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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Calculate Hamiltonian from Lagrangian for electromagnetic field

I am unable to derive the Hamiltonian for the electromagnetic field, starting out with the Lagrangian $$ \mathcal{L}=-\frac{1}{4}F^{\mu\nu}F_{\mu\nu}-\frac{1}{2}\partial_\nu A^\nu \partial_\mu A^\mu ...
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1answer
116 views

Why Gauss' law is applied?

Why Gauss' law is applied? Why is there a need of finding electric field by Gauss' law if we can find the electric field through Coulomb's law? or has it got more applications than Coulomb's law?
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103 views

Why doesn't a particle's velocity effect the strength exerted on it by an electric field?

Here is what I know: $F = E q = m a$ so $a = \frac{E q}{ m}$ and we can increase the acceleration ($a$) of a particle in an electric field ($E$) by either decreasing its mass ($m$) or ...
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2answers
262 views

How is electric flux related to permittivity?

How is Gauss' law related to permittivity? I know that it equals $1/\epsilon_0$ times the magnitude of the charge enclosed. But, I'm unable to understand what it actually means. Can someone ...
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268 views

Electromagnetic duality

A key aspect of modern physics is the mapping of theories or different descriptions of a theory into a one-to-one correspondence. As I am trying to further understand the electromagnetic field tensor, ...
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1answer
118 views

Why will an accelerated electron emit a photon? [duplicate]

Why will an accelerated electron (or a charge) emit a photon? For example normal matter even if it accelerate or not will emit a photon.
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2answers
341 views

Why would an electron in an orbit be accelerating continuously and would thus radiate away its energy and fall into the nucleus in a classical model? [duplicate]

I was reading this answer by madame anna v: You are right, the planetary model of the atom does not make sense when one considers the electromagnetic forces involved. The electron in an orbit is ...
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2answers
115 views

How far could one fire a charged particle in space?

I've wondered how far would be able to send a concentrated beam of electrons in space. The reason is would we be able to launch a magnetic ring with an electron absorbing material on one side and just ...
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1answer
74 views

Does the Lorentz force applied to a current carring wire by a magnetic field act in the negative or positive direction of the right hand rule?

If I say that I am calculating the Lorentz force $F$ applied to a wire carrying a current $i$ at a point $P$ in a magnetic field $B$, would the actual force be opposite of that given by the right hand ...
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1answer
64 views

Cause of radiation resistance

I was reading Radiation Damping and light scattering from Feynman's lectures on physics. The following excerpts are from chapter 32, Vol I What is this radiation resistance due to? Let us take a ...
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60 views

Lorentz force helix

I have a few questions about the Lorentz force. Is it possible to create a Lorentz force helix in a cloud chamber and if so how would this be achieved? How would the pressure inside the helix be ...
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3answers
315 views

Is a real life electric shield possible? [closed]

I got this question from playing games like Halo and Borderlands (I know kinda dumb but raised a good question) in which the primary protection is an electric shield. Now I'm wondering if it would be ...
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3answers
120 views

Does electromagnetic radiation make sense in one dimension?

I'm trying to do a simple simulation of a 1D charged quantum particle, which gets irradiated by an electromagnetic wave — in context of non-relativistic QM. The Schrödinger equation for such a ...
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0answers
47 views

Do these steps demonstrate that acceleration of charged particle is proportional to current?

One formulation of Maxwell's Gauss Law for electric field is: $$\bigtriangledown E = 4 \pi k \rho $$ This can be worked into the Divergence Theorem as follows: $$\int\int_{A} F_\perp \:dA= 4\pi k ...
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2answers
135 views

In Jackson's expression for the electrostatic Green function, why is the Laplacian taken with respect to the primed coordinates?

Jackson writes, The function $1/|\mathbf{x} - \mathbf{x}'|$ is only one of a class of functions depending on the variables $\mathbf{x}$ and $\mathbf{x}'$, and called Green functions, which satisfy ...
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191 views
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473 views

Deriving the Electromagnetic Tensor

The electromagnetic tensor is given as: How do you derive this? And how come there is a partial derivative in front of $A_\mu$? Do you multiply the derivatives or what?
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1answer
107 views

What happens when molten metal cools within a strong magnetic field?

What happens when molten metal cools within a strong magnetic field? I don't know what more say, don't remember anything relevant to this in my uni text books.
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3answers
318 views

How exactly does WiTricity's wireless power transfer using resonance work?

The WiTricity group has made a lot of buzz but no internet article explains the exact method that they are using. Does any one know how it works?
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1answer
95 views

Motion in a Paul trap: $2n$th harmonic with larger amplitude than $n$th harmonic

Using a Paul trap, we captured the motion of a light charged particle (based on a rotating potential applied by AC current). Our rotational frequency was 50 Hz, and so when used FFT on the data, we ...
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2answers
236 views

Motion of charge in magnetic field with drag force [closed]

Say you have a charged particle in a region that contains a fluid that will produce a drag force that goes as $F=-kv$ where $v$ is the speed and $k$ is some constant. The region also contains a ...
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2answers
869 views

Do two magnets stacked on top of each other repel/attract stronger than just one magnet?

In designing a switch, I made it such that it "springs" back via two neodymium magnets (one in button repelling one in switch). I've found the magnets are too weak and don't spring back. I've resorted ...
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1answer
73 views

In a non auto-ranging digital multimeter, what do the increasing numbers in the Ohm section represent?

For my science fair project I am measuring the resistance of nichrome wires under different conditions. I am using an Innova 3300 digital multimeter to measure the resistance. Other websites have told ...
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1answer
145 views

Traditional Kirchoff voltage law in AC circuit?

The traditional (not taking into account phasor addition or complex addition) application of Kirchoff Voltage law, i.e. $\Sigma\Delta V=0$ along a loop, does not work for AC circuits. We can sum the ...
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1answer
53 views

Calculating Energy & Small functional time scale

I have an electric motor that can apply a pull force of $3000 \;\mathrm{lb}$ (electric winch), it draws $180 \;\mathrm{A}$ at $12 \;\mathrm{V}$. I understand that power $P = I \cdot V = 2.1 ...
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4answers
3k views

Why is current a scalar quantity?

Current has both magnitude and direction. As per the definition of vector defined in encyclopedia, current should be a vector quantity. But, we know that current is a scalar quantity. What is the ...
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196 views

explain how does Rectenna work to someone with some college level physics

I understand that rectifying antenna (rectenna) is supposed to convert electromagnetic energy to electric current however I do not understand how it's really working. I do get that it's kind of like ...
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217 views

Derivation of Lagrangian density for an infinite classical dielectric in interaction with the EM field

I am tasked with reading and reproducing all the steps in J.J. Hopfield's 1958 paper "Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals". Embarrassingly I am stuck ...
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61 views

Magnetic Field Given Magnetization

We are working on problem 6.12 from Griffiths Electrodynamics. It says that we have an infinitely long cylinder of frozen in magnetization of $M=ks\hat z$. We are trying to find the field. We have ...
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74 views

Lenz's Law and Eddy Currents

You can determine the direction of eddy currents according to Lenz's law. E.g. If a metal sheet is losing flux into the page, it will experience induced eddy currents in a clockwise direction to ...
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30 views

Opposite field's acting on a soft ferromagnetic material

To external fields(H, - H) not equal to each other are acting on that material. Based on the hysteresis loop what would be the result? The hysteresis area of that materials is very narrow(i.e easy to ...
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1answer
576 views

Axial forces on a solenoid windings

I understand that the windings in a solenoid experience a Lorentz force $\mathbf{f} = \mathbf{J} \times \mathbf{B}$, which tend to cause an outward pressure where $\mathbf{B}$ is directed along the ...
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2answers
870 views

1 Tesla electromagnet?

Is it possible to create a powerful electromagnet at home? With use of a ferromagnet it seems so... Using the following formula: $B(Tesla)= k\mu_0nI$. I understand some ferromagnet's like iron could ...
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4answers
213 views

Is voltage important when creating a magnetic field?

Would a DC circuit that has a high current and low voltage have a powerful magnetic field? I'm trying to create a powerful solenoid. In order to create a powerful magnetic field, I'm focusing more on ...
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410 views

Does a conductor of total charge zero placed in a uniform external electric field experience net force?

The question I have in mind is: If we place a conductor (arbitrary shape) of total charge zero in a uniform external electric field $\textbf{E}_0$, does it experience any net force? Why (not)? Now I ...
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47 views

Help in understand Magnetostatic Energy

$$E_{\mathrm{ms}} = \frac{1}{2}\mu_0 \int_V \mathbf{M} \cdot \mathbf{H}_{\mathrm{ms}} d^3 r$$ I can't understand this formula, what is the magnetostatic stored potential energy?! What does it show? ...
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58 views

Noether's theorem in the realm of superfluids

In 1969 Keith Moffat showed helicity conservation for ideal fluids such as liquid Helium. This work is proving seminal in our understanding of turbulent flows and viscous fluids. In the case where ...
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148 views

Do electromagnetic fields gravitate?

It's well known that electromagnetic fields contains energy but do they gravitate ? When we talk about the composition of the universe it's now accepted that the 74 % is dark energy , the 22 % is ...
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0answers
81 views

Potential of a charged disc brought above the z=0 plane at an arbitrary point

Potential of a charged disc can be obtained easily. If we want to calculate the potential at an arbitrary point we should just write: $$ \phi(z_0)=\frac{\sigma ...
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1answer
100 views

Does there exist electric field around all the substances?

A system of two equal and opposite charges separated by a certain distance is called an electric dipole. Electric dipole moment ($p$) is defined as the product of either charge ($q$) and the ...
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1answer
205 views

Why Inox Steel doesn't interact with magnets?

My dad has a HUGE magnet on his workshop. I love magnets, and when I saw it, I asked him what it was for. His reply was: "I don't know why, but inox steel bolts don't get attracted to it, so I use ...
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0answers
51 views

Help Understanding Equation for Characteristic Time of Induced Magnetic Field

I am reading this book, the part in particular about Eddy-current separation starting at Page 246, in it there is an equation for calculating the "characteristic time with which the induced magnetic ...
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1answer
157 views

Why does a light wave invert at a boundary with greater index of refraction?

Is there a reason why a EM wave reflects invertedly when it meets a boundary point with a greater index of refraction. In the case of ropes, if remember correctly, the reason why it inverts is to ...
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238 views

1-dimension radiation problem [closed]

A positive charge $q$ is fired head-on at a distant positive charge $Q$ ( which is held stationary ), with an initial velocity $v_{0}$. It comes in, decelerates to $v=0$, and returns out to ...
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1answer
62 views

Does zero change in magnetic flux always imply zero emf induced?

If you have a uniform B field, with a finite piece of wire inside it. Assuming the B field spans all space and the wire cannot leave the field. Are you able to create an emf by moving the wire ? I ...
3
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2answers
126 views

Classical electrodynamics formulated in terms of forces

The Newton's law of universal gravitation is described in terms of a force, which is produced by an action at a distance. It also can be described using the concept of a field, and that would be an ...
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121 views

What is the more fundamental quantity? The electromagnetic field F or the potential A?

Read somewhere that potential is a more fundamental quantity than EM field because if the latter is more fundamental then gauge transformation will reduce to nothing more than a mathematical trick. I ...
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222 views

Is there a Hamiltonian for the (classical) electromagnetic field? If so, how can it be derived from the Lagrangian?

The classical Lagrangian for the electromagnetic field is $$\mathcal{L} = -\frac{1}{4\mu_0} F^{\mu \nu} F_{\mu \nu} + J^\mu A_\mu.$$ Is there also a Hamiltonian? If so, how to derive it? I know how ...
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77 views

$E$ and $B$ fields in Axial Gauge

I am trying to compute the $\vec{E}$ and $\vec{B}$ fields in the Axial gauge ($n \cdot \vec{A}=0$) where $n^2=1$, but I'm having trouble seeing the usefulness/how it simplifies the equations.
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43 views

Boundary Condition for Generating Evanescent Waves

I'm reading through Novotny and Hecht's book on Nano-Optics (Principles of Nano-Optics), and I've come across a subtlety in the boundary conditions for evanescent wave generation (via total internal ...