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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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 ...
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2answers
74 views

Why do we feel hot because of sunlight? [closed]

sunlight , light generally , is an electromagnetic wave which turns into heat when it contacts a matter (solid,liquid,etc..) is that right ?
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1answer
204 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"
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0answers
43 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 ...
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2answers
58 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 ...
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19 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 ...
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1answer
331 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 ...
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1answer
143 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 ...
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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 ...
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3answers
83 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 ...
3
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1answer
111 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 ...
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1answer
136 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 ...
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4answers
90 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 ...
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2answers
176 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 ...
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2answers
160 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?
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1answer
39 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 ...
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3answers
421 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? ...
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2answers
63 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 ...
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1answer
36 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 ...
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1answer
47 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 ...
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1answer
226 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. ...
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3answers
440 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 ...
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4answers
121 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 ...
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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 & ...
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3answers
232 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 ...
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2answers
94 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 ...
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1answer
32 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 ...
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1answer
5k 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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1answer
78 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 ...
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0answers
25 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.
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2answers
53 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 ...
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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 ...
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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
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3answers
241 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 ...
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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 ...
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5answers
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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 ...
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1answer
248 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 ...
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2answers
39 views

Why does a voltmeter give a positive reading for a path in the same direction as the electric field?

Straight from my textbook: If the direction of the path from initial location to final location is the same as the direction of the electric field, the potential difference is negative. Yet a ...
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0answers
31 views

EM field of free moving electrons?

If one put two wires parallel and in a little distance to each over and let through between them slow moving electrons, do one measure a current in the wires? Of course the electrons don't hit the ...
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2answers
165 views

Drift Speed and Current in Two Different Inertial Frames

We have a long, cylindrical wire carrying a constant current I in an inertial frame. At a distance of R from the center of the wire, the magnitude of magnetic field is $μI/2πR$. What is the magnitude ...
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2answers
48 views

Are magnetic field lines level sets?

I have been learning a bit about level sets. After doing this, I looked at a diagram of magnetic field lines and noticed they don't intersect rather like the lines on closed curve level sets. My ...
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1answer
83 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 ...
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1answer
64 views

Charge of a moving particle [duplicate]

Is there an experiment that measures the electric charge of a moving particle, therefore proving "experimentally" that it is indeed the same as a static particle?
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2answers
378 views

Electric field from current without Maxwell's law of induction

A long, straight wire carries a current that decreases linearly with time. What is the direction of the induced electric field outside the wire? I would interpret this as follows: a current ...
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1answer
29 views

In a noiseless environment, how accurate do today's transmitters send EM waves?

Suppose that there is no external noise in the environment. How accurate are today's TEM wave transmitters in such a case? So if we want to send $200\cos(1000\pi t)$, can transmitters send exactly ...
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2answers
111 views

Do Ampère's and the the Biot-Savart law give different answers for the magnetic field of a circular loop? [duplicate]

I have been trying to calculate the magnetic field due to a circular loop using Ampère's circuital law. However, I am unable to do the same. Whenever I apply the law, while integrating length ...
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3answers
14k views

Is it correct to say “like poles attract, unlike poles repel” while two magnets are placed such that one is inside another?

As we know a solenoid is considered as a electromagnet(magnet) if there's a current flowing through it. if a soft iron core is placed inside the solenoid, the former get magnetised. Consider the ...
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1answer
37 views

The force on the northern hemisphere

I am reading Griffiths' Introduction to Electrodynamics. On page 364, example 8.2 (4th edition), he calculates the force on the northern hemisphere of a ball with total charge $Q$ spread uniformly. ...
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1answer
28 views

Induced electric field

Let's consider a thin cylinder of radius $r$ with a charge in is outer surface. It is made of an isolator. Let the magnetic field be parallel to its axis. If the magnetic field changed by $dB$ in time ...
6
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2answers
249 views

Gauge invariance and Bohm-Aharonov effect

I am confused with the Bohm-Aharonov effect: though quantum mechanics is said to be gauge invariant, the presence of a solenoid imposes a gauge. I used to think that a phase shift did not change ...