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.

learn more… | top users | synonyms (6)

3
votes
2answers
28 views

Current loop and direction ambiguity of the magnetic moment

Consider a circular loop in the XY-plane which carries a current $I$. Then it behaves as a magnetic dipole with moment $\textbf{m}=I\int d\textbf{S}$ where $\int d\textbf{S}$ is the area of the loop ...
4
votes
3answers
483 views

Poynting's theorem - energy conservation

Poynting's theorem: $$\int_V\left(\vec{E}\cdot\vec{J}\right)\,\mathrm dV = -\dfrac{\partial}{\partial t}\int_V\dfrac{1}{2}\left(\epsilon_0 E^2 + \dfrac{1}{\mu_0}B^2\right)\,\mathrm dV - ...
0
votes
0answers
21 views

How much energy would be required to remotely induce a signal in a wired network (e.g. Ethernet)?

How much energy would be required to remotely induce a signal in a wired network, such as Ethernet? Assume, for the sake of discussion, that the wired network has a single cable of length $l$ (eg 10 ...
3
votes
4answers
160 views

How can we prove charge invariance under Lorentz Transformation?

We have gravitational force between two massive particles and we have electromagnetic force between two charged particles. When special relativity suggests that mass is not an invariant quantity why ...
0
votes
0answers
10 views

How does Dual-polariasation weather radar detect raindrops that have a larger H to V ratio

I was reading this document on dual polarisation weather radar - http://www.wdtb.noaa.gov/courses/dualpol/documents/DualPolRadarPrinciples.pdf and looking at how this can be used to detect when water ...
0
votes
1answer
10 views

A media in which the electrical displacement vector is not causal

I recently did an electrodynamics homework problem in which we showed that in a certain model (Lorentz-Drude), where the permittivity of free space $\epsilon$ was dependent upon the angular frequency ...
0
votes
0answers
29 views

Why does magnetism persists much longer than electric charge?

I remember Feynman explained two things In contrast to magnetic property, electrically charged bodies quickly attract some garbage and loose their charge. Meantime all observers observe the same ...
0
votes
0answers
27 views

Net magnetic field at a point

Given 5 wires ( $w_0, w_1,w_2,w_3,w_4$) all separated equally by a distance ($r$) from one another like this diagram: Applied a voltage two each wire individually and current flows like so: ...
0
votes
0answers
18 views

Resistance of a plate of smaller plates made up of different materials?

This problem propped in my head yesterday related to a project I'm working on and I just can't figure out how to crack it! If we have a large plate made up of nxn smaller plates, what is the ...
0
votes
1answer
25 views

I want to use the complex susceptibility to prove that E dot J is independent of the real part

As stated above, I want to use $\chi$=$\chi'$-i$\chi''$ to prove that E$\cdot$J is related to $\chi''$ and independent of $\chi'$. I need to do this using a monochromatic field ...
0
votes
1answer
34 views

Finding energy stored in spherical capacitor [closed]

What will be energy stored in a spherical capacitor with the shells having unequal charges?
0
votes
1answer
41 views

Why do bonds with positive and negative charges not collapse?

I previously believed that the reason positive and negative charges don't constantly attract until they collapse is due to a repulsive strong force at small distances. However, in a textbook by ...
0
votes
2answers
23 views

Is Emf induced in the coil when magnetic Flux passing through it is zero? If so then what is the explanation?

I thought Emf would not be induced if the coil doesn't cut the magnetic Flux . But since the graph in my paper shows Emf is induced when magnetic Flux is zero , I got confused. I thought since the ...
0
votes
0answers
20 views

Explanation of crystal formation in a Fitzroy's storm glass with the change in weather conditions

I was recently reading about Admiral Robert FitzRoy's storm glass and how it was used to predict the weather conditions based on the formation of camphor crystals in a sealed glass tube containing ...
-1
votes
1answer
25 views

Generating electricity in space with magnets

Assuming we have a very strong magnet of strength B and of uniform mass in space inside a coil of copper wire of turns N. Also, assuming that the magnet spins at the centre of the coil with no ...
0
votes
0answers
25 views

What would be the best material to reflect radio waves?

I was thinking about what would be needed to build a parabolic dish and I inevitably came to what material to use. From what I could gather after a quick search on the internet, it would seem that any ...
-1
votes
0answers
31 views

How can there be E and B fields inside waveguides?

A hollow waveguide that is made of conducting material allows for the existence of an electromagnetic wave inside it which propagates from one end to the other. However, conductors are known to ...
0
votes
1answer
54 views

What is time badwidth product for pulses with GDD?

For a gaussian pulse the time bandwidth product is dwdt = 0.4. What is the time-bandwidth product for chirped pulse with group delay dispersion (GDD) added only ?
2
votes
1answer
54 views

What causes electromagnetic waves to propagate in free space?

In free space, $\rho=0$ and $J=0$, so there are no electromagnetic sources/sinks. Maxwell's equations thus reduce to: $\nabla\cdot E = 0$ $\nabla\cdot B = 0$ $\nabla\times E = -\frac{\partial ...
0
votes
0answers
18 views

What does relative permitivity of medium mean and what does it depend on?

Does relative permitivity have someting to do with the polarization of medium , like for example how in inside the capacitor magnitude of electric field would be less due to a smaller opposing field ...
0
votes
2answers
32 views

Why does a rotating aluminium disc slow down when a current carrying coil with iron core is held above it?

What I don't understand is it the Emf induced that slows down the disc or is it a force.
-1
votes
1answer
38 views

How can current remain same when potential difference decreases?

According to Ohm's law, $$\text{potential difference} = \text{resistance}\times\text{current}$$ Also, current remains the same everywhere. Now when current passes through a bulb, the electric ...
0
votes
1answer
32 views

Why does the Emf generated by a magnet going through rings of coil generate a sinusoidal curve of Emf vs time?

I am having trouble writing the reason for the part of the curve to cut the x axis at Emf = 0 and when reason for curve to form max and min peaks.
0
votes
0answers
13 views

the inner self-inductance of coaxial cable using integration

I'm trying to calculate the inner inductance of a coaxial cable using integration knowing that the answer is : $L=μl/8π$ i get the same answer when i take a cylindrical shell of thickness $dr$ but the ...
0
votes
0answers
23 views

Topics important in electrodynamics for particle physics

Say I'm going to study Griffith's electrodynamics, what are the important topics in it, for particle physics? Like Maxwell equations etc.
0
votes
0answers
27 views

Is it possible that an heavier object is attracted with a higher speed than a lighter by a magnet?

Imagine there is a magnet with a lot of Tesla power. At a big distance from it are two balls of iron one of 10kg and an other of 100kg. Now I suppose that the lighter one will initialy be attracted ...
0
votes
3answers
77 views

Confusion on relativistic electromagnetism of two charge particles

When I was introduced to the relativistic nature of electromagnetism, the following scenario is introduced: Two protons are fired such that they travel at the same velocity in parallel directions ...
1
vote
1answer
37 views

What is the difference between emu and esu?

Ok...so this is the statement given in my book: a) In C.G.S System the unit of charge is electrostatic unit of charge (E.S.U). It is also called Stat Coulomb (StatC). b) In C.G.S system, the unit ...
1
vote
2answers
63 views

Two Jumping Rings Go Higher than One. Why?

When I do the Thomson's jumping ring demo with one aluminum ring it rises up in the air to some height. When I place two identical rings on top of each other and repeat the demo, both rings rise ...
0
votes
1answer
24 views

Is it possible to express Fleming's Left Hand Rule and Right Hand Rule in terms of vectors?

I recently studied Fleming's Right Hand Rule and Fleming's Right Hand Rule for electromagnetism (For locating direction of Force, Magnetic Field and Current). Using my hands to find the direction is ...
0
votes
1answer
54 views

Derivatives involving four vectors [closed]

The Schrödinger lagrangian for complex fields is $$L=\frac{1}{2m}(D_i \psi)^* Di \psi - \frac{i}{2} \left[\psi ^* D_0 \psi - (D_o \psi)^* \right] - \frac{1}{4}F^{\mu \nu}F_{\mu \nu}$$ Where ...
0
votes
0answers
29 views

Electric field of a non-uniformly charged quarter circle

I was studying for my Electromagnetism test when I stumbled upon this problem and haven't been quite able to do it correctly, mostly because I can't find any symmetry for me to calculate the Electric ...
0
votes
0answers
25 views

Ampere right hand screw rule or Maxwell screw rule is the more correct name of the right hand grip rule?

Wikipedia states ampere right hand screw rule Some textbooks state maxwell screw rule Which one is the more correct name in representing the (colloquial) Right hand grip rule?
0
votes
1answer
38 views

Coulomb gauge and vector identites

consider a coulomb gauge and the following volume integration: $$\int d^3x{\dot{A}.\nabla A}$$ How can we show that this is zero in coulomb gauge? (A is a vector potential) this is my attempt at ...
0
votes
1answer
51 views

if the voltage is the driving force of the current, how can it lead or lag?

Correct me if I'm wrong on this, but up till now I've assumed that the voltage across a resistor could be seen as both the energy lost by a current passing through it and the energy needed for a ...
0
votes
2answers
38 views

Wave reflection

I have a simple question on wave reflection. I know that if I have a progressive monochromatic EM wave and a mirror, the reflected wave will be opposite in phase on the mirror to assure a total E ...
1
vote
1answer
38 views

Why don't stars re-emit the photons they absorb, thus restoring a continuous emission spectrum?

If you shine white light through a gas, electrons can absorb sufficiently energetic photons to reach higher excited states. This produces gaps in the spectrum and it's how Helium was discovered. So ...
0
votes
1answer
32 views

Moving point charge --> using Maxwell induction integral to find B-field

I'm trying to wrap my head around this problem. A point charge is moving with a uniform speed v. The question requires that I show how the Maxwell induction integral can be used to find the magnetic ...
0
votes
1answer
36 views

Newtons corpuscular theory

I am learning about the history and evolution of certain physics theories, one being Newtons corpuscular theory. I am reading that Newton predicted the corpuscles, which make up light would travel ...
0
votes
0answers
24 views

Is the permeability of plasma equal to the permeability of free space?

I learned that in Maxwell equations for plasma, the permeability is equal to the permeability of free space so that means the relative permeability is equal to the unit tensor. Also we have always ...
2
votes
1answer
73 views

When exactly do we substitute $\epsilon_0 \rightarrow \epsilon$ and $\mu_o \rightarrow \mu $?

If everything is embedded into vacuum, why aren't Maxwells Equations always with $\mu_o$ and $\epsilon_o$? When exactly do we have to make the substitution $\epsilon_0 \rightarrow \epsilon$ and ...
0
votes
0answers
21 views

Which formulas would tell me the gradient of an electromagnetic field at an arbitrary distance from a pole? [duplicate]

I'm a newbie to physics and was wondering where I can read about electromagnetic gradients. From what I understand (and my intuition) electromagnetic fields create force gradients around its poles. ...
2
votes
2answers
107 views

Batteries Do Not Supply Electrons?

I've read that a battery does not supply electrons, it establishes the electric field that exerts force on the electrons in the wires. This makes total sense to me for AC, but not for DC. The way I ...
0
votes
0answers
38 views

Deflection of cathode rays in a uniform magnetic field

A beam of cathode rays moves from left to right in a plane of the paper and it enters into a uniform magnetic field acting perpendicular to the plane of the paper and inwards . Now the cathode rays ...
0
votes
2answers
21 views

Link between energy per unit frequency and derivative of the energy in regard of frequency

I take a simple example to illustrate my question. I consider $\frac{du}{d \omega}$. It physically represents the volumic energy per unit interval of frequencies (imagine a system with ...
0
votes
1answer
32 views

why is the vertical component of the phasor an imaginary number?

I was learning about the phasor representation of AC circuits, and on adding two out of phase phasors they were each resolved in to a horizontal and vertical component to facilitate the addition. What ...
1
vote
2answers
55 views

Are AC waves actually a sin function or just appear in their alternating nature as sinusoidal waves?

I mean do we use the graph of a sin wave just as a representation of how the current varies, or are the actual instantaneous values of current really an output of a sin function of the angle the ...
0
votes
1answer
25 views

How would the Compton Effect be if an electron absorbs 100% of the energy of the incident photon? [duplicate]

How can I prove that the relativistic energy and the impulse cannot be conserved if an electron, which is essentially free, absorbs all the energy of the incident photon?
3
votes
0answers
26 views

How does the electromagnetic field of an electron and a rotating ball of charge behave in a co-rotating reference frame?

First time poster, hope I'm not breaking any rules. Basically I'm curious about how far the classical analogy of an electron as a rotating ball of charge can be stretched. The situation I'm ...
0
votes
0answers
39 views

How is it possible to have an electromagnetic wave inside a superconductor if E = 0 and B = 0 inside it?

I am studying "Wave guides" from the book "Introduction to electrodynamics" by Griffiths. We study the propagation of electromagnetic field inside a superconductor cylinder. We can prove that E = 0 ...