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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magnetic effects of electric current

If a magnet cant have one pole then in electric motor working we two magnet symbolized with names N and S. Are the single poled or bipolar? and what is the effect of magnets in working of DC motor.
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39 views

Boundary condition for E field

My book says that the boundary condition for the E field is: $$\hat{n} \times (\textbf{E}_1 - \textbf{E}_2) = 0 $$ and then concludes that the above condition can be summarized by the statement, "The ...
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1answer
62 views

What is the metric of a constant electromagnetic (pure electric or pure magnetic) field?

For example, imagine a magnetic field $B_x$ directing in $\hat{x}$ direction filling all the space. What is its associated metric field? I can construct the electromagnetic stress-energy tensor for ...
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50 views

How to solve Laplace equation in a domain with one boundary along a curve?

Is there a way to solve the 2D Laplace equation $\frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} =0$ on $0 <x<\infty$ and $0 < y < \infty$, such that the domain is ...
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47 views

Tesla's Egg of Columbus [closed]

trying to make Tesla's egg of Columbus for a science project (https://www.youtube.com/watch?v=pG6KPtpEl7M). I have managed to cast a small rod of solid aluminium into a solid egg shaped piece of ...
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1answer
59 views

Weight of magnetic objects and polarization [closed]

I am an Iranian engineer and have a question which no one has answered me yet. In weighing a strong magnet, there is a difference between the weights from different polarization directions, why is ...
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0answers
28 views

Force between magnets with different sizes

I would like to know how to calculate pulling or pushing force between two the same shaped magnet but with different sizes. Specifically, I'm looking for cylindrical and bar magnets. I tried solve ...
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1answer
54 views

Finding the magnetic field given the electric field [closed]

I'm reading a book on Electricity and Magnetism. I was solving some problems and found this very interesting one: WARNING: The next problems don't have to be solved applying Maxwell's equations; ...
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1answer
56 views

Is the Biot Savart Law applicable only for continuous currents?

There is a widely known formula for the magnetic field due to a moving charged particle. $$\frac{\mu_0}{4\pi} q \frac{\vec{v}\times\vec{r}}{r^3} $$ The usual derivation is as follows. $$ dB = ...
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50 views

question about Blandford-Znajek original paper

I'm trying to follow the original Blandford-Znajek paper (found here), and I can't find how equations 2.1, 2.2 and 2.3 are obtained: 2.1 refers to "the charge density required to ensure that $E \cdot ...
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1answer
28 views

why induced emf in primary coil is only caused from self inductance but not mutual inductace

A is primary coil and C is secondary coil A is connect to a battery while there is no source connected to C When the current in A varies with rate $\frac{di_0}{dt}$ there is induced emf in B that ...
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2answers
111 views

How does a photon mediate both electric attraction and repulsion?

The answer to this question probably lies in QFT, which I know just enough about to appreciate my current lack of understanding of the subject, if you follow me. About a year ago I asked our ...
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3answers
43 views

Antenna direction

I have a router with a wifi antenna that can be turned in any angle. I wonder what difference does the direction of the antenna make to the electromagnetic signals propagation? Where is the signal ...
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1answer
92 views

Typo in Griffiths' electrodynamics

We are referring to the second equality in equation (9.24) in section 9.1.3 Boundary Conditions: Reflection and Transmission. $$ \frac{\lambda_1}{\lambda_2} = \frac{k_2}{k_1} = \frac{v_1}{v_2}. ...
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1answer
79 views

Does the skin effect, eddy current / hysteresis losses contribute to Johnson noise in an inductor?

Based on my very basic understanding of the Johnson noise, it's not just a DC phenomena, but should change with frequency in a system, where there is a frequency dependent, real component to the ...
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73 views

Lorentz force on superconductor vortices

I am reading a paper on superconductivity and in it it says that the vortices are subject to a Lorentz force given by $$\vec{F}_L=\frac{\Phi_0\vec{I} \times \vec{H}}{cH}$$ Here $\vec{F}_L$ is the ...
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1answer
44 views

Determining wave vector

If I have an $\mathbf{E}$ field: $$ \mathbf{E}_1 = (\hat x 2e^{j\pi/2} + \hat y5)e^{-j4z} $$ How do I find the wave vector $\vec{k}$? If I multiply through I get $ \mathbf{E}_1 = \hat x ...
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19 views

Induced Electric Field inside Faraday Cage

Imagine there is a hollow toroidal conducting chamber and a solenoid in the middle. When we power the coil, does the induced electric field "propagate" inside the chamber? My prediction is that the ...
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1answer
20 views

E field using cylindrical coordinates

Can someone explain why, when I am going to calculate the $\vec{E}$ or $\vec{B}$ field of a charged ring in its axis (using cylindrical coordinates), the position of source field is $(R,0,0)$ and not ...
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2answers
104 views

Two different electrostatic charges in rotation

It's a theoretical problem. I mean I drawn a circle and I consider this circle exist. I would like to understand how works this device composed of a circle and two charges: Two charges fixed on a ...
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0answers
29 views

Electromagnetic perfection required

An electromagnet consists of turns and current, the more of each the better up to saturation but how is the maximum saturation achieved? The lower the resistance the better for current but this will ...
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1answer
19 views

How does static protection really work?

I've read that while working with circuits you want your body to stay at the same voltage as the components you are trying to protect. So you're supposed to connect those wrist things to one of the ...
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1answer
35 views

As the magnetic field propagates does it have a momentum that can be felt by another magnetic field of the same charge?

If I have a solenoid and a permanent magnet, and I placed the magnet an inch away from the solenoid, oriented in a position where the magnet would be repelled as the solenoid is turned on. Is it a ...
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25 views

Do electrons move easier depending on the way a TIG tungsten head is grinded?

I'm in a TIG welding course, and during the course we (inevitably) messed up a few (suicidal) tungsten heads by grazing/suicide dive into the molten puddle during operation. Standard procedure is to ...
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2answers
102 views

Could some astronomical objects have superconducting properties?

The colder it is, the more efficient the superconductivity process works. And as we know, if there is no star nearby, space gets pretty cold. I do appreciate that many condensed, burnt out, stars ...
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Magnetic dipole near black hole

Usually it is said that black holes cannot have electric or magnetic dipole, only electric charge and angular momentum are allowed quantities besides mass So, it would seem that black holes behave as ...
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1answer
32 views

Electricity on magnetic powder

I had the idea of, what if you ground up some magnets into a fine powder, what would happen with the powered, and how would it act? After some google searches, it seems that this isn't done very ...
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2answers
38 views

Electron moving in constant magnetic field

I'm revisiting an old problem which is to show that the trajectory of an electron in a constant magnetic field $\textbf{B} = (0,0,B)$ is a helix. I started with the Lorentz force equation $$ ...
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2answers
65 views

Converting Pendulum into Electricity? [closed]

I've been thinking about this, I want to use this as my science project. The two viable solutions I've thought of so far are magnet or rotary based. Pendulum clocks could be powered once a day and ...
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13 views

Analytical Calculations For a Stable Electret in Motion in Interplanetary Space. Determining the magnetic field

We know that a charging capacitor has, associated with the increasing E-field, a magnetic field. Now please consider the case of a very strong dipole or E field produced in an electret. Let us say ...
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1answer
79 views

Finding the magnetic moment of a permanent magnet to calculate the torque when exposed to a magnetic field

A permanent magnet has a magnetic moment $m$ associated with it. When exposed to a magnetic field $B$ the torque acting on the magnet can be calculated as $\tau = m \times B$. I am trying to ...
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2answers
84 views

Wave number of $\mathbf{E}$ field

If I have an $\mathbf{E}$ field: $$ \mathbf{E}_1 = x E_0 e^{-j(y-z) } $$ I think I can find its wave vector direction by finding the $\mathbf{H}$ field and then solving for the Poynting vector ...
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Question about Meissner Effect video

In this video, what material is on top of the superconductor disk that keeping it cold? Is it just a circular chunk of dry ice? http://www.youtube.com/watch?v=Ws6AAhTw7RA I'm planning on recreating ...
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1answer
40 views

Does an electron beam always repel electrons outside the beam?

Having read that question: Magnetic force as a relativistic effect? And quoting from the answer: http://physics.stackexchange.com/a/143901/7743 If you want to analyze things in the electrons' ...
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2answers
62 views

The notion of fixing a gauge

I don't understand the notion of gauge fixing; can we choose any gauge or are there some restrictions? For example why can we choose $\nabla\phi = 0$ here: Determine the Electric field using ...
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42 views

Determine the Electric field using magnetic vector potential

How to determine $\vec{E}(t)$ form the magnetic vector potential ? when $\vec{B} = \frac{\mu_0i(t)}{2\pi r}\vec{e_\phi}$ is the magnetic field at the point given by : $M(r,\phi,z)$ ( cylindrical ...
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How to calculate magnetic flux density in gap

I have the following setup (where the relative permeability of the ring is $\mu_{r}$): I am struggling to work out how to calculate the magnetic flux density $\vec{B}$ in the gap in the ...
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40 views

Ampere's law and magnetic circuit

For magnetic circuit there is an equation NI=HL. This equation is obtained by applying Ampere's law. H is magnetic field intensity and L is the length of the circuit. In following picture the equation ...
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1answer
29 views

How to write magnetic dipole transition hamiltonian using ladder operator?

The magnetic dipole transition Hamiltonian is $\hat{H}=\frac{e}{2m_ec}\hat{\mathbf{B}}\cdot\hat{\mathbf{L}}$ How do I express it in terms of ladder operator $\hat{L}_+$, $\hat{L}_-$, and the ...
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1answer
46 views

Explanation of van der Waals force

Can someone please explain why the van der Waals force is attractive instead of repulsive? My understanding is that is has to do with the electron cloud creating a dipole, but I can't seem to ...
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18 views

$H$ field in Faraday's law problem?

In my textbook it says, Consider a rectangular loop in the vicinity of an infinitely long wire carrying a current $I(t)$. To calculate the magnetic field, we can use the quasistatic approach, which ...
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1answer
25 views

Polarization type/direction of $E$ field

I have an E field given as $E(z) = [xE_a - yjE_b]\exp(-jkz)$ and I know the answer is that the field is right-hand circularly/elliptically polarized depending on whether $E_a = E_b$. Can someone show ...
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42 views

Torque on a wire loop from straight wire [closed]

I need to show that the torque on this wire loop is $$N = \frac{\mu_0}{\pi} 2abdII' \frac{(b^2+d^2)\sin(\theta)}{b^4+d^4-2b^2d^2\cos(2\theta)}$$ The where $\theta$, is the angle between the loop ...
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1answer
70 views

Calculating electric field using a given magnetic field equation (Maxwell-Faraday law)

The following magnetic field is given (and nothing else is known): $$ \begin{cases} B_0 \frac{t}{T} & \quad\quad\quad r < R \\ 0 & \quad\quad\quad r > R \end{cases} $$ I need to ...
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4answers
86 views

Photon field interaction

I want to know is there interaction between photon and electric or magnetic field? In other word can I deflect photons by electric or magnetic field? If it is possible, under which condition one can ...
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2answers
79 views

Do transformers lose energy?

EDIT: The title should rather be how/why transformers lose energy My idea of a transformer is that it is composed of two separate wire windings around some metal core. The purpose is to ...
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How do I evaluate this general magnetic dipole equation for this given setup?

This shouldn't be too hard a question (mostly focused on vector multiplication) but I'm still not confident in my answer. Basically, I am looking at the force between two magnetic dipoles and using ...
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2answers
203 views

Does a rotating disk develop a potential difference between the centre and rim?

This stems from thinking about the question If a perfect conductor where to move what happens to the electrons?. Suppose we have a rotating disk with no external magnetic field, so this is not a ...
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2answers
93 views

If a perfect conductor were to move, what happens to the electrons?

Theoretically, if a perfect conductor were to move (or start spinning), would the electrons inside it remain where they were or be 'dragged' along with the conductor?
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1answer
79 views

Obtaining quantum Hamiltonian for charged particle from path integral formulation

I was working on Shankar 8.6.4, which is about obtaining in one dimension the Hamiltonian operator of a charged particle from the path integral formulation. First, I get the propagator over a time ...