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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What is the meaning of dielectric constant with AC fields?

I understand that in the DC case for parallel plates, a dielectric constant tells you by what factor the electric field gets reduced in the dielectric material vs. in vacuum. Does the same ...
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2answers
55 views

Is there any other momentum besides the Poynting momentum stored in an electromagnetic field?

I am having some conceptual difficulties with energy and momentum stored in the EM field. The force density at a point is $\rho E + j\times B$ Because of conservation of momentum, and because the ...
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2answers
54 views

What is $c$ in the Lorentz force expression?

The usual Lorentz force expression I am familiar with is this: $$\vec F=q(\vec E+\vec v \times \vec B)$$ I have seen some other versions lately that include an extra factor $1/c$: $$\vec ...
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36 views

What are diamagnets and paramagnets?

A paragraph in my text reads: An atom or ion with all electrons paired is not attracted by strong magnets and is termed 'diamagnetic'. In contrast, spins do not cancel when unpaired electrons are ...
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183 views

CGS Units for Magnetism

Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
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51 views

Experiments or phenomenon to falsify the existence of monopoles?

This is a related, but opposite question to this one I have heard about a lot of things regarding elementary and GUT magnetic monopoles, as well the quasiparticle monopoles in spin ice Since there's ...
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91 views

Doppler shift and special relativity

I have been listening to an audio course on Special Relativity. In the course the teacher repeatedly says that the "laws of physics are equally valid for all uniformly moving frames of reference". ...
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1answer
230 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 ...
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10 views

how to build an anti proximity alarm [migrated]

I am trying to build an alarm that works in an opposite way to a security alarm in a shop. In shops RF tags on items set off an alarm as you walk through a detector. I wanted to build alarm that ...
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24 views

The image current due to moving charges and a current carrying thin wire

Suppose we have a plane conductor and positive charged particles of mass m are moving parallel to the plane conductor at a distance d. The trajectory can be assumed to be straight. An image current ...
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2answers
214 views

Are electron fields and photon fields part of the same field in QED?

I know in classical field theory we have the electromagnetic field. And Maxwell's equations show how electromagnetic radiation can propagate through empty space. I also have been reading about QED ...
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1answer
102 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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16 views

Magnetization of a type I superconductor

How could I derive an expression for the magnetization of a spherical type I superconductor? I've searched in many sites but I haven't found any way to proceed.
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639 views

How exactly does a solar flare cause a power line surge?

There are occasional large scale electrical blackouts caused by solar flares. Quebec Solar Flare Blackout My question is: what affects the severity of the problem? It obviously depends on the ...
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1answer
228 views

Remote EEG detection / reading

I recently noted that Philips, 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 ...
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3answers
437 views

Why can't electrons fall into the nucleus?

I read a book on pop sci book on quantum mechanics and the author said that electrons do not fall into the nucleus due to quantum mechanics- which principles suggest this (I think it was Heisenberg's ...
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1answer
1k 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. ...
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2answers
291 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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1answer
35 views

Distance of electromagnetic waves based on frequency

I would like to ask your opinion regarding this matter: Based on the relation of frequency and wavelengths, the higher the frequency, the shorter the wavelengths and therefore the distance of travel ...
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2answers
26 views

Quality Factor in a Parallel LC Circuit

I was wondering if there is a notion of a quality factor in a parallel LC circuit, since there is no resistance. One can show that this circuit has a resonance frequency as follows: Impedance: ...
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1answer
2k views

How can a magnet have a greater magnetic field strength than a bigger, heavier and more powerful magnet made of the exact same material?

Here are some informations I found on the magnetic field strength (also called "magnetic flux density" or "magnetic induction", commonly denoted as B and expressed in tesla or in gauss) of an average ...
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1answer
48 views

If magnetic field cannot change velocity, what changes the velocity of a compass needle?

I am really confused after reading the properties of magnetic field. I thought magnets could speed up things when they attract them.
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1answer
56 views

Advanced (undergraduate) book on electromagnetism [duplicate]

I am an undergrad student and i have been through a first easy course on electrodynamics and i have used the book of Griffiths which let me satisfied.But the next two courses I will be taking are ...
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1answer
17 views

understanding shceme field lines - magnetic field

I would like to know if the magnetic field is in "the x-y plane". Does this mean the field lines are actually parallel to the visible line x or y? Or does it mean the field lines are parallel to the ...
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1answer
16 views

Electric force between 2 rods with different lengths carrying different currents

What is the electric force between tow rods with different lengths that carry currents $I_1$ and $I_2$? Indeed, by the use of the Biot-Savart law I obtained a relation to calculate it, but there is ...
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16 views

Lifting capacity of superconductors

How does the strength of the magnetic field the superconductor is exposed to affect its ability to levitate/lock heavier objects? Or is there no relationship (for superconductors) between magnetic ...
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2answers
61 views

Are spherical coordinates distances or angles?

I've become confused about spherical coordinates when dealing with electric fields. The way I always understood spherical coordinates is something like the below picture. To define a vector, you give ...
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2answers
107 views

Solution of simple problems using only Maxwell equations in differential form

Solve simple electrostatic or magnetostatic problems using only Maxwell equations. For example: In every book there is an excercise to find a magnetic field outside a thin wire of radius $a$ with ...
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44 views

How do I calculate the energy flux for this toroid?

My question is how do i calculate the total energy flux through the walls of the torus in this situation? The Poynting flux is given by $\vec{N} = \frac{1}{\mu} ( \vec{E} \times \vec{B})$ but the ...
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2answers
120 views

Far Field Diffraction of EM waves: what does the zero frequency signify?

If you have a system of independently radiating electrons/point-charges, the far field distribution of the EM waves can be approximated by the fraunhoffer diffraction integral, or simply by the ...
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With what frequency triangle swings if it shuns from equilibrium? [on hold]

Guide V-shaped is with no friction in the swivel bearings A and B (see Fig.) Of the magnet with a vertical magnetic field of 0.8 T. The weight of the guide is 50 g, the length AB 4 cm, and the length ...
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11 views

What voltage is induced between the endpoints of bar, if we swing it by an amplitude [on hold]

After a long horizontal conductor carrying current of 10 A is vertically suspended 2m long insulated conductive rod. We swing it by an amplitude of 10 cm. (a) in the direction of the wire; (b) in ...
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2answers
138 views

Electromagnetic reaction force?

The classical (retarded) Lienard-Wiechert scalar and vector potentials describe the electromagnetic field due to an arbitrarily moving electric point charge. Thus given the motion of electron $A$ one ...
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2answers
175 views

Use of Amperian loop to calculate the $B$-field inside a co-axial wire

The problem asks to calculate the magnetic field at point $a$ of a co-axial cable with a wire at the center surrounded by a rubber layer, and then another hollowed cylindrical wire followed by a ...
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1answer
30 views

Maxwell, speed of light and the electric permittivity and magnetic constants

When Maxwell found that the already-measured speed of light could also be derived from the already-measured electric and magnetic constants, what were the electric and magnetic constants measured to ...
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3answers
324 views

Polarization vector of dielectric electrostatics

I know two laws for Polarization vector of a dielectric material $$\vec P = N q \vec{\Delta L}$$ Where $N$ is the number of dipoles per meter cube, $\vec{\Delta L}$ is the vector that represents the ...
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1answer
48 views

Electromagnetic cooling

Heat means stored kinetic energy in matter. Microwave owens and induction heaters works on increasing kinetic energy on matter. Is it possible to make inverse of this heaters like cooler. Can we ...
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1answer
172 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
83 views

Why does aluminium foil spark in the microwave? [duplicate]

I don't understand fully why aluminium foil sparks in a microwave oven? I think what is happening is the oven sets up an electric field, and the field induces a current in the foil (how?), and ...
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6k views

Does a positive or negative charge attract a neutral object?

Three objects are brought close to each other, two at a time. When objects A and B are brought together, they attract. When objects B and C are brought together, they repel. From this, we conclude ...
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21 views

Is the Electric field around a loop on conducting wire always constant?

If a wire is subject to some arbitrarily changing magnetic flux inducing an electric field within, what is the E-field equal to? $\oint \vec{E} \cdot \vec{dl} = -\frac{d}{dt} \int \vec{B} \cdot ...
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24 views

Uniform field through a disc

Let us say that we have a magnetic field of the form $B_0 \sin(\omega t) \hat e_z$ in all space. Now let us say we put a conducting disc within this field. I want to find the electric field within ...
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1answer
91 views

Constant magnetic field attenuation by µ-metal (mu-metal)

I am interested in the magnetic field attenuation by µ-metal. Specifically shielding from earth's magnetic field $\boldsymbol{H}_E$ through closed cylindrical layers of such a metal, yielding an ...
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1answer
26 views

Why isn't any current induced in a rectangular loop when it is moved in a uniform magnetic field with constant velocity?

It is said that the magnetic flux passing through the coil remains constant. But when the rectangular loop is moved out of field with constant velocity,then the magnetic flux decreases. Thus current ...
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1answer
43 views

Why do magnets vibrate near an AC source?

Why do they vibrate and simply not get pulled closer or further from the source of emf? This may be related to the shape, but if I take a small N45 disc magnet and put it near an electrical ...
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1answer
37 views

Question about Gauss' law and dipoles

I've got a question about dipoles and Gauss' law: Consider an electric dipole $p=ql$ in free space, centred at the origin of a Cartesian reference frame. Using Gauss’s Law, discuss if the electric ...
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18 views

What is strength of an electromagnet at a distance?

I want to levitate a permanent magnet between two electromagnets. My two governing equations are: $$F = \frac{B^{2}A}{{2\mu_{0}}}\tag{1}$$ $$B = \frac{NI\mu}{L}\tag{2}$$ Where $x$ is the distance ...
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24 views

In a system with a decaying magnetic field, is the induced E-field around the wire constant?

Below is an example problem that I know how to solve, particularly when the current is switched off and the magnetic field is allowed to decay. However, in systems typically like these, is it a far ...
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1answer
107 views

Magnetic field of a static disk magnet

I have a magnetic disk (Radius R, height h) that creates magnetic field lines (axisymmetrical). I simulated the field lines, exported the data and am now trying to fit a function into the data to have ...