What is Em: Definition and 1000 Discussions

An em is a unit in the field of typography, equal to the currently specified point size. For example, one em in a 16-point typeface is 16 points. Therefore, this unit is the same for all typefaces at a given point size.The em dash — and em space   are each one em wide.
Typographic measurements using this unit are frequently expressed in decimal notation (e.g., 0.7 em) or as fractions of 100 or 1000 (e.g., 70/100 em or 700/1000 em). The name em was originally a reference to the width of the capital M in the typeface and size being used, which was often the same as the point size.

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  1. G

    Relativistic charged particle in a constant, uniform EM field

    I have to find pμ(τ) of a particle of mass m and charge q with v(0) = (vx(0), vy(0), vz(0)) in a electric field E parallel to the y-axis and a magnetic field B parallel to z axis, both constant and uniform, with E = B. Here follows what I have done (see pictures below): I wrote 4 differential...
  2. L

    Can a Tiny Antenna Transmit Long Wavelength EM Waves?

    Can you transmit em wave with antenna much smaller than its wavelength? For example. ELF antenna is very long. Can you make one small enough to fit in the pocket by the device constructing the long wavelength part by part?
  3. U

    I Boundary conditions and discontinuity of EM fields

    Premise: everything that follows is done in the frequency domain. Boundary conditions If there are superficial currents (electric and magnetic) impressed on the boundary between two media, we have these discontinuities for the tangential components of the fields...
  4. sergiokapone

    I Derivation of Geodesics Eq from EM Tensor of Point Particle

    The energy-momentum tensor of a free particle with mass ##m## moving along its worldline ##x^\mu (\tau )## is \begin{equation} T^{\mu\nu}(y^\sigma)=m\int d \tau \frac{\delta^{(4) }(y^\sigma-x^\sigma(\tau ))}{\sqrt{-g}}\frac{dx^\mu}{d\tau}\frac{dx^\nu}{d\tau}.\tag{2} \end{equation} The covariant...
  5. Y

    I What will eventually happen to all EM waves / photons?

    I'm not an expert in this matter, and at best only aware of some superficial facts and a layman's understanding of them. So please forgive me for any ignorant mistakes in my thoughts, and kindly point them out to me. Going by the Lambda-CDM model, the expansion of the Universe will eventually...
  6. Z

    The Energy of a Continuous Charge Distribution (Griffiths EM Sect. 2.4.3 3rd ed)

    I'm working through Griffiths EM 3rd ed. in section 2.4.2 (point charge distribution) and 2.4.3 (continuous charge distribution). I understand from the section on point charge distributions that when we add up all the work (excluding the work necessary in creating the charge itself), one clever...
  7. W

    Geodesics and Motion in an EM Field

    I've also attached my attempt as a pdf file. My main issue seems to be I only get one A partial term. Any help would be appreciated.
  8. W

    I Are EM fields clouds of photons?

    Personally, I have trouble visualizing a flock of birds without the birds. I have the same trouble trying to visualize an EM field without the photons. More than once, I have been hounded off of the ham radio forums for mentioning the word "photon". As one ham radio operator put it, "We don't...
  9. U

    If DC current is chopped on it's way to a load, what happens to the EM?

    I am curious what happens if you draw DC power to a load, but as it approaches the load you chop the load connection say with a SSR or switch. The power will be traveling near the speed of light, following the wire toward the load. When the load is chopped by a switch just before the pulse...
  10. G

    Do intensity flickers result from the nature of EM waves?

    Since light intensity is proportional to the amplitude of the EM wave, and wave amplitudes undulate up and down, does this result in natural intensity flickering of observed light? For visible light, the frequency is extremely high, but it might be more easily observable in ELF waves.
  11. ubergewehr273

    B Doubt on an EM problem regarding gauss law

    There's this problem 2.18 in the book "Introduction to electrodynamics" by Griffith. The problem says the following, "Two spheres, each of radius R and carrying uniform charge densities ##+\rho## and ##-\rho##, respectively, are placed so that they partially overlap (Image_01). Call the vector...
  12. gibberingmouther

    EM Wave Amplitudes and Fictional Force Field

    So, my game is coming along. My psychic energy shielding protects against EM radiation. The energy used for shielding gets depleted based on the type of EM radiation (the wavelength) and according to the amplitude of the radiation the energy shielding is exposed to. I can't find many numbers...
  13. A

    EM radiation strength wrt time

    So I see EM radiation , visible spectrum including being portrayed as two sinewave vectors each perpendicular to other one being that of the E field the other being the B field, the field is carried by photons, let's take the visible spectrum as an example. So having a specific frequency the...
  14. C

    Service drop wire running to my house and EM fields?

    Hi, I have a case of elevated AC Magnetic field in the house, strongest nearest the electric meter and along the service drop line connecting the house to the street power lines. Should that service drop line measure no current or fields when I shut off my main circuit breaker? it seems shutting...
  15. S

    Is the EM field the medium for light waves

    EM waves can propagate through empty space, but there is also the EM field. Is this field really empty space or is there something that exists that the light wave disturbs, like some kind of fluid? In areas of space where values of the field is 0, is this like empty space or more like calm...
  16. C

    I Visualizing the Attached EM Field for Free Electron in QFT

    My understanding of the QFT model of a free electron is that there is a localized higher energy level in the electron matter field which couples to the EM field in two ways: (1) the coupling allows the electron matter field to 'feel' a force from an outside EM field and accelerate in response...
  17. R

    Reversing the Faraday effect to get an induced current

    Hi all! I was thinking if it is possible to revert the Faraday effect. If I have a magnetic field, it interacts with EM wave (light) by changing its polarisation. Can I got a variable magnetic field interacting with polarised light so that I can get induced current in a coil? I can understand...
  18. P

    I Is the electromagnetic field of an electron in QFT a real physical field?

    According to the QFT, each electron creates an electromagnetic field around itself. Is this field real or is it just a virtual mathematical field?
  19. iVenky

    From Maxwell's equations to EM waves

    Hi, I just finished studying Maxwell's equations. Based on my understanding, when you solve maxwell's equation, you get the wave equation and it simplies to in a charge and current-free region. I understand that these two equations are similar to an equation of a wave in space. What I am...
  20. S

    B Is the classical EM wave a single photon?

    Does a single photon travel in two different waves at once? If photons are particles like the Photoelectric Effect, Compton Scattering, and Blackbody radiation all suggest, how do polarizing filters block light completely? Is a particle from a radio antenna actually that large in size?
  21. M

    I EM Lagrangian: Question on $(\partial_\mu A^\mu)^2$ Term

    The EM Lagrangian is $$\mathcal{L} = -\frac{1}{2}[(\partial_\mu A_\nu)(\partial^\mu A^\nu) - (\partial_\mu A_\nu)(\partial^\nu A^\mu)]$$ In the QFT notes from Tong the EM Lagrangian is written in the form $$\mathcal{L} = -\frac{1}{2}[(\partial_\mu A_\nu)(\partial^\mu A^\nu) - (\partial_\mu...
  22. C

    I QFT Interpretation of Electron and Attached EM Field

    This is an elementary question on visualizing the interaction of an electron with the surrounding EM field in QFT. I believe in QFT the electron is viewed as an excitation of the Electron matter field with an associated coupling constant between the electron field and EM field (say q) - q...
  23. A

    EM Wave Reflection and Refraction: Understanding the Fresnel Formalism

    For this question I want to clarify that 5k which is the electric field component perpendicular to the incident plane ( the xy plane) will be continuous for reflection and refraction , For which none of the options seems correct ,am I right?? The component of magnetic field perpendicular to...
  24. P

    B Is the Strong Force Really Stronger Than the Electromagnetic Force?

    So I am aware that the strong force is the strongest force with the electromagnetic force being the second strongest force. I am wondering how we go about the process of deciding which force is stronger. I am visualizing a helium nucleus; two protons, two neutrons. The two protons are...
  25. R

    Are all EM waves produced by accelerating charges?

    So, a static charge at rest produces an electric field, but no magnetic field. A charge moving with constant velocity produces both electric and magnetic fields. Why is it that accelerating charges are the source of all electromagnetic radiation? How would one go about showing this using...
  26. F

    I Expectation Maximization (EM) : find all parameters

    I am tackling a technique to determine the parameters of a Moffat Point Spread Function (PSF) defined by: ## \text {PSF} (r, c) = \bigg (1 + \dfrac {r ^ 2 + c ^ 2} {\alpha ^ 2} \bigg) ^ {- \beta} ## with the parameter "(r, c) =" line, column "(not necessarily integers). The observation of a...
  27. tworitdash

    Classical Which classic book to follow for Advanced EM?

    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)...
  28. M

    Trying to buy quality EM meter and

    Hi all! Not sure if this is the right place but I am having really hard time getting these questions answered in other places as well. I became interested in EM topic when power company decided to build a 2x110kV power line near my house. I would like to approach this problem rationally with...
  29. bob012345

    Very Low Frequency EM Waves (now: Magnetic Launcher)

    Is it possible to make very low frequency EM waves, say 10 KHz, without an antenna the size of Texas? Something more human sized? Thanks.
  30. B

    Why Do Electromagnetic Waves Propagate Independently of Their Source?

    Homework Statement Use Maxwell's equations to elaborate an coherent explication of why electromagnetic waves propagate independently of the source that produces them. Homework Equations Maxwell's equations in vacuum: ##\nabla * E=0## ##\nabla * B=0## ##\nabla \times E = -\frac{\partial...
  31. S

    Piezoelectric Effect and EM Radiation

    It's my understanding that deforming a piezoelectric crystal causes electric charges to build up on the outer faces of the material. What I would like to know is if these charges behave like the free charges in a metal. More specifically, does deforming a piezoelectric crystal effect how em...
  32. I

    Normal incidence of EM wave - p & s polarization convention?

    Hi. I'm reading a paper "Transmission of light through a single rectangular hole in a real metal" and the author refers to the incident light shown below as "p-polarized" without further specification. Note that ax > ay. Is there any convention in regarding a certain polarization as...
  33. Jaden159

    A Is energy contained in matter wave equals hv like EM waves?

    h is plank constant and v is frequency. I was using this to derive the TDSE. But I ran into problem because to substitute k^2 in E=h^2/8mpi^2 * k^2, I can use single derivative of psi squared or double derivative, both of which tend to give the correct answer. So, is my assumption of energy...
  34. D

    What is the basis for beach erosion control methods?

    https://www.msn.com/en-ph/news/technology/egypts-most-famous-pyramid-has-a-mystery-power/ar-BBLlYJ5?ocid=spartanntp How true is this?
  35. S

    Frequency and energy of EM radiation

    Am I correct that the direct proportionality between the energy and frequency of an EM wave can be obtained from classical electromagnetism? Of course there's the Planck–Einstein relation ##E = h \nu## for a photon, but that entails QM. I'm wondering about the relation ##E \propto \nu## for an...
  36. H

    B Maxwell's EM Theory vs Principle of Relativity

    What was the problem between Maxwell's EM theory and the principle of relativity? Why went the theory against the principle? I understand that the EM theory says that Light was a wave and ether is it's medium. On the other hand the principle of relativity says that there is no state of...
  37. G

    I Help required regarding deriving E-L equations for EM

    I am a retired High School teacher trying to use tensors in getting the Euler-Lagrange equations from the em lagrangian density. I attached a document in my post since I am not fluent in writing LaTex. Can anyone, please check my work. Thanks.
  38. Monsterboy

    How does the velocity of air molecules affect EM waves?

    I would like to know how exactly or if the velocity of air molecules affect the light i.e electromagnetic waves passing through it. Ignoring the effect of pressure and/or temperature differences in the air which might also affect the light (due to changes in refractive index).
  39. WeiShan Ng

    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...
  40. Z

    How was Newtonian relativity ruled out in EM propagation?

    What I've read on the Michelson/Morley experiment explains that it made the idea of the luminiferous aether seem less likely, but I don't think I've ever seen an explanation of why everyone didn't just assume that light follows normal Newtonian relativity. What I mean is this: according to...
  41. P

    Why is there interference if EM waves don't interact?

    Radio waves pass through everywhere without interacting with each other and that enables us to hear different phone calls and radios without disturbance. However, we do hear some noise sometimes because different signals interfere with each other. How are both of the last two statements true (if...
  42. snoopies622

    I Can one get E=mc^2 using classical EM theory?

    I'm wondering if one can arrive at E=mc^2 using only the physics of the late 19th century, in the following way: As light waves pass over an electrically charged particle, they push it in the direction of the wave motion, transferring both (kinetic) energy and momentum to the particle. Let's...
  43. W

    EM: Evaluating a Poynting vector

    Homework Statement I reference problem 9.10 Purcell's Electricity and Magnetism (3rd ed). A very thin straight wire carries current ##I## from infinity radially inward onto a conducting shell with radius R. Show that the total flux of the Poynting vector away from an imaginary tube of radius...
  44. G

    How do we visualize EM radiation?

    What is the best analogy to explain electromagnetic fields? I have seen the depictions of em radiation as perpendicular waves. Do we exist in a huge sphere of em radiation in which waves such as cell phone signals and radio broadcasts can be thought of as spherical ripples that emanate at the...
  45. W

    Superposition of Plane EM Waves Using Complex Notation

    Homework Statement I have a simple problem relating to the superposition of plane EM waves that I'd to try out using complex notation. Could anyone run through the work to see if my understanding is right? Many thanks in advance! The incident E bit of the wave is $$\vec{E}_I = E_0 \sin(ky -...
  46. Sahar ali

    I The 6th Spectral Series: Wavelength & EM Spectrum

    we know the five spectral series of Lyman, Balmer, Paschen, bracket, and Pfund their wavelength and also the part of EM spectrum they fall in, my question is why do we neglect the 6th series in the spectrum? and in what part of EM spectrum the 6th series exist and what could be its wavelength...
  47. Buzz Bloom

    What fraction of long wavelength EM gets through a small hole?

    Assume a source of EM radiation at wave length λ hits a barrier with a small circular hole of diameter d << λ. What fraction of the radiated power (watts) that hits the hole passes through it? Does it depend on the thickness of the barrier? I understand that after passing through the hole, the...
  48. P

    A Are vacuum EM modes circularly polarized according to QED?

    I'm talking of the virtual EM modes of vacuum...
  49. ?

    EM fields of two opposite moving point charges

    I wrote and solved this problem but am having serious doubts about the answer I obtained. Homework Statement Two point charges \pm q move along the z-axis with velocity \pm v. If they are at the origin when t=0, what is the electric field magnitude a distance r from the z-axis? Homework...
  50. binbagsss

    A What is the importance of EM measurements in astrophysics?

    I was asked this in a PhD interview and hit a blank, it seemed to open...
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