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. D

    Preparing for Modern Physics: Mechanics & EM Review

    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...
  2. S

    Understanding EM Wave Attenuation in Conductors and Dielectrics

    Hi, I have been reading my textbook and online but I'm not sure if I understand. An EM wave attenuates in a conductor but not in a dielectric? If that is correct what does the impedance of a dielectric represent, since I thought impedance would be associated with an energy loss...
  3. michael879

    EM Stress-Energy Tensor Derivation: Understanding the Symmetry and Conditions

    Can someone please walk me through (or provide a link that does) the derivation of the EM stress-energy tensor? I get all the concepts I'm just a little confused on some of the details. Basically, you have the definition of the stress energy tensor in terms of the lagrangian, and the condition...
  4. H

    Device for recording EM signals

    I'm looking for a device that can record EM signals of unknown frequencies (unknown to me) emitted by devices. For example if I point my remote control at it and start pressing buttons it should record intermittent IR frequency surges. Or for example if I build an emitter to modulate an...
  5. T

    Special relativity adandons Maxwell's mechanical interpretation of EM?

    page 61 in Introduction To Electrodynamics (3rd ed, Griffiths) writes: " What exactly is an electric field? ... I encourage you to think of the field as a "real" physical entity, filling the space in the neighborhood of any electric charge. Maxwell himself came to believe that electric and...
  6. V

    Maxwell's Derivation of EM Wave Momentum: E/c

    How did maxwell derive the momentum of electromagnetic wave to be E/c ?
  7. D

    Magnetization in Classical EM: Bound Electric vs. Magnetic Charges

    I have been trying to remember if in classical EM it is equivalent to describe magnetization through bound electric currents A. \vec{j_b} = \nabla \times \vec M \vec{k_b} = \vec M \times \vec{\hat{n}} OR bound magnetic charges B. \rho_b = -\nabla \cdot \vec M \sigma_b = \vec M \cdot...
  8. S

    How Do You Calculate the Number of Photons from Sunlight on Earth?

    Homework Statement Light from the sun arrives at the earth, an average of 1.5*1011 m away, at the rate of 1.4*103 W⁄m2 of area perpendicular to the direction of the light. Assume that sunlight is monochromatic with a frequency of 5.0*1014 Hz. (a) How many photons fall per second on...
  9. S

    EM: induction in a moving conductor

    Homework Statement An infinitely long wire contains a current I. A metal rod with length L moves with speed v. Determine the emf in the rod. http://img227.imageshack.us/img227/5149/52966616.jpg With \epsilon = -\frac{d\Phi_b}{dt}, I would have said emf = 0 at first. The current I generates...
  10. S

    EM: Gauss' law for electricity

    Homework Statement A solid sphere of radius R carries a volume charge density \rho = \rho_0e^{r/R}, where \rho_0 is a constant and r is the distance from the center. Find an expression for the electric field strength at the sphere's surface. Homework Equations \int\vec{E}.d\vec{A} =...
  11. A

    Gravitation & Electromagnetic Forces: Interdependence?

    Gravitation and Electromagnetic forces are independent, correct? That implies.. Gravitation would exist even if Electromagnetic forces didn't. But, electromagnetic forces are responsible for existence of atoms.. and atoms are what make up the mass, no? And Gravitation exists because of Mass...
  12. U

    Uncovering the Mystery of Neutrino Interactions with Electromagnetic Force

    While neutrinos have no electric charge they have magnetic moment and will interact with magnetic field causing precession of its spin.http://prl.aps.org/abstract/PRL/v45/i12/p963_1" So if neutrinos can interact with virtual photons what makes it impossible to interact with real photons...
  13. S

    EM radiation => Self propagating wave ?

    Both Wikipedia and my educators called electromagnetic radiation a self propagating wave. I would suggest that the word self be removed from wikipedia at least. There is no self in a wave. Even the word "coupled" or "causally related" are risky WRT the grav and mag components as I don't...
  14. C

    Testing Failed midsem EM exam, to drop or not to drop?

    I'm doing an electromagnetic fields and waves course. The first part of the course was mainly about transmission line's, using the Smith chart and a host of equations that comes with transmission lines...we had a mid semester exam and i got 13/30, which i wasn't happy about but i didn't put in...
  15. B

    EM field lines in non-isotropic media

    Electromagnetic field lines in non-isotropic media Hello, We are using time domain reflectometry (TDR) in soils to measure soil moisture. I want to know how the electromagnetic fields are oriented when the soil is not isotropic, i.e some parts of the soil consist of moist or wetter parts...
  16. A

    Are Static EM Fields in QED Composed of Virtual Photons?

    Hello, How is a static magnetic or electric field understood in terms of QED. Is it correct to interpret them as a collection of photons? If so, would these photons be real or virtual? Thanks in advance.
  17. H_man

    Power Loss Of An EM Wave Hitting A Cu Wire

    Hi all, :rolleyes: I am trying to calculate the loss in power of a wave which is partially reflected and partially transmitted from a wire. The skin depth of the wire is many times the thickness of the wire. My first attempt to solve this problem was to consider...
  18. K

    EM and Gravitational binding energy for quarks

    Hi guys, If I have a \Sigma triplet of baryons, how do I calculate the EM and gravitational binding energy of these baryons? (assuming there is 1fm of distance between quarks and that each quark has 1/3 the mass of the \Sigma), I guess I have to use the EM and Gravitational Potential...
  19. K

    EM Waves Penetration: Frequency Effects & Atmospheric Limitations

    When an EM wave has a higher frequency, it is able to penetrate materials more easily. However, why does a microwave able to penetrate haze while visible light can't? Why visible light is able to enter our atmosphere while those of gamma can't?
  20. N

    Does an EM wave bend in a B-field?

    I read somewhere that Thomson (1897) concluded that the electron was not an EM wave because it bended in a magnetic field and that it had been proven that EM waves did not do this. Is this true?
  21. T

    Is There a Maximum Wavelength for Electromagnetic Radiation?

    Is there any lower limit on the frequency of electromagnetic radiation? I imagine that beyond a certain frequency it becomes impossible to detect, but is there anything in the maxwell equations that establishes a maximum wavelength (besides the limits of the size of the universe)? Thanks
  22. L

    Welcome! Ask Away: How Electrons Orbit Without EM Waves

    Hi Everyone! I'm new here and like this forum. Can anyone tell me how the electron moving roundly in atom doesn't emit EM waves while changing it's speed?
  23. B

    How to study EM wave interactions via internet?

    I would like to do some reading about how EM waves interact with each other via internet. Upon googling "EM wave interaction," I got some general links about EM radiation and some book titles but I didn't see any websites. Does anyone know one or more good online sources that give an overview...
  24. X

    Frequency & Wavelenght of EM wave

    Frequency & Wavelenght of EM wave! Hi..on some other thread i found this link showing animation of propagation EM wave: http://www.molphys.leidenuniv.nl/monos/smo/index.html?basics/light_anim.htm it has cleared my one doubt, because i used to think that EM wave is like an invisible line...
  25. E

    Spin Coil in EM System for DC Current?

    You have a coil of wire inside a solid cylinder of iron or some other magnetic material. There is a DC current going through the coil of wire so you have an electromagnet. If you were to spin the outer shell, would the coil also spin in the same direction. My first thought when approaching...
  26. H

    Narrow EM wave beam - how it propagates

    Consider a narrow beam of EM waves that propagates in the Z direction and is concentrated near the YZ plane, so its intensity fades rapidly as we move away from the YZ plane in the X direction. Let the E field be in the X direction. Consider a cube with edges parallel to the X, Y and Z axes...
  27. M

    Finding the average energy density of an EM wave in a magnetic field

    Hi, I've not posted on here before but I'm trying to keep on top of work over the summer and I'm having some real problems with this question Homework Statement Consider the plane polarised EM wave in a source free vacuum with magnetic field B = (1,1,0)B0cos(kz-wt) where B0 = 0.001T. Find...
  28. G

    Exploring EM Diffusion and Scattering by a Rod: Analytical Possibilities

    [EM] Scattering by a rod Hello everyone :) I'm interrested by the diffusion of an electromagnetic field by a rod with rectangular section i.e. a parallelepiped with one infinite dimension. Does anyone know if that's already been done ? If it is even possible analytically ? Thanks in...
  29. M

    Comp Sci Fortran program to calculate texas hold em odds

    The instructions are to calculate the odds of winning if you are dealt King and 9 of clubs, what are the odds of winning? I am finding myself spending more time trying to figure out texas hold em (never played before) than writing the program. So far I found a list on wiki that breaks down the...
  30. D

    Selection Rules for EM Dipole Radiation

    hey, I was asking myself a few questions about the selection rules for EM dipole radiation which occurs if electrons "jump" into lower bound states according to the selection rules. now I know that the full explanation about matrix elements of the dipole operator comes from fermi's golden...
  31. V

    Analysis of current flow and EM fields in seawater?

    Hi, I have a few queries regarding the flow of electric current through (sea)water... In general, can the flow of electricity through seawater be treated the same as through a metal conductor? I am wondering whether the fact that it is ions rather than electrons that carry the charge has...
  32. C

    The medium that light travels through is the EM field

    I remember my teacher saying that the medium that light travels through is the EM field , is this correct , if so then what is the medium that the electron , or neutrino or gluon travels through . Or what is the medium that a gravitational wave travels through .
  33. A

    Polarization of EM wave - does the E vector trace an ellipse w.r.t space as well ?

    Let us consider the Electric field components of a polarized EM wave . [PLAIN]http://www.cdeep.iitb.ac.in/nptel/Electrical%20&%20Comm%20Engg/Transmission%20Lines%20and%20EM%20Waves/graphics/CHAP%204__255.png. Now if we fix the value of z (for convenience take z=0) and consider the locus of...
  34. N

    Can maxwell's EM theory explain the photoelectric effect?

    i know that einstein explained it in terms of particles but can maxwells EM waves explain the photoelectric effect aswell?
  35. pellman

    EM vector potential - covariant or contravariant?

    Are potentials appearing in the Maxwell equations the components of a contravariant vector or a covariant vector? Let us be specific. metric is (+,-,-,-) . Let us write the potentials which appear in the Maxwell equations as \Phi and \vec{A}=(A_x,A_y,A_z) Is it then the case that...
  36. N

    Confusion on Source of EM Radiation? (segment Serway)

    Goodday. In my introductory course of Physics, we use Serway. I've typed over a little piece from the book that I find quite vague. It is giving an example of (after deducing the plane wave EM equations) EM radiation. It uses the following antenna: The positive and negative sides are...
  37. R

    Can sound wave also interference just like EM wave

    its my exam question.. i am not so sure... but i put it can't because sound is a wave of vibration which it can diffract but not interence... but i don't know.. can sound wave interference? cheers
  38. maverick280857

    Stationary Position of a 3D Harmonic Oscillator in a constant EM field

    Hi, I have to find the 'stationary position' of a particle of mass m and charge q which moves in an isotropic 3D harmonic oscillator with natural frequency \omega_{0}, in a region containing a uniform electric field \boldsymbol{E} = E_{0}\hat{x} and a uniform magnetic field \boldsymbol{B} =...
  39. I

    Hertz and polarization of EM waves

    Hi, I'm having difficulty understanding exactly how the reciever loop detects the EM waves in this experiment and I can't find any definitive information online. My understanding is that since EM waves are transverse, to be absorbed by the receiving electrodes, the length of the molecule chains...
  40. fluidistic

    Non-Polarized EM Wave: Example & Explanation

    Can someone give me an example of a non polarized EM wave? I've heard that a light bulb would produces EM waves not polarized because the E fields of each waves aren't in the same direction. This I can understand. But in the case of a single EM wave, how do one gets a non polarized wave? I...
  41. J

    Understanding Resonance in Electromagnetism (LRC Circuits)

    Hi everyone, I've dealt with resonance (normal modes of a system) in a fair amount of detail, but it was in the context of a mechanics course. Now, I'm trying to extent this concept to electromagnetism, particularly LRC circuits, and I'm having a really hard time wrapping my head around it. I...
  42. snoopies622

    Can Maxwell's equations alone explain the momentum of EM radiation?

    I've read that even before the 20th century physicists realized that light carries momentum, and that - although experiment is the ultimate arbiter in science - one can arrive at this conclusion by studying Maxwell's equations alone. If this is the case, could someone give me an outline of the...
  43. fluidistic

    Sum of 2 EM Waves w/ Same Phase & Amp but Diff Freq

    Homework Statement Describe the sum of two EM waves that have the same initial phase and same amplitude but different frequencies such that \omega _1 >> \omega _2.Homework Equations E=E_0 \cos (kx -\omega t + \alpha).The Attempt at a Solution I summed them up and reached, after an...
  44. fluidistic

    Prove: EM Waves Addition Superposition Equivalence

    Homework Statement Show that the superposition of three waves with frequencies \omega _c, \omega _c + \omega _m and \omega _c - \omega _ m and same amplitude are equivalent to another wave of frequency \omega _c which is modulated by a sinusoidal wave with frequency \omega _m, i.e. E=E_0 \left...
  45. K

    Time reversal symmetry breaking in EM

    I have come across a problem I am trying to understand, and hoping someone here has some insight. Basically, when writing down different solutions for an EM field from given sources, there seems to be a problem from the standpoint of time symmetry. From my understanding, if you reverse time, the...
  46. fluidistic

    Can the amplitude of an EM wave affect its physical size?

    In the case of plane waves, E is orthogonal to B and they're both orthogonal to the direction of propagation, call it k. I'm not sure I'm picturing well what such an EM wave is. For instance I know that E and B oscillates with respect to time. Without looking to quantum electrodynamics that...
  47. S

    Max Distance to Detect EM Wave: Find r w/ 0.0050 V Induced EMF

    Homework Statement A radio transmission tower radiates electromagnetic waves uniformly in all directions with an average total power Pav = 70, 000 W at a frequency f = 98 MHz. A radio receiver uses the induced emf in a single circular wire loop of radius r = 5.0 cm to detect a radio...
  48. K

    A concept issue about EM field energy

    About whether energy is stored in the charge or the field, in Griffiths EM textbook, i found this: "...(for electrostatics) it is unnecessary to worry about where the energy is located. In the context of radiation theory it is useful(and in General Relativity it is essential) to regard the...
  49. C

    Neutrino-AntiNeutrino Collisions: EM Field?

    When a neutrino and an anti-neutrino collide they produce a photon , from what field does this photon come from. Photons are excitations of an EM field correct. or what about neutron anti-neutron collisions .
  50. D

    What Lies Beyond the EM Spectrum?

    Quick question from a complete science ignoramus. The electromagnetic spectrum is described as a continuum, correct? So, given the scales of frequencies that our science is familiar with, The high end would be gamma rays, with frequencies of 300 EHz, the low end being extremely low frequency...
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