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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What happens if a current flows through a wire wrapped around a magnetised material?

A cylinder of radius $a$ has a uniform magnetisation along its axis, which results in zero bound volume current density and a bound surface current density $M\hat{\phi}$. A coil with $N$ turns and ...
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30 views

Light reflecting off a sphere

Suppose light of momentum $-P\hat{\textbf{k}}$ is shone on a sphere. What is the momentum transferred onto the sphere? I think the transferred momentum upon reflection is given by $(2P\hat{\textbf{k}...
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30 views

Relationship between magnetic force acted upon a diamagnetic substance and the magnetic field applied to it

I have a sample of bismuth that I have applied a measured B-field to through the use of a solenoid electromagnet. I've been trying to find a formula relating the B-field to the force that the bismuth ...
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22 views

The frequency induced by the alternating current

Let set up the experiment as followed. A string is stretched at two ends. Two magnet poles N and S are placed under and above the string. Now we let an alternating current go through the string with ...
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3answers
164 views

Does waving a magnet around create light?

If I wave around a bar magnet, the magnetic field in the space around it changes. Is this enough to go through the whole speed of light derivation implying that the motion creates an electromagnetic ...
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48 views
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59 views

Magnetic field of a permanent magnet - numerical calculation

I'd like to understand methods for calculating the magnetic field generated by a magnetized piece of material, e.g. a permanent magnet. I was wondering about the approach described here as it seems ...
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1answer
39 views

EM Wave equation and dispersion in a plasma

I am currently studying for qualifiers and I am really stuck on this one practice problem and I want to understand why I am having such a difficult time. The idea is that we have a plasma with free ...
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2answers
42 views

Skin effect explained in terms of the flow of charges?

Can the skin effect for em waves incident on a conductor be explained by the movement of free charges within the conductor (given that they as $t\rightarrow \infty$, $\rho\rightarrow0$)? And if so how?...
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2answers
585 views

Dielectric tensor vs. conductivity tensor in (cold) plasmas

I'm studying Waves in Cold plasmas right now, but I guess my question is generalizable. It's about the 4th Maxwell Equation in polarizable / conductive media: $\nabla \times H = \frac{1}{c} \frac{\...
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2answers
85 views

Direction of torque due to magnetic moment

I have a problem with determining the direction of torque due to magnetic moment on a current loop. Since the torque is defined as $$ \tau = p_m \times B $$ so I guess the direction of the torque ...
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1answer
283 views

Electromagnetic inertia due to advanced radiation?

The scalar potential $\phi$ and vector potential $A$ at a distance $r$ from a charge $q$ are given approximately by $$\phi = \frac{q}{r}$$ $$\mathbf{A} = \frac{q\mathbf v}{r}$$ where the constants ...
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1answer
49 views

Auxiliary magnetic field identically zero?

Consider a 3D space that's filled up completely with a material of fixed magnetization $\mathbf{M}$ (i.e. everywhere in space the magnetization points in the same direction and has the same magnitude)....
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56 views

Electric field due to a current carrying loop

I want to know how I can calculate the radiated electric field in the far zone of the loop. Knowing that the loop is in center at the origin of a x-y plane Knowing that the loop as a time-harmonic ...
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1answer
194 views

Why does the skin effect cause current to flow at surface of a conductor?

I have read that due to the skin effect we only need to coat e.g. cables with a good conductor to make them effective. I understand that when a em wave is incident on a conductor it only propagates so ...
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38 views

How to Create a Strong Electromagnetic Field?

For a project, I need to make a strong electromagnetic field, but it has to be light, small, but about 1-2 teslas strong. The attached image is a simple drawing shows the direction I need to generate ...
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1answer
3k views

Deriving the Electromagnetic Tensor

The electromagnetic tensor is given as: \begin{equation} F_{\mu \nu} = \partial_\mu A_\nu - \partial_\nu A_\mu \end{equation} How do you derive this? And how come there is a partial derivative in ...
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0answers
72 views

Which of Landau's books are suited for undergraduates? [closed]

Seeing how well-received Landau's books are, I want to know if any of them are suited for undergraduates.
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67 views

Problem in computing self-induction for a wire with zero diameter

This is quoted form Edward Purcell's Electricity and Magnetism; it tells about the problem of computing magnetic flux during self-induction for a wire with zero-diameter: You may think that one of ...
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1answer
46 views

Difference between flux of wire loop and wire in a uniform magnetic field

How come when a wire loop sweeps out an area in a uniform magnetic field and moves with constant velocity there is no induced current because there is no change in flux. However, when a straight wire ...
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2answers
112 views

Electric Field “at” the surface of a conductor

It has been pointed out to me that the Electric field exactly on the surface of the conductor is conventionally taken to be $E=\frac{\sigma}{2\epsilon_0}$; does this come from taking the midpoint of ...
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3answers
212 views

stationary electron in a time varying field

A moving electron in a stationary magnetic field will experience magnetic Lorentz force given by $q(\vec v\times\vec B)$. But will a stationary electron placed in a time varying magnetic field ...
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0answers
32 views

How useful would mass EM communication be in Space? [closed]

Considering a Science Fiction question, it occured to me that in "populated" space, with no atmosphere, you might expect to find more EM interference than on Earth. Basically, if you are outside of ...
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1answer
4k views

How is Gauss' Law (integral form) arrived at from Coulomb's Law, and how is the differential form arrived at from that?

On a similar note: when using Gauss' Law, do you even begin with Coulomb's law, or does one take it as given that flux is the surface integral of the Electric field in the direction of the normal to ...
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1answer
224 views

Electric field a distance z above the midpoint of a straight line segment

In Griffiths there's an example to evaluate the Electric field a distance z above the midpoint of a straight line segment of length 2L. Which carries a uniform charge $\lambda$. In that calculation, ...
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18 views

Increase Phase Velocity by Losing Power

I just read some words from the book Optical Waveguide Theory by Snyder, Allan W., p226. It says that However, although it is possible for the phase velocity of a mode to exceed the maximum speed ...
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2answers
39 views

Why is flux linked between a bar magnet and a coil same as the flux due to bar magnet from radius of coil to infinity?

Why is flux linked between a bar magnet and a coil same as the flux due to bar magnet extending from radius of coil upto infinity? It has been stated without any reasoning in this video. at about 1 ...
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73 views

Curl of a simple magnetic field and resulting current distribution

I've been doing some thinking lately and here is my question: If one imagines that there is an auxiliary magnetic field $H$ whose spatial dependence is given by equation: $$H(x,y,z)=y\hat{i}$$ ...
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1answer
49 views

How does this approximation of the $B$-field of a charged rotating disk for $z\gg R$ make sense?

Consider a disk of radius $R$ rotating with angular velocity $\omega$ and with a surface charge density $\sigma$. I have shown that the magnetic field as a function of the distance along its axis (z) ...
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5answers
660 views

EMF in an Open circuit?

If a wire that is connected to a circuit passes a magnetic field, it would induce $EMF$ that would allow current to flow, and generate an opposing force. Now what if the wire is not connected to a ...
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3answers
157 views

How is complex permittivity measured?

Complex relative permittivity is defined as $$\epsilon_r = \frac{\epsilon(\omega)}{\epsilon_0}=\epsilon_r^{\prime}(\omega) + i\epsilon_r^{\prime\prime}(\omega) = \epsilon_r^{\prime}(\omega) + \frac{i\...
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1answer
25 views

Force from a large conducting plate [closed]

How can I find the distance to a test-charge, hoovering below a conducting plate? Newtons laws applies. $$ $$ According to this information, I conclude that the force is equal zero. It is not moving....
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1answer
17k views

Dielectric constant or permittivity of metals

I'm wondering what the dielectric constant or permittivity of metals is --particularly copper. Do metals have an infinite permittivity?
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27 views

Looking for materials with specific magnetic properties

To begin with, I am not a native English speaker and have very limited knowledge regarding magnetism and want to apologize in advance for probable use of incorrect terms. In the picture, M ...
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1answer
66 views

Solved Gauss' Law for $\vec{E}$ without boundary conditions?

Why can I solve for the electric field of a point charge Q at the origin without boundary conditions? $\nabla\cdot\vec{E}=\rho/\varepsilon_0 = \delta(\vec{r})/\varepsilon_0$ is a 1st order ...
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1answer
81 views

Magnetic field felt by an observer “orbiting” a proton

Classically, if one has an electron orbiting a proton, how can the magnetic field felt by an observer with the same instantaneous velocity as the electron be calculated? It seems that I may find the ...
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4answers
559 views

Is the electromagnetic mass real?

In his Lectures on Physics vol II Ch.28-2 Feynman calculates the field momentum of a moving charged sphere with charge $q$, radius $a$ and velocity $\mathbf{v}$. He finds that the total momentum in ...
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4answers
737 views

The problem of self-force on point charges

Allow me to preface this by stating that I am a high school student interested in physics and self-studying using a variety of resources, both on- and off-line, primarily GSU's HyperPhysics website, ...
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1answer
61 views

Experiment: releasing the copper bar above a permanent magnet

A classic experiment shows that when we drop a magnet through a copper coil long tube, there is eddy current induced in the coil. Then, due to lenz law, the motion of the magnet will be opposed and it ...
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2answers
293 views

Communication back in time?

On the wikipedia page for time travel, it mentions that Robert Forward said (theoretically, unknown since we don't have a working theory of quantum gravity) that if a heavy nucleus was subjected to an ...
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2answers
2k views

What's the relation between virtual photons and electromagnetic potentials?

Given that: 1) virtual photons mediate the electric and magnetic force fields 2) the magnetic field is the curl of the magnetic vector potential 3) the electric field is the negative gradient of ...
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1answer
98 views

Why does an electron lose all of its energy in a resistor?

Why does the electron uses nearly all of its energy for one resistor in a simple circuit made of a battery, wire, and resistor? I really need to know what happens exactly at the atomic level in the ...
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3answers
102 views

If we consider the electric field to act upon charges with a force, how does it stay in line with Newton's laws?

This should be a relatively simple question. Let's say we have a constant electric field $\textbf{E}$ in a conductor. The electric current density would then be $$\textbf{j}=\rho \textbf{v}$$ where ...
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1answer
199 views

Biot-Savart for current carrying wire

A homework question has made me doubt my understanding of the application of the Biot-Savart law. Question: The magnetic field 43.0 cm away from a long, straight wire carrying current 5.00 A is ...
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1answer
5k views

Force from solenoid

I'd like to approximate the force from a solenoid, or at the very least find a formula which is proportional to the force so that I can experimentally find the constant for my particular case. ...
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1answer
64 views

Electro magnetic Wave? [duplicate]

I understand the fact that a static charge produces an electric field around it. I accept the fact(without questioning) that a moving charge produces a magnetic field. Question 1 : If a moving ...
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0answers
37 views

How exactly are the relative strengths of gravity and electromagnetism quantified? [duplicate]

I've often heard it said that gravity is much weaker than electromagnetism, and after looking at several questions on SE, I feel that I've got at least a qualitative handle on the concept -- gravity ...
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2answers
44k views

Effects of a very large magnetic field on the human body

Ever since reading about the NHMFL I have always wondered about this and asked several people without getting a good satisfactory answer. My question is, considering the simplest case let's say a ...
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1answer
76 views

Why does a toroidal coil transformer work - it should not! [closed]

A time variant current through a coil wound around a toroid core will produce a time variant magnetic field within the volume of the toroid, but no field external to that volume. Since a time variant ...