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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4
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2answers
45 views

Why does an oscillating electric field couple more strongly to electrons than to ions?

As stated in Principles of Plasma Discharges and Materials Processing, by M. A. Lieberman and A. J. Lichtenberg, about capacitively coupled plasmas: (...) the light and heavy charged particles in ...
0
votes
1answer
31 views

Electric field inside an insulator's cavity [closed]

I'm trying to find the electric field inside insulating sphere's cavity, and I don't know how. However, if there was an conductor I'd know that $E=0$ from Gauss's Law.
4
votes
1answer
96 views

What is relationship between electromagnetic mass and rest mass?

Is there a direct equation which compares rest mass $m_°$ and electromagnetic mass $m_{em}$? Nothing on web I found. $m_{em} = \frac{4 E_{em}}{3c^2}$ 4/3 problem The final solution of the ...
0
votes
1answer
25 views

is it possible to isolate and pair electromagnetic interaction? [closed]

is it possible for two electromagnets to interact each other, ie. repulsive or attractive forces to be exerted to each other, without being exerted to other metallic items or other electromagnetic ...
0
votes
1answer
28 views

What would happen if a magnet was pushed towards a copper tube in space?

So the question was inspired by one of curious comments on youtube. What exactly would happen if you pushed a strong neodymium magnet towards the copper tube (aiming for the opening in an effort to ...
0
votes
1answer
47 views

Fermi energy of electron gas with electrostatic interaction

I have been given the following exam question and am unsure how I would go about solving it: Consider the case of a one-dimensional metal, consisting of a chain of $N$ positive charges $+q$ ...
25
votes
7answers
3k views

Why are the electric force and magnetic force classified as electromagnetism?

I confuse the four kinds of fundamental interactions, so I think the electric force and magnetic force should not be classified as a big class called electromagnetism. Here is my evidence: The ...
6
votes
1answer
256 views

Confusion in Maxwell's derivation of Ampere's Force Law - Part II

I am reading Maxwell's "a treatise on electricity and magnetism, Volume 2, page 156" about "Ampere's Force Law". I have some confusion in the following pages: My question is of two parts: 1. ...
1
vote
0answers
42 views

advantages/disadvantages of a cylindrical microwave oven cavity in comparison to a rectangular cavity

Why are rectangular cavities preferred for standard microwave ovens with a frequency of 2.45 Ghz? What is the reason that you can hardly find a circular cavity? What are the disadvantages of ...
3
votes
0answers
48 views

Why is diproton unstable? [duplicate]

Diproton is an isotope of helium without any neutrons. It commonly forms in the Sun, where protons are fused constantly. However, it is extremely unstable, and will revert back to two protons almost ...
0
votes
0answers
18 views

What is the physical meaning of a magnetic conduction current?

In electrodynamics, it is possible to have an electric conduction current, whereby $J=\sigma_e E$, with $J$ being the current, $\sigma_e$ the electrical conductivity and $E$ the electric field (this ...
1
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2answers
63 views

Two Black Holes held stationary by EM forces

If two black holes with large enough mass (so that the tidal forces are minimal and the intersection is large) that are held apart by like charges (saddle point stability). Imagine the black holes in ...
0
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0answers
11 views

Magnetic field of a current loop on a boundary between materials

How can I find the magnetic field of a current loop on a flat boundary between two materials given their magnetic permeabilities $μ_1$ and $μ_2$ and a field $\overline{H_0}(\overline{r})$ which the ...
3
votes
1answer
30 views

Reaching equilibrium in a blackbody and light-matter interaction

Suppose we have a metallic cavity maintained at a fixed temperature. Suppose we start with any distribution of radiation that is not in equilibrium with the container. Gradually, when the equilibrium ...
-3
votes
1answer
21 views

Mechanism by which a capacitor is charged, using surface charges

I want to gain a deeper understanding as to how a capacitor actually gets charged. Consider a situation in which an uncharged parallel plate capacitor is connected to a battery using two wires. I ...
8
votes
2answers
560 views

Why do we use gauges in Maxwell equation?

While solving the Maxwell's equation we often use the Lorenz or Coulomb gauge, but why is that? Are the equations unsolvable if the gauge is not fixed? Or is it just for the simplicity?
0
votes
3answers
132 views

Why geologist do not consider molten iron in the magma to be the source of Earth's magnetic field?

This was a question that came in a exam. How come is this possible? Actually, a question on the source of Earth's magnetic field was asked here on physics SE, which matches my opinion. The ...
-1
votes
1answer
65 views

How I can prove refractive index in an environment is this? [closed]

in "Applied Quantum Mechanics" by A.F.J. Levi, is a problem that I couldn't solve it can any solve it: if electrical filed is: $$ \mathbf{E}\left(\mathbf{r}, \omega \right) ...
5
votes
3answers
889 views

What are the differences between the differential and integral forms of (e.g. Maxwell's) equations?

I would like to understand what has to be differential and integral form of the same function, for example the famous equations of James Clerk Maxwell: How to know where to apply each way? Excuse ...
1
vote
1answer
55 views

Tensor notation of Maxwell's equations

Tensor notation of Maxwell's equation read So when we explicitly try to find the Maxwell's equation from the above tensor equation we only get gauss law and curl of B. The div.B=0 and curl of E are ...
0
votes
1answer
34 views

surface charge on a resistive wire in dc circuit

I want to understand how does the energy transfered from battery to the resistor in a simple dc circuit . I readed that it is due to the surface charge the battery creats on the wire . So why this ...
-1
votes
2answers
78 views

Is electromotive force really a force? [duplicate]

As far the definition goes emf of electromotive force is basically potential difference. It even has dimensions of potential. Then why is it called a force?
0
votes
2answers
39 views

Zero electric field in empty region of hollow conductor

A hollow conducting shell having a net charge of +Q has a point charge -Q placed at its centre. From the diagram, it looks like Suppose the radius of the inner shell from the origin is a and the ...
4
votes
1answer
60 views

Is diamagnetism a static or dynamic effect?

When we put a diamagnetic material in the presence of an external magnetic field $\vec B_0$, the magnetic field inside the material decreases to $$\vec B=(1+\chi_m)\vec B_0,$$ where the magnetic ...
2
votes
1answer
97 views

Confusion in Maxwell's derivation of Ampere's Force Law

I am reading Maxwell's "a treatise on electricity and magnetism, Volume 2, page 155" about "Ampere's Force Law". I have some confusion in the following pages: On the top of the page, Maxwell says: ...
2
votes
1answer
88 views

Can light be a spinor?

A recent discovery suggests that photons can have half-integer spins. This seems to contradict the well understood notion that photons are vector (1-form) fields What does this mean for the ...
0
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0answers
25 views

The velocity of the charged particle in the magnetic field [duplicate]

Enrico Fermi considered that moving charged particle gains energy in the magnetic field, so its velocity increases. According to the Lorentz Law $\vec F=q\vec v\times\vec B$. Work is $A=\vec ...
-1
votes
1answer
38 views

Maximum electric field of a circular ring

How do you differentiate the equation for electric field of uniform ring $$ E_x = \frac{kxQ}{(x^2+r^2)^{3/2}} $$to get the maximum at a point? My book says $x = \frac{r}{\sqrt2}$. I tried ...
2
votes
1answer
72 views

Electric field due to a time and space varying current density?

Let us say I have a current density: $$\vec J\equiv\vec J(\vec r, t)$$ This will produce both a time varying magnetic field and a net charge density. Both these effects produce an electric field. If ...
0
votes
2answers
55 views

Does a rotating electric field vector (in circular light) moves slower than the speed of light?

We can imagine circular light as a combination of two rotating vectors moving through space: the electric field vector that´s rotating together in sync with the magnetic field vector. As the vector ...
0
votes
1answer
34 views

Can circular light be produced with more than one vector, tracing out multi stranded strand helixes in space? [closed]

Is it possible to construct circular light with more than one vector tracing out a one strand helix in space? For example, circular light with two vectors in opposite directions (on the same line), ...
0
votes
0answers
17 views

Magnetostatic field inside a wire and current density

Right now I'm studying magnetostatics an I ran across a problem when trying to understand a question in a book witch led me to thing that the steady-state current density inside a wire is not ...
0
votes
2answers
40 views

Why doesn't the group velocity of a plasma EM wave equal the phase velocity here?

For plasma EM waves we have the dispersion relation $$\omega^2=\omega_p^2+c^2k^2$$ where the plasma frequency $$\omega_p^2=\frac{n_e e^2}{\epsilon_0 m_e}$$ One can show that $v_p v_g=c^2$, i.e., the ...
0
votes
0answers
7 views

Does charge injection from electrodes increases the total charge in a (semi)conductive channel?

When a (semi)conductor is connected to a voltage bias, charge is injected from the electrodes. When steady state is reached (constant current flow), does the total charge in the (semi)conductor is ...
0
votes
1answer
52 views

B field of an EM pulse

I am trying to find the B field associated with the following E field (to eventually find the effect it has on a charged particle): $$E(\vec{x},t)= \Re[E_0(z-ct)]e^{ik_0(z-ct)}$$ Where $E_0$ is an ...
1
vote
1answer
34 views

Using 2 pair of like pole magnet side by side will increase magnetic field?

I would like to know if using two pairs of magnets with like poles side by side will strengthen the magnetic field? As I understand, placing 2 pair of magnetic with like poles side by side will dry ...
0
votes
1answer
19 views

Do we use rms values or peak value for calculating power of a transformer?

Do we use rms values or peak value for calculating power of a transformer? I have an A levels exam tomorrow and I wasn't able to find any kind of explanation in the book.
0
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0answers
22 views

Purcell's approach to Larmor's formula, assumption?

In 'Electricity and Magnetism' By E.M.Purcell, a derivation is given of Larmor formula (a version of which can be found here). I will give a brief overview here: A particle is considered at ...
3
votes
2answers
80 views

Does a source of photon emission “sense” if the photon is absorbed? [closed]

Can the absorption of a photon be 'felt' by the source that emitted it? At least, if we assume that it emits a steady stream of photons? Is there a back reaction of some kind on the source?
0
votes
0answers
39 views

Rotating magnet and magnetic field intensity change?

if magnetic disc is rotating on it's axis super fast and electric field is produced(because of time varying magnetic field),does this means that in the frame of non moving observer magnetic field no ...
1
vote
1answer
59 views

what do we mean by speed of light dependent on direction?

I have a statement in textbook saying "When the speed of light is independent of direction,the secondary waves are spherical". Can someone tell when it is dependent on direction and how the secondary ...
4
votes
0answers
36 views

Special Relativity, refractive index and catching up with a wave

Einstein was partially motivated by the following: With Maxwell's equations, a plane wave is a sinusoidal wave that varies in space in time and moving with speed $c$. These variations are linked by ...
0
votes
0answers
16 views

How can I approximate calculate the magnetic force between electromagnets?

I try to find an approximate amount of force exerted between two electro magnets. The below site could've helped me but the problem is that the below just talks about the number of coil turns and so ...
1
vote
2answers
23 views

Levitation inside a solenoid

Is it possible to levitate something inside a solenoid? If yes, is it possible in any direction (horizontal, vertical)? Also, is it possible when an object is traveling through? Thanks in advance!
0
votes
2answers
58 views

Am I wrong about the right hand grip rule?

The right hand grip rule (also known as right hand screw rule) tells you the direction of a magnetic field due to a current. If you point your thumb in the direction of the current, your fingers will ...
0
votes
1answer
33 views

Why don't conductive objects change color when electric dipoles are induced in them?

I was playing around with a comb, my hair, and pieces of paper when it occurred to me that inducing a dipole on the pieces of paper meant that many valence electrons previously on the side closer to ...
1
vote
2answers
124 views

Calculating the rate of change of Magnetic Field strength [closed]

I'm a bit stuck on this question (which is homework so hints are more welcome than outright answers). The question is: A very long wire carrying a current I is moving with speed v towards a small ...
1
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1answer
13 views

Light Ray component field strength

When looking at a diagram of the electric and magnetic fields oscilating down the time axis of a light ray diagram the field amplitude and frequency of each component is shown equal.Are they infact ...
0
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1answer
33 views

What is the interrelationship among PD, T1 and T2 in relation to MRI

Proton Density (PD), Spin - Lattice Relaxation Time (T1), Spin - Spin Relaxation Time (T2) are three major characteristics by which we can distinguish different types of tissues and all three are ...
0
votes
0answers
24 views

Do EM waves create action in receiver antenna without reaction?

If EM waves such as radio waves, can induce electrons in the receiver antenna to move, and electrons have a small amount of mass, isn't this action without a reaction? Is it possible to use the ...