2
votes
0answers
31 views

Conceptual question on helmholtz equation

Given the homogeneous Helmholtz Equation $[\triangledown ^2 + \kappa^2] E= 0$, I understand $\kappa$ represents the wavenumber $\omega_0 / c$. In light of this, what physical meaning can be taken from ...
0
votes
1answer
55 views

Why does the magnetic force exerted by two charged particles on each other do no work?

An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system during motion, one ignores the magnetic force of one on another. ...
0
votes
0answers
31 views

Will there be an induced emf in this situation?

Here is a (horrible) illustration: The first ring has a voltage $V_1$ applied to it through the voltage source on the left. Ring 1 has resistance $R_1$ whereas ring 2 has resistance $R_2$. Will ...
0
votes
1answer
21 views

Finding electric field of a line charge [on hold]

I have a bit of an issue setting up the integral for finding an electric field of a particular scenario. The scenario is that a line charge of length R is a distance R from the left side of the ...
3
votes
1answer
92 views

Force exerted by light on a moving mirror

Consider a light with energy density E shining uniformly over a mirror. The mirror has an area A. The mirror is moving at with a velocity β. Calculate the force that the photons exert on the ...
1
vote
2answers
49 views

DC Generator with magnet as rotor

DC generators convert the AC current in them by split ring commutators right and the graph of the current will be like this but the question is how would be the graph if the magnet is the rotor and ...
1
vote
0answers
45 views

How to calculate the Induced Voltage Given Spinning Conducting Loop? Intro to Electromagnetics problem [closed]

I'm having trouble figuring out a different variation of this problem. On my homework, I'm asked to find the induce emf for the following B field: $$\vec{B}=50\vec{a_y}$$ I was able to figure out the ...
0
votes
1answer
27 views

Estimating the rotational speed of DC motor

What is the best strategy to roughly estimate the RPM of a simple DC motor as seen in this video?. I want to estimate the max possible rotation of the coil that has $n$ number of turns and has moment ...
0
votes
2answers
37 views

Net Electrostatic Field inside a Conductor is $\vec{0}$

I've heard an explanation for this, and my professor wasn't really able to clarify my questions, so I was hoping someone could help: Suppose there is an electrostatic field $\vec{E_{net}}\ne\vec{0}$, ...
0
votes
2answers
86 views

Non-stationary capacitor

A common exercise in non-stationary electromagnetism is to find the electric and the magnetic field generated by a capacitor with round plates, if the potential difference between the plates varies in ...
1
vote
1answer
78 views

Finding the magnetic vector potential by calculus of variations

Given the functional $$F[A]=\int_{\mathbb{R}^3}\{\frac{1}{2\mu(x)}|\nabla\times\vec{A}|^2-\vec{J}\cdot\vec{A}\}d^3x$$ with $\vec{A}$ is a candidate vector potential for the field ...
2
votes
1answer
54 views

Current in an RC circuit inside in a magnetic field

Does my question make any sense? Context: The capacitor is charged with charge $Q_0$. There is a magnetic field $B=B_0 \hat{z}$ perpendicular to the monitor's screen. The length of the (AB) wire is ...
4
votes
2answers
81 views

Can we explicitly solve the Hamilton–Jacobi equation for a particle in a uniform magnetic field?

HJE for nonrelativistic charged particle in an electromagnetic field is $$\frac{1}{2m}\left(\nabla S - q\mathbf{A}\right)^2 + q\phi + \frac{\partial S}{\partial t} = 0.$$ For a uniform magnetic ...
1
vote
2answers
41 views

Prove Biot-Savart law, assuming that $\vec{A}=\frac I c\int \frac{d\vec{L}}{r}$

Prove Biot-Savart law, assuming that $$\vec{A}=\frac I c\int \frac{d\vec{L}}{r}$$ $$\vec{B}=\nabla\times \vec{A}$$ Any hint on what to do next?
0
votes
1answer
84 views

How to compute speed without knowing mass or charge values?

A gold nucleus is 460 fm (1 fm = 10^(-15) m) from a proton, which initially is at rest. When the proton is released, it speeds away because of the repulsion that it experiences due to the charge on ...
0
votes
1answer
30 views

Alignment of a system in equilibrium and its potential energy

Consider a flexible free conductor which is placed near a strong long bar magnet. How will the the conductor arrange itself if current is passed through it? I asked my professor: he gave me a hint ...
0
votes
1answer
21 views

Torque due to a magnetic field

We have two infinite perpendicular wires, each carrying a current "i" and the distance between them is l. If both the wires are free to move, how will they arrange themselves because of the torque ...
0
votes
1answer
43 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 ...
2
votes
1answer
75 views

Looking for a simple proof of symmetry of linear susceptibility tensor

Getting a diagonal susceptibility tensor: Define the linear susceptibility tensor as $\chi_{ij}$: $P_i = \epsilon_0\chi_{ij}E_j$, using standard notation for the electric field and the polarization. ...
1
vote
1answer
54 views

Coefficient matrix of quadratic Lagrangian

I've been studying path integrals from Weinbergs QToF vol 1. He says that when the $\mathcal{L_0}$ is quadratic in fields we can always write free term $I_0$ in the generalized quadratic form ...
1
vote
0answers
21 views

Very short physics question (electromagnetic field): Just draw the figure of the problem? [closed]

Can you please draw me the figure of the problem? I dont want the solution,just the figure? A straight, stiff, horizontal wire of length 25 cm and mass 50 g is connected to a source of emf by light, ...
3
votes
3answers
60 views

Ampere's law and external currents

In Ampere's law the current outside the curve taken is not included in the expression. Does this mean that only the currents crossing the area bounded by the curve taken contribute to the magnetic ...
1
vote
1answer
63 views

An electromagnetic induction problem [closed]

The question goes like this : A thin non conducting horizontal disc of mass $m$ having total charge $q$ distributed uniformly over its surface, can rotate freely about its own axis. Initially ...
1
vote
1answer
54 views

Calculating the magnetic field due to a rotating ring using Biot–Savart law

I wish to calculate the magnetic field due to a charged ring rotating about an axis perpendicular to the plane of the ring and passing through a point on its circumference $(P)$. I arrive at something ...
2
votes
1answer
49 views

Proving the equivalency of the potential energy of a system of charges and the work required to assemble a system of charges

This is a very cool, and highly beneficial problem in my opinion. I feel as though truly understanding this proof would broaden anyone's conceptual understanding of electric potential. My textbook ...
1
vote
1answer
45 views

Flaw in the radius of a spherical electron model (classical electron radius)

My textbook asks me to derive an equation for the potential energy ($U$) of sphere ($r_0$) filled with an electric charge of uniform density ($\rho$), expressed in terms of the total charge $Q$. The ...
1
vote
1answer
62 views

In which loop does the induced emf remain constant during the passage out of the magnetic field region?

A rectangular loop and a circular loop are moving out of a uniform magnetic field to a field-free region with a constant velocity V as shown in the figure. Explain in which loop do you expect the ...
1
vote
1answer
90 views

What would Maxwell's Equations be if we had magnetic charges and magnetic currents?

Mind you, we still have electric charge and electric currents. But, what would Maxwell's equations look like if we had to take magnetic charges and magnetic currents into consideration? Would there be ...
0
votes
1answer
100 views

Post Problem Analysis - Rod in a cylindrical time varying magnetic field?

A metal rod of length $l$ is placed (as shown in figure) in a cylindrical time varying magnetic field. Find the potential difference across it. My answer: As electric fields as forming circles ...
0
votes
0answers
48 views

Electromagnetic waves and boundaries

I am reading about dielectric boundaries and electromagnetic plane waves following griffiths ch7. When considering a boundary at z=0 with electric fields perpendicular to the plane of incidence it ...
0
votes
1answer
42 views

Magnetic field above center of current-carrying loop

A current $ I = 2A $ flows in a circular loop coil of radius $R = 10 cm$. Evaluate the magnitude of the magnetic field (in Tesla) at a point $P$ on the axis of the coil, and a distance $z=5cm$ away ...
2
votes
3answers
162 views

Describing a circular current loop as delta functions

It would be really nice to see how Jackson got eqn 5.33 on his example problem for finding the vector potential of a circular current loop $$ ...
-1
votes
1answer
43 views

Determine the electric field [closed]

A loop of wire is put in a changing magnetic field. The magnetic flux through the loop is given by $4t(t+2)$. The loop is connected to a parallel plate capacitor that has a plate separation of 15mm. ...
0
votes
1answer
80 views

Minimum separation problem

I am trying to solve this problem where two point charges are infinite distance apart(horizontally) but separated by some finite distance vertically. Both are positively charged and projected ...
1
vote
2answers
54 views

How do I find the perpendicular velocity of a particle to a varying magnetic field?

I am trying to find the component of velocity perpendicular to a magnetic field. This was easy when the magnetic field was static and pointing in only one direction (the $z$ axis), but now I need to ...
0
votes
0answers
32 views

Finite rod aligned with a time varying magnetic field

A finite length rod is aligned parallel with a magnetic field that has a changing magnitude (let's say a sinusoidal change), given the conductivity and permeability of the rod and assume a large ...
0
votes
2answers
102 views

Electrical force between two objects

I tried to solve the following problem: There are 2 objects . The object m1 with charge q and the object m2 with charge q.(same charge).The object m2 is connected with a rope to the ceiling. at the ...
1
vote
0answers
28 views

ODE envolving current [closed]

I have to do this exercise: 9. The current $I(t)$ at time $t$ flowing in an electric circuit obeys the differential equation $$I''(t) + RI'(t) + I(t) = \sin \omega t$$ where $R$ and ...
0
votes
1answer
45 views

Maxwell calculations that predicted the generation of waves (further use for wireless telegraphs)

At this point in this documentary about the history of electricity: https://www.youtube.com/watch?v=oPnS2WO2_0k&t=4m40s the guy says the Maxwell calculations predicted the generation of certain ...
0
votes
1answer
30 views

Determine stationary angular velocity of wheel with circuit in magnetic field

I have a wheel (free to spin around the $z-$axis) with four spokes that is connected by sliding contacts to a circuit with $U_0 = 0,72V$. Also, there is a B-Field parallel to the $z-$axis For the ...
1
vote
0answers
42 views

Why a transistor is activated when I touch its base with my finger? [closed]

Is my body charged eletrically? If so, how much? Will it work with all types of transistors?
1
vote
0answers
105 views

A question on an exercise from Gravitation by Misner, Thorne and Wheeler

My question is on problem 4.1 of Gravitation. In a generic case of electric field and magnetic field(i.e not $E=0$ or $B=0$ or $E$ and $B$ perpendicular), define the direction $\hat{n}$ unit vector , ...
1
vote
2answers
44 views

Uniformity in a solenoid

I know the magnetic field strength increases as the number of turns in the solenoid increases. However, I've learnt the field inside the solenoid is usually nearly uniform. So, does the number of ...
1
vote
3answers
169 views

Why is the inner product between divergence-free current$\vec{J}$, (satisfied $\nabla\cdot\vec{J}=0$) and a gradient field$\nabla \varphi$ zero?

I read a statement saying that the inner product between divergence-free current and a gradient field is zero. Divergence-free surface current is $\nabla\cdot\vec{J}=0$, and $\vec{J}$ could be ...
5
votes
2answers
154 views

Direction of current flow

In one of my homework questions, I was given the following setup. I was asked to determine if the induced current flows from X to Y, or from Y to X. From Lenz's Law I know that the current in the ...
0
votes
0answers
37 views

Torque per unit length on infinite rotating charged cylinder

For homework I have the following question, but I am stuck on finding the torque on the cylinder. An infinite cylinder of radius $R$ carries a uniform surface charge $\sigma$. We start rotating the ...
1
vote
2answers
68 views

Electric Potential of a sphere given electric field

I'm studying for a course in electromagnetism, and I've been given an electric field for which I need to find the associated scalar potential. The field is the field generated by a sphere of radius ...
0
votes
1answer
38 views

Potential in the middle of a square [closed]

Find the electric potential in the middle of a square with side length $a$ and charge $Q$. If I put the origin in the middle of the square, for the potential I get: ...
3
votes
1answer
107 views

Solving electromagnetic vector field using the Lagrangian

Given an action of the form \begin{equation}S=-\frac{1}{4}\int d^4x\eta^{\mu\nu}\eta^{\lambda\rho}F_{\mu\lambda}F_{\nu\rho}\end{equation} where ...
1
vote
0answers
37 views

Is the dielectric constant of an insulator always positive?

I know there are many examples of metals with a negative dielectric constant at optical frequencies. But what about insulators?