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
1answer
140 views

Magnetic fields in transformers and back emf?

i am having some trouble with understanding the magnetic fields inside transformers and how they relate to back emf. My first query is: is the magnetic field produced inside the soft iron core ...
2
votes
1answer
53 views

How does an infrared thermometer actually calculate temperature?

I am slightly confused about infrared radiation and the equations related to it. $P = A \epsilon \sigma T^4$ (1) and $B_{\lambda}(\lambda,T) = \frac{2hc^2}{\lambda^5} \frac{1}{e^{\frac{hc}{\lambda ...
3
votes
3answers
332 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 ...
0
votes
0answers
11 views

Is there a magnetic force affecting point charges outside of a toroidal coil with sinusoidal current?

To clarify my question a bit more, if we have a toroidal coil with sinusoidal current flowing through wires around the core of the toroid, is there a magnetic force that affects charges outside of the ...
4
votes
2answers
229 views

Maxwell's equations invariant under all linear transformations?

Maxwell's equations in tensor notation read: \begin{align} \partial_\mu F^{\mu\nu} &= J^\nu \\ \partial_{[\lambda}F_{\mu\nu]} &= 0 \end{align} Consider doing a general coordinate ...
15
votes
5answers
3k views

Is the EmDrive, or “Relativity Drive” possible?

In 2006, New Scientist magazine published an article titled Relativity drive: The end of wings and wheels1 [1] about the EmDrive [Wikipedia] which stirred up a fair degree of controversy and some ...
0
votes
0answers
47 views

How do I obtain the metric tensor of a constant electric or magnetic field? [on hold]

Consider the empty space with a constant magnetic or electric field. From the Faraday or the Maxwell tensor that describes this situation, how do I obtain its metric tensor? I could try the Einstein ...
1
vote
1answer
63 views

Shouldn't the currents be time-continuous here?

My intuition is that the current upon an inductor (say, a solenoid) will always be time-continuous, without "sudden changes". But below is a case that seemingly contradicts this point of view. There ...
5
votes
2answers
2k views

How to calculate required current and specifications for a “wet coil” to generate x Tesla of magnetic field?

A water management project requires a "wet coil" (coil will be submerged in aqueous media) designed to generate a steady-state electromagnetic field of adjustable magnetic magnetic flux density at the ...
0
votes
2answers
70 views

Why do we feel hot because of sunlight? [on hold]

sunlight , light generally , is an electromagnetic wave which turns into heat when it contacts a matter (solid,liquid,etc..) is that right ?
1
vote
1answer
192 views

Ampere's circuital law for finite current carrying wire

When I was studying about Ampere's circuital law. Then there comes a question in my mind that "whether this law is applicable for finite current carrying wire or not"
3
votes
0answers
41 views

MRI and precession

A lot of explanations of the quantum mechanics of MRI discuss the precession of a proton in an external magnetic field, for example here: http://www.physicscentral.com/explore/action/mri.cfm Doing ...
0
votes
2answers
55 views

why do electromagnetic waves have no charge?

i would have thought that because the electric and magnetic fields oscillate, the charge could be positive or negative between 0 and 1 inclusive at any one point in time. i cannot see any explanation ...
1
vote
2answers
61 views

What makes waves propagate?

Why do electromagnetic waves propagate? I have searched a lot about EM waves, but I am still unable to understand what is driving them. Could you explain?
0
votes
0answers
15 views

langevin theory of diamagnetism

According to langevin diamagnetic theory when an electron orbiting the nucleus is subjected to an uniform external magnetic field (say perpendicular to plane of orbit) then the angular frequency of ...
1
vote
1answer
326 views

How is the current flow perpendicular to the wire?

This answer gives a great explanation of how surface charge builds up to force the current to move perpendicular to the wire: http://physics.stackexchange.com/a/102936/41086 However, it fails to ...
3
votes
1answer
137 views

Could the Faraday's rotating wire experiment be explained without using Lorentz force?

I read Faraday's rotating wire experiment which resulted in the invention of the homopolar motor. He was clever enough to think it that way and make the wire rotate around one of the poles of the ...
0
votes
0answers
9 views

Relation between frequency, speed, and coverage range

From http://superuser.com/a/901075/9265 about wireless network: Higher frequency signals degrade over shorter distances, but can carry data more quickly. I wonder what theorem/theory in Physics ...
0
votes
0answers
15 views

Electromagnetic wave reflection [closed]

Is there any Rf reflector(In Practical) with attenuation greater than -120 dB? I have searched for a Rf reflector with attenuation of -100 dB(MAC 2200) but now i need a reflector with attenuation ...
0
votes
3answers
79 views

Where is the energy stored in an inductor?

In an inductor, Most text books say that the $(1/2)Li^2$ is stored in the magnetic field. But is there another way to explain this? In a capacitor I understand that all the energy that the battery ...
2
votes
1answer
108 views

How can Drift Velocity of an electron causes change?

Assume we have a conductor that has current flowing and its placed in a magnetic field so that it experiences the Lorentz force, and it gains Kinetic energy due to that force. Would drift velocity ...
2
votes
1answer
121 views

Why is there no induced electric field in the experiment (Faraday's Law)

Below are three circuit diagrams for each of Faraday's experiments that allowed Faraday to come up with Faraday's Law. In Griffiths' Introduction to Electrodynamics Griffiths states (on page 302 of ...
0
votes
0answers
16 views

Gaussian cgs units [closed]

Gaussian representation of Electric charge in M L T does not prove that charge is a fundamental quantity. He also says in cgs says that Permittivity of free space is dimensionless. Is he right?
0
votes
4answers
88 views

Induction and electromagnetic fields

I've got a few questions on induction and electromagnetic fields. My current understanding of induction and electromagnetic fields is that, when electricity/current flows through a wire, it creates an ...
0
votes
2answers
169 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 ...
3
votes
2answers
158 views

How to calculate the force acting on a magnet due to a solenoid

Can I assume both magnet and solenoid as magnetic dipole and use Coulombs law to find the force acting the magnet?
1
vote
1answer
37 views

Question regarding charge and acceleration

From a stationary charge electrostatic fields arise. From a moving charge, magnetostatic fields arise. From an accelerating charge, EM waves arise. So i wonder -- what about a non-constantly ...
5
votes
3answers
417 views

Why possibility for X-ray to excite inner electrons higher than outer electrons?

It seems X-ray absorption spectroscopy is usually ascribed to the interation between photons and inner electrons. Does it mean inner electrons are much preferred by X-ray photons to outer electrons? ...
0
votes
2answers
62 views

When does $\mathbf n\times(\nabla V_2-\nabla V_1)=0$ imply $V_1=V_2$

I was reading a paper on electrohydrodynamics which has the following sentence (in my own words): At the interface/boundary, the requirement of continuity of the tangential component of the ...
0
votes
1answer
34 views

Current direction and magnetism

So using the right hand rule, I understand current is clockwise. Because I read the magnetic field emerges from the north pole, I thought the answer was a, that the north pole is on the top. But the ...
2
votes
1answer
46 views

Special relativity, electromagnetic fields, charge and Q

Is it true, were Coulomb's constant k to be several orders of magnitude smaller, that there would be no (or increasingly negligible) magnetic fields generated by moving charges? The reason being the ...
0
votes
1answer
218 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. ...
8
votes
3answers
418 views

Explanation of Lorentz-Force

In high-school level books (for example the german standard text: "Dorn-Bader") I have often seen an explanation of the Lorentz force as on the following picture: The textbooks consider the ...
4
votes
4answers
105 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
1answer
77 views

Can the physical properties of the EM field be described directly from the 4-gauge potential?

I'm trying to make an argument that classically, the EM field is considered a more 'real' physical quantity than the potentials, and am tempted to say that the fact that the field carries energy & ...
1
vote
3answers
224 views

What does it mean that a magnetic field's flux vanishes through any closed surface?

I'm reading the Britannica guide to Electricity and Magnetism, and I came across the following quote: A fundamental property of a magnetic field is that its flux through any closed surface ...
0
votes
2answers
90 views

Magnet arcs assemly

Is it possible to join multiple Neodymium magnet arcs (shown blow), which are radially magnetized, to create a cylindrical ring unity that is also radially magnetized? If it is not possible, how to ...
1
vote
1answer
26 views

Neutral $K$ and $B$ mesons decay to 2 photons?

The neutral pion $\pi^0$ decays almost exclusively to 2 photons, $\pi^0 \rightarrow \gamma \gamma$, which got me thinking: Can we have $K^0 \rightarrow \gamma \gamma$ and $B^0 \rightarrow \gamma ...
7
votes
1answer
4k 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 ...
1
vote
1answer
77 views

What is the sum of the electrons' magnetic moments in a wire?

The electron has a magnetic moment. This magnetic moment will be influenced in a electric field; the magnetic moments will be more or less aligned. During the movement of an electron in a wire under ...
6
votes
2answers
3k 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 ...
0
votes
0answers
22 views

Magnets quantum locking/levitating [duplicate]

How does cooling a magnet allow it to quantum lock/levitate? I have seen it in videos but do not know how it works.
0
votes
2answers
39 views

Is there no electric field inside a conductor?

I came across this statement while studying electric currents and I am confused: "There is no electric field inside a conductor. Hence no current can flow through it". Is there a fallacy in this ...
1
vote
1answer
45 views

Magnetic moment of an iron-core solenoid

I'm currently developing a Simulink model for the attitude control system (ACS) of an undergrad-developed CubeSat. The ACS uses magnetorquers for attitude actuation. The magnetorquers are iron-nickel ...
-1
votes
0answers
7 views

Superconductivity induced by magnetic flux or temperature fluctuations (+/-)

Does anyone know of an experiment with magnetic flux rotations that can make a difference in local temperature? I would really appreciate it, I need it for my theoretical model of some things and ...
1
vote
0answers
12 views

Eddy currents are out of phase with respect to the field generated by a coil?

I have a coil from which a sinusoidal current (low frequency, few kHz) should generate a precise AC magnetic field in the surrounding space. Another coil intercepts this field and the corresponding ...
8
votes
3answers
232 views

Why are permanent magnets permanent?

Let me see if I get it right. When an iron bar is attracted by a permanent magnet it becomes a magnet itself because all of its magnetic domains start to point in the same direction. When the iron bar ...
6
votes
4answers
2k views

Relativity and Current in Wire

If an observer is stationary relative to a current-carrying wire in which electrons are moving, why does the observer measure the density of moving electrons to be the same as the density of electrons ...
16
votes
5answers
15k views

Phase shift of 180 degrees on reflection from optically denser medium

Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave,when reflected from an optically denser medium? I tried searching for it ...
2
votes
1answer
246 views

Understanding Poynting's Theorem intuitively

I understand that, broadly speaking, Poynting's theorem is a statement of conservation of energy. Energy density of a volume of current and charge decreases proportionally to work done on charges ...