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. Q

    Phase invariance of an EM wave in special relativity

    Homework Statement So I'm trying to show for a specific, given EM plane wave in vacuum that kx - \omega t = k' x' - \omega' t' but I'm running into some difficulties. I'm hoping someone can show me where I'm going wrong. Here's the setup: In the lab frame K, a plane EM wave traveling in...
  2. T

    A few questions on electricity and magnetism and EM fields

    Hello: Many many years ago I walked away from community college with an associates degree in physics, and despite my fondness for the subject, I had to put it aside for other things. I recently happened upon something that lead me to realize how much I had forgotten, and so I've decided to...
  3. S

    EM Theory: Refractive index of water

    Homework Statement Problem: Sea water has k = 80 in the low frequency limit. Yet its refractive index is around 1.34. Explain the discrepancy Homework Equations For a non magnetic dielectric medium, the absolute refractive index in the low frequency range, is given by : n = √k...
  4. W

    Even Hitler rants about Jackson EM book

    Guys, I found this Downfall parody video about how much Hitler hates Jackson EM book: Apparently, even he can't stand Jackson after one full semester of EM course. :D
  5. Disconnected

    The amplitude of EM radiation

    Aight, so I have this problem where I think about something and then I understand it, but then instead of stopping thinking and understanding it, I keep thinking and stop understanding it. Unfortunately then my poor brain is so confruzed that I am unable to backtrack to understanding, and I...
  6. S

    Why do electromagnetic waves oscillate?

    Hi everyone, I have a specific question regarding the nature of electromagnetic waves. I understand he basics of EM waves (frequency, amplitude, wavelength, etc.) and that 'it' is a pair of oscillating fields; one electric and one magnetic, perpendicular to the direction of travel. What I...
  7. H

    Exploring the Mysteries of EM Waves in Space

    Hi, I am not a student of physics, asking the question as it came to mind as I read certain articles: 1. When EM Waves is emitted by a source say star it spreads in vacuum. How does the waves Stretch? What fills in the gaps as the sphere of spread grows? If light emission is continuous...
  8. ElijahRockers

    Visualizing EM Waves: Learn Basics in 9 Days

    Perhaps this is going in the wrong forum, but the Math and Science Learning Materials forum would not let me create a new topic. I am not necessarily worrying about a specific question; in fact I am able to use the equations and formulas relevant to electromagnetic waves in my physics course...
  9. G

    Spherical EM wave or one or more photons?

    My understanding is that the EM field at r.t generated by a radiating source can be described as the amplitude of the EM fields at r, at time t. Is there a corresponding photon associated with that wave? A unit surface area at large r from the source will have less energy passing through it...
  10. Q

    Understanding EM Wave Equations

    hi, i have a problem understanding why the wave equations are as such if wave is moving left, it is f(z,t) = Acos(kz + wt - d) if wave moving right , f(z,t) = Acos(-kz -wt + d) finally i don't know what this represent f(z,t) = Acos(kz - wt + d) where A is constant , k is wave...
  11. D

    How can particles undergo EM interactions *and* have definite strong isospin?

    I am deeply confused about the following and I'd really appreciate it if anyone could help! Consider a charged hadron such as a proton. Amongst the state-independent properties that define a proton are strong isospin Iz=1/2 and charge Q=e. Now, the total Hamiltonian for a proton is Hs...
  12. J

    Discussion of A. Neumaier's claim that classical EM can violate Bell's theorem

    Discussion continued from this post on another thread... Bell's proof does not involve any notions of "particles" or "waves" whatsoever, it only involves observed experimental results combined with the idea that the theory generating them is local realistic. Again, do you agree or disagree...
  13. X

    EM waves, Light, radio and all that.

    I find EM waves very intriguing. But i get more confused and new questions pop up in my head every time i read something about it. I have a few basic questions. 1. Wavelength: As wavelength is distance between two peaks, how can we measure this distance in cm, meter etc. as physically there...
  14. M

    EM Wave - Where Does the Energy Go?

    For an EM wave in vacuum, we know the energy density is given by1/2 e E^2 for the electric field, with a similar expression for the magnetic (e is permittivity of vacuum). E^2 implies that the energy oscillates as a cosine squares function if we represent the E field as E_0.cos [ kx -...
  15. P

    Diff between aether & EM field

    What's the main difference between the concepts of aether and that of the EM field? They both seem, to my unpracticed eye, to be everywhere and the medium that allows light.
  16. F

    EM waves and conducting surfaces

    Firstly, I'm a bit confused about EM wave propagation. Take the picture you see everywhere illustrating the perpendicularity of E and B in a traveling EM wave (like this http://web.onetel.net.uk/~gdsexyboy/em_wave.jpg) -- does that actually illustrate the magnitudes of E and B at a particular...
  17. J

    Why are EM waves blocked by non-conductors?

    My father-in-law uses a USB internet stick for his internet since he lives in The Middle of Nowhere where there is no cable nor DSL. His problem is that reception from his basement office is very poor. However, reception from the second floor of his house is okay... I don't understand this...
  18. W

    Average intensity problem? related to EM waves?

    Homework Statement The helium-neon lasers that we use in lab produce light waves with an average power of 0.85 mW. Suppose that at a distance of 1.0 m from the laser the beam has a diameter of 1.1 mm. a) What is the average intensity of the laser beam 1.0 m from the laser? b) What is the...
  19. M

    Momentum of Charged Particle in EM Field Explained

    Can someone demonstrate how the momentum of a charged particle in a time-varying electromagnetic field is given by p - qA where A is the vector magnetic potential? I've always wondered :-) Cheers!
  20. Matterwave

    Transforming the EM field tensor

    Homework Statement So, I'm asked to find how the fields (E, and B) transform by transforming the electromagnetic field tensor. The transformations are a) rotation around y axis, and b) boost along z. Homework Equations F'_{\mu\nu}=\Lambda^\mu_\rho \Lambda^\nu_\sigma F_{\rho\sigma} The...
  21. M

    Question about energy in EM waves

    The instantaneous energy density of a region of space of an EM wave is: u = \epsilon0E2 [J/m^3] hence the average energy density is: uavg = (1/2)\epsilon0E02 [J/m^3] uavg = <S> / c [J/m^3] Is this equal to the wave's average http://en.wikipedia.org/wiki/Radiation_pressure"...
  22. A

    Why Does Dipole Radiation Shift from Radial to Theta Hat Direction?

    Hello everyone, I'm a bit confused about electric dipole radiation. In my E&M book "Intro to Electrodynamics" by David Griffiths, it states that the electric field from an oscillating dipole is in theta hat direction. Mathematically I have the proof as to why in my book, but conceptually I'm...
  23. Y

    Need to find an EM book between Griffiths and Jackson.

    I am a self studyer and I studied "Field and Wave Electromagnetics" by David K Cheng and "Introduction to Electrodynamics" by david Griffiths. Cheng is more for EE where it put a lot of effort in phasor, transmission lines, Smith Chart. Griffiths is more on the physics side that cover in more...
  24. D

    Do Fresnell equations apply to other waves than EM waves?

    When I read about Fresnel equations, I see that it is based on boundary equations determined by Maxwell's equations. So I would think these equations only apply to EM waves. But is there a similar behavior in other waves like sound waves in water? Would I want to use or modify Fresnel...
  25. A

    If light is quantized, why are EM spectrum and Blackbody spectrum continuous?

    If light is quantized, and is given out in packets, why are the EM wave spectrum and the black body spectrum continuous? I am very confused, can someone offer some explanation? Any input is greatly appreciated.
  26. R

    EM Waves and Dielectric interfaces

    Homework Statement Re‐write the formula E = Eo cos (k• r −ωt) for each of the three waves, in terms of the angles θi, θr and θt and the dielectric constants ni and nt (for the materials on the left and the right) in place of k The incident wave is in the xy plane only as is therefore...
  27. P

    Does waving a magnet back and fourth produce an EM wave?

    If you take a permanent magnet and wave it back and fourth, or even just move it along in one direction, this seems like it should create an electromagnetic wave. If so, how would one compute the frequency, and could you possibly detect it with a radio if you shielded it from other noise?
  28. G

    Advanced EM problem involving summation of 1/Cosh

    We solve Poisson's equation in cartesian coords for a region bounded by planes forming a box. Some of the planes are grounded. The lengths of the box are L1, L2, L3. There is no charge distribution. Let a=mPi/L2 b=nPi/L3 The solutions goes like: sinh(sqrt(a^2 +...
  29. J

    Are the E and B fields in phase in an EM wave?

    I keep looking at a sketch of the mutually perpendicular electric & magnetic fields in an EM wave. The sketch (and my text) says they are in phase, i.e. their maximum values occur simultaneously, ... ...but the discussion centers around the changing E field producing the B field and...
  30. S

    The Doppler Effect For EM Waves

    Homework Statement A star is moving away from Earth at a speed of 2.4 x 10^8 m/s. Light of wavelength 480 nm is emitted by the star. What is the wavelength as measured by an Earth observer?Homework Equations fo=fs(1 - vrel/c) I substracted because they the star is moving away from the earth...
  31. snoopies622

    EM Field Lagrangian: What Defines It?

    According to this site http://quantummechanics.ucsd.edu/ph130a/130_notes/node453.html a good choice of Lagrangian for the electromagnetic field is L = - \frac {1}{4} F_{\mu\nu}F_{\mu\nu} + \frac {1}{c} j_\mu A_\mu where F_{\mu \nu} = \frac {\partial A_\nu}{\partial...
  32. L

    My question concerns electromagnetic (EM) wave propagation.

    my question concerns electromagnetic (EM) wave propagation. consider a two layer medium where the first layer has constant conductivity $\sigma_0$ and the second layer constant conductivity $\sigma_1$. If a EM wave propagates from the first medium into the second one boundary conditions have...
  33. R

    Explaining a Strange Phenomenon: EM Field & PC Fan

    Hello. I need an explanation for a strange phenomenon ... I use a fan (PC type) powered by mains (220 V / 50 Hz in my country). At a distance of about 3', I get a sin EM field (sure it is not by mains cords) at a stable frequency of 200 Hz synchronized on main frequency. Even during power on...
  34. N

    Can an Electromagnetic Wave Charge a Battery?

    This is a theoretical question not a homework question, so if this is the wrong section to post this in please let me know. But my friend has this idea that if he sent an electromagnetic wave at a certain frequency at a surface it could create a current and this in turn could charge a battery...
  35. K

    What does it mean by independent (in gauge fixing of EM field)

    What does it mean by "independent"(in gauge fixing of EM field) In my textbook, it gives the Coulomb gauge \phi = 0,\nabla A = 0 and says they will kill two degrees of freedom of the four potential and leave two independent components. I understand \phi = 0 will kill one degree of freedom...
  36. G

    Exploring the Possibility of an EM Stress-Energy Tensor with Equal Sigma-Values

    Is it possible for an EM stress-energy tensor such as this: [PLAIN]http://www3.telus.net/public/kots1906/emtensor.jpg to exist, where \sigma_{xx} = \sigma_{yy} ?
  37. P

    EM response function of the Phase Action of a BCS superconductor

    EM response function of the "Phase Action" of a BCS superconductor Hello, I am looking for a paper in which people calculated the EM response of phase action of A BCS SC. In the book "Condensed Matter Field Theory" by Altland and Simons, on page 393 they mention such a thing in the discussion...
  38. D

    EM Wave Boundary Layer Question

    I understand application of Snell's law for transition from one medium to another but I have a question regarding this model. When an electromagnetic wave transitions from air into a conductive medium does the wavelength change instantaneously as the theory seems to imply or is there a boundary...
  39. E

    Exploring the Limits of Electromagnetic Energy Density: A Scientific Inquiry

    Hi yall, I was just wondering, seeing as how photons are massless, would there be a limit to the amount of electromagnetic energy you can fit in a given (finite) area? Of course excluding the practical reasons that would stop you. And if there is a limit, how come? Because black holes are...
  40. R

    What is the Mistake in Deriving the EM Hamiltonian?

    For some reason I can't derive the Hamiltonian from the Lagrangian for the E&M field. Here's what I have (using +--- metric): \begin{equation*} \begin{split} \mathcal L=\frac{-1}{4}F_{ \mu \nu}F^{ \mu \nu} \\ \Pi^\mu=\frac{\delta \mathcal L}{\delta \dot{A_\mu}}=-F^{0 \mu} \\...
  41. H

    EM waves amplitude's effecting penetration?

    If you held frequency constant, and changed the amplitude of EM waves coming from a source, then would the distance into a material the EM waves would penetrate change?
  42. S

    Understanding EM Fields: Storage and Transmission of Energy

    I have been told that 1. EM fields contain energy. 2. EM fields can store energy, e.g. capacitors. 3. EM fields can transmit energy, e.g. via waves. How are 1. and 2. any different from each other?
  43. K

    Why according to classical, electron circuling around the nucleus will emit EM wave?

    why according to classical, electron circuling around the nucleus will emit EM wave and lose energy and collapse? I know electron undergoes circular acceleration, but why emit EM wave?
  44. R

    EM Lagrangian in terms of E and B

    What's the most persuasive argument for using the potential phi and A as independent deegres of freedom in the electromagnetic Lagrangian instead of the more physical field E and B? Why does the cannonical approach break down for E and B?
  45. S

    Self-replicating fields in EM radiation?

    I see everywhere the explanation that electromagnetic radiation self-propagates this way: a changing magnetic field induces a changing electric field, which in turn induces a changing magnetic field and so on and so on. I do not understand that. I do not have a problem with saying (it is a...
  46. morrobay

    E = Em sin(kx-wt) and I = T+R what wavelength is reflected wave

    When a light wave enters a medium the electric field value of the wave is smaller in the medium. With the incident wave = transmission wave + reflected wave. If the E field of the reflected wave is known . Can the wavelength of the reflected wave be obtained from these relationships : E = Em...
  47. pellman

    What is the EM Lagrangian in curved spacetime?

    In flat space time the Lagrangian for the EM potential is (neglecting the source term) \mathcal{L}_{flat}=-\frac{1}{16\pi}(\partial^{\mu}A^{\nu}-\partial^{\nu}A^{\mu})(\partial_{\mu}A_{\nu}-\partial_{\nu}A_{\mu}) which is a scalar for flat spacetime. I would have expected the...
  48. M

    Separating Electricity and Magnetism from EM Waves

    Electricity and magnetism had been unified under Maxwell's four famous equations. But I wonder if there any method to separate the this two components from an EM wave? Will there be any effects if this two components are separated? Hope you all can help me to solve this problem^^ Thank...
  49. S

    Please explain human EM interference with my XMAS lights

    I am aware that the human body has electrical properties, But was baffled by what I witnessed Christmas Eve. I had a malfunctioning electric candle in the window. I picked it up to check the tightness of the bulb, and it immediately came on. I set it down and it went off. Three times this...
  50. D

    Reviewing Mechanics & EM for Modern Physics: Priority Topics

    I'm taking Modern Physics in the spring semester (starts Jan 10). It's been over 5 years since I took Mechanics and EM. Which topics from these courses should I review in preparation for Modern Physics? I plan to review everything, however this will take me longer than the time available before...
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