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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8
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2answers
793 views

No magnetic dipole moment for photon

Electrically neutral particles such as neutrinos can have nonvanishing magnetic dipole moments. Spin-1 particles, e.g., deuterium nuclei, can also have dipole moments. Googling seems to show that the ...
4
votes
1answer
116 views

What is the intensity of this light?

I am struggling with a derivation that calculates the cross sections for Mie scattering and since the incident light is considered to be a x-polarized plane wave I thought that we would have $$I_i = ...
10
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2answers
2k views

Is it possible to levitate a toy maglev train using only permanent magnets as long as the train stays in motion?

I am fascinated with magnets, and specifically the idea of magnetic levitation. I recently purchased a Levitron toy maglev train and have enjoyed finding the perfect sweet spot at which it levitates, ...
1
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1answer
592 views

Definition of complex permittivity

I'm not sure if this is the appropriate forum for my question as I actually am studying this as part of electrical engineering and I don't actually study physics. Nonetheless, I shall ask and if need ...
2
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1answer
495 views

Graphene batteries/super capacitors

A while ago, there was some news about micro-scale graphene-based supercapacitors and these devices can charge and discharge a hundred to a thousand times faster than standard batteries. Question: ...
0
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1answer
60 views

What would happen if you attached a wire to an electrically charged sphere?

If you have a sphere covered in electrons, and you connected a copper wire to it, what would happen? The copper wire's other end is not connected to anything and assume that the copper wire is ...
6
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6answers
2k views

Why does electricity need wires to flow?

If you drop a really heavy ball the ball's gravitational potential energy will turn into kinetic energy. If you place the same ball in the pool, the ball will still fall. A lot of kinetic energy will ...
6
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3answers
17k 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 ...
0
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1answer
3k views

Work done by magnetic field [duplicate]

I know Lorentz force don't do any work. but I want to know whether any type of magnetic field do a work or not.
2
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2answers
2k views

Why do circuits work so fast?

Drift velocity (explained to me as how fast the electrons are moving) is really slow. My book says the electrons move at around 10 mm/ s. If electrons move so slowly how do circuits work so fast? If ...
0
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2answers
2k views

How to transform mechanical work into electrical energy without using piezoelectricity?

can someone help me with the following issue. I need a method for transforming mechanical work into electrical energy without using piezoelectricity. I have such kind of mechanical forces (like on the ...
1
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2answers
422 views

Electric potential due to circular disk

Relevant diagram is available here. The circular disk of radius $a$ lies in the $xy$ plane and carries surface charge density of $\sigma (s, \phi) = s^{2}cos\phi $, where $(s,\phi)$ are in ...
0
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1answer
4k views

Why does pushing a magnet inside a solenoid produce current?

If you push a bar magnet inside a solenoid, a current is produced. But why is that? I mean, the wire is being moved along the magnetic field, so taking the cross product: $\vec{F} = ...
1
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1answer
229 views

Magnet alignment within solenoid

Imagine I have a solenoid connected to a power supply. Solenoid produces an electromagnetic field. Now I take a permanent magnet and place it inside the solenoid. How will the magnet align itself ...
6
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3answers
783 views

How do I calculate electric fields due to currents of magnetic dipoles?

Short version of my question: Do dipole currents cause fields? I think currents of aligned magnetic dipoles cause an electric field, but I don't know how to calculate this field except in the ...
1
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2answers
2k views

Why does the thickness of a wire affect resistance?

For small thicknesses of wire, it's pretty obvious why resistance affects thickness. (The electronics squeeze to get through). But after a certain thickness shouldn't the thickness become irrelevant? ...
1
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1answer
4k views

How to find the direction of the magnetic field for an infinite conducting wire?

We've got two long straight wires carrying current of 5A and placed along x and y axis respectively current flows in direction of positive axes we have to find magnetic field at a) (1 m,1 m) b) (-1 ...
0
votes
1answer
1k views

Work done on a moving particle in electric field

This is one of the "fast answer" exercises I've been given to train (should be answerable in around 6-7 minutes). I can only think of a very long-round way to solve this. The question is as following: ...
0
votes
1answer
3k views

what is the specific cause of permanent magnetism [duplicate]

Why can't we answer this simple question? Where does the magnetic field of a permanent magnet come from? What is different about a magnetizable atom that allows it? Why is it perpetual? Or is it ...
1
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2answers
161 views

Understanding the basics of electromagnetic induction

Suppose two rings are kept facing each other and that one ring have some current which increases constantly. Will the other ring be attracted or repelled? Does this also depend on how they are kept?
10
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1answer
775 views

How can I explain or demonstrate the Lorentz Force to a 12 year old?

I'm a space physicist. I've been working with a group of school students (aged between around 10 and 12) exploring the Sun, the Earth and the solar system. We've talked about some basics of magnetism ...
4
votes
2answers
151 views

Applicability of the concept of voltage in electrodynamic circuits

In electrostatics, we have $$\nabla \times \vec{E} = 0$$. Hence, we can define a scalar potential $V$, where $$\vec{E} = -\nabla V$$. We know from Faraday's law that $$\nabla \times \vec{E} = ...
0
votes
1answer
97 views

What type of magnetic fields does a Hall effect semi-conductor pick up on?

What type of magnetic fields does a Hall effect semi-conductor pick up on? AC or DC fields? How would one go about building a device that measures AC Magnetic fields?
2
votes
1answer
3k views

Physical difference between Lorenz and Coulomb gauge-fixing conditions

Lorenz and Coulomb gauge-fixing conditions. What is physical difference between these two gauge-fixing conditions? Mathematical expression are clear but how to we choose one of these means what they ...
3
votes
1answer
357 views

How to introduce the electromagnetic field in Quantum Field Theory?

There are many ways to introduce the electromagnrtic field in Quantum Field Theory(QFT), such as canonical quantization method which introduces the creation and annihilation oprators by treating the ...
2
votes
0answers
159 views

Doubts about the Aharonov-Bohm effect

In F. Schwabl, Quantum Mechanics p.148 it is explained that if we have a particle in an electromagnetic field given by potentials $\varphi$ and $\mathbf{A}$ with wave function $\psi$, then a gauge ...
1
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2answers
102 views

Time evolution of the worldlines of 2 particles

Suppose I have a lab frame that is freely falling in a gravitational field of the Earth -- assume non-homogeneity-- and a uniform constant electric field. There are 2 test particles in the frame -- ...
2
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0answers
88 views

Mie Scattering for spheres with constant dipole moment

I was wondering whether there exists a theory that describes Mie Scattering for spheres that have a constant dipole moment. Since there are theories that describe Mie scattering in the case of a ...
2
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4answers
6k views

The relation between Gauss's law and Coulomb law and why is it important that the electric field decrease proportionally to $\frac{1}{r^{2}}$?

My question relates to the third MIT's video lecture about Electricity and Magnetism, specifically from $21:18-22:00$ : http://youtu.be/XaaP1bWFjDA?t=21m18s I have watched the development of Gauss's ...
3
votes
2answers
345 views

Why is the radial direction the preferred one in spherical symmetry?

I am learning about electricity and magnetism by watching MIT video lectures. In the lecture about Gauss's law, while trying to calculate the flux through a sphere with charge in it, the lecturer ...
1
vote
1answer
535 views

General solution to the Helmholtz wave equation with complex-valued frequency in cylinderical coordinates

The Helmholtz equation is expressed as $$\nabla^2 \psi + \lambda \psi = 0$$. This equation occurs, for eg., after taking the Fourier transform (with respect to the time coordinate) of the wave ...
2
votes
1answer
402 views

Current Density and the Dipole Moment $\int _v \vec{J} dV = d \vec{p}/dt$

Show that: $$\int _v \vec{J} dV = \frac{d \vec{p}}{dt}$$ Here's my attempt: $$\vec{p} = \int _v \rho r dV \rightarrow \frac{d}{dt} \int _v \rho r dV = \int _v \vec{J} dV = \int _v \frac{\partial ...
1
vote
1answer
215 views

Reflection, transmission, absorption…how to calculate them?

I was wondering whether there is an equation that enables me to calculate the reflection, transmission, absorption and polarization, when the electric field everywhere is given? Consider this: You ...
1
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2answers
276 views

Sliding force less or equal?

Why is the force required to slide a magnet off a steel plate A LOT less than the force required to directly pull it off? The force required to pull the magnet can be: 20lb While the force required ...
0
votes
2answers
329 views

Elementary (fundamental) properties in electricity

I tend to believe that there are two elementary properties in electricity: Electric charge Coulomb's force I think that I can express any other entity in electricity using just these two (by means ...
2
votes
2answers
986 views

Is the standard explanation for the ring launcher incomplete?

Related: Faraday's law in a ring The ring launcher is a standard introductory physics demonstration that I assume almost everyone has seen (if not, YouTube it). The explanation of why the ring is ...
1
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0answers
46 views

I've been reading that it's possible to create a “mostly magnetic” wave, and I have a few questions

Would it be possible with the "mostly magnetic wave" to have it behave in such a way that it would be undetectable by radio triangulation? I read about the monument at the CIA headquarters that was ...
3
votes
2answers
555 views

Electromagnetic Momentum

My book says : The fact that electromagnetic radiation of energy carried momentum was known from classical theory and from the experiments of Nichols and Hull in 1903. This relation is also consistent ...
1
vote
1answer
102 views

Single directional electric field insulator?

Is there any material, (kind of like a one way mirror), which allows an Electric Field to pass through from one direction, but not from the other? Thanks. Edit: As Ali has pointed out, one way ...
2
votes
1answer
71 views

Radiation interactions: how is the transition from the “electric” regimen to “particle-like” regimen?

When we study the interaction of the electromagnetic radiation with free electrons we can find two different approaches in the literature: for low frequency (RF, light...) a classical view is used and ...
2
votes
1answer
2k views

Why this perpetuum mobile can't be possible? [duplicate]

I know that this won't work but I'm asking Why? Becuase as far as the vehicle POV - there is a force which drags him to the right. Isnt $F=ma$ applies here? What is that im missing?
2
votes
3answers
1k views

Why does a dielectric have a frequency dependent resistivity?

This question has come about because of my discussion with Steve B in the link below. Related: Why is glass much more transparent than water? For conductors, I can clearly see how resistivity ...
1
vote
1answer
336 views

Understand equations of a conducting sphere

Can somebody explain to me, when the following two equations (equations 2.48 and 2.50 in this document) are applicable and what $\Phi_s$ and $\Phi$ actually are? The thing is, I want to find general ...
0
votes
1answer
778 views

Electric field from conductive to dielectric media

I am interested in the main difference between transitions from electric fields from Conductive to Conductive/ Dielectric to Dielectric and Dielectric/Conductive media. What are the boundary ...
2
votes
0answers
103 views

How the dielectric rigidity depends on the shape of material?

A research article says that, "The conical shape improves the dielectric rigidity", but how I am not able to understand.
0
votes
0answers
108 views

Potential energy: Electric field two spherical charges

I want to determine the potential energy of two equally charged spherical charges by using the equation: $V_{pot}= \int_V \frac{1}{2} \epsilon_0 E^2 dV$ and therefore I was wondering what I has to ...
2
votes
0answers
172 views

Charge above a conductor; effects due to Lorentz force law for moving charges

Currently working through a practice preliminary examination problem. I have your standard charge situated a distance d from a infinite conductor(lets say in the $\hat{z}$ direction and neglecting ...
11
votes
6answers
5k views

What is canonical momentum?

What does the canonical momentum $\textbf{p}=m\textbf{v}+e\textbf{A}$ mean? Is it just momentum accounting for electromagnetic effects?
6
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2answers
859 views

How does one show using QED that same/opposite electric charges repel/attract each other, respectively?

Why do same charges repel each other and opposite charges attract each other (please explain the phenomenon using real laws of nature (QED) not with the approximation model)?
0
votes
1answer
3k views

Potential energy of the dipole-dipole interaction for two parallel dipole moments

I am looking for an equation that gives me the potential energy of the interaction between two parallel dipoles.