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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33 views

A commutation between curl and integral

I have been struggling to understand the only derivation of Ampère's law from the Biot-Savart law for a tridimensional distribution of current (which, needless to say, is not the case of a linear ...
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0answers
8 views

Magnetic flux density off the axis of a plane current loop

is it possible to find a general expression for the B field at any point (including off the axis) for a planar loop of wire using the law of Biot-Savart?
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1answer
306 views

Proof of Ampère's law from the Biot-Savart law for tridimensional current distributions

Let us assume the validity of the Biot-Savart law for a tridimensional distribution of current:$$\mathbf{B}(\mathbf{x})=\frac{\mu_0}{4\pi}\int_V ...
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1answer
29 views

Why is Kirchoff's Voltage Rule not satisfied when the circuit is shorted?

For example if I have a simple circuit with a battery and a resistor, and then the resistor is shorted, Kirchoff's Rule is no longer satisfied. Since the current through the resistor is 0, the voltage ...
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1answer
19 views

Will a charge attract or repel bar magnet?

I read in many answers that magnetic fields are basically electrostatic fields arose due to motion of charges usually. It gave me a question, Let there is a charge of 1C in vicinity, and a bar ...
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13 views
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1answer
236 views

Boltzmann Distribution of Electrons in Confining Potential

I have a particle simulation wherein many non-interacting electrons are trapped in a electric potential well. I would expect, and therefore I initialize according to this, that the electrons would ...
2
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0answers
10 views

Ferromagnetism and Domain Theory

According to the domain theory, Ferromagnetic substance when kept in the external strong magnetic field, it undergoes magnetisation by Rotation and becomes a permanent magnet thereby even after ...
4
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2answers
48 views

Why are magnets not in form of hollow spheres

If magnets were in form of follow spheres, outer surface and inner surface being either north or south pole, then I cannot imagine what would be the field be like. But in that case, every magnet would ...
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1answer
651 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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2answers
64 views
+50

Relating Poyntings theorem to Lenz and Faraday's law?

In system's similar to a motor, where the armature begins to accelerate simultaneously there is induced $-\epsilon$ to reduce the applied current(hence the applied power $P(t)$ is also reduced), or ...
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0answers
18 views

Electromagnetic Wave Propagation [on hold]

!http://i.imgur.com/xADz3GB.png Hello I was doing this long problem linked above and I am unsure about certain parts. Thanks for anyone who is willing to help! From my calculations: It is a good ...
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0answers
17 views

Electric flux boundary condition

I dont' get why following equations hold isn't equation (3-121b) should be $D_{2n} - D_{1n} = \rho_s$ instead? Since $D_{2n}$ is going outward of the gaussian surface whereas $D_{1n}$ is ...
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1answer
674 views

Quick introduction to electromagnetism / Maxwell's equations [duplicate]

Possible Duplicate: Electrodynamics textbook that emphasizes applications I am a graduate student in applied mathematics and I am looking for a concise introduction to Maxwell's equations / ...
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1answer
28 views

Plane waves intensity

Our professor said that a plane wave has the same intensity in the plane perpendicular to it's propagation. I can't really make sense of how that is possible. I mean, shouldn't the intensity decrease ...
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0answers
28 views

Do gravitational waves interact with the Earth's magnetic field?

Do gravitational waves cause a minor vibration of the Earth's magnetic field, and therefore a minor (if undetectable) electrical inductance?
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0answers
27 views

Does an electromagnet use more energy when it is repelling another magnet?

Kind of a strange question. Is the amount of energy used to power an electromagnet directly proportional to the amount of work an electromagnet does? That is, if I were to power an electromagnet ...
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3answers
68 views

According to Ohm's Law, shouldn't there be a zero electric potential within a capacitor since the current is 0?

If V = I*R, then how does a capacitor store electric potential? The current across a capacitor is 0.
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7answers
12k views

What is Electromotive force (EMF)? How is it related to potential difference?

What is Electromotive force (EMF)? How is it related to the potential difference? Is it created by the potential difference in any conductor? Is it a process? Why is it called force? Does writing EMF ...
0
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0answers
16 views

induction of voltage from power lines to telephone lines [on hold]

A pair of isolated parallel power lines of 1 cm radius with a separation of 90 cm have a voltage difference of 440 V. These power lines are 60 cm above a pair of telephone lines. The telephone lines ...
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1answer
74 views

Is Lorentz Invariance necessary for Maxwell Equations?

An example shows that Maxwell Equations do not need Galilean invariance, therefore Lorentz Invariance is not needed for Maxwell Equations. The example: In a train moving at a constant speed relative ...
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2answers
518 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
20 views

Is the magnetic dipole moment in charged particles the more survivable property than the electric charge?

After asking about Which of the properties of particles are intrinsic properties and why? it was commented "Charge, spin, lepton number, baryon number definitely. Mass maybe." and "is magnetic dipole ...
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3answers
323 views

Currents induced by rotation of a coil about magnetic field lines

If the coil on the left is rotated about the magnetic field lines, will there be an induced current? I know that the flux concerns only the component of the magnetic field perpendicular to the ...
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0answers
9 views

Superconductive contour

We have a superconductive contour in the shape of circle with radius $a$ . Inductance of contour is $L$, when the contour is out of magnetic field, there's no current in it. What's the current in the ...
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1answer
33 views

Please help me with this doubt from rotational dynamics

Torque to a magnetic field on a current carrying loop is same with respect to any point in space,is the same true for all force couples acting on any body?
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3answers
14k views

Do magnets lose their magnetism?

I recently bought some buckyballs, considered to be the world's best selling desk toy. Essentially, they are little, spherical magnets that can form interesting shapes when a bunch of them are used ...
0
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1answer
18 views

Electric field in a cavity of a conductor

I have a spherical conductor with a cavity. A point charge +q is placed in the cavity. Now, on the outside of the sphere, I bring a charge of -q close to the sphere. I understand that the amount of ...
4
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1answer
3k views

Force on Earth due to Sun's radiation pressure

I have been asked by my Classical Electrodynamics professor to calculate the force that the Sun exerts in the Earth's surface due to its radiation pressure supposing that all radiation is absorbed and ...
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1answer
290 views

Magnets and speed of light

I am in no way a physicist but I do have a fascination with physics. My question is if magnets are being explored / studied as a potential source to achieve the speed of light and if that is even ...
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2answers
41 views

Why can only moving metals (aluminium) reflects a magnetic field of a magnet?

A maglev train is riding because the magnets in the train are reflecting their magnetic field on the aluminium rail, so it is lifted up. Also when you rotate a aluminium plate and you hold a magnet ...
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0answers
10 views

Electromagnetic Induction, specifically induced voltage

Why is it that if a conductor 'cuts through' (what does this mean?) a magnetic field that a potential difference is produced? I read this recently in a textbook...
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1answer
116 views

How to interpret the complex index of refraction?

The index of refraction which represents how much light gets refracted when entering a medium is defined as $$n = \frac{c}{v}$$ I have seen it stated in several places, such as here, that we can ...
4
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4answers
116 views

How do they find the energy of a photon?

Is the actual energy of a photon ever measured? How is it done? I read that a photon is usually identified by diffraction, that means its wavelength is measured, is that right? In this way we ...
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0answers
37 views

Electric Filed on a point on a sphere [on hold]

I really want to understand the concept of those problems. Thanks.
5
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1answer
312 views

Biot-Savart law from Ampère's with multivariate calculus

Let us assume the validity of Ampère's circuital law $$\oint_{\gamma}\mathbf{B}\cdot d\mathbf{x}=\mu_0 I_{\text{linked}}$$where $\mathbf{B}$ is the magnetic field, $\gamma$ a closed path linking the ...
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5answers
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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0answers
20 views

Electromagnetic field tensor [on hold]

How will we derive the component of electromagnetic field tensor by using the electromagnetic field tensor formula; what is the procedure? Can anyone tell me about how I would derive $F_{00}, ...
4
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3answers
148 views

Does the force between two point charges change when inertial reference frame changes?

The two point charges are as below: Then the Coulomb force between them is: $$F_1=\frac{q^2}{4\pi\epsilon_0 r^2}$$ When the inertial reference frame moves left at $v$(or the point charges move ...
2
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2answers
1k views

Can an insulator have magnetic poles?

Is it possible to make an insulator have two poles like a magnet? If yes, how to make it?
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1answer
122 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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2answers
34 views

How can space strech a wave?

Because of the expanding of the universe a wave from a far galaxy or star can be streched to more red. But how is space doing that? There must be a kind of 'entanglement' between the wave and space. ...
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2answers
85 views

How does magnetism work? (Not asking what magnetism does)

To clarify, if I asked "how does electricity work" I would want "electrons flow from a negatively charged area to a positively charged area", not "it powers electronics like phones" as an answer. The ...
0
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1answer
23 views

Zero or non zero electrical potential energy of the system

Say you have this scenario: and you wanted to find the electric potential energy of the +q charge Now if you were to find the potential energy of +Q and +q system and add that to the potential ...
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1answer
54 views

What does radiofrequency really do in NMR & MRI?

In NMR, the protons’ frequencies are affected by the magnet which makes them precess around its magnetic field so now all protons will have different frequencies according to their environment ...
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0answers
15 views

Why oscillating charges produce magnetic field? [duplicate]

Electron is a charged particle. As far as I know, an electron has an electric field when it is at rest. But when it starts "moving", it creates a magnetic field. How does that happen? How does the ...
2
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2answers
153 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
16 views

calculation od stacked plate capacitor capacitance [on hold]

how we calculate the capacitance of a stacked plate capacitor ? is it the summation of the capacitance of each capacitor $Ci$ ?
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2answers
600 views

How does the magnetic field get induced in car's tire?

I was amazed to see how my car's tire started attracting the nuts and bolts, how in the world did it induce the magnetic field? Never heard of such phenomena!!!
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0answers
28 views

Electric Field From a point P

A rod of length $L$ and total charge $Q$ creates an electric field at a point $P$, $x$ units away. We need the electric field at this point. I am working on a physics problem for homework and the set ...