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.

learn more… | top users | synonyms (6)

0
votes
4answers
328 views

Who foots the (magnetic) energy bill?

Gravitational attraction and electrostatic attraction/repulsion are intrinsic properties of matter, any particle (electron, proton) for some unknown reason can produce KE at a distance. But magnetic ...
0
votes
2answers
168 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 ...
1
vote
1answer
86 views

The strength and control over the strength of electromagnets, correlation between the strength and weight of the magnets

I am currently working on a project that aims to use the attraction & repulsion between two electromagnets as an alternative to weights, from a physiotherapy and rehabilitation perspective to help ...
0
votes
1answer
55 views

Einsteins thought experiment on travelling with a light wave

This might be better suited to the science.history.SE; but I thought I would try here first. Einstein reportedly considered a thought experiment where one considers travelling alongside a light wave; ...
0
votes
2answers
116 views

Superconducting electromagnets?

Is it true that superconducting electromagnets don't need any power? So... energy to created a magnetic field via a superconductor would be zero? Since resistance is zero, does that also mean that ...
4
votes
2answers
3k views

How can a conductor be grounded yet there are induced charges on it?

A classic example for the method of images is the following, quoted from Griffiths's Introduction to Electrodynamics, page 121: Suppose a point charge $q$ is held a distance $d$ above an infinite ...
1
vote
1answer
71 views

How to prove the following relation $H=\frac{E\times v}{c}$ [closed]

Using CGS units, how can we prove the following relation $$H=\frac{E\times v}{c}$$ where H is magnetic field, E is electric field and v is velocity
0
votes
1answer
468 views

Why is the Ohm's law $j=\sigma E$ accurate? [closed]

Ohm's law $j=\sigma\cdot E=\frac{Q}{A}\cdot \frac{F}{Q}=...?$ $$j=\rho\cdot v \\ =\frac{q}{V}\cdot \frac{s}{t}=\frac{1}{A}\cdot I$$ So why is $j=\sigma E$?
8
votes
2answers
4k views

How do superconducting materials float in magnetic field?

The movie Avatar got me interested in the subject, but so far I only found sophisticated articles loaded with unfamiliar words. Is there a simple way to explain how magnetic field affects ...
11
votes
3answers
20k views

How does Hendo hoverboards achieve the self-propelling motion? What is the MFA?

From what I understand of the idea behind Hendo hoverboards, they use four disc shaped hover-engines to generate a self-propelling motion. I also understand that it uses Lenz's law and eddy currents. ...
0
votes
2answers
128 views

What kinds of behavioural anomalies can a zero-field-cooled (ZFC) / field-cooled (FC) split indicate?

If a material shows a spiltting in the ZFC and FC curves, is it necessarily superparamagnetic, or could there be any other reason for the irreversibility? I have heard spin glasses also show ZFC-FC ...
5
votes
5answers
244 views

A current through a wire produces a magnetic field around it. Is the reverse possible?

If somehow a magnetic field around a wire can be made to exist that is identical to the magnetic field produced when a current passes through the wire, will a current be produced in the wire?A thought ...
0
votes
0answers
11 views

What is the role of the Cowling, Pedersen and Hall resistivities (or conductivities) in plasma physics?

The current density $\vec{j}$ can be computed as $\vec{j} = \vec{\vec{\sigma}} \vec{E}$, where $\vec{\vec{\sigma}}$ is the conductivity tensor and $\vec{E}$ is the electric field. This conductivity ...
4
votes
4answers
463 views

How do electrons actually move in a circuit?

Last year, we were taught about electricity, about how electrons move in a closed circuit. But as our teacher had not taught us about electric fields yet, she gave us a simplified model of motion of ...
-3
votes
3answers
202 views

Why do electrons and positrons exhibit opposite helical motion in a magnetic field?

When you throw an electron through a solenoid, it moves helically around the field lines, as per this schoolphysics illustration: © Keith Gibbs 2013 Then if we were to throw a positron through the ...
3
votes
2answers
13k views

Dirichlet and Neumann Boundary condition: physical example

Can anybody tell me some practical/physical example where we use Dirichlet and Neumann Boundary condition. Is it possible to use both conditions together at the same region? If we have a cylindrical ...
2
votes
2answers
122 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 ...
2
votes
2answers
68 views

Showing that the magnetic field inside an infinite current carrying cylinder is zero

I'm doing some self studying of introductory physics, and was working a question from a textbook that has several parts. The first few parts asked me to show that the magnetic field of an infinite ...
0
votes
2answers
40 views

Physics of color: why do visible colors absorb all colors except the color itself?

It's funny I cannot answer this question, but I really don't have a very good physical intuition for how color works. I know that sunlight and (say) light from lightbulb is a superposition of all ...
1
vote
1answer
80 views

Why curvature produced by electric field in spacetime is so small?

The electric field due to charge is $E=\frac{Q}{r^2}$. Why is the curvature in spacetime produced by this electric field so small? Let's say we want to calculate curvature(in spacetime) produced by ...
1
vote
1answer
248 views

Calculating the magnetic field from rectangular conductor?

I want to apply the Biot-Savart law to calculate the magnetic field at a point created by current flowing through a square/rectangular conductor. More specifically, a trace on a printed circuit board. ...
3
votes
2answers
182 views

What is the area in Faraday's law if we have only a piece of metal moving in a magnetic field?

If a piece of metal of length $l$ is moving with a speed $v$ in a region where there is a uniform magnetic field $B$ perpendicular to it, there will be a potential difference across its terminals ...
1
vote
1answer
163 views

Free Electromagnetic field in Lorenz gauge

To get rid of the extra term in the QED Lagrangian we need to redefine the electromagnetic four-vector: $A^{\mu} \rightarrow A^{\mu} - \frac{1}{c} \partial_{\mu} a(x)$ where $a(x)$ is the function ...
0
votes
2answers
29 views

The source and sink phenomenological description of charge - how realistic is it?

I've heard over and over an electron described as a source of the electric field, but that is a misleading term. Source makes one thing of a tap, with a constant flow of something coming out of it. ...
3
votes
1answer
210 views

Magnetic Levitation : Confusion with Lenz's Law

Considering the above diagram, I am failing to see why the aluminium ring floats upwards rather than oscillates. I understand Faraday's law, but I am struggling with Lenz's law - as there is an ...
14
votes
0answers
503 views

Do intergalactic magnetic fields imply an Open Universe?

According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty ...
0
votes
0answers
5 views

Dipole moment and magnetic power of permanent magmets(circular or disc shaped)

Is any circular or disk shaped permanent magnet has its dipole magnetic moment and if answer is no,then why and how are the behaves as magnetic materials or say that they show magnetic power?
0
votes
2answers
83 views

Most fundamental quantity in magnetostatics and magnetodynamics [closed]

In both elctrostatics and electrodynamics, the electric charge is defined as the most fundamental quantity. What is the most fundamental quantity in case of magnetostatics and magnetodynamics ?
3
votes
3answers
288 views

Electromagnetism boundary condition

The dielectric slab surrounded by air as shown in diagram has permittivity $\epsilon$ different from air and permeability almost same as air. Electric field everywhere within the slab is given to be ...
11
votes
3answers
131 views

Can a magnet damage a compass?

I've heard the claim before that a magnet can ruin a compass, and was about to repeat it to my son when I realized it sounds like complete nonsense. Googling turned up such unsubstantiated and ...
2
votes
3answers
125 views

General relativity without curvature?

Is there a reformulation of general relativity without curved space time, just with fields (like classical E&M)? Edit: removed the part about E&M with curvature (multiple posts).
0
votes
2answers
146 views

How come the length of a wire does not affects on the circuit current?

Today we started learning about the electromagnetic induction. Out teacher gave us the following explanation: Suppose we have a conduction frame inside a magnetic field $\vec{B}$ going towards the ...
-1
votes
0answers
14 views

How to divert water flow by magnetic field or static electricity or electricity?

Is it possible to make a magnetic or electric plate that prevent hitting water to itself or at least divert it? Which force is most effective, magnetic field or static electricity or electricity?
3
votes
2answers
2k views

Why is bandwidth, range of frequencies, important when sending wave signals, such as in radio?

So in wired/wireless networking and radio, signals are sent in form of wave. Then the concept of bandwidth comes in, which is the difference between highest frequency and lowest frequency in a signal. ...
-4
votes
3answers
105 views

Can we say that gravity(indirectly) is responsible for motion of electrons around nucleus? [closed]

From Wikipedia But because general relativity dictates that the presence of electromagnetic fields (or energy/matter in general) induce curvature in spacetime From Wikipedia An ...
2
votes
1answer
124 views

Divergence Theorem, mathematical approach to Gauss's Law?

Let $D$ be a compact region in $\mathbb{R}^3$ with a smooth boundary $S$. Assume $0 \in \text{Int}(D)$. If an electric charge of magnitude $q$ is placed at $0$, the resulting force field is ...
1
vote
1answer
48 views

Using ion beam to create strong magnetic field

Consider, I want to make very strong magnetic field in some spot in space ( focus ) but I cannot put any solid material ( like metal conductor, or superconductor ) close to that spot (e.g. because ...
3
votes
1answer
156 views

$U(1)$ 5-dimensional Kaluza-Klein topological defects

Five-dimensional Kaluza-Klein theory is well-known to predict that the electromagnetic field can be described as a curled additional dimension over four-dimensional spacetime. That is, you only need ...
2
votes
2answers
104 views

Why does larger permittivity of a medium cause light to propagate slower?

I was wondering about what physically happens when light is transmitted through a non-magnetic medium. Specifically, I’m trying to visualize how materials slow down light as the electromagnetic wave ...
4
votes
2answers
210 views

Looking for lists of student difficulties with electromagnetism, circuits (DC and AC)

I'm trying to help with an Instructor's Guide for an introductory calc-based physics text, and it would really help if I could locate some lists of typical student difficulties with topics like ... ...
0
votes
0answers
31 views

Electromagnetic Waves and Magnets

Does an electromagnetic wave have an affect on a permanent magnet or on a piece of metal? When light hits a regular bar magnet is there a magnetic push or pull on it? And Does an electromagnetic ...
0
votes
2answers
41 views

Energy in conductors with $\vec{E}$-field

The question is deceptively simple. Suppose I have a uniform circular wire in which I have created the a E-field by some mean. We that without the source the electric field should not be there, so if ...
0
votes
0answers
121 views

Electrical Resistance of Spherical, Contacting Colloids

So, I work a lot with Magneto-Rheological (MR) fluids - which are essentially electrolyte solutions with suspended, paramagnetic colloids/particles. Once a magnetic field is applied to such a fluid, ...
2
votes
3answers
59 views

Observing a photon during flight

When I was reading about the double-slit experiment in quantum mechanics, everything seems to make sense in terms of the waves and the interference pattern, but if thinking more about this ...
0
votes
3answers
47 views

Rate of change of current in an Inductor

Considering an Inductor in a DC circuit: when the switch is first closed there is a change in current through the inductor, which induces an emf in the opposite direction (Lenz's Law). My question is ...
0
votes
0answers
5 views

What does magnetic flowmeters measure?

What parameter does the magnetic flowmeter actually measure? Velocity of the conducting liquid flowing through it or the amount of conducting liquid directly? The volume of liquid can be calculated ...
2
votes
2answers
302 views

What are the specific electronic properties that make an atom ferromagnetic versus simply paramagnetic?

As I understand it, paramagnetism is similar in its short-term effect to ferromagnetism (spins of the electrons line up with the magnetic field, etc.), though apparently the effect is weaker. What is ...
0
votes
0answers
19 views

What is direction of induced current in a rotating rod pivoted at one end in a magnetic field?

This is rod of length l, placed on metallic ring of the same radius as rod's length. This set up is placed in a magnetic field perpendicular to the rod and into the plane. It is very clear that an ...
10
votes
3answers
5k views

Can superconducting magnets fly (or repel the earth's core)?

If a superconducting magnet and appropriate power supply had just enough $I\cdot s$ (current $\cdot$ length) so that when it was perpendicular to the earth's magnetic field, the force of the ...
0
votes
2answers
41 views

An IE IRODOV Electrodynamics problem

A ring (radius R) is given a negative charge -q and at the centre a positive charge +q is kept. We have to calculate the electric field on the axis of the ring at a distance X from the centre. Take ...