say I have a parallel plate capacitor. How would I find the electric field at a certain point INSIDE the capacitor (inside the dielectric let's say). From what I understand, the flux of the ...
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1answer
5k views

Repulsion of the pieces of a broken magnet [duplicate]

Possible Duplicate: Why does it seem like a broken magnet's poles flip? I have experienced that if we break a bar magnet into two pieces and try to bring those broken faces together it ...
2
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1answer
2k views

Electrodynamics textbooks that emphasize applications

Please recommend undergraduate-level textbooks on electrodynamics which emphasize practical applications and real life examples. Please describe the book's level and contents and its intended audience ...
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183 views

Should $E$ and $B$ change with Gravity?

Lets examine a typical GR metric: $$ds^2=g_{00}dt^2-g_{11}dx^2-g_{22}dy^2-g_{33}dz^2$$ The "d" going with ds has its correct meaning when the path is specified with respect to a one dimensional ...
0
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2answers
192 views

Is the magnetic field a feature of our universe, or is it a consequence of the electric field?

Is the magnetic field a feature of our universe, or is it a side effect of the electric field? In other words: if we were to simulate in a computer a system of moving charged particles, taking in ...
5
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3answers
779 views

Is photon emission possible without electrons changing energy levels?

Does molecular vibrational transition and consequent emission of infrared radiation involve electrons changing energy level? In wikipedia, about vibronic transitions it says "Most processes leading to ...
0
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1answer
668 views

Potential of a spherical conductor next to a point charge

Griffiths, when he's talking about the method of images, shows how to calculate the potential distribution when given a point charge that is outside of a spherical conductor which is held at a fixed ...
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4answers
3k views

Can the Lorentz force expression be derived from Maxwell's equations?

The electromagnetic force on a charge $e$ is $$F=e(E+v\times B),$$ the Lorentz force. But, is this a separate assumption added to the full Maxwell's equations? (the result of some empirical ...
2
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2answers
1k views

How are magnetic fields transmitted?

A common analogy for gravity is the ball-on-a-rubber-sheet model. In this model, mass distorts spacetime and creates a 'valley' into which other mass can fall. Is this same principal valid for ...
2
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2answers
427 views

Orientation of Magnetic Dipoles

Does a magnetic dipole (in a permanent magnet) tend to align with the B-field or with the H-field? The current loop (Ampère) model of the magnetic dipole suggests the former, while the ...
3
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2answers
1k views

Energy in an EM wave should depend on frequency

I just finished reading Feynman's Lectures on Physics vol.I, §34-9: "The momentum of light". The author explains that there is a relation between the wave 4-vector $k^{\mu}$ and the energy-momentum ...
2
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4answers
484 views

Gauss' law - changes in the magnitude of E field inside the closed surface

Gauss's law says that the flux through a closed surface which contains neither a sink nor a source will be zero. It's quite clear that all field lines will have to exit somehow, but the strength of ...
5
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2answers
494 views

Is there any idea why the electric charges of electron and muon are equal?

Is there any idea explaining why the electric charges of electron and muon are equal? Edit: The total charge of a particle is proportional to the integral of its own electric field flow through the ...
21
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3answers
1k views

Why do we need 12 atoms to store 1 bit of data?

Recent research at IBM has found a way to store 1 bit of data in 12 atoms. While that is a big accomplishment compared to what we have today, it does seem like a waste to a non-physics eye like me. ...
6
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4answers
7k views

Understanding the diagrams of electromagnetic waves

I'm having trouble understanding the diagrams of elctromagnetic waves. I have no problem with any concept in classical mechanics, and I think this can be answered without any relativity (which I ...
6
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4answers
874 views

What is the physical definition of causality?

Maxwell's equations give a physical relationship between the electric and magnetic fields $\vec E$, $\vec B$ at the same time, which some interpret as changes in one causes changes in the other etc. I ...
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1answer
2k views

What happens if I crush a neodymium magnet into powder?

There are a couple questions here. Will it retain its magnetism? Can I add the powder to the iron powder currently in my "magnetic" silly putty to make it truly magnetic? And finally, how best do I ...
2
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1answer
320 views

Complex Potentials, Potentials and Fields

Suppose an electric field $E=-\nabla \psi$ where $\psi=-Q\ln r$ where $Q$ is just some constant and I have found its harmonic conjugate to be $-Q\theta+c$ where $c$ is some constant. What does it say ...
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2answers
3k views

Why can't electromagnetic radiation from magnets be used for apparent perpetual motion?

If electromagnetic radiation represents a transfer of energy, then does a permanent magnet represent unlimited energy, and if so, why can't magnets be used for perpetual motion? Even if permanent ...