# Tag Info

1 vote

### Ohm and Fourier's law with Seebeck effect - derivation?

Check Landau & Lifshitz volumes 8 "Electrodynamics of Continuous Media" chapter III ("constant current"), paragraph named "Thermoelectric Phenomena" for the origin of ...
• 234

### Does direct current exist in an infinite straight thin wire?

Mathematically, the electromagnetic energy per unit length is divergent for an infinite wire supporting a finite electromagnetic field. The characteristic impedance is infinite. Thus, in principle, ...
• 6,171

### Does direct current exist in an infinite straight thin wire?

The integral on the LHS is not zero. Use Stokes theorem, and realize that because $|{\bf H}|\sim 1/r$, the boundary contribution is independent of the distance from the wire.
• 43.3k
Accepted

### What should be the direction of moving electron in magnetic field?

The picture in your book contains an error. The magnetic field has to go into the other direction. Otherwise the electric and magnetic force cannot cancel each other and thus the experimental setup ...
• 48

### Can ionized air be used as a conductive medium for eddy brakes?

Your question is a good one, and has been a subject of research for a number of years in the field of magnetohydrodynamics, or MHD (convenient search term). This is a frightfully complicated field ...
• 76.4k

### What is electric potential really a measure of?

Electric potential is defined by the amount of work energy required to move a unit of electric charge from a reference point to a specific point in an electric field.
Accepted

### Can we choose the Coulomb gauge if we're in a gauge where the gradient of the scalar potential is zero?

You're asking whether we can impose both $\phi = 0$ and $\nabla \cdot \mathbf{A} = 0$ simultaneously. This will not be possible in any situation where $\rho \neq 0$, since if both conditions on the ...
• 39.9k

### What should be the direction of moving electron in magnetic field?

If you use the conventional current direction you have to use your right hand. The left hand is used, if you take the first finger in direction of the moving electron. So your text should not just say ...
• 4,030
1 vote
Accepted

### What exactly does it mean by gauge-invariant "operators"?

The whole point of the quantization of gauge theories is that we don't really want a realization of the gauge symmetry in the quantum theory because the gauge symmetry is unphysical (see also this ...
• 111k

### Does a constant magnetic field induce an emf in space irrespective of what is inside the space?

It matters not what the material of the rod is. If the rod was a metal there would be a migration of the free/mobile electrons and in an insulator the motion of the rod through the magnetic field ...
• 80.9k

### Magnetic field lines permeability in vacuum

I think the problem you're having is that you're thinking of the field as a thing that travels through space, when it's nothing of the sort. The field is a property that exists everywhere, always. ...
• 20.8k
1 vote

### Does the quantum mechanical wave function work as a charge distribution?

Schrödinger initially believed that the wave function is related to actual charge distribution density, rather than to probability density. Some objections were raised to such interpretation. One of ...
• 24.6k

### Does the quantum mechanical wave function work as a charge distribution?

The charge distribution is related to the probability of the electron being in a given location. That is, the amplitude-squared of the wave fucnction, $\bar{\psi} \psi$. Under some circumstances ...
• 454

### Is it correct using Larmor Formula for identifying a non-inertial reference frame?

In principle, Larmor's formula for the radiated energy: $$\frac{\text{d}E}{\text{d}t} = \frac{q^2 a^2}{6 \pi \varepsilon_0 c^3}$$ provides a method for an inertial observer to detect relative ...
• 29.6k
1 vote

### Understanding the role played by moving charge in electric field and why charges reside on surface only in current flowing conductor too

First think of a neutral wire - a metal with diameter r and conductivity $\sigma$ and is very long. One end of the wire is grounded, and at the other end of the wire there is a switch that can connect ...
• 2,701
1 vote
Accepted

### Relative permeability: why does $\mu_{air} = \mu_0$?

Ferromagnetic materials can have large $\mu$. But for paramagnetic and diamagnetic materials, a typical permeability is $\mu_r = \frac{\mu}{\mu_0} ≈ 1 \pm 10^{-5}$. I don’t know of a good ...
• 75.7k

### Can we just consider thermionic emission as a kind of thermoelectricity?

Feel free to call things what you wish. Just make sure you define your terms clearly in your publications, or people will be mighty confused.
• 11k

### What produces the electromagnetic fields in the light wave when photons are chargeless?

Electric charges are not the only thing that can generate electric fields. Changing magnetic fields can also create electric fields. Also, changing electric fields creat changing magnetic fields. So ...
• 9,895
Accepted

### The Maxwell equations in 4-dimensional notation for vacuum and covariant component of electromagnetic tensor

Not quite. Using $c=1$ we have $A_\mu= (\phi, -{\bf A})$ where ${\bf A}$ is the usual three vector vector potential. So you should have $A_\mu= (\phi, A_i)$ with the covariant component $A_i=-A^i$. ...
• 43.3k

### How to show monopole term in magnetic vector potential is 0?

Here is a simple proof using a corollary of the Divergence Theorem. I'll assume $\nabla \times \textbf{B} = \mu_0 \textbf{J}$, meaning $\frac{\partial \textbf{E}}{\partial t} = 0$ (electrostatic ...
Accepted

### Magnetic explosions on Sun

Magnetic fields in an ionized plasma have more complicated behaviors than magnetic fields in free space. A lone charged particle in free space will undergo “cyclotron motion” in the plane which is ...
• 75.7k

### Magnetic explosions on Sun

https://www.nasa.gov/feature/goddard/2019/nasa-s-sdo-sees-new-kind-of-magnetic-explosion-on-sun and https://iopscience.iop.org/article/10.3847/1538-4357/ab4a0c when the voltage has exceeded the LEL ...

### What is rotation of magnetization?

One can think of a domain as a region where many atoms line up so that the magnetic field they produce add together to produce a resultant magnetic field. Imagine that magnetic field as being produced ...
• 80.9k

### Circuit in a $B$-field question

The induced voltage will definitely not cancel in general. That is the entire point of magnetic induction. Unless you specify that the field is constant and the wires don't move, it is actually quite ...
• 117
Accepted

### Circuit in a $B$-field question

The induce voltage is calculated by the change of magnetic flux, if neither the area nor B changes you have no induced voltage
• 4,030

### How electromagnetic train works?

Complete physics of levitating train is nicely animated and explained in this film: https://www.youtube.com/watch?v=XjwF-STGtfE There are various films on the Internet, but for me the description is ...
• 31

### How electromagnetic train works?

You are correct. Without an active feedback mechanism that senses the distance between the train and the track the two would stick together just like ordinary magnets. There is even a mathematical ...
• 117
1 vote

### Inductor connected to an AC source

To formalize all the previous answers, model the switch by a step function: $$H(t) =\begin{cases}0 & t< 0\\1 & t \geq 0\end{cases}$$ And use it to limit the power source to positive t. ...
• 1,625
Accepted

### What is electric potential really a measure of?

I like to think of electric potential as "pressure". By Ohm's law, $$I = V/R$$ Here, $V$ is the potenial difference. The current carrying wire lets in more current, if the potential ...
• 1,805

### Is the magnetic field outside a solenoid almost null for AC current?

Bio-Savart does not apply to AC currents. It is a law of magnetostatics. For AC currents we have to use the full Maxwell equations or a retarded potential formula like Jefimenko's equations. In ...
• 117

### What is electric potential really a measure of?

It is a scalar function whose gradient gives the force a charge would experience were it placed at a point. It is a function which measures the change in energy if a charge were to be brought from ...
• 287

### What is electric potential really a measure of?

Electric potential is the ability of a system to perform work on a charge. There do not have to be other charges in the system itself.
• 117
1 vote
Accepted

• 23.9k
1 vote

• 70.9k

### Electric field due to charge within a sphere

Well Electric field will be the part you have plus simple culomb potential. The 1st part is for '0' solutions of poisson equation i.e charge density is zero part which will give you that part and ...