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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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 ...
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1answer
75 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 ...
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0answers
26 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 ...
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0answers
20 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 ...
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35 views

Does photon polarization take place without scattering processes?

Behind two perpendicular crossed polarisation filters (for visible light) the intensity of light will be zero. After installing a third filter between the two others non parallel to each of them some ...
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34 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 $I$ ...
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63 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 ...
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16 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 ...
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539 views

why is the magnetic field circular

According to relativity, If magnetic field is just an electric field viewed from a different frame of reference, why is the magnetic field around the wire is circular?
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41 views

Magnetic Susceptibility

maybe very silly question :( In Magnetism: \begin{equation} \ B=H+4\pi M (SI) \end{equation} \begin{equation} \ B=\mu(H+M) (CGS) \end{equation} in these formulas M is Volume Magnetisation ...
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1answer
33 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 ...
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1answer
31 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 ...
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45 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 ...
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1answer
44 views

Electric field in a hollow object

I am currently visiting a course about electrodynamics. In my last lecture it was said that if a hollow sphere is inside of a bigger sphere, but only in the bigger sphere there are charges, the ...
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1answer
21 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 ...
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1answer
61 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 ...
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2answers
65 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 ...
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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 ...
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1answer
25 views

Explanation for electrostatic energy expression

My question is about the electrostatic energy $We$ expression for $n$ point charges. I just can't figure out where the factor "$\frac{1}{2}$" came from: $We=\frac{1}{2} \sum_{i=1}^{n} q_iV_i$
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3answers
59 views

Why use different arguments to plot a sine wave?

In my Electronics class was given some examples of sine wave graphs that represent voltage in respect to time, $v(t) = Asin(wt)$ and other graphs whose $x$ axis is $wt$ instead of $t$, like in the ...
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1answer
35 views

Signs in derivation of capacitor discharge differential equation

In deriving the discharge current for a capacitor I have seen two different approaches: By Kirchhoff's law we have: $$ \begin{align} 0 &= I R + \frac{Q}{C}\\ \implies 0 &= \dot I R + ...
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24 views

Effective Area of Isotropic Antenna: Explanation?

I'm reading some introduction to antenna theory and I've often puzzled on the equation: $$ A_{eff} = \frac{\lambda^2}{4\pi} $$ which relates the effective area by which an antenna captures radiation ...
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6answers
675 views

Is electromagnetic vector field a sum of E and B?

I have a hard time to fully understand (classical) electromagnetic field theory with respect to Helmholtz's decomposition. Let me start from Helmholtz's theorem: Any vector field of class ...
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0answers
32 views

Faraday's law in free space explaining away the constant vector?

Let's say that I have a plane electromagnetic wave travailing in free space, and I know the electric field part to be $\vec E$. If I am using Faraday's law to get the magnetic field part I will get ...
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0answers
21 views

Why does this 'Quantum Pinned' superconductor allow easy repositioning

I'm confused by videos such as this (popular demonstration of 'Quantum Levitation'): https://www.youtube.com/watch?v=Ws6AAhTw7RA So my current understanding of superconductors is that when in the ...
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2answers
51 views

Auto spinning turbine generator

This might be dumb question but I'm so curious to know if this actually works or is impossible. I was researching how you could generate electricity from magnets and copper wire and also have read how ...
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71 views

Does an accelerating electron also produce a constant magnetic field in addition to an electromagnetic wave?

I've been reading about the basics of electromagnetism, from a layman's point of view, how static charges are the source of the electric field and how constant moving charges are a source of a magnet ...
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1answer
31 views

Computing energy loss for a hysteresis curve

Let a hysteresis loop be as shown in the figure, with $B_0$ the applied external field due to a current $I$ in a toroid, and $B_M$ the additional field due to the ferromagnetic material. Hence we ...
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1answer
47 views

Is the Lorentz force expression valid for magnetic field created by a magnetic monopole?

Will the Lorentz force expression be valid for a magnetic field created by a magnetic monopole? I haven't seen any derivation of Lorentz force expression yet and I don't know whether it was derived ...
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1answer
44 views

Electro-magnetic radiation

What happens when a free electron and an electromagnetic wave interacts? Does it vibrate or move along the direction of the electromagnetic wave?
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55 views

Electromagnetic Induction in two coils placed in vicinity of each other

First read the question (No I'm not saying you to solve it): Now what I think: As we know $V\propto -dI/dt$, so $I\propto -\int Vdt$ or negative of area bound. First area is increasing first fastly ...
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1answer
87 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 - ...
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17 views

How can DC-magnetic fields be produced with the aid of currents?

How can DC-magnetic fields be produced with the aid of currents? What's a possible theoretical explanation for this, and how does one achieve this in the real world? What are the maximum fields that ...
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1answer
23 views

Energy Loss during Sharing of Charge between two Capacitors

It is fairly easy to show that there is always a loss energy when two capacitors share their charge to attain the same common potential, but is it with the same ease that one can explain why it ...
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1answer
24 views

How to approximate the force on a magnet below a coil

o x o x Coil with 4 turns o x o x _ | | | | Neodymium magnet |_| I need to know the ...
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2answers
291 views

Integral vs differential forms of Maxwell's equations

As stated in this post, the integral and differential Maxwell equations should be identical. However, in a text I was reading it states that The integral forms of Maxwell’s equations describe the ...
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3answers
87 views

Why does the electric field vanish at infinity?

When $r \rightarrow \infty$, $E \rightarrow 0$ for a point charge or set of charges or a finite charge distribution. While this seems obvious, I cannot find a reason why this is true when inspecting ...
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5answers
1k views

Why does a ring falling through a magnetic field experience an upward force?

The Problem states: A metallic ring of Mass $M$ and radius $r$ falls freely under the influence of gravity in the direction along the negative Z-axis. A magnetic field $B_z = B_0(1-z\lambda)$ ...
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1answer
172 views

What happens in a gas of magnets?

This SMBC comic asks what happens if you make a gas of magnetic particles: I was wondering whether anyone has run into actual examples of this or something like it. A classical example similar to ...
3
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1answer
81 views

Biot-Savart law from relativity

Few days ago I came by a derivation of STR from Bio-Savart law. Since then I have been trying to derive Bio-Savart law from STR. The derivation mentioned previously used two parallel current-carrying ...
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1answer
38 views

Does the direction of the magnetic field inside a solenoid depends on the direction which it is turned?

Today trying to explain some magnetic stuff, just came across with a simple (at least seems) question. My question is brief: does the direction of the magnetic field inside a solenoid depends on the ...
0
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1answer
39 views

Dielectric constant of Transition Metal Dichalcogenides (TMD)

Why is the in-plane dielectric constant of transition metal dichalcogenides larger than the out-of-plane dielectric constant? Is this because of the spacing between monolayers of the TMD? Why do ...
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2answers
68 views

Finding net magnetic and electric force on charged particle [closed]

This is from my textbook, it is not an assigned problem, but I want to understand. It says: Consider the situation in the figure, in which there is a uniform electric field in the x direction ...
3
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3answers
74 views

Do electrons in the conduction band have a quantum number?

Sorry if this is a silly question, but I haven't seen the concept anywhere and was wondering why. If valence electrons can have a quantum number, why can't conduction electrons? Is there something ...
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1answer
32 views

Law of Conservation of Energy and a moving bar magnet through a coil? [closed]

How would you do this question? What I've currently attempted is applying the Law of Conservation of Energy formula where $E_i = E_f$, and I've accounted for gravitational potential energy and ...
0
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1answer
27 views

There is some attempt to build a magnetic flywheel (reservoir of motion)

We know how flywheel works! There is some attempt to substitute the flywheel-friction mechanism for some magnetic torque ? Exist some mechanism that uses thermal cycle of gases for generate magnetic ...
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1answer
18 views

Average induction voltage

I've got a basic question on the method of computing the average induction voltage. Here is the task: A conductor loop with the area $A_1 = 50\text{cm}^2$ in a magnetic field with the flux density of ...
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0answers
52 views

Motion of a charged particle in a “solid” charged sphere (accounting for radiation)

Consider a particle (point charge) with charge $q$ and mass $m$ that crosses into a uniformly charged sphere (with charge $Q$ and radius $R$). The trajectory of the particle is a diameter of the ...
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33 views

Refractive index and electric susceptibility

Suppose we have a complex refractive index $n_{ref}=n+ik$ whose value is given at a precise frequency $\omega_l$ from experimental data. We know that the imaginary part is responsible for the ...
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2answers
29 views

What determines the alignment of an object levitating in a magnetic field?

Can you explain why sometimes (more often) a levitating object is not horizontal and sometimes it is perfectly horizontal?