What is Electromagnetism: Definition and 842 Discussions

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. 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...
  2. 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...
  3. 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.
  4. 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...
  5. 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...
  6. 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...
  7. 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...
  8. 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.
  9. 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...
  10. 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...
  11. 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...
  12. 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...
  13. S

    I "Strange contradiction" that Maxwell found and resolved

    In "The Strange Story of the Quantum", Banesh Hoffmann writes: What was that contradiction?
  14. 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...
  15. 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...
  16. 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...
  17. 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!
  18. 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...
  19. 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}...
  20. 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...
  21. 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...
  22. 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...
  23. 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...
  24. 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...
  25. 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...
  26. 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...
  27. 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...
  28. 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...
  29. 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...
  30. 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
  31. 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...
  32. 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...
  33. G

    I Current through U-wire (rail gun): Violation of (naive) 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...
  34. 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...
  35. 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...
  36. 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?
  37. 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...
  38. Hak

    Coaxial semi-infinite solenoid and superconducting disc

    I have no idea how to deal with the problem. Do you have any hints, please?
  39. Hak

    Inductance Puzzle involving coplanar conducting rings

    I solved the first point as ##L_n = \frac{\mu_0 n R}{2}##. How to proceed for the other point?
  40. cemtu

    I How does the electrical field become bent instead of following a straight line?

    Why is electrical field bent like that rather than following a straight line across to the relevant point over there?
  41. cutielollipop

    Electromagnetism question -- Forces between two current carrying wires

    Here is the question. I just wanted to confirm and see if I'm understanding the question clearly. For 3a) I said the first wire would have the magnetic field going in a counter clock wise direction and the second wire would have a magnetic field going in a clockwise direction using the right...
  42. B

    I Power Loss Due to An Eddy Current

    Hi there! Recently, I am studying this kind of power loss from the following link: https://www.electricalvolt.com/2019/08/eddy-current-loss-formula/?expand_article=1 Just to summarize an idea, Supposed that we got a material, which is penetrated by a magnetic flux. The material will generate...
  43. Z

    Understanding example in Wikipedia entry for open circuit voltage

    Consider the circuit (Wikipedia, Open Circuit Voltage) I am having a bit of difficulty understanding the steps and concepts here. I redrew the circuit as follows The red square is the same piece of the circuit with the dotted lines around it in the original diagram (not sure what this is...
  44. Z

    How do we obtain an ampere-hour rating given voltage and watt-hour?

    The solution is thus 0.69 ampere-hours, or 690 mA-hours. Now, as far as I can tell, an ampere-hour is unit of charge. That is we can convert it to coulombs (C). For example, if we have a current of 1A for 1h, then we have 1 C/s x 3600 s = 3600 C = 1 ampere-hour. This is my guess since the book...
  45. Z

    I Understanding dissipation of energy in a resistor through the Drude model

    In section 4.8 entitled "Energy Dissipation in Current Flow" of Purcell and Morin's Electricity and Magnetism, there is the following snippet The model in question is the Drude model I believe (though the book does not seem to give the model any name). There are some paragraphs like this in...
  46. S

    I The vector math of relative motion of wire-loop & bar magnet

    I was watching this video about how the problem of a wire-loop moving relative to a bar magnet: The case of presuming that the wire-loop is fixed seems to be that the magnetic flux (along the surface normal to the direction of the centerline - call it C) through the wire-loop is changing in...
  47. Matthew_

    I Help with Canonical Poisson Brackets & EM Field

    We were introduced the lagrangian for a particle moving in an eletromagnetic field (for context, this was a brief introduction before dealing with Zeeman effect) as $$\mathcal{L}=\dfrac{m}{2}(\dot{x}^2_1+\dot{x}^2_2+\dot{x}^2_3)-q\varphi+\dfrac{q}{c}\vec{A}\cdot\dot{\vec{x}}.$$ A...