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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Magnetic field due to far away square loop current and due to infinitely long axis, Biot-Savart [closed]

in two of my 5 homework problems I'm not very confident in my solution, and my TAs have been on strike for 3 weeks so I could use some help, thanks haha Griffiths is the textbook we use, so I ...
2
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2answers
60 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
32 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
24 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
58 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
34 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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20 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
649 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
27 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
20 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
39 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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60 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
20 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
43 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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0answers
37 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
79 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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1answer
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
17 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
23 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 ...
4
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2answers
270 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
82 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
154 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 ...
4
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1answer
74 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
28 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 ...
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1answer
33 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
55 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 ...
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3answers
65 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
31 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 ...
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1answer
26 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
49 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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0answers
24 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
28 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?
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0answers
30 views

Experiments or phenomenon to look/deduce for the nonexistence of monopoles?

This is a related, but opposite question to this one I have heard about a lot of things regarding elementary and GUT magnetic monopoles, as well the quasiparticle monopoles in spin ice Since there's ...
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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.
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1answer
52 views

Question on boundary condition for Maxwell's Equations and Coulomb's law

When deriving Coulomb's law using the differential forms of Maxwell's equation, the boundary condition that $\phi = 0 $ at infinity is also used. From $\nabla × E = 0, E = \nabla \phi$ for some ...
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2answers
128 views

EM wave in Real life

everyone I'm new here, but not so new in physics. I have read many articles about EM wave to find what I'm searching for and nothing still. I have seen many pictures, animations and videos about EM ...
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1answer
24 views

Poisson equation in non-homogenous media

If the dielectric constant be different in x, y, and z directions, how the Poisson equation would change. Is it right to say: $$ \epsilon_{xx} d^2V/dx^2 + \epsilon_{yy} d^2V/dy^2 + \epsilon_{zz} ...
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1answer
20 views

The electric tension connected to an X-ray tube

The maximal frequency from an X-ray tube is 8.4 ·1018 Hz And I'm trying to figure out the tension, voltage, connected to the x-ray tube So far I have hf max = eU where e = ?? Elemtary charge of an ...
3
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0answers
69 views

Can I hide a charge behind a black hole?

Suppose that you are standing on one side of a black hole. I'm standing directly opposite you, on the other side of the BH, and I'm holding a charged particle. Is it possible for us to be positioned ...
30
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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
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3answers
108 views

Is it possible to create a beam of light with frequency of 0?

Is it possible to create a beam of light with frequency of 0? So this would involve photon(s) that move forward without fluctuating with any frequency. If yes, how could this be done? Also, ...
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1answer
19 views

currents in an isolated metal exposed to an alternating uniform electric field

For example, let's look at a metal sphere between a capacitor's (infinite) plates. Let's run an AC current through the capacitor, resulting in an oscillating electric field. The charges will move in ...
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2answers
58 views

Gravity and the Speed of Light

Let's assume that I am on an airplane that is at about 4,000 altitude and now let's also assume that I am standing on one of the wings with a light torch, if I point the light torch below to the ...
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0answers
16 views

Designing a capacitor and electrical double layer capacitor

Hello! based on the image above, I am wondering if my assumptions are correct that i can make a capacitor using this design (1) and (2) are "infinitely" long layers (1) layer is made up of same ...
2
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0answers
23 views

Calculating the magnetic permeability of a known plasma

I have a plasma, of which I know the composition, temperature, exact electron/ion/particle densities. How can I then calculate a theoretical permeability of the plasma. Is this even possible? I ...
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2answers
86 views

Crazy? Generate unusable electricity

I know that sound crazy, but today I am wondering if there is a way to generate a current that is unusable by almost all electic/electronic devices? The idea would be to generate that current and ...
2
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1answer
37 views

EM wave generated with DC battery and coil?

I did below experiment and wanted to confirm the spark is in deed Electro magnetic wave. 1) Took coil of wire , wound it over a iron rod. 2) connected one end of coil to +ve terminal of battery and ...