# What is Elecrtomagnetism: Definition and 136 Discussions

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1. ### Line of charge and conducting sphere (method of images)

I was thinking of using the sphere and point charge as an analog, but is quite diferent from what i have seen
2. ### I A math confusion in deriving the curl of magnetic field from Biot-Savart

I am recently reading "Introduction to Electrodynamics, Forth Edition, David J. Griffiths " and have a problem with the derive of the curl of a magnetic field from Biot-Savart law. The images of pages (p.232~p233) are in the following: The second term in 5.55(page 233) is 0. I had known...
3. ### Magnets with Halbach array in an AC generator

As a project, I am demonstrating electromagnetic induction by making an AC generator. The magnets I currently have access to are very strong neodymimium countersunk magnets, but I noticed that they utilise a Halbach array. I am aware that this means the magnet has alternating N-S poles spaced...

Hi All, I’d like you to weigh in on a debate I’ve been having. I’ve been thinking about a Faraday Cage (FC from now on) but from the opposite perspective. Sure it stops EMF getting in. But does that also mean, if there are items emitting EMF in there, that gets trapped within the room. Taking...
5. ### How is converted the energy of a E.M. wave in a conductor

I'm thinking about how the energy is conserved when a E.M. wave pass through a conductor. If a E.M. pass through a conductor, the electrons must move "oscillated", thus the energy from the E.M. wave is converted to kinematic energy. Another way I see that is the E.M wave must generate a current...
6. ### I Tangential electric force at a surface

Suppose you have an infinite plane of charge. If the surface charge density is uniform, would the tangential electric force always be zero, even if it is not a conductor nor static? My thought process for this is that if you look at each point charge and draw the electric field lines, then at...
7. ### I Outright understanding L/R inductor time constant

How is the real understanding, when an external constant E potential (voltage) is imposed/applied on a LR circuit, that is being charged as the characteristic L/R inductor time constant: the greater R the shorter time inductor get (full) charged This absolutely independent to the E; it could...
8. ### What chapters can I skip when self-studying Griffiths electrodynamics?

I'm currently studying Griffiths electrodynamics on my own and I want to be done with it once and for all. I however don't know if all the chapters are important for the rest of physics and which ones can be skipped without loss of continuity. Can someone give me some insight?
9. ### Magnets near a current carrying wire

Hi! I'm trying to understand electromagnetism. So I had a question: if we place a bar magnets on the low friction surface near vertical wire, then switch on the current, what will happen? As I been told they will align with the magnetic lines, and then nothing will happen. I also suggest that...

Summary:: Figure b also shows that there is no negative sign in Faraday's Law. How do I know when to include the negative sign?
11. ### Minimum seperation of two electrons moving toward each other

[Mentor Note -- Two threads started by partners in a class have been merged into this one thread, since they are working on a shared solution to turn in]
12. ### Green's reciprocity theorem about current density and magnetic field

I have to prove three equations above. For first two equations, I've been thought and made reasonable answer by using a definition of the electricfield. However, for third, I can't use a definition of a magnetic field due to the cross product Like J_2 X J_1 X (r_2 - r_1). I think three of 'em...
13. ### Solenoid pendulum frequency

The flux enclosed by the loop consisting of the solenoid, wires and conducting rod at an angle θ is Φ = blBsinθ, then using small angle approximations and differentiating the induced emf can be found. I know that there must be some torque opposing the motion but am unsure how to proceed.
14. ### DC Motor Efficiency

So I am doing an experiement about DC motors, and I would like to know why the efficiency plot looks like a parabola. What is the structure behind that. Thank you in advance
15. ### Electric field in a rotating rod in a magnetic field

The first part of the problem seems easy enough, the free electrons in the wire would move in a circle owing to an electric field that would be induced in the rod which would provide the centripetal force for the same (Please correct me if I am wrong). So we have $$eE=mω^2x$$, where e is the...
16. ### I Linear Induction Motor

I am currently trying to create a linear induction motor for fun and am having some trouble getting it to start oscillating or move at all. I am using this video as a reference... I am using 3D printed PLA as the structure for the copper to wind around, 26 GA Craftware USA copper wire, 5/8"...
17. ### B Light without a source

Can electromagnetic radiations exist on their own without any source (i.e light existing on its own without flashlight). If light can't exist on its own so sources are necessary for light or any EM radiations in order to exist, how about the EM fields?
18. ### Need help with Axial flux generator

I am sure axial are used in wind turbines so i will use it as my example. Say you are using the wind turbine to generate power but its getting to windy and you want to slow down the turbine to bring it to a stop. What i want to know is ... If you can cut off the power that's been generated...
19. ### Do concentric coils steal power from one another?

The amount of power generated by a coil in a changing magnetic field is determined by the area perpendicular to the field, the field strength, and how quickly those values change over time, and is multiplied by the number of loops in the coil. But that made me wonder what would happen if one...
20. ### Displacements in a system of two masses connected by three springs

Figure : Option of question : Solution attempt :
21. ### I Moving charges in electrostatics

According to a popular book on electrodynamics a special case of electrostatics is- ''source charges are stationary (though the test charge may be moving)''. My question is- now that the test charge is moving, how is it a special case of electrostatics anymore? Also many times we deal with...
22. ### Magnetic field generated by a current in a wire - special relativity

First I wrote in ##S'##, by using Gauss theorem $$\int_{\Sigma} \underline E' \cdot \hat n d\Sigma = \frac Q {\varepsilon_0} \rightarrow E'(r)2\pi rH=\frac{\lambda'H}{\varepsilon_0}$$ $$\underline E'(\underline r)=\frac{\lambda'}{2\pi\varepsilon_0r}\hat r$$ Its components are...
23. ### Is the magnetic flux density B constant?

Summary:: Is the magnetic flux density B constant? Is the magnetic flux constant? I am working on a project design for Uni and I am stuck. In a magnetic circuit is either the magnetic flux or the magnetic flux density B constant? This magnetic circuit has all different cross section areas and...
24. ### Trying to understand electric and magnetic fields as 4-vectors

I was trying to show that the field transformation equations do hold when considering electric and magnetic fields as 4-vectors. To start off, I obtained the temporal and spatial components of ##E^{\alpha}## and ##B^{\alpha}##. The expressions are obtained from the following equations...

26. ### Does moving an insulating cylinder produce a magnetic field?

Suppose that we have an insulating cylinder with ##\rho_q##. If i move the cylinder towards ##+\hat{n}##, will it produce a magnetic field? My assumption is that since we have an insulator, then the electrons are bound and there cannot be a current, thus a magnetic field is not produced. Also...
27. ### I Stratton-Chu solution, special case

I will try to ask the question, saving as much calculations as possible, so as not to weigh down those who want to try to help me. Starting from the general electromagnetic problem in empty space, taken as a domain a volume V delimited by a closed surface S, Elliot (1) shows how the field (i.e...
28. ### Electromagnetic plane waves from a current sheet

I have an infinite sheet (in lossless, homogeneous medium) of time-harmonic current in ##yz##-plane at ##x=−d##. The current density on this sheet is given by $$\mathbf{J}=\hat{z}J_0\delta(x+d)$$ ##δ(x+d)## is delta function. Moreover, there is a perfect electric conductor (PEC) half space at...
29. ### B How a Secondary Resistor affects the Transformer Primary Side?

Let's assume a 2:1 transformer which has a 100V Source connected on the primary circuit and has no/negligible resistance, on the secondary circuit a 5 Ohms resistor is connected. Using the 'Impedance Transfer/Reflection' method, the primary circuit would act as if there was a 25 Ohms resistor...
30. ### Potential difference in a 2 disk system (Capacitor)

Homework Statement A capacitor consists of two large metal disks placed a distance ##s## apart. The radius of each disk is R ## (R \gg s)## and the thickness of each disk is ##t##. The disk on the left has a net charge of ##+Q## and the disk on the right has a net charge of ##-Q##. Calculate...
31. ### Moving charges in a moving frame of reference

Hi. If 2 bodies with charge q are in rest then both have electric force ##F_1=\frac{q*q*k_q}{|\vec{r}|^2}##. But in another frame of reference, that is moving with velocity v relative to first frame of reference, they feel both magnetic and electric force...
32. ### Finding the electrostatic potential of a square sheet.

Homework Statement Consider a uniform surface charge density σ on a square of unit area. (a) Compute the electrostatic potential Φ along the line normal to the center of the square. My current attempt at a solution (image attached) is either incomplete or is simply wrong but I am unable to...
33. ### Electromagnetic Field vs Electromagnetic Wave

When there is electric charge, then there is an electric field in space aorund it. Or when the electric charge is moving (without acceleration), then it is produced magnetic field in a space around it. Both of these fields permeated to infinity according to Maxwell theory. But how fast...
34. ### A New Covariant QED representation of the E.M. field

90 years have gone by since P.A.M. Dirac published his equation in 1928. Some of its most basic consequences however are only discovered just now. (At least I have never encountered this before). We present the Covariant QED representation of the Electromagnetic field. 1 - Definition of the...

The book should have the following content. I want to refer a classic book which explains every detail. 1) Ohmic losses at high frequencies 2) Potentials and Green's functions 3) Image theorem 4) Fields radiated by sources in the far field region 5) Equivalence and reciprocity theorems 6)...
36. ### Electric field of "half" an infinite charged sheet

Homework Statement A charged sheet with charge density ##\sigma## is described by ##-\infty<x<0,-\infty<y<\infty, z = 0##. Find the electric field at ##(0,0,z)##. Homework Equations Electric field of continuous density charged body from the Coulomb law: E = \frac{1}{4\pi...
37. ### Mutual inductance coefficient with so little info

Homework Statement I have the following circuit: The two inductors are connected in series are characterized by internal resistances R1 and R2 and self-inductances L11 and L22. The magnetic coupling factor between the inductors is k = 0.75. The inductors carry the same current i. What is...
38. ### I Why different cutoff frequencies for TE and TM in a medium?

Today in my electro-magnetics class, we were told that the cut-off frequencies of TE and TM waves on a co-axial cable are different. As far as I understand, it says that if there is an electric field with one frequency projected on it and if we know that the problem can be simplified if we see...

I recently learned about Electric displacement field and capacitors, and I have a question that how was the formula derived shown below (blue circle part)? Thanks!
40. ### Electric displacement field

Hello, I'm going through electrodynamics by griffiths.. I'm unable to understand the case 1) for no free charge and 2)when curl of P(polarisation ) is zero at the boundary Then what can we comment about D the displacement field having both it's divergence and curl zero at the boundary. Please...
41. ### Magnetic field created by a current carrying wire

Hi, I studied the Maxwell laws and the Biot Savart law and I found something I cannot answer. If you have a finite wire carrying current (let say 5m long) and you want to determine the magnetic induction vector due to it at some point that has distance r from the wire, you have 2 options I...
42. ### Maxwell equation are derived in which coordinate system

Ignoring special relativity theory，maxwell equation are deduced in which coordinate system?In most electrodynamics textbook,maxwell equation are deduced without specifying which coordinate we are using.For example,when we are solving poisson equation in static case，it seems we can freely choose...
43. ### A How are curvature and field strength exactly the same?

I am watching these lecture series by Fredric Schuller. [Curvature and torsion on principal bundles - Lec 24 - Frederic Schuller] @minute 34:00 In this part he discusses the Lie algebra valued one and two forms on the principal bundle that are pulled back to the base manifold. He shows...
44. ### Find angular momentum of EM field in terms of q and ##\Phi##

Homework Statement A point charge q sits at the origin. A magnetic field ##\mathbf{B} (\mathbf{r})=B(x,y)\mathbf{\hat{z}}## fills all of space. The problem asks us to write down an expression for the total electromagnetic field angular momentum ##\bf{L_{EM}}##, in terms of q and the magnetic...
45. ### Electromagnetism homework check

Hello. I have an assignment to submit and I really want to get full marks/know where I went wrong If anyone has a tiny bit of free time would you mind please checking my homework? Even if you just check 1 page I will be so grateful. Thanks again https://imgur.com/a/rHVdjzn
46. ### What is the second-order Born approximation?

Homework Statement Equation (10.30) in Jackson is the first-order Born approximation. What is the second-order Born approximation? Homework Equations The Attempt at a Solution I can get the first-order Born approximation in Jackson's textbook. If I want to obtain the second-order (or n-th...
47. ### Polarizibility, Electric Field & Force

Homework Statement A neutral atom with known polarizability α is located at the origin. A point charge Q is situated on the y-axis a large distance d from the atom. (The atom therefore becomes polarized due to the electric field of the point charge.) (a) Find the electric field due to the atom...
48. ### Verification of Ampere-Maxwell Law

Homework Statement A current I flows along a wire toward a point charge, causing the charge to increase with time. Consider a spherical surface S centred at the charge, with a tiny hole where the wire is – see figure below. The circumference C of this hole is the boundary of the surface S...
49. ### Using Gauss's Law to Calculate Electric Field of a Charged Thread

Hello, Can not Gauss's Law be used to calculate the electric field generated by a uniformly charged finite thread? I suppose it is because I can not consider the electric field constant (always going to the same direction), and for this I would have to do it by parts (the lateral flow, and the...
50. ### I Counterexample to the Poynting theorem

The counter-example is as follows: We have a rectangular toroid ferrite(ring ferrite), magnetized in a closed loop around the ring. We put capacitor plates on top and bottom surfaces, with suitable direction. Now the Poynting vector points inwards or outwards. We look at a cylindrical surface...