# What is Electromagetism: Definition and 282 Discussions

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1. ### I Deriving the Curl of the Magnetic Field, Role of the Nabla Operator

We know that the magnetic field can be written in the following way: $$\nabla_{\vec r}\times \vec B(\vec r) =\frac 1 c \nabla_{\vec r} \times\int d^3\vec r_q\ \vec j(\vec r_q) \times \frac {\vec r-\vec r_q}{|\vec r-\vec r_q|^3}$$ and, using the ##BAC-CAB## identity, the curl of this...
2. ### Electric field external to Conducting Hollow Sphere with charge inside

I have read Griffiths' Chapter 2 sections on Conductors. According to it, (if I understood it correctly) if the charge is put inside the cavity of a conductor, then the equal and opposite total charge will be induced surrounding the cavity. This charge and the total charge induced surrounding...
3. ### Creating a simulator for an electromagnetic mass driver

I am trying to build a simulator for designing a mass driver. What I am essentially doing is given an iron rod and a coil of wire, calculate the force on the iron rod at various positions along the coil axis. Ideally, I want to use the various forces to calculate the final velocity of the rod...
4. ### Two different dielectrics between parallel-plate capacitor

We have a parallel plate capacitor with two different dielectrics It seems to be the case that the potential difference on each half of the capacitor is the same. Initially, the electric field was ##\vec{E_0}=\frac{2\sigma_+}{\epsilon_0}\hat{j}##. If we were to insert a single dielectric...
5. ### Derivation Of Torque On Current Loop Due To Uniform Magnetic Field

I can derive it for a circular loop: $$dF=BI\sin\phi\ dl=BIr\sin\phi\ d\phi$$ Torque on quarter circle when field is parallel to plane of loop=$$\tau=\int^{(\pi/2)}_0 BI \ dl \sin\phi (r\sin\phi)$$$$=\int^{(\pi/2)}_0 BIr^2 \sin^2\phi\ d\phi$$ Net torque=##4\tau=BIA## If magnetic field is at any...
6. ### A confused Compass Needle between two solenoids

Original Question: If a small magnetic compass needle was placed at point Z, what direction would its north pole point? Explain your answer My try: Since the current flowed in an anticlockwise loop in the first wire => North pole was formed inside the loop => Outside the wire near the point...
7. ### Syntax Error? x and y components of E field in unit vector form

I am trying to work through the MIT free coursework to study Physics before next semester and I am having a heck of a time with the strict answer syntax, or I am having a fundamental issue with the physics, of course. The problem is from "Week 1: Electrostatics Problem Solving Practice 1 W1PS4...
8. ### Electric field between capacitor plates when a dielectric slab is inserted

my sir gave us the above question as homeork,our task to derive why it happens so I couldnt get it right. So today he gave the answer to it. we were also a..sked to find the induced charges on the dielectric surface. . E ext : electric field between capacitor plates when no dielectric was put E...
9. ### Using Gauss' law to find the induced surface charge density ##\sigma##

My attempt: The electric field in the interior of a conductor is ##0##. By symmetry, the electric field is directed radially outward. Take the Gaussian surface as the thin cylindrical shell of radius ##\rm 3\ cm## and length ##L##. ##\displaystyle\oint\limits\vec{E}\cdot d\vec{A} =...

12. ### I Any free MHD software recomended?

I'm currently searhing for a magnetohydrodynamics simulator to study how different molten metals interact with variating magnetic fields, is there any free (preferebly) simulator that someone recomends in particular? And also, is there any way to use Autodesk's CFD to create a MHD simulation?
13. ### I Rate of work done by F on charge carrier with average velocity v

In the book Electricity and Magnetism by Purcell and Morin, there is the following snippet My question is how to arrive at ##\vec{F}\cdot\vec{v}## as the rate that work is done?
14. ### I Magnetic field at a point along the solenoid's axis but outside the solenoid

For a personal project, I need to calculate the EMF induced by a solenoid, the problem is, that the secondary circuit where the eddy currents are formed are outside the solenoid's length but still on it's axis. The problem comes when i need to calculate the vector magnetic field inductance at a...
15. ### I Is total flux linkage λ=dΨ*Ienclosed/I or λ=N*Ψ?

In Sadiku, he used the formula dλ=dΨ * Ienclosed/I to determine the total flux linkage for coaxial cable for ρ<a and for a<ρ<b, but I applied this formula for the solenoid and it didn't work, the way that works for the solenoid is by using λ=N*Ψ. So why we multiply by Ienclosed/I in the coaxial...
16. ### MIT OCW, 8.02 Electromagnetism: Potential for an Electric Dipole

Here is a depiction of the problem a) The potential at any point P due to a charge q is given by ##\frac{kq}{r}=\frac{kq}{\lvert \vec{r}_s-\vec{r}_P \rvert}##, where ##r## is the distance from the charge to point P, which is the length of the vector difference between ##\vec{r}_s##, the...
17. ### B Confusing diagram of a rotating coil in a magnetic field

Hello all, I am currently studying for a physics a-level qualification in the UK, I use the AQA specification and I am having trouble understanding this image representing a scenario I found in my textbook. The first image in the three part diagram shows this rotating coil and to me, it makes...
18. ### Electromagnet Polarity Switching for Reversed Motion

I am trying to understand the differences in design between a traditional solenoid with a ferrous plunger vs linear motors that use either induction or permanent magnets. From my understanding, a traditional solenoid, whether DC or AC, cannot fire in both directions with polarity switching since...

Hello All: read a paper related to electromagnetic force and its applications in acceleration of charges particles , some thing came up in it , they drive a force applied on the particle called Faraday force = [1/2B]* [dB/dt]*m*v B magnetic field , m mass of the particle , v the velocity of...
20. ### A How to define ##\nabla \cdot D ## at the interface between dielectrics

It is believed that Maxwell equations (together with other relations depending on the materials) are sufficient to account for any electromagnetic macroscopic effect. The problem is that, for a Maxwell equation to hold, it must at least be defined. Consider for example the case of two...
21. ### Schools What are some good science fair project ideas or resources?

I am currently a sophomore in HS, and I have a science fair project next year. I want to do something interesting- I don't want to put in barely any effort and do something that everyone has done, like 'growing plants in different light levels'. I would ideally like to do something with...
22. ### A Exploring the Electric Field of a Moving Charged Spherical Shell

The electric field inside a charged spherical shell moving inertially is, per Gauss's law, zero. If the spherical shell is accelerated, the field inside is not zero anymore, but it gains a non-null component along the direction of the acceleration, as mentioned, for example, in this paper. The...
23. ### Chapter 2 Griffiths EM Problem: E-Field from a charged ring

Hi. I have solved the problem below as shown in the attached image. However I'm at a loss to figure out where I am making a mistake, and I know it is indeed a big goof up. Requesting guidance over identification and rectification of this big goof up. (Edit- I can solve this problem in the...
24. ### Seeking Guidance to Find Surface & Volume Bound Charges of a Half Cone

This was a trivial question I had (which I posted here on the PF EM Forum: https://www.physicsforums.com/threads/bound-charges-polarisation-of-a-half-cone.1015308/). As I received no response on the above link I decided to post the same as a self formulated HW problem. Below I have attached an...
25. ### I EM equations - am I missing something?

Summary:: There seems to be a mismatch, in the "Maxwell's" equations, between the number of equations and number of variables. I was trying to play around with the equations for Electromagnetism and noticed something unusual. When expanded, there are 8 equations, 6 unknown variables, and 4...
26. ### I Measurement of charge in motion

In Purcell's E&M Section5.3 "Measurement of charge in motion", he said when a charge is in motion, the force on test charges may not be in the direction of radius vector r. And in next paragraph, he defined Q by averaging over all directions. However, he just measured the radial component of...
27. ### I Something about retarded potentials for oscillating electric dipole

In a problem of an oscillating electric dipole, under appropriate conditions, one can find, for the potential vector calculated at the point ##\vec{r}##, the expression ##\vec{A}=\hat{k}\frac{\mu_0I_0d}{4\pi}\frac{cos(\omega(t-r/c))}{r}## where: ##\hat{k}## is the direction of the ##z-axis##...
28. ### B Magnetic pendulum and electric energy....

While reading about electromagnetism from the OpenStax books with my son (and doing some experiments), he asked this question. Suppose I hang a pendulum and make it oscillate inside a coil connected to a Galvanometer as shown in the schematic diagram: Hopefully the image is clear enough. His...
29. ### Solving Electric Field Vector Problems Using Gauss's Law

I'm preparing for exam but it seems I can't find problems similar to this on the internet. Here I will apply Gauss's law on the electric field vector to get the charge density. but the problem is that I can't find similar examples on the internet that uses direct vectors on Maxwell's equations...
30. ### I What is the cause of electrical disturbances?

The electromagnetic field can carry waves, some of them are light, others are used in radio broadcast. These oscillatory waves are distinguished by their frequency. The usual pickup that we get from electric currents in the circuits in the walls of the building has a frequency of about 100...
31. ### Force on a charge centered around a rotating magnet

Charge will experience a rotating magnetic field around it. What will be electric field ( If any ) at the centre, generated by rotation of magnet ?
32. ### Proving modified Maxwell action is gauge invariant

I want to show that the action staying the same action after taking ##A^\mu \to A^\mu + \partial ^\mu \chi##, for the first term I suceeded in showing the invariance using the fact ##[\partial ^ \mu , \partial ^\nu]=0## but for the second term I'm getting: ##\epsilon^{\alpha\mu\nu}A_\alpha...
33. ### B Are silicon iron sheets really necessary for a commercial transformer?

Hi, everyone. I just finished studying the principle on which a transformer works. It relies on Faraday's law of induction. And my high school physics book uses the following picture for illustration: Roughly speaking, the...
34. ### Electric field acting on the source charge

I am reading Griffith's textbook on EM. There is a problem asking to find the force acting on the northern hemisphere by the southern hemisphere of a uniformly charged sphere. The solution idea is to find the expression of the E field by Gauss's law and integrate the force over the northern...
35. ### Electromagnetism - movement of a coil in a magnetic field

So my idea was that to reach the equilibrium position, the final moment of force has to be 0 (so in the end the forces will “eliminate” each other). And I found the equation Fm=B*I*l*sinα, which should characterize the force, which affects wire with the current in a magnetic field, and Fleming’s...
36. ### Find maximum current in a coil using oscilloscopes and Faraday's Law?

The experiment consists of a large field coil (connected to a current source) surrounding a coplanar and coaxial small detector coil in the center of the field coil connected to the oscilloscope. 1. Matching Current v. Time Graphs to Oscilloscope Graphs Example of one pair of graphs (I'll...

48. ### Refraction of Light: Myth or Reality?

In my latest 10th grade physics lesson, we were learning about the refraction of light. I decided to share what I knew about why light slows down in a vacuum, which is, in short, because the electric field of the electromagnetic wave exerts a force on the charged electrons of a medium, which in...
49. ### Radiation detector - cylindrical ionising chamber

Let r = position of the electron = 6mm - 36.8μm; λ = mean free path traversed. Integrate E(r) = Q/(2πϵLr) between the two shells gives: V = [Q/(2πϵL)]*log(r/(r-λ)) I know that the question is asking for the voltage at which the electron energy will get to 23eV, but i am unsure how to get rid...
50. ### Electric field direction on a grounded conducting sphere

I am required to find the direction of the electric field on the surface of a grounded conducting sphere in the proximity of a point charge ##+q##. The distance between the center of the sphere and the point charge is ##d## and using the method of images we find that the charge of the sphere is...