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. F

    EM fields. Angular Momentum seems not to be Conserved

    EM fields. Angular Momentum seems not to be Conserved! Hi everyone.. This is a paradox which as been stated in Feynman Lectures 2 (Chapter 17-4). What Feynman is telling is, there is a plastic plate which is free to rotate about an axis passing through the center of the disc, there are...
  2. B

    Inductive interference from two EM coils (aka a transformer)

    I've been wondering about this for a long time. The little black box that you plug into the wall so you can power your electronic devices contains a general run-of-the-mill transformer. But if the power is completely shut off to the device: not draining any current waiting for a power button...
  3. M

    Does typical Sun Light usually contain all EM spectrum?

    Are all the frequencies of the Electromagnetic spectrum in a beam of light before being filtered by the Earth's ozone? Beyond ROYGBIV though.. so like infrared rays, microwaves, xrays, etc
  4. I

    How Does Electronic Susceptibility Affect EM Signal Propagation in a Dielectric Medium?

    In general the electronic susceptibility, χ, is a function of frequency and is complex for a dielectric medium. So what are the implications of this for the propagation of EM signals in a dielectric medium? Since the electronic susceptibility varies with refractive index, and the refractive...
  5. P

    Special relativity the EM stress energy tensor

    Homework Statement Using the expression below for the stress energy tensor of the em field, show that it has zero trace. Homework Equations T^{\mu\nu}=F^{\mu}_{ \alpha}F^{\alpha\nu}+\frac{1}{4}\eta^{\mu\nu}F_{\beta\gamma}F^{\beta\gamma} F is the faraday tensor and eta is the...
  6. S

    What is the extent of the electric field in electromagnetic waves?

    Hello. Just a noob question about electromagnetic waves. An electron "wiggles" and sends out a change in electric field, which creates a change in magnetic field, which creates a change in electric field etc etc. My question is, because the photon travels to virtually infinite distances...
  7. T

    Help Layman Understand EM Wave Propagation

    Hello everyone. I have tried to do as much research as my layman mind will allow on how an electromagnetic wave propagates in relation to how a sound wave for example does. I understand that an acoustic wave is longitudinal and works on compression and that a light wave is a transverse wave...
  8. K

    Does the Sign of the wt Term in EM Wave Propagation Matter?

    We were taught in the vibrations and waves lecture course that the solution to the wave equation for traveling waves is of the form ψ(z,t) = Acos(wt-kz). In the Electromagnestism course we learned that EM waves are traveling waves and have the solution E = E0cos(kz-wt). I know that changing...
  9. S

    How to define the b.c.s for the EM field of a perfectly contucting surface?

    If you have a 3 dimensional perfectly conducting body the conditions at the boundary for the EM field is as follows: \boldsymbol{E}_{\parallel} = 0, B_{\perp} = 0, E_{\perp} = \frac{\sigma}{\epsilon_0}, \boldsymbol{B}_{\parallel} = \mu_0 \boldsymbol{j} \times \boldsymbol{\hat{n}} where \sigma...
  10. R

    What Determines the Electric Field at Points Along an EM Wave?

    Homework Statement There is an EM wave (http://www.geo.mtu.edu/rs/back/spectrum/) that has theoretical points all along the thick line that runs along both the electric/magnetic waves. What could you say about the electric field at these points? Homework Equations None The Attempt...
  11. Dale

    Maxwell's Eqns: EM Basis Functions Explored

    Are there any sets of basis functions that are particularly useful for Maxwell's equations? I was thinking about Fourier just because it is the first basis I always think of, but I don't know that it would actually be a convenient basis. For example, I don't know that curl or divergence would...
  12. S

    Calculating Electric Field for an FM Antenna in an EM Wave

    Homework Statement A 1.10m long FM antenna is oriented parallel to the electric field of an EM wave. How large must the electric field be to produce a 1.28 mV voltage between the ends of the antenna? Homework Equations u = EoErms^2 I = uc Intensity = Power / area The Attempt at a...
  13. Q

    Atomic Interactions with External EM Fields

    I did not quite understand the "stimulation" part. How does the photon coming near the excited atom force this atom to emit another, identical (from the physical properties point of view) photon? More precisely, how do the stimulating photon and the atom interact?
  14. pellman

    Is EM theory in curved spacetime the same as unification ?

    Is EM theory in curved spacetime the same as "unification"? I am wanting to learn about classical EM theory in curved spacetime (just curious) and I found this old thread containing some references https://www.physicsforums.com/showthread.php?t=3950 Is simply talking about the EM field in...
  15. L

    Scalar potential in EM and Newton's Law

    Hi,all, problem is: in Electromagnetism, we introduced a scalar potential psi, such that: E = - grad Psi and In Newton's law, there is also a scar potential U, such that: F= -grad U My question is, one is the gradient of a scalar field give a field and a force?! Can anyone...
  16. G

    Visualizing daily life through broader EM spectrum

    Sorry if this isn't the best forum for this question; putting it here due to a gut feeling cos my brain wouldn't tell me where to put it. I'm thinking of an idea for an art project. I'd like to somehow depict what a boring, every-day scene might look like if our eyes could see more of the EM...
  17. E

    Photon absorption via changing EM field

    Does it work like that.. that a changing EM field in-phase (actually, like.. 180 degre (spell?) out-of phase) with the incoming photon would absorb it? .. when sound waves of 180 degre.. (spell) out of phase with each other would cancel each other out. The same thing would happen with EM...
  18. K

    Does E=cB Hold for All EM Waves?

    I know for spherical or plane EM wave, there's relation E=cB, and we can prove it by the explicit expression of these two kinds of wave. But does E=cB hold for all EM waves, e.g. all possible wavelike solutions of maxwell's equation?
  19. Z

    Differences Between EM Waves & Pulses: Comprehensive Analysis

    What is the difference between EM Waves and EM Pulses and how are they related to one another? When trying to look up info on this, I always get a full article on either just one or the other so that no comparison is ever made between one and the other, the definitions alone seem a little...
  20. D

    E-Fields vs B-Fields in EM Wave Disturbances

    I'm learning that in the electrodynamics of circuits and charges, an E-field is very different from a B-field. But in Maxwell's equations for a disturbance in the electromagnetic field, where a changing electric field causes a changing magnetic field, which in turn causes a changing electric...
  21. S

    Exploring EM Wave Transmissions: Experiment & Wavelength

    Homework Statement a) A microwave transmitter T and receiver R are placed side by side facing two sheets of material M (aluminium) and N (hardboard). A very small signal is registered by R; what can you deduce about the experimental set up? b) When M is moved towards N a series of maxia and...
  22. K

    DB loss for a 3GHZ EM wave traveling through 2 meters of a medium

    Homework Statement What is the dB loss for a 3GHZ EM wave traveling through 2 meters of a medium with ϵ=1.5ϵ_0 and loss tangent = 9E-4? Homework Equations Umm...I'm actually not sure. I can't find anything really relating these things at all. The Attempt at a Solution My first...
  23. R

    Can EM Waves Vibrate Powders? Exploring Physical Effects

    Can man-made EM waves produce physical vibrations of objects? Can man-made EM waves vibrate powders like sand or talc? I'm trying to understand if any types of electronic frequencies can cause physical vibrations...thanks.
  24. M

    Is F. Abeles' 1950 paper on EM waves in stratified media still accessible?

    I've been trying to search for this seminal paper on EM waves in stratified media. 'Investigations on the propagation of sinusoidal electromagnetic waves in stratified media', by F. Abeles Ann. Phys. (Paris) 5, 596, 706 (1950). I cannot find it in any database. Can anyone please tell me...
  25. pellman

    Delta function representation from EM theory

    Claim: \nabla \cdot \frac{\hat{e}_r}{r^2}=4\pi\delta^3(\vec{x}) Anyone know of a proof of this? (or a reference which covers it?) We need to show that \frac{1}{4\pi}\int_0^R{(\nabla \cdot \frac{\hat{e}_r}{r^2})f(r)dr=f(0). The claimed identity can be seen in the solution for...
  26. L

    Why are Maxwell's Equations a complete description of EM?

    It is a well-known fact that Maxwell's Equations, along with Lorentz's Force Law, form a complete description of classical electromagnetism. But why is that? I mean, I can understand that Lorentz's Law is necessary for describing the interaction between matter and electromagnetic fields, and I...
  27. Q

    Frequency of Radiating EM Waves from a Moving Electron on a Unit Circle

    Let's assume that electron is moving around the unit circle. Using Maxwell equations show what is the frequency of radiating EM waves. How to start with it? What's the form of known variables - current density and charge density?
  28. C

    How bright are radio and other EM signals?

    How "bright" are radio and other EM signals? Greetings, I am wondering how "bright" man made EM signals such as radio are. That is, if they were in the visible range, how would they compare to visible light sources? Would they be barely visible? Would they be daytime sky bright in every...
  29. pellman

    Eqs of motion for 2-body problem in EM?

    What are the equations of motion for two charges where the two charges are the only sources for the EM field? (No background field) What I'm looking for is given two particles of mass m_1 and m_2 with respective position vectors x_1 and x_2, what are f_1 and f_2 such that...
  30. C

    EM wave polarized in the time direction?

    Is it possible to polarize an EM wave in the time direction as you would in any of the spatial dimentions?
  31. B

    Interaction of EM waves with media at boundaries

    Please see the diagram attached, I don't understand how my lecturer is resolving the E-field in the perpendicular case. He obtains: E_i = \bold{y} e^{-i\omega(t - \frac{n_1}{c} (-xsin \theta_i - z cos \theta_i))} How has he obtained the (-xsin \theta_i - z cos \theta_i) part from that...
  32. M

    Vacuum Polarization as EM Wave.

    Vacuum polarization is when an EM field causes the virtual particle pairs around it to become polarized like a dipole. The most common example is with an electron in vacuum, but a transmitting radio antenna could do it as well. But, if this was with an oscillating signal, it would create waves...
  33. K

    Is the range of EM forc affected by Black Hole Gravity

    The electromagnetic field is mediated by virtual photons according to QED. These virtual photons, by virtue of being massive, should respond to Gravity. Therefore, in the presence of strong gravity such as that of a black hole range of EM force should be affected because very few virtual photons...
  34. M

    EM waves in matter (electrodynamics)

    I'm having some trouble understanding this module. It would be great if anyone could help. In a homogeneous nonconduction region where μr = 1, find εr and ω if E=30(pi)e[i(ωt-4/3y)] in z direction H=0.1e[i(ωt-4/3y)] in x direction I am to understand that for a homognous nonconduction region...
  35. J

    Conservation of energy in EM with point charges

    This is probably a product of how I was taught, but I am unsure of the status of conservation of energy in classical EM with point particles. Here is the background: When I was taught in undergrad, general arguments were used to show that there needs to be a back-reaction on a point particle...
  36. P

    Power reflectoin coefficient of EM Radiation on a good conductor

    Homework Statement http://i258.photobucket.com/albums/hh258/Plutoniummatt/Untitled-1-2.jpg Homework Equations Pwr Ref Coeff: (Z'-Z)/(Z'+Z) all squared I maybe made a mistake but i can't spot it. And its bugging me I did square the whole thing afterward for the power...but...
  37. C

    Shouldn't a DC current emit EM waves?

    So in a DC current we have a relative uniform movement of electrons in one direction. If we would have a very narrow coil around the dc current carrying wire, (like an atom wide), every time an electron pass trough the plan of the coil, should induce a voltage in the coil, so theoretically we...
  38. H

    Solving EM Problem with Variation of Parameters

    I am trying to solve the following equation using the variation of parameters method d2x/dt2-(q2Bz2/m2)x=qEx/m I have put x1=cos(t) and x2=sin(t) into the Wronskian method. Can someone tell me if these are the correct functions to use, or should I be using exponential functions. Any...
  39. R

    Energy density of grav. field - analagous to EM

    The electromagnetic energy density is well-defined through the square of the Maxwell Field tensor. Why cannot such a quantity be defined for the grav. field?
  40. A

    Polarization of EM Waves: Understanding Its Role in Electrodynamics

    i read this "in electrodynamics, polarization characterizes em waves, such as light, by specifying the direction of the wave's electric field" in a book. i really didn't get it clear & does the magnetic filed nothing to do with polarization?
  41. J

    Solving Classical EM Gauges: Lagrangian, Lorenz, Coulomb

    Everytime I try to work out the Lagrangian for EM in different gauges, it gets messy really quick. Maybe there is some trick to simplify the process that I do not know, but either way I'd appreciate some suggestions. Starting point: For a point particle a (non-relativistic) Lagrangian...
  42. M

    Lattice Vibrations and em waves

    Can EM waves falling on a solid contribute towards lattice vibrations? If yes, then i)when is the energy used in lattice vibrations, ii)when is it used in excitation of electron into higher energy level, and finally, iii)when is the energy utilized for slight vibration of the electron...
  43. G

    Light as an EM Wave: Effects on Charges

    Since light is a type of EM wave, wouldn't that mean that when you flash a beam of light on a system of charges, you're exerting electric and magnetic fields onto the system?
  44. fluidistic

    Lorentz force (EM), what happens if

    I'm wondering what happens (or if it makes sense) if in the formula \mathbf{F} = q (\mathbf{E} + \mathbf{v} \times \mathbf{B}) we replace v by Lorentz factor, that is \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}. I realize that in the new formula I'm cross-producting a scalar with a vector, but I could...
  45. kelvin490

    Will EM wave be emitted from simple circuit?

    Even in simple circuit, there must be some "turning point", for example the electric wire is bent somewhere. Therefore when electron pass through there is centripetal acceleration. I have heard that accelearation of electrons emits EM wave, so will EM wave be emitted from these positions?
  46. A

    Studying Study EM Theory: No Classical Mechanics Required

    Studying EM Theory?? Hi All, Is it possible to study Electromagnetic theory without knowing the concepts of classical mechanics. I have studied Newtonian mechanics but does not have any idea about Lagrange and Hamiltonian mechanics. I am concentrating on the below topics in...
  47. D

    EM Calculus Struggles? Get Help Here!

    I'm having a lot of trouble with basic EM (calculus based) because for some reason, the material simply isn't clicking with me. We're using Ohanian/Markert's Physics for Scientists and Engineers and I'm finding it to be very lacking. I'm wondering if anyone has any better alternative suggestions...
  48. A

    Understanding Polarisation in Electromagnetic Waves

    Homework Statement "Polarisation" is defined as the cofinement of the vibrations of the wave in only one plane and the removal of the vibrations in the other perpendicualar plane of the electromagnetic wave. But the e.m. wave is defined as a wave in which the electic vetors are restricted...
  49. P

    Visualizing EM Waves: 3D Model & Resonant Cavity Simulation in Java Applet

    I am trying to code a java applet to give a physical representation of an EM wave, and how it will react in a resonant cavity. However, I am having a very hard time finding a good physical representation of what an EM wave actually looks like. For example, If I take a 2 meter wave, place it...
  50. E

    Does string theory predict additional long range forces gravity EM

    Does string theory predict additional long range forces gravity EM? One of the most touted features of String theory is that it correct predicts and even requires 2 infinite long range forces, gravity and E&M. I've heard that string theory though also predicts additional non-observed long...
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