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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7k views

Electric field due to nonconducting plastic sheets [closed]

Two very large, nonconducting plastic sheets, each 10.0 cm thick, carry uniform charge densities $\sigma_1, \sigma_2, \sigma_3$ and $\sigma_4$ on their surfaces (the four surfaces are in the ...
2
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2answers
1k views

Does a charging capacitor emit an electromagnetic wave?

Assume you charge a (parallel plate) capacitor. This establishes an electric field (the $\mathbf E$ vector points from one plate to the other) and a circular magnetic field (the $\mathbf B$ vector ...
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0answers
303 views

How to find the electric field at a point based on a uniformly charged surface

What is the general solution to finding the electric field at a point based on some (or multiple) charged surfaces. I know that we can perform a line/surface integral if a charge is close to a wire or ...
2
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1answer
83 views

Close electric field lines in wave guides

In a wave guide, graphics of propagation of Transversal Magnetic modes show closed field lines for the electric field. For example, for a rectangular guide: $E_x (x,y,z) = \frac {-j\beta m \pi}{a ...
2
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1answer
187 views

For someone who only studied electromagnetism, what is the modern way to explain electromagnetic fields?

After reading most of the electromagnetism chapters of Feynman's lectures on physics, I would like to understand in more detail, at least an idea, of what causes the electromagnetic fields. Not sure ...
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1answer
143 views

How can there be a path to ground with thick shoes and a carpet?

I'm connecting a test light to one pin of an halogen lamp. When I touch the metallic part on the back of the test light, the light glows, as it is supposed to. However, I have thick shoes and I am ...
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1answer
151 views

Where does the photon's energy come from in X-ray bremsstrahlung?

In an X-ray tube, bremsstrahlung is generated by accelerating electrons toward a target metal. As a photon is emitted, something loses energy. I suppose that the target metal remains fixed and is ...
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1answer
3k views

What is the source of Earth's magnetic field?

I saw on Discovery channel that the source of Earth's magnetic field is the molten iron and metals in the earth's core. The spinning of these liquid metals produces the same effect as electric current ...
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1answer
587 views

Hole in the disc shaped magnet

Let's suppose we have a magnetic disc. One half is magnetic north pole and the other half is magnetic south pole. Now suppose we drill a hole in the disc. Does anybody has an idea how the magnetic ...
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4answers
587 views

How is this classical “paradox” resolved in electromagnetism?

A magnet and a coil move relative to each other. In the frame of reference of the magnet, there is a magnetic field and consequently a force acting on the charges in the coil according to the Lorentz ...
3
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1answer
65 views

generation of arbitrary potentials

Suppose you have as many electrically charged particles as needed (even countably many) and consider the open unit ball centered at some point in space. For every continuous real valued function on ...
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1answer
260 views

Do Maxwell’s Equations contain the Ideal Gas Law?

Maxwell’s Equations contain some other Laws as special cases. Does it possibly include the Ideal Gas Law as well? – the idea being that material substances can be modeled by appropriate subsets of the ...
3
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2answers
556 views

Will Gauss's law still hold in case of deviation from inverse square law?

Assume a spherical metallic shell over which a charge $Q$ is distributed uniformly. Applying Gauss's law $\displaystyle\oint\textbf{E}\cdot d\textbf{a}=\frac{Q_{\text{enc.}}}{\epsilon_0}$ by ...
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1answer
647 views

Would the north poles of two magnets repel each other if a weaker south pole was inserted between them?

My son asked me this question and I was stumped - my intuition says that is the south pole was strong enough the attraction between the north and south poles would outweigh the repulsion between the ...
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1answer
573 views

Magnetization of coin on a railway track

The rumor was you could make a magnet by leaving a piece of iron on a train track. The train going over it would magnetize it. Is it true?
2
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1answer
875 views

How does one calculate the force applied on an object by a magnetic field?

I've tried very hard to find an answer to this question, and every path leads me to an abstract discussion of fundamental forces. Therefore, I will propose two very specific scenarios and see if they ...
6
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2answers
1k views

Lots of little questions about radiation of LC circuits

I'm trying to get a more intuitive understanding of resonant inductive coupling. It's supposed be a more efficient way to transfer electrical energy wirelessly, because the coils are only coupled by ...
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1answer
203 views

Violation of conservation of energy and potential energy between objects

I would like to clarify my question. I have numbered them to be independent questions For any conservative fields, $\vec{F} = -\nabla U$. Which means the restoring force is opposite to the ...
2
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2answers
2k views

Physical meaning of the energy density of an electrostatic field

I understand the physical meaning of electrostatic energy of a system of charges (or a distribution with given density) as the energy stored in the system while working to carry the charges from ...
0
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1answer
435 views

electric dipole momentum calculation

I'm studying the linear electric susceptibility, using Schroedinger equation and perturbation theory of the interaction potential $$V=-\mu \cdot E$$ and the book arrive to an expression where ...
0
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3answers
227 views

What is the physical reason a $+5V$ equipotential coutour cannot intersect a $-5V$ equipotential coutour?

Now I've been told that equipotential contours with different values can never intersect. That is, if one level is 5V and one is -5V, they can't intersect. This make sense to me mathematically (one ...
2
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2answers
410 views

Which symmetry is associated with conservation of flux?

Which symmetry is associated with conservation of flux (e.g., in electromagnetism)? For example, when working with Gauss's law in electromagnetism, net flux through an arbitrary volume element ...
5
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2answers
470 views

Why do physicists believe protons and electrons are present in equal numbers?

I tended to consider that negative and positive charges are present in equal numbers in the universe to be a known, obvious fact. But is it so? How can we rule out the possibility that there is some ...
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1answer
346 views

Gauss's Law vs Newton's Law

This is thought experiment. I couldn't get a good answer because I keep getting negative mass. Gauss's Law say that eletric field is proportional to charge, how much charged is enclosed. Newton's ...
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2answers
672 views

On Electromagnetic Self Energy

In the process of pair annihilation an electron and a positron annihilate each other to produce a pair of photons, conserving momentum and energy. As the oppositely charged particles approach each ...
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1answer
207 views

Quantum Locking and gravity

Can a device be constructed of a super conducting disk connected to a electromagnet that generates a field such that the disc locks "in" the field generated by the electromagnet such that the device ...
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0answers
172 views

Energy loss rate for periodic movement

How to prove that if a particle performs cyclical motion then its energy loss rate averaged over the period equals averaged radiation intensity? The energy loss rate is the quantity of energy that ...
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2answers
973 views

Can quantum communication really replace electromagnetic waves for telecommunication medium in future?

Currently I am planning to get masters degree. So I am thinking about a subject in which I have to get masters degree. Following are my questions to leading physicists.. Which technology is the ...
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3answers
234 views

Can the field generated by a magnet domain extend to infinity?

As a thought experiment let us assume that we have isolated a magnetic domain. This domain is of finite size and we know its dimensions. Assuming that we can measure an infinitesimal field, will there ...
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1answer
3k views

Can microwaves affect wifi - and can they harm us?

I listen to the radio via my iPad with wifi. When I switch the microwave oven on, the radio cuts out. When the microwave oven is finished, the radio comes back on. (This is 100% reproducible!) So - ...
4
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2answers
380 views

Is there a magnetic field that satisfies $\nabla \times B = 0$?

In electrostatics Maxwell's equations for the magnetic field are $\nabla \cdot B = 0$ and $\nabla \times B = \mu_0 J$ Now, take $B = xi-yj$, where $i$ and $j$ are the usual unit vectors, then one ...
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1answer
469 views

Counting the number of modes

The exercise is the following Show that the number $N(\lambda) \, \mathrm d \lambda$ of standing electromagnetic waves (modes) in a large cube of volume $V$ with wavelengts within the interval ...
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1answer
549 views

Absorption / Control / Reduction of EMF radiation using passive antennae & matched load

Wanted to understand the physics behind usage of passive antennae and matched load combination, to absorb, control and reduce the Electromagnetic-Field (s.a. due to microwave radiation from cellular ...
4
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5answers
753 views

Why are $\mu_0$ and $\epsilon_0$, which appear in electrostatics and magnetostatics, related to the speed of light which appears in electrodynamics?

$\epsilon_0$ and $\mu_0$ appear in electrostatics and magnetostatics. When we include time varying fields we have electrodynamics and the appearance of c which turns out to be related to $\epsilon_0$ ...
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1answer
3k views

Can you magnetize iron with a hammer?

We know that a piece of ferromagnet, such as iron, can be magnetized by putting in a strong magnetic field to get domains parallel to the field grow. I also remember from pop. culture and MacGyver ...
2
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0answers
217 views

Solar wind and the Earth's magnetic field

I have again an old question from a comprehensive exam I took a couple of months ago. Lucky for me one could pick 5 out of 8 questions, because on some of the problems I didn't even know how to start. ...
10
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3answers
5k views

What is reason for electronic compass calibration

Most GPS receivers and smart phones contain an "electronic compass", which I understand is generally a Hall effect magnetometer. These devices generally require "calibration", which involves waving ...
3
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2answers
148 views

Derivatives of delta function and equation of continuity for a single charge…

For a single charge $e$ with position vector $\textbf R$, the charge density $\rho$ and and current density $\textbf{j}$ are fiven by: \begin{equation} \rho(\textbf{r},t)= ...
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3answers
1k views

How do electromagnetic waves carry quantised energy?

If an electron oscillates about a mean position, it will create a time varying electric filed which in turn will create a time varying magnetic field and so on to create an electromagnetic wave. How ...
4
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1answer
1k views

Magnetic fields and friction

If I was to pass a metal object through a magnetic field would there be any friction?
4
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3answers
796 views

Second law of thermodynamics and a bunch of magnets

Say I put a bunch of powerful square magnets on a nearly frictionless table in a disordered fashion. The second law of thermodynamics states that the system shall spontaneously get more disordered, ...
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1answer
123 views

Different results for Magnetic field using different methods

In calculating the magnetic field created by this current at the center point of the loop using Biot-Savart and using the vector potential will there be a difference? If so what is it and why? ...
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0answers
245 views

The effect of an EMP on circuits

Which of these is the current (also, what about voltage?) induced by an EMP pulse within a circuit most dependent on? The area of the region enclosed by the circuit The orientation of that area with ...
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2answers
280 views

Point charge potential (sign problem)

I'm a bit embarrassed, but I'm not able to compute the electric potential at point $P$ (at a distance $R$ from the origin) generated by a positive unitary point charge in the origin with the right ...
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0answers
368 views

Skin effect and currents

Here in this picture you can see I_W which is induced by H. But why I_W is not vice versa? Because of $$rot \, \vec B = \mu_0 \, \left( \varepsilon_0 \frac{\partial E}{\partial t} + \vec j ...
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0answers
272 views

Levitation rotation speed involving laser acceleration, pyrolytic graphite and a vacuum

The experment would involve a small NIB magnet levitating between or on the diamagnetic material pyrolytic graphite, unlike other forms of levitation this doesn't require power to run such as ...
2
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1answer
1k views

Understanding the Seebeck effect

Thermoelectricity is, as I understand it, the difference in voltage between the hot and cold ends of two dissimilar materials. If two materials are connected at two different junctions, the hot ...
0
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1answer
169 views

Electrophorus: Why can't we use a magnet in substituion of a charged body to induce the polarization?

Since a magnetic field can induce a current in a coil, moving electrons from one side to another. Why isn't possible to use the same principle in a electrophorus using one magnet instead of charged ...
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1answer
412 views

What is the magnetic susceptibility of NdFeB?

NdFeB is one of the most popular materials used for making permanent magnets. Yet I could find no link or reference reporting a decent value for the magnetic susceptibility of NdFeB. Magnetic ...
4
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2answers
240 views

interaction at a distance: how it works

In classical physics, I know the forces due to gravity and electrostatics. I know a lot of work has been done, but don't know the status. Is there a generally accepted theory of how the force is ...