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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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 ...
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1answer
28 views

Given a Magnetic Field vector, what is the Electric Field, charge density, & current density?

I tried searching for this before having to ask, so please forgive me if I missed a previous question similar to mine. This is a Grad class, so I may be unnecessarily too careful in not assuming ...
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1answer
38 views

Fields and the way they should be looked at

When I think about the electrostatic interaction and interaction of charged particles having uniform velocities sometimes I do find it giving rise to a picture of superluminal communication which ...
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20 views

Anyone know of a flow chart or list of common/useful consequences of Maxwell's equations?

I just recently started to appreciate the Maxwell equations. I had never really take the time to study them but I feel like I'm finally more familiar with them. I've noticed that it seems like a lot ...
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190 views

Energy stored in fields

How to intuitively think of energy stored in a magnetic/electric field? Kindly answer in a bit simple terms without referring to mathematics.
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34 views

Why doesn't a levitating superconductor generate usable electricity?

Why doesn't a super cooled superconductor generate usable electricity due to the phenomenon of electromagnetic induction when placed upon a magnet? I basically mean, why can't it create electricity by ...
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33 views

Finding current density?

I found (from here) that current density can be found with the formula: $$\vec j= \frac{dI}{ds} \vec a$$ where $s$ is the cross sectional area centred on the point we are considering and with a ...
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2answers
49 views

Divergence of vector potential

I was given the vector potential $$\vec A (\vec r) = - \vec a \times \nabla \frac{1}{r}$$ with a constant vector $\vec a$. Now, I found the $\vec B$ field which is I think $- \vec a \frac{2}{r^3}$, ...
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72 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 ...
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57 views

Divergence of the Poynting vector with static fields

The Poynting vector is an energy flux density, and applying the divergence theorem shows that: $$\int_V (\nabla \cdot \vec{S}) dV = \oint_{\partial V} \vec{S} \cdot d\vec{a} = -\frac{\partial ...
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196 views

Is there inductance to a DC circuit?

When a DC circuit is carrying current, large amounts or small, is there induced-emf due to the inductance? Or is it only applied to AC circuits?
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128 views

Are magnetic hysteresis losses relevant to alternating currents flowing in a wire?

Say we have an AC in a magnetically lossy material, like iron. Because of iron's relatively high permeability, skin effect will be more pronounced than it is in say, copper, so this iron wire isn't so ...
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61 views

Different forms of the Einstein field equation

I am working my way through the wonderfully written introduction "General relativity for mathematicians" by Sachs & Wu. I am indeed a mathematics student and find this book to be well suited to ...
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1answer
40 views

If a cube magnet were a die, how would it be numbered?

I have a number of cube-shaped magnets. If I start with 64 cubes all in one long chain and begin arbitrarily "folding over" groups of them so that I am left with a 4x4x4 cube, I rarely get the same ...
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3answers
594 views

What was meant by the 'ponderomotive force' as understood by Minkowski?

Skimming through Minkowski's famous 1907 paper, he uses the term ponderomotive force. What does he mean by this?
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1answer
14 views

Plastic and Iron - static and magnetic

I'm wondering if when you have a ball that is made out of plastic but has an inner core of metal. Would that ball if given friction produce static? Or would the static from the outerside of the ...
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63 views

Momentum around an accelerated electron

Assume that an electron is accelerated along the +x-axis. The electron will radiate electromagnetic energy and momentum in every direction. But it seems to me that the EM momentum it radiates in ...
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1answer
72 views

Is lasering some natural phenomenon soemwhere?

Laser is actually the simplest kind of light, from a physical point of view. Many people think it is some rocket science simply because it is not natural but takes some human efforts to create. So, ...
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2answers
36 views

What happens to electric field of a bar magnet?

Electromagtic waves say that Magnetic field and electric field exist orthogonal to each other. Also all electric field has some magnetic field and vice-versa (As I understand). In that case what is ...
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31 views

Creating permanent magnets with minimal resources?

Okay, so this is kind of a goof-off question, but it's part of an of educational game that I've been playing with myself for years. If you don't want the background then skip to the horizontal rule, ...
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30 views

Why is no work done by magnetic forces in a uniform B and E field (perpendicular to one another)?

Say $B$ points in the $x$ direction, and $E$ points in the $z$ direction. Assuming a charged particle begins at rest at the origin, its motion will be a cycloid that progresses indefinitely in the ...
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1answer
17 views

$E \times B$ drift plasma physics

Do you need to have an electric field in order to have an $\mathbf E \times \mathbf B$ drift? I'm confused because it's seems that there is an induced E-field from the charge separation due to forces ...
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1answer
79 views

Earnshaw Theorem for ionic solids

Is a single molecule of sodium chloride (say) or a cluster of molecules of NaCl unstable, although macroscopically NaCl is in fact, stable? How can I reason this based on Earnshaw's theorem?
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192 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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1answer
155 views

Dielectric tensor vs. conductivity tensor in (cold) plasmas

I'm studying Waves in Cold plasmas right now, but I guess my question is generalizable. It's about the 4th Maxwell Equation in polarizable / conductive media: $\nabla \times H = \frac{1}{c} ...
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3answers
87 views

Induced EMF of a rectangular loop should be zero?

Considering the shape of a rectangular loop in a changing magnetic field: The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. ...
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2answers
95 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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2answers
67 views

How to demagnetize a magnet temporarily?

How to demagnetize a magnet temporarily? Is it possible making a permanent magnet demagnetize for a short span of time? Or at least long time but temporarily!
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9 views

Interaction between the induced field of a current carrying with an exterior field?

If a current carrying wire is placed inside a magnetic field(B), and the wire creates it's own magnetic field($B_w$), will $B_w$ be in opposition to the applied external magnetic field(B)? Will there ...
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1answer
166 views

Closed circuit integral, magnetic vector potential for straight line segment

Looking at a worked example in Chengs Field and Wave Electromagnetics (newest ed, p 236) about Biot-Savarts law. A conducting wire of length $2L$ is carrying the current $I$. Find $\textbf{B}$ by ...
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1answer
94 views

Dielectric boundary

I am trying to determine why electric field may be confined to a certain region if there is a large difference in the permitivity for example if electric field flows through water and then reaches a ...
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1answer
25 views

What is the physical interpretation of the free current ($(K_f\times \hat{n})$)?

The diagram below, taken from Griffiths is of an Amperian loop on a surface current in matter. Why is it that the free current is perpendicular to the surface current?
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41 views

Electric Flux through cube from point charge

"A point charge is located at the origin. Calculate the flux of E through a cube centered on the origin and aligned with the Cartesian axes. Evaluate the surface integral directly and verify that the ...
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1answer
35 views

why the electromagnetic wave transmits momentum in the direction of the movement?

Ordinarily a transverse wave does not transmit momentum, as it is the case of the well-known sea wave phenomenon. But the electromagnetic radiation has both fields transversal to the movement of the ...
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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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67 views

Attributes of polarized light

Which of the following is true for a polarized light? The electric vector is constant in its module and direction The electric vector oscillates in every plane that passes through the ...
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19 views

Earth's magnetic effect on my magnet

My magnet seems to work independently of, and unaffected by, the Earth's magnetic field, yet the magnetic field of my magnet changes as I introduce some other similar magnets of varied sizes into its ...
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26 views

Is there any material that a magnetic field cannot penetrate? [duplicate]

Is there any way to build an enclosure that protects its contents from magnetic fields more effectively than simply keeping sources of magnetic fields away by the same distance the box would?
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51 views

Why do we lose signal in elevators? [duplicate]

Whenever I am talking on my phone and walk into the elevator the call drops as soon as the doors close, and also the WiFi signal completely stops. Why does this happen? Note: I am asking this ...
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1answer
103 views

How does back-emf oppose the input-current?

A circuit, has current $A$ flowing at a certain $V$. When there is a change in magnetic-flux, based on Faraday's law of induction & Lenz's law, we know that there is change in Potential ...
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9 views

Magnetic field generated by wires with magnetic cores

Describe the magnetic field generated by alternating current as it passes through a wire's special core of tiny, metal-coated, cylindrical rare earth magnets? Examples of real-world consequences of ...
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3answers
117 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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15 views

Phased non linear array antenna - First Sidelobe

I have a problem I cannot seem to solve and I REALLY need some help. It's about phased-array antennas whose dipoles are not equally spaced, not equally phased, not equally fed (amplitude). Let's ...
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1answer
61 views

How to propagate a planar e/m field in free space using plane waves?

I read this great answer to this question: Numerical software to manipulate a light beam in its plane wave representation? The main thing that I am trying to clear in my head is the following: ...
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1answer
596 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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3answers
124 views

What does amplitude in wavelength of light physically mean?What oscilates with time in photon?

Like amplitude in wavelength of water waves signify the displacement of water particles about their mean position.
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1answer
78 views

Does $v \cdot B$ carry physically meaning information?

Given two vectors $v$ and $B$, $\frac{v \cdot B}{|B|}$ yields the magnitude of the projection of $v$ onto $B$ and $\frac{|v \times B|}{|B|}$ the magnitude of the rejection of $v$ from $B$. The force ...
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1answer
44 views

Forces on a wire moving in a magnetic field?

When a wire is placed inside a magnetic field and current starts to flow in that wire, that wire experiences the Lorentz force = $IL \times B$ and beings to move. Aside from induced motional - ...
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3answers
962 views

Why does a dielectric have a frequency dependent resistivity?

This question has come about because of my discussion with Steve B in the link below. Related: Why is glass much more transparent than water? For conductors, I can clearly see how resistivity ...
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42 views

A question about induced fields

This is something that I just realized bothers me, after thinking about it too much. Faradays law tells us a changing magnetic field creates a circulating electric field. This electric field has a ...