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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Interaction Between Two Bar Magnets

How would one describe the interaction between two bar magnets? In the E&M course I took we described quite a few very basic electric interactions, but things that I would have thought of as being ...
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2answers
803 views

What is the force between two magnetic dipoles?

What is the force between two magnetic dipoles? If I have two current loops parallel to each other with currents $I_1$ and $I_2$ and radii $R_1$ and $R_2$ a distance $z$ from each other, what is the ...
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1answer
46 views
+50

Sequence of E and B field in radio waves and in single photons

In antenna technology we distinguish between nearfield and widefield. In the nearfield the electric and the magnetic fields are shifted by 90°. If you look closer you can see that there are two ...
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3answers
21 views

Electromagnetic Theory

As, using the fact that E*A stays constant for a point charge ( E= electric field, A=area of sphere centered on charge touching the point where we have to measure E) , we can prove that E falls by ...
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0answers
9 views

Magnetic Force on a Ferromagenetic Material

I am currently working on a project involving solenoids, and I needed a force(Newtons, not a measure of magnetic field strength) equation. What I came up with after some digging around on the ...
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1answer
313 views

Dielectric slab counter-intuitive formula?

For the calculation of force on one of the two equal(in magnitude, opposite in sign) point charges separated by $r$ with a dielectric slab of dielectric constant $K$ and width $d$ in between, the ...
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2answers
119 views

What happens when you pass a current through a coil made of a ferromagnetic substance?

I know when you pass a current through a length of copper coil or a solenoid, there is the induction of a magnetic flux. But what of the coil made of the ferromagnetic material, the permanent type ...
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2answers
46 views

amplification of magnetic field

can we by any means amplify magnetic signal as we can with electric signal. As both electric and magnetic field can be represented in the form of a wave the analogy seems to be natural. I want the ...
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4answers
100 views

Is magnetic levitation a good way to vibrationally isolate a speaker from its support?

I recently heard about this company planning to sell a magnetically levitating bluetooth speaker. I understand that the levitation is partly motivated by visual aesthetics, but on the website the ...
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4answers
268 views

Are metals more heavy due to the Earth's magnetic field?

Non-metal objects are attracted to the Earth due to gravity. So the weight of non-metal objects can be only dependent on their mass. On the other hand metals can be attracted to the Earth's magnetic ...
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1answer
41 views

Teflon and induction cookware

I remember once getting new Teflon (non-stick) cookware; however, when I tried this new cookware on my induction cooker the cookware did not heat up. My regular steel cookware worked just fine on the ...
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2answers
106 views

Drift Speed and Current in Two Different Inertial Frames

We have a long, cylindrical wire carrying a constant current I in an inertial frame. At a distance of R from the center of the wire, the magnitude of magnetic field is $μI/2πR$. What is the magnitude ...
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1answer
103 views

about field gradient

I read the term field gradient in most of the article about magnetic field. I search it online but most of the explanation is about the math. I wonder in physics, what the gradient field really mean? ...
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1answer
289 views

If we have a current $I$ flowing down a wire, why must the net bound current be zero?

Say we are dealing with a wire that has a current $I$ flowing through it, i.e. $I$ is the free current. Why must it then hold that the net bound current, that is, the bound volume current, $J_b$, and ...
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1answer
54 views

How can I boost the FM reception of a smartphone? [on hold]

How can I improve the FM reception on my Nokia smartphone? I have tried by removing the insulation on earphone wires and connecting the earphone wires to another wire placed high on roof but it has ...
13
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4answers
1k views

Are voltages discrete when we zoom in enough

Voltages are often thought of as continuous physical quantities. I was wondering whether by zooming in a lot, they are discrete. Can all voltages (potential differences) that exist in reality be ...
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1answer
383 views

Proof of equality of the integral and differential form of Maxwell's equation

Just curious, can anyone show how the integral and differential form of Maxwell's equation is equivalent? (While it is conceptually obvious, I am thinking rigorous mathematical proof may be useful in ...
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5answers
10k views

How do moving charges produce magnetic fields?

I'm tutoring high school students. I've always taught them that: A charged particle moving without acceleration produces an electric as well as a magnetic field. It produces an electric field ...
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6answers
94 views

Do electrons flow only on the surface of a wire?

Since the $\vec{E}$ field inside a "perfect" conductor is zero, do the electrons(the current) flow only on the outer surface? This has bothered me since I studied electromagnetism. Thank you for ...
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6answers
1k views

Why aren't there compression waves in electromagnetic fields?

I just started learning about optics, and in the book I'm reading they explain how the electrical field caused by a single charged particle could be described by a series of field lines, and compare ...
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1answer
44 views

What physical forces allows for electromagnetic induction

In electromagnetic induction, what force is actually doing the work? what physical force actually drives the electrons around the circuit? Let's say we have a coil and an increasing magnetic field ...
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2answers
99 views

Does a simple copper wire with an AC current create EM waves?

I want to know if a household AC current flowing through a simple copper wire will radiate EM waves? If yes, up to what range and can they be called radio waves?
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1answer
89 views

Boltzmann Distribution of Electrons in Confining Potential

I have a particle simulation wherein many non-interacting electrons are trapped in a electric potential well. I would expect, and therefore I initialize according to this, that the electrons would ...
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2answers
50 views

Current Density

I'm trying to understand the definition given on my electromagnetism course for the current density. More specifically, I want to know why, as defined below, the current density is given the name ...
2
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1answer
90 views

Boundary conditions in Electrostatics

If I have a grounded conducting material, then I know that $\phi=0$ inside this material, no matter what the electric configuration in the surrounding will be. Now I have a conducting material that ...
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0answers
24 views

calculating electrostatic force [on hold]

I am trying to answer this question: Calculate the electrostatic force on a metal ion carrying a charge of $3.2$x$10^-$$^1$$^ 9\ \text{C}$, and which is in an ionic solution between two parallel ...
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2answers
444 views

How do I express the interaction energy between two charged spheres?

Consider two identical insulating spheres each with radius $R$ and uniform charge $Q$ through their volume. They are separated from their centers by a distance of $d>2R$. Here is my general ...
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0answers
18 views

Very short physics question (electromagnetic field): Just draw the figure of the problem? [on hold]

Can you please draw me the figure of the problem? I dont want the solution,just the figure? A straight, stiff, horizontal wire of length 25 cm and mass 50 g is connected to a source of emf by light, ...
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1answer
100 views

Why is the magnetic field stronger at the edges of a bar magnet?

I have been performing an experiment at school in which I test the force on an iron surface from the magnetic field of an electromagnet. The electromagnet has a rectangular iron core. The theory ...
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0answers
18 views

Splitting of degenerate energy levels with a perturbed particle in a box

Suppose you have a particle in a square box $[0,L]\times[0,L]$. As the box is a square, the (2,1) and (1,2) eigenfunctions will have the same energy. If you were to apply an oscillating electric field ...
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0answers
25 views

Smallest view-able object by radio wave magnification with metamaterial?

What is the theoretical and in practice microscopic maximum/best resolution of a meta material radio wave lens (assuming diffraction or beyond diffraction techniques)? I'd like to use diffraction or ...
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1answer
37 views

How much power you might need to generate dangerous EMP shock?

While playing with voltage multiplication and capacitors, I began to think if my experiments could damage my electrical equipment (eg. mobile phone) by creating strong magnetic fields over a short ...
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1answer
130 views

Total Momentum From a Standing Electromagnetic Wave

How does one show the momentum imparted to a perfect conducting resonance cavity (boundary) of any shape by a classical standing electromagnetic wave inside is zero? It should be by conservation of ...
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2answers
2k 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 ...
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0answers
24 views

Poynting Vector Volume Integral Inside a Cavity

Given an electromagnetic wave in resonance mode in a vacuum cavity inside a perfect conductor, on the boundary, the parallel component of $E$ field vanishes, and the perpendicular of component of $B$ ...
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0answers
19 views

Physical reason for magnetic mirroring?

I understand the mathematical derivation of magnetic mirroring, which usually starts from the conservation of the magnetic dipole moment (e.g. in a plasma). But physically: a mirrored particle is ...
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1answer
137 views

Earnshaw's theorem and springs

Earnshaw's theorem states that the Laplacian of the potential energy of a small charge moving through a landscape full of static negative and/or positive charges (and gravity) is zero. Thus you can't ...
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1answer
48 views

Transformer ratios - 1:2 vs 50:100

I am only the equivalent of a high school student, so please, if possible, don't answer this question with anything too complex or really advanced university level. I am very happy to research new ...
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0answers
28 views

Circuit breaker Trips during thunderstorm

The circuit breaker at the electrical mains trips at home when there is a thunderstorm outside. Why does this occur?
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1answer
84 views

What determines the magnitude of force of an electromagnet?

Let's assume that there is an electromagnet with $B = 1 Tesla$ , what is the magnitude of the force it could generate on other dipoles equal to it's surface area? Can the magnitude of force (in ...
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3answers
56 views

Ampere's law and external currents

In Ampere's law the current outside the curve taken is not included in the expression. Does this mean that only the currents crossing the area bounded by the curve taken contribute to the magnetic ...
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1answer
115 views

What is absorption of an electromagnetic wave?

Can any one explain the absorption/transmission/reflection of electromagnetic waves in the wave form? It is generally said that the atom absorbs/reflects/transmits photon. But can this phenomena be ...
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1answer
24 views

Magnetic force and relative frame

The magnetic field for to a moving charge depend on its velocity (Biot Savart's law). My question is that is it then not frame dependent? If it is, it means if a man is walking and other is standing ...
3
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1answer
119 views

EmDrive Cavity modes

The EMDrive described in patent application GB 2493361 is a simple section of a sphere surrounded by a superconducting cooling system. I have attempted to solve the oscillation modes possible and ...
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0answers
26 views

Ampere's law question regarding magnetic field [closed]

In amperes law the current outside the curve taken is not included in the expression u.i does this mean that the magnetic field calculated by using this law gives contribution of only the currents ...
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1answer
21 views

Single Photon emission from positronium in strong magnetic fields; how is momentum conserved?

I recently came across a paper (which I will not describe in detail, however, the authors examine the probability of positronium producing a single photon, while under the influence of a strong ...
2
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2answers
171 views

AC Electromagnets

Could someone help explain the uses of AC electromagnets. Wherever I look it says that DC electromagnets create stronger magnetic fields. I understand why AC electromagnets could be used in ...
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2answers
72 views

Electromagnetic induction of an uniform magnetic field

EDIT: Thanks to the first answer, I may see that there is differences between a giant solenoid and a completely uniform magnetic field. Therefore it would be great if one can explain to me both the ...
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2answers
76 views

Determining the minimum pull force of a magnet required to hold it in place

If I place a magnet on the underside of a metal object so that it is held there by its attraction to the metal, how can I determine the minimum pull force the magnet needs to have in order to keep it ...
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2answers
68 views

Curious about this perpetuum concept

I know it's not possible to build a perpetual motion machine, but I still got this concept running in my mind. If you take a (longer)copper coil, and insert an axis through it(right-angled) with a ...