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)

0
votes
2answers
86 views

Origin of the Lorentz force from the point view of relativity?

I'm a physicist, when I'm working on the quantum spin hall effect, I recollected the high-school knowledge on Lorentz force and try to explain the origin of it, but didn't get it in the first glance. ...
0
votes
1answer
25 views

Magnetic field due to sheet current [closed]

Suppose we have current flowing along very long thin sheet with current per unit length as $\lambda$ so what would would be magnetic field at distance $r$ from this sheet? Using Ampere Law ...
0
votes
1answer
8 views

Magnetic field sensor readings varying depending on polarity

I was playing around with one of these Hall effect magnetic field sensors when I noticed something I thought was strange. I held a cylindrical magnet several centimeters from the tip of the sensor and ...
0
votes
1answer
29 views

Potential Difference and Electric field due to changing magnetic fields

Suppose above Frame is kept in time dependent magnetic field $B=\frac{B_0t}{T}$ where $T,B_0$ are constant , then What will be potential difference between Point $O , A$ and $A , B$ . I know that ...
-1
votes
0answers
14 views

Electromagnetism field of an infinite surface of charge [closed]

Charge is distributed uniformly through out an infinitely long cylinder of radius R. Show that E at a distance r from the cylinder axis (r
0
votes
3answers
172 views

Polarization vector of dielectric electrostatics

I know two laws for Polarization vector of a dielectric material $$\vec P = N q \vec{\Delta L}$$ Where N is the number of dipoles per meter cube, $\vec{\Delta L}$ is the vector that represents the ...
3
votes
1answer
128 views

Are magnetic hysteresis losses relevant to alternating currents flowing in a wire?

Say we have an AC in a magnetically lossy material, like iron. Because of iron's relatively high permeability, skin effect will be more pronounced than it is in say, copper, so this iron wire isn't so ...
2
votes
1answer
65 views

What does it physically mean to take the curl of the curl of a field (wave equation derivation)?

What does it physically mean to take the curl of the curl of a field in the derivation of the electromagnetic wave equation from Maxwell's equations, as presented here, on Wikipedia? Why was it a ...
1
vote
0answers
24 views

How does a synchronous motor know to increase current when the mechanical load is increased?

Consider a very simple singe phase synchronous motor, such as the one shown in figure 1. This motor will not be self starting, but if an AC voltage is applied and the permanent magnet is given an ...
2
votes
2answers
146 views

Does electric field cause changing magnetic field also?

According to Faraday's law, changing magnetic field causes eletric field (imprecise wording, but generally accurate). My question is, can Faraday's law be interpreted in opposite way - that is, ...
5
votes
3answers
13k views

Is it correct to say “like poles attract, unlike poles repel” while two magnets are placed such that one is inside another?

As we know a solenoid is considered as a electromagnet(magnet) if there's a current flowing through it. if a soft iron core is placed inside the solenoid, the former get magnetised. Consider the ...
1
vote
2answers
190 views

Faraday's law and “infinite induction”

I may have confused after thinking too much about Faraday's law. If an emf is induced in a circuit due to some changing magnetic field, the induced current will be in a direction such that the ...
0
votes
0answers
18 views

Weizsäcker–Williams approximation

I'm having some trouble understanding the Weizsäcker-Williams approximation What I think it is is the following: I have a charged particle at high speed, close to the speed of light, at this speed ...
1
vote
1answer
707 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. ...
1
vote
1answer
30 views

Ampere's law, do I include the electric field causing the current?

Let's say I have a long, straight wire with a time varying current, $I$ through it. Now if I take a circular Amperian path around this loop wire (and concentric with it) there is both a current $I$ ...
0
votes
0answers
12 views

How to calculate the capacity of an electroscope out of the geometry and why is it independent on the pointer deflection?

How can one calculate the capacity of an elektroscope like the following from the geometry of the setup? How does this formula implies that the capacity does not (approximatly) depend on the ...
9
votes
4answers
8k views

How do you determine power flow direction in a transmission line?

Below is a picture of a typical transmission line(about 200 kV). Is there a simple physics experiment which can be performed safely near the line, to determine the power flow direction. Or in other ...
4
votes
2answers
59 views

How is the conservation of momentum satisfied in long-range attraction such as electromagnetism and gravity?

I'm not a physicist, but my understanding is that electromagnetism (including attraction between opposite charges) is mediated by the photon, and gravity is probably (hypothetized to be?) mediated by ...
1
vote
3answers
127 views

The propagation of electric field

In case of a charged particle which is travelling at a uniform velocity, the electric field due to it at a given point doesn't change instantaneously . The reason for this delay in change of electric ...
1
vote
2answers
125 views

How does electromotive force (emf) influence electrostatic force in a circuit?

I am learning about emf and I am using university physics of Hugh D. Young which states that when the emf source is not part of a circuit the non-electrostatic force of the source moves charge form ...
1
vote
1answer
87 views

Magnetic field of a static disk magnet

I have a magnetic disk (Radius R, height h) that creates magnetic field lines (axisymmetrical). I simulated the field lines, exported the data and am now trying to fit a function into the data to have ...
2
votes
2answers
128 views

How can a magnetic field accelerate particles if it cannot do work?

A varying magnetic field can accelerate charge particles, but it is said that a magnetic field can't do any work so it should not be able to speed up charged particles, right? How is this apparent ...
1
vote
3answers
108 views

Induced EMF of a rectangular loop should be zero?

Considering the shape of a rectangular loop in a changing magnetic field: The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. ...
-3
votes
1answer
54 views

Production of electric field [closed]

How does varying magnetic field produce an electric field? Is it a natural phenomenon having no reason? i want the process/steps of producing of an electric field by a varying magnetic field.
3
votes
1answer
2k views

The electric field of a conductive sphere containing a charge - grounded vs not grounded

Let's suppose we have a sphere but unlike theoretical ones it'll have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge $q$ ...
2
votes
1answer
23 views

Question regarding Van De Graff generator Belt

I have made a VDG generator with a rubber band as the belt and a glass roller. It doesnt seem to work because I think the rubber band may be conductive. I was thinking of using other materials for ...
30
votes
4answers
2k views

How wrong are the classical Maxwell's equations (as compared to QED)?

Now, I don't really mean to say that Maxwell's equations are wrong. I know Maxwell's equations are very accurate when it comes to predicting physical phenomena, but going through high school and now ...
2
votes
1answer
32 views

What's the strength of a $B$-field inside a magnet?

I understand that the strength of a magnetic field decreases as you get further away from the source of the field. However, whats the strength of the field within a magnet? Is there a field in a ...
1
vote
2answers
118 views

Use of Amperian Loop to Calculate the $B$-field inside a co-axial wire

The problem asks to calculate the magnetic field at point $a$ of a co-axial cable with a wire at the center surrounded by a rubber layer, and then another hollowed cylindrical wire followed by a ...
1
vote
1answer
185 views

Closed circuit integral, magnetic vector potential for straight line segment

Looking at a worked example in Chengs Field and Wave Electromagnetics (newest ed, p 236) about Biot-Savarts law. A conducting wire of length $2L$ is carrying the current $I$. Find $\textbf{B}$ by ...
1
vote
0answers
39 views

Aether/Ether in Electrodynamics?

From reading Maxwell's original papers on the formulation on the dynamics of electromagnetism, I saw that Maxwell kept mentioning aether in his paper. I know today in modern physics the Michelson ...
1
vote
1answer
82 views

Forces on a wire moving in a magnetic field?

When a wire is placed inside a magnetic field and current starts to flow in that wire, that wire experiences the Lorentz force = $IL \times B$ and beings to move. Aside from induced motional - ...
0
votes
1answer
19 views

Monochromatic wave between two conducting planes

I'm trying to do a question which reads: Perfectly conducting planes are positioned at $y=0$ and $y=a$. Show that a monochromatic wave may propagate between the plates in the direction $z$ if the ...
1
vote
1answer
278 views

Question about superconductivity

A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
0
votes
1answer
104 views

Dielectric boundary

I am trying to determine why electric field may be confined to a certain region if there is a large difference in the permitivity for example if electric field flows through water and then reaches a ...
1
vote
1answer
106 views

Force of a solenoid on a ferromagnetic material

As I was trying to find an answer to another problem, I was informed that the equation I was using would not work. The equation I was using was $$F = (NI)^2\mu_0\frac{\text{Area}}{2g^2}$$ If this ...
10
votes
7answers
10k views

Electromagnetic fields vs electromagnetic radiation

As I understand, light is what is more generally called "electromagnetic radiation", right? The energy radiated by a star, by an antenna, by a light bulb, by your cell phone, etc.. are all the same ...
8
votes
3answers
2k views

Maxwells Equation from Electromagnetic Lagrangian

In Heaviside-Lorentz units the Maxwell's equations are: $$\nabla \cdot \vec{E} = \rho $$ $$ \nabla \times \vec{B} - \frac{\partial \vec{E}}{\partial t} = \vec{J}$$ $$ \nabla \times \vec{E} + ...
0
votes
1answer
54 views

The role of sine function in the Biot-Savart Law

$$ d\vec{B}~=~\frac{\mu_0}{4\pi}\frac{I d\vec{\ell}\times \vec{r}}{r^3}. $$ If we change the $ d\vec{\ell}\times \vec{r} $ with the $ d\vec{\ell}\times$r . sin(Ө) = $ d\vec{\ell}\times$R in order ...
3
votes
2answers
246 views

What happens when you pass a current through a coil made of a ferromagnetic substance?

I know when you pass a current through a length of copper coil or a solenoid, there is the induction of a magnetic flux. But what of the coil made of the ferromagnetic material, the permanent type ...
2
votes
2answers
61 views

Classical Hall effect when current has neutral charge

Suppose I have a current of both negative and positive charges(I know that there is also current from only negative and only positive charges,I'm not confused) along an infinite wire of square ...
2
votes
2answers
593 views

How do magnets work?

I've read a classbook on the field theory (including EM): it perfectly describes quantitive patterns in EM-theory, but I have no luck understanding how and why it works. I mean, magnetic substances ...
5
votes
1answer
796 views

Why doesn't the electric field inside a wire in a circuit fall off with distance from the battery?

We studied electric fields due to point charges. The magnitude of these fields decreases with the square of the distance from the point charge. It seems to me that we could treat the positive ...
1
vote
1answer
93 views

Ampère's law and superconductors

I have been reading about superconductors and emerge me a inquietude about the explanation of existence of currents inside the superconductor while their magnetic field (inside too) is zero. Based in ...
0
votes
1answer
33 views

Why does a current loop generate the same magnetic field as a flat permanent magnet?

Imagine a current loop perpendicular to the screen pointed up and down with the current flowing out of the top end. Next to it, imagine a flat permanent magnet lying in the plane the screen with the ...
1
vote
2answers
173 views

Understanding fields and their correlation to forces

I seem to be confused between the concept of a "force", and a field. Now let's assume there is a magnetic field of $1$ $\mathrm{Tesla}$, what does that mean in relation to force? Finally, if field is ...
1
vote
4answers
5k views

Transformer: primary side & secondary side current 180 degree out of phase

I am a novice in electrical engineering. I notice that in transformer the secondary side current & current referred to as primary are 180 degree out of phase from each other. But why it is so, I ...
5
votes
3answers
792 views

Why does the electric field dominate in light?

I read a book on the wave property of light where the author mentioned that the electric field, instead of magnetic field, dominates the light property. I don't understand why. In Maxwell's theory, a ...
2
votes
2answers
2k views

Properties to select suitable materials for making permanent magnets

I have read that soft iron is suitable for making a permanent magnet. Because it is required for permanent magnet to have high coercivity and high retentivity. Same text also said earlier that soft ...
0
votes
1answer
82 views

How is the current equation calculated from Ginzburg-Landau (GL) free energy?

In the Ginzburg-Landau theory, we can get the current expression from GL free energy: $$F = \int dV \left \{\alpha |\psi|^2 + \frac{\beta}{2}|\psi|^4 + \frac{1}{2m^*} \mid (\frac{\hbar}{i}\nabla - ...