Electromagnetism Definition and 852 Threads

Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force is carried by electromagnetic fields composed of electric fields and magnetic fields, and it is responsible for electromagnetic radiation such as light. It is one of the four fundamental interactions (commonly called forces) in nature, together with the strong interaction, the weak interaction, and gravitation. At high energy, the weak force and electromagnetic force are unified as a single electroweak force.

Electromagnetic phenomena are defined in terms of the electromagnetic force, sometimes called the Lorentz force, which includes both electricity and magnetism as different manifestations of the same phenomenon. The electromagnetic force plays a major role in determining the internal properties of most objects encountered in daily life. The electromagnetic attraction between atomic nuclei and their orbital electrons holds atoms together. Electromagnetic forces are responsible for the chemical bonds between atoms which create molecules, and intermolecular forces. The electromagnetic force governs all chemical processes, which arise from interactions between the electrons of neighboring atoms. Electromagnetism is very widely used in modern technology, and electromagnetic theory is the basis of electric power engineering and electronics including digital technology.
There are numerous mathematical descriptions of the electromagnetic field. Most prominently, Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents.
The theoretical implications of electromagnetism, particularly the establishment of the speed of light based on properties of the "medium" of propagation (permeability and permittivity), led to the development of special relativity by Albert Einstein in 1905.

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  1. Kostik

    A Electric and magnetic field lines in a plane wave of finite extent

    In an infinite plane wave propagating in the ##z## direction, the momentum density is ##\mathbf{p}=(4π)^{-1}(\mathbf{E} × \mathbf{B})## which points in the ##z## direction; therefore, the angular momentum density about the ##z##-axis ##\mathbf{L} = \mathbf{r} × \mathbf{p}## has no...
  2. A

    I Is This Correct Description of Magnetic Saturation?

    I believe that if a put current through a coil of wire and if I have unlimited power to force through that current and if the coil can accept unlimited power without being degraded, I could produce a magnetic field in a vacuum of unlimited strength-is that correct? But supposedly if I applied...
  3. J

    I Mother of all equations for the Lorentz force

    David J. Griffiths Introduction to Electrodynamics page 460: Lorentz force equation invariance leads to different Lorentz force values in different inertial frames. Is this a problem for conservation of momentum? More specifically conservation of angular momentum?
  4. Z

    How to calculate quality factor for RLC circuit?

    Using Faraday's law we have $$\mathcal{E}=\oint\vec{E}\cdot d\vec{l}=\frac{Q}{C}+IR=-L\dot{I}\tag{1}$$ where ##I=\dot{Q}##. After rearranging the expression we get $$\ddot{Q}+\frac{R}{L}\dot{Q}+\frac{1}{LC}Q=0\tag{2}$$ $$\ddot{Q}+\gamma\dot{Q}+\omega_0^2Q=0\tag{3}$$ If the system is...
  5. R

    Laplace equation not provided in simulation

    I'm following this tutorial I noticed that he provided the boundary values in FEMM but he didn't provide the Laplace equation ##\dfrac{\partial^2 V}{\partial x^2} + \dfrac{\partial^2 V}{\partial y^2} = 0## for the field but it is still corrected simulated? or is it not necessary to provide it...
  6. Jovin Sanga

    Hello Physicists! I'm a new member

    I found Physics Forums via Google search. I'm a Tanzanian citizen pursuing a bachelor's degree in Physics (minor in Economics) at The Open University of Tanzania. I'm a first year undergrad. I'm glad to be a part of a huge community of like-minded people. I'm especially passionate about...
  7. R

    I Can anyone tell me what these formulas in electromagnetism are called?

    I know what ##\Phi## and B are, I think they are the magnetic flux and its density. I think ##\mu## is the permeability. But I dont know what ##R_c## and MMF are and how are these formulas deduced.
  8. thejuanestevez

    Electromagnetism - Infinite plane of charge & Tension

    TL;DR Summary: Ping-pong ball hanging static from infinite plane of charge and a string Really struggling with this question. I'm not sure if I have set up the free body diagram correctly and don't know how to set up the x and y components
  9. Heisenberg7

    B What is the difference between B and H?

    A week ago, I started studying electromagnetism. I was introduced to a few new concepts and one of them was H. Now, in my book, they defined H as just magnetic field strength and B as magnetic induction. The thing is, I don't understand what those terms really are (in a physical way), let alone...
  10. Heisenberg7

    Found the Ideal Physics Community for Olympiad Prep?

    For the past few months, I've been on a look out for the best physics community on the Internet and I've just come across this one. My primary goal is to gain as much knowledge as possible in the area of classical mechanics and electromagnetism in a year. I'm fairly new to magnetism, but I can't...
  11. Z

    How to understand typo in MIT OCW chapter on Poynting vector?

    Here is a chapter from MIT OCW's 8.02 Electromagnetism course. At the end of page 14 is section 13.6 "Poynting Vector". The calculations I am interested in are on page 15. There is a passage that seems to have a typo in it. Let me try to show why despite recognizing a typo I am unsure of what...
  12. Z

    Direction of travel of a plane wave given direction of electric field

    Apparently, the direction of wave propagation is the direction of ##\vec{E}\times\vec{B}##. From what I have seen so far, given Maxwell's equations, the set of solutions giving plane waves has the characteristics that 1) electric field has only a component in the ##y## direction 2) magnetic...
  13. Z

    Mathematical representation of a pulse on a rope

    My initial thought was to model the wave as $$y(x,t)=Ae^{-B(x-t)^2}$$ This question is part of an automated grading system and the above entry is considered incorrect. I think I need to incorporate the information that the speed of the wave is ##v## somehow.
  14. P

    New to website but not physics

    Hello everyone. I am new to this website but not new to physics. I took physics in high school and college, but I did forget a lot of what I learned. I graduated from college with a degree in Electronics Engineering. I graduated in 1997. The areas of physics that I am the most interested in is...
  15. L

    Find the magnetic induction vector

    I can find the magnetic induction vector of the first conductor at a given point using the formula (its 6,667*10^-7 Tl) but I don’t understand what needs to be done with the second conductor. I have come across similar problems in which, however, the distance from the second conductor to the...
  16. Priyo137

    Studying How to study for Electromagnetism or something else in high school level?

    I have been taught topics in high school circling around Newtonian mechanics and some basics of work and energy, waves, geometric optics, current and circuits and some poor electrostatics and unclear concepts of modern physics. I realize that I have significant weak areas in Physics and I aim...
  17. A

    B Understanding the electric field

    I’ve been trying to get the proper understanding of electric field. Fine I get the definition: any charge changes space around itself and thus generates electric field that acts with force on any object that’s relatively close to the charge. But first from the first, how can the FIELD act with...
  18. mw3565

    I'm a person who loves physics.

    I'm an ordinary college student who likes physics, engineering mathematics, and electromagnetism. I'm not sure because it's my first time participating in an overseas forum, not a domestic one, but I look forward to your kind cooperation.
  19. J

    I Question about Activating Solenoids that are facing each other

    In this situation I would have two solenoids facing each other, such that both ends are north for example, and when activated they are actively experiencing repulsion, I know that the magnetic field of both would decrease in strength, but would there be any affect on the electrical input of each...
  20. vera1

    Total Internal Reflection and Transmitted Wavelength

    In my electrodynamcis assignment I'm being asked to derive the wavelength of the normally polarised wave transmitted through a glass/air interface as a function of ##n_1## (the refractive index of the first medium) using the concept of phase continuity and the fact that maxima should be equal at...
  21. Z

    Ion moving through electric potential difference and magnetic field

    My question is about item (b). For item (a) we have uniform circular motion in the regions with uniform magnetic field. $$\vec{F}_{B_1}=qv\hat{\theta}_1\times B_1(-\hat{k})=-qv_1B_1\hat{r}_1=-mR_1\theta'^2\hat{r}_1\tag{1}$$ $$B_1=\frac{mv_1}{R_1q}\tag{2}$$ A similar calculation for the...
  22. Z

    Origin of repulsive force on a magnet approaching conducting ring?

    Between ##t_1## and ##t_2## the magnetic flux is positive and increasing. Thus, we have a negative emf and from the point of view of the little stick figure above, the induced current is clockwise. It is not clear to me where the repulsive force on the approaching magnet comes from. The...
  23. S

    I "Strange contradiction" that Maxwell found and resolved

    In "The Strange Story of the Quantum", Banesh Hoffmann writes: What was that contradiction?
  24. N

    A Coupling torsion to electromagnetism and torsion tensor decomposition

    When extending general relativity to include electromagnetism, several authors (e.g. Novello, Sabbata ecc.) assume that the traceless part of the torsion tensor vanishes or is deliberately set to zero. Then, either the trace or axial part of the torsion is used in association with the...
  25. E

    Electromagnetism problem: Merging of 2 charged drops of mercury

    I originally thought that this problem was simple, and it still seems like it is, but there are conflicting solutions and I don't know which is correct. So I first solved for R1 and R2 using V=kQ/r where R1 is 0.514 and R2 is 0.54. My original thought was volume is conserved so V1 + V2 = V3 and...
  26. B

    Planar Coil Rotating in a Magnetic Field

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  27. deuteron

    I Expanding around 0 for multipole expansion

    For the multipole expansion of the electric potential, we expand ## \frac 1 {|\vec r-\vec r'|}## in the following way: $$\frac 1 {|\vec r-\vec r'|} = \frac 1{\sqrt{(\vec r-\vec r')^2}} =\frac 1 {[ \vec r^2 +\vec r'^2 -2\vec r\cdot \vec r']^{-\frac 12}}=\frac 1{[\vec r^2 + \vec r'^2 - 2|\vec...
  28. A

    I Question about electromagnetism and frames of reference

    in this text: my question is in highlighted line: "The two rods have the same length (in S) and contain the same number of charges." why? Considering that the negative rod has movement, it should have a shorter length than the positive rod according to a relativity!
  29. deuteron

    I Dispersion Relation in Different Media

    In my lectures, we have derived the dispersion relation $$ |\vec k|^2 = \frac {n^2 \omega^2}{c^2}$$ by substituting in a plane wave solution for the electromagnetic wave, into the wave equation derived from the Maxwell equations $$\Delta\vec E= \mu_0\epsilon_0 \frac {\partial^2 \vec...
  30. Z

    Linearizing a circuit with a nonlinear element

    I think I managed to solve the entire problem, as I show below. My main doubt is about item (e), the incremental circuit. Part (a) Using the node method and KCL we reach $$\frac{v_I-v_A}{2}=10(1-e^{-v_A/5})\tag{1}$$ Part (b) We can simplify (1) to $$v_A=5\ln{\left ( \frac{20}{v_A+20-v_I}...
  31. Z

    What does a Zener diode do in a circuit with non-ideal voltage source?

    Part (a) The circuit in figure 1b is linear. It is a simple voltage divider circuit. The relationship between a voltage source ##V_I## and output voltage ##V_O## is $$V_O=\frac{R_LR_{IN}}{R_L+R_{IN}}V_I$$ This relationship is true individually and independently for the DC voltage source and...
  32. Z

    Worst-case power consumed by a circuit with MOSFETs

    Here is the circuit. Note that no current flows between the left and right sides of the circuit: their only relationship happens through the MOSFET that is parallel to B. There are eight cases to consider: all the combinations of ON/OFF for the three MOSFETs. Here is a summary of the eight...
  33. Z

    Two concentric conducting spherical shells and resistor in between

    The first thing I thought about was the relationship ##\vec{J}=\frac{\vec{E}}{\rho_r}## which is a statement of Ohm's law. That is, current density is proportional to electric field and the constant of proportionality is the reciprocal of resistivity ##\rho_r##, which is the same as...
  34. deuteron

    I Why Does the Negative Sign Appear in the Vector Potential Equation?

    We have motivated the derivation of the vector potential in the following way: However, I cannot understand where the ##-## sign in the second equality came from. I thought that it was because the gradient was with respect to the ##y##-variable, and then using the product rule one could...
  35. Z

    Minimizing the Energy of Two Conductors Very Far Apart

    The work done by the electric field when we bring a charge ##dq## from an infinite distance to the surface of a shell with radius ##r## is $$dW=\int_{\infty}^r \frac{Qdq}{4\pi\epsilon_0 r^2}dr=-\frac{Qdq}{4\pi\epsilon_0r}\tag{1}$$ The work done by the electric field to charge a spherical shell...
  36. Z

    Charge on inner/outer surfaces of two large parallel conducting plates

    Let me first think about a simpler case. Suppose we have a capacitor. That is, the two plates have charges of equal magnitude and opposite signs. Consider the purple rectangle which represents a Gaussian pillbox. The electric field due to one of the plates individually has field lines...
  37. D

    Direction of the magnetic needle

    I drew the magnetic field lines. The setup was like this: The needle below AB was in the same plane so above AB we get the magnetic field pointing inwards then looping all over Ab from behind the emanating from below AB i.e. pointing outwards. The needle is kept at that point from which the...
  38. D

    Magnetic field pointing into a normal magnetized compass needle

    I was just thinking that if we keep the wire in, suppose, XZ plane and the magnetized needle also in XZ plane. Then in which direction will the needle point? we're going to have either +j cap or -j cap direction by drawing out the tangent at the point where the needle is kept. But a needle could...
  39. D

    Deduction about Magnetic Poles surrounding a Conductor

    As shown in the diagram, a copper conductor is placed over two stretched copper wires whose ends are connected to a D.C. supply. What should be the magnetic poles at points A and B lying on either side of the conductor to experience the force in the upward direction?-------------------- My...
  40. P

    B What's the difference between these two curves for AC Generator?

  41. deuteron

    I Why don't we talk about the E & H fields instead of E & B fields?

    We have the following constitutive relations: $$ \vec D= \epsilon_0 \vec E +\vec P$$ $$\vec B=\mu_0\vec H + \vec M$$ And Maxwell's equations are: $$\nabla\cdot\vec D = \rho$$ $$\nabla\cdot \vec B=0$$ $$\nabla\times\vec E=-\frac{\partial\vec B}{\partial t}$$ $$\nabla\times\vec H=\vec j...
  42. ka_reem13

    Electromagnetism question: Current flowing between concentric spheres

    I know that my solution is time dependant, and I initially tried to use a capacitor model of sorts, but I realised as it was filled with a conductive medium, I cannot use a capacitor model. So now I am very stuck on this
  43. T

    I Alternating current in a perfect conductor

    Premise: the electric field inside a perfect conductor is zero. The boundary conditions indicate that the tangential component is continuous, so the tangential component at the surface of the conductor is also zero. In conclusion, the electric field is perpendicular at the surface of a perfect...
  44. L

    I What causes the unexpected annihilation point in the Magic-Tee configuration?

    I have some problems understanding the magic-tee. There is a configuration for the E and H arm, where the signal output is blocked. As far as I understand you should be able to set one arm to 0 and the other to 1/4 of a wavelength, so the reflected wave's phase will be shifted by pi compared to...
  45. G

    I Does a Railgun's Current Violate Conservation of Momentum?

    Hi. I had a question about railguns, but I think I can formulate the underlying problem more clearly and concisely, hence I'm opening a different thread. Consider the following rigid arrangement of three pieces of wire and two parallel capacitor plates: There's an open switch somewhere in the...
  46. Q

    I What causes skin effect in AC currents?

    My assumption has been it is the electromagnetic field starting from the center of the wire that pushes the electrons outward. However, this would also be true of a DC current, but it isn't. So why does an AC current cause electrons to move toward the skin of a wire? I don't recall ever seeing...
  47. B

    I Can the Last Maxwell's Equation Explain Polarization of a Wire's Insulator?

    Hi there! Recently, I have been reading about polarization of a wire's insulator. First of all, I want to see a connection between the last Maxwell's Equation: $$\nabla\times\\B\ =\mu_0\ J\ +\mu_0\ \epsilon_0\ \frac{\partial E}{\partial t}$$ and the polarization. So I draw a simple cartoon...
  48. AntonioJ

    Potential associated with a conservative force field F

    Given the potential energy, the force is obtained as F = -∇U(r). A conservative force field F is associated with a potential f by F = ∇f. Does the first expression arise from this last one? If so, with -∇U(r), would one obtain the electric field E instead of the force F?
  49. A

    B Designing a current sensor for 50mA up to several dozen Amps

    Hi everyone .with your help I would like to understand if there is the possibility of creating a current sensor with the method that I illustrate below. it has to measure from 50mA up to a few dozen A, I need maximum precision and linearity.it is similar to the current transformers that already...
  50. Y

    Faraday's law -- Confusion about which Area to use in calculations

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