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. S

    Physics Book on EM Waves, Radiation

    Hey, I'm new here but I was hoping you guys could help me out. I am currently studying Waves mainly using MIT's course 8.03 - Vibrations & Waves. I am now at a point where I have almost finished reading AP French and entering EM Waves. Unfortunately French decided (for whatever reason) to not...
  2. S

    Biological effects of HF EM fields/radiation

    Hey all, Random question, but is there anyone in the microwave/radio frequency industry that also wonders the biological effects of constantly being exposed to it? From what I have been told, the effects are not scientifically sound, and as such, the majority of people I have met treat that...
  3. T

    Why do mechanical and EM waves in the same way?

    Mechanical waves (sound, water waves) and EM waves both undergo diffraction. But the actual physical processes involved in the two are totally different. EM waves are produced by accelerated charges while mechanical waves are tiny displacements of molecules of the medium in which the wave is...
  4. H

    Relation between photons and EM wave frequency

    Do individual photons have some attributes which relate to EM wave frequency? In other words, is there any difference in photons composing a red and blue beam of light?
  5. O

    EM Wave Reflection: Why Parabolic Reflectors Differ from Mirrors

    Why do parabolic reflectors for microwaves (e.g. satellite TV) not reflect light like a mirror? Why do things like spoons, which are presumably conductive, reflect light? Thank you
  6. R

    Can a sinusoidal EM wave exist in nature?

    I saw an example of a hypothetical EM wave that had constant E and B fields. Is that possible? How would it be produced? And wouldn't such a wave have an infinite wavelength?
  7. S

    Explaining Electric and Magnetic Fields in EM Waves

    Hi, I've read that the speed of em waves in the vacuum is the electric field component frac the magnetic field component, why and how can this be explained more easily ? Thanks
  8. T

    Q&A: EM Orbits & Tides: Attraction & Repulsion?

    1. Can a magnet orbit another magnet (of opposite charge) despite the fact that their masses are insufficient to generate the orbit; i.e., their orbits are caused only by the EM force? 2. Would/could the EM attraction between them generate tidal effects on the two magnets? 3. If you had two...
  9. T

    Exploring the Limits of Modeling EM Forces as Spatial Curvature

    Why can't EM attraction/repulsion be modeled as spatial curvature the way gravity can be? And for that matter, why can't the strong and weak nuclear forces be modeled that way either? Or can they?
  10. B

    Physical Interpretation of EM Field Lagrangian

    Using differential forms and their picture interpretations, I wonder if it's possible to give a nice geometric & physical motivation for the form of the Electromagnetic Lagrangian density? The Lagrangian for the electromagnetic field without current sources in terms of differential forms is F...
  11. K

    Why EM and weak interaction are unified

    In SM, people say the ##SU(2)\otimes U(1)## theory unify the EM and weak interaction. I understand that the two interactions both originate from the electroweak theory in a mixed way. But does this mean they are unified? Since when we talk about unification like GUT, we require the couplings run...
  12. U

    EM Momentum,Hidden Momentum,Centre of Energy Theorem and Lorentz Force

    I recently read Griffiths paper on Hidden momentum, and still didn't found it complete. Following is the short summary. The usual setup of current carrying loop and a charge lying nearby, according to the paper, Shockley and James presented the problem, that is, when we let the current die...
  13. binbagsss

    EM , magnetic field at centre of a carrying circular loop

    The question is to find the magnetic field at the centre of a current carrying circular loop of radius R, where the current = I Okay so I'm trying to do this by both Amp's Law and Biot Savarts Law, and I can't get my answers to agree. First method - Biot Savarts Law...
  14. F

    How Fast Do EM Waves Travel in Ferromagnetic Materials?

    Homework Statement An EM wave with frequency 87Mhz travels in an insulating ferromagnetic material with \mu_0 \mu_r = 1000 and \epsilon_0 \epsilon_r = 10 - What is the speed of the EM wave in the material. Homework Equations v=(\sqrt{\mu_0 \mu_r \epsilon_0 \epsilon_r})^{-1} The Attempt at a...
  15. B

    Can Charge and EM Energy Change Metal Structure

    Hello Everyone, This is my first post. Background: I am an audio guy and seek to understand how cables contribute to the sound of my stereo. Issue: Do audio cables such as power cords, interconnects, and speaker cables "break in" as they are used? Typically, with high-end cables, users...
  16. F

    Does non-sinusoidal electric/magnetic fields generate EM waves

    why all EM waves are sinusoidal ? or this is not true at all, i.e., EM waves of other waveforms do exist ? the above 2 questions are among the most vague in classical EM theory, of which i studied many topics in collage as an electrical engineering student, and i liked the subject so i kept...
  17. mesa

    What function was used for current record of decimal places of EM cnst

    The record for the number of decimal places of accuracy for the Euler-Mascheroni constant stands at just over 29,000,000,000 decimal places set by Alexander J. Yee & Raymond Chan back in 2009. Does anyone know what function they used to set this record? ***EDIT*** On further searching the...
  18. F

    Does the Drude model predict the emission of em waves?

    The Drude model of electrical conduction has the charge carriers being accelerated by an electric field and then interacting with the lattice ions. The cycle is then repeated. An unbound charged particle if accelerated emits electromagnetic waves. Does that mean that a prediction of the Drude...
  19. E

    Kalman Filter EM decrease log likelihood

    Hi all I am using the Kalman Filter with an EM algorithm, (schumway and stoffer). From my understanding the log likelihood should monotonically increase. In some instances however I obtain a decrease in the log likelihood, What can I infer from this? Has anyone experience of these...
  20. binbagsss

    Plane EM wave in a vacuum, quick identity question

    Okay the question is, given a plane electromagnetic wave in a vacuum given by E=(Ex,Ey,Ez)exp^{(i(k_{x}x+k_{y}y+k_{z}z-wt)} and B=(Bx,By,Bz)exp^{(i(k_{x}x+k_{y}y+k_{z}z-wt)} , where k = (kx,ky,kz), to show that kXE=wB. So I'm mainly fine with the method. I can see the maxwell's equaion...
  21. V

    EM Waves & Forces: Does a Particle Feel Force?

    Would a particle kept in the path of an electromagnetic wave experience a force? Both magnetic and electric field are present, so the equation of motion may be complex, but i have never seen my textbook refer to the the possibility of an em wave causing a charged particle to move.
  22. V

    Can Electromagnetic Fields Be Directionalized Beyond the Halbach Array?

    Hi all- I am working on a hobby project to manipulate top load streamers by applying EM fields to the top load. I cringe to reference it, but think of Red Alert 2 on a much, *MUCH* smaller scale. The streamers are roughly 295KHz at 300kV (9kVA NST supply, 8.5turns primary coil & 1042turn...
  23. L

    Can High Frequency Signals Flow Without a Medium?

    EM waves do not need a medium for propagation. But isn't high frequency signal (like USB) flowing in a wire also electromagnetic. How come it needs copper as a medium.
  24. D

    EM wave described using a sine function.

    I'm going through my notes and I don't understand how they have included position in an equation to describe an EM wave. The equation is of the form http://upload.wikimedia.org/math/f/6/3/f6386c1751b91ec23c7123b15a11b52f.png [Asin(kx-ωt)]. This equation is just stated in my notes and there is no...
  25. Crazymechanic

    Can EM Waves Levitate Objects in a Transparent Plasma?

    Hi, recently I saw a great video about some scientists levitating small objects in mid air by trapping them in overlapping 180 degrees out of phase sine waveforms. It is easy to see that air is the medium through which sound can travel and sounditself being the vibration of air traps these...
  26. U

    Solve EM Wave Vacuum Homework: Prove Relation, Find B, Reflect Wave Form

    Homework Statement EM Waves propagating through vacuum towards the x-direction approaches a conducting surface at x = 0. Part (a): Prove the following relations. Find the boundary conditions for E and B if charge density σ and current density s exists on the surface. Part (b): Find B in...
  27. U

    EM Waves in Transmission lines

    Homework Statement (a) Find the capacitance per unit length and find the inductance per unit length of a transmission wire above a infinite conducting plane. I used the image method (b) Show that EM waves can propagate, and find their speed and the characteristic impedance of the line...
  28. A

    Microwave oven EM wave amplitude

    I don't really know how to quantify EM waves when it comes to amplitude, but I understand that MRI rf waves are measured in units of tesla. What is the amplitude of a typical microwave oven EM wave in Tesla (mT, uT)??
  29. C

    Compactification effects in Kaluza-K theory - Gauge away EM?

    I was first considering to post this in the GR section on the forum, but as I've understood it, compactification is essential in string theory, so I thought that perhaps you guys know the subject better also in Kaluza-Klein theory. Compactification in Kaluza-Klein theory as I understand it is...
  30. S

    From Poynting vector to EM wave

    After reading Griffiths' book, I find the explanation from poynting vector to sinusodial EM wave pretty blurred. EM wave has two requirements. 1. E / B = c 2. E and B are sinusodial How could we ensure the Poynyig vector ultimately will produce the form satisfying the two points mentioned? Or...
  31. K

    Em field 6 degrees of freedoms & 4-potential

    What does it mean that the electromagnetic field strength includes six degrees of freedom (three for the electric field strength, three for the magnetic field strength), whereas the four-potential includes only three degrees of freedom (two for the photon, one for the Higgs boson)? What is the...
  32. B

    Does using the em field generated by flowing elctricity cause V drop?

    I was wondering if you were to use the em field caused by electricity flowing through a wire or coil to push a magnetic or object would you effect the flow of power in the line. Like if you were to wrap a one phase of transmission lines around a solenoid type device to use the em field to push...
  33. A

    Is a surface plasmon an EM wave?

    From my understanding, surface plasmons (SPs) are oscillations of electrons caused by incident EM waves. A photon from the EM wave can couple with a plasmon to create a surface plasmon polariton (SPP). Some books/websites use the words SPP and SP interchangeably, so I am confused as to whether...
  34. N

    Prove perpendicularity of E field and direction of EM wave

    Homework Statement Show that the E and B field of a plane EM wave are both perpendicular to its direction of propagation in a non-dispersive and transparent medium. Homework Equations Maxwell's equatons.The Attempt at a Solution Well, I know how to easily show that E and B fields are...
  35. N

    Moire pattern and interference of plane EM waves

    Homework Statement Look at the Moire pattern in "Oppgave 5" (assignment 5) in the attached pdf file. The Moire pattern you see is supposed to represent the interference between two plane EM waves whose directional ##\vec{k}## vectors form an angle ##\theta## with each other. Anyway, the main...
  36. C

    Dispersion Relation for EM Waves: Skin Depths in Good and Poor Conductors

    Homework Statement The following dispersion relation is obtained by substituting a plane wave solution for ##\mathbf{B}## into $$\nabla^2 \mathbf{B} = \mu \epsilon \frac{\partial^2 \mathbf{B}}{\partial^2 t} + \mu \sigma \frac{\partial \mathbf{B}}{\partial t}:$$ $$\hat{k} = (\mu \epsilon w^2 +...
  37. D

    Given an EM Wave find different values

    Homework Statement The magnetic field intensity of an electromagnetic wave that propagates in vacuum is described with the following phasor notation: (H_{z} = H_{0}e^{+jkx}u_{z}) if the previous equation is hard to read maybe this will be easier: Hz=H0e^(+jkx)Uz. Assuming that the frequency of...
  38. N

    Can Maxwell's equations be quantized in the presence of a source particle?

    Hello everyone, I have been wondering about the quantization of Maxwell's equation in free space, but now suppose that we have a source particle, fermion for example, now the field equation is a mixture of both fermion field and photon field so my question is whether you can get out of this by...
  39. J

    The Effects of Water on EM Waveforms

    What effect does water have on electromagnetic waveforms? Does metal attract or reflect EM waveforms (specifically rf)?
  40. Z

    Derive EM Field in a 1D PC Hill Equation from Maxwell's Eq's.

    Homework Statement Derive from Maxwell's equations these Hill equations for 's' and 'p' mode waves; s\hspace{3mm} modes: E(r,t) = \Psi_{s}(z)e^{i(\beta x - \omega t)}y \\ \hspace{10mm}Hill\, Equation for\, \Psi_{s}(z)\\ \hspace{17mm} \dfrac{d^{2}\Psi_{s}(z)}{dz^{2}} +...
  41. K

    EM waves- self propagating in a vacuum?

    First of all, I understand that an EM wave is created by a charged particle that is moving. What I don't understand is how it propagates itself when traveling through a vacuum as there is no medium for it to interact with. Can this be explained by anything other than photon theory? I really...
  42. K

    Do Quarks Create EM Waves? Exploring the Connection

    Do quarks cause EM waves?
  43. N

    Electric field Phase change upon rarer to denser reflection of an EM

    Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave,when reflected from an optically denser medium and also why the phase remains same on reflection from a denser to rarer medium? please try to explain on an atomic/sub...
  44. Jameson

    MHB Log likelihood function and EM algorithm

    I'm going to start by asking about an example from class and then hopefully use that to work on the problem I need to solve. Here is an example: Let's say we have a multinomial distribution $x \sim M(n;.5+.25\theta,0.25(1-\theta),0.25(1-\theta),0.5\theta)$. The likelihood function of $\theta$...
  45. N

    Advanced EM Field Book Using Differential Forms

    Hey guys, I am wondering whether there is any book out there that approaches EM field using differential form and on the same or more advanced than Jackson, I have a solid knowledge of differential form and algebraic topology, thanks :D
  46. K

    Gravitational waves analogous to photons and EM radiation?

    Are gravitons and gravitational waves analogous to photons and EM radiation?
  47. H_man

    In acoustics is there something analogous to skin depth in EM?

    Hi all The reflection and transmission of a pressure wave at a boundary depends on the relative impedance of the two media. I'm pretty new to acoustics but the textbook I've been looking at doesn't seem to specify that the such calculations are only valid when the medium is at least a few...
  48. S

    Dielectric & Magnetic Nanoparticles with EM field

    Hi all, Homework Statement 1. What are the impacts of EM field on Aluminium Ferrite nanoparticles (carry both dielectric & magnetic features), compare to EM field applied on Aluminium oxide (pure dielectric)? 2. If solenoid is used to generate magnetic field; what is it use to generate...
  49. R

    Energy Of A Single Photon In Em Radiation?

    Energy Of A "Single" Photon In Em Radiation? Is the energy of all photons in em radiation same? That is, say light differs from radio waves only in the number of photons per second
  50. S

    EM waves: energy and power related to frequency

    Hello, I was reading articles about EMR and health and I realized I never got how "output power" is related to frequency. Let's talk about energy, the rule is: the higher the frequency (shorter wavelength) the higher is the energy transported by the wave. Now power, power is the energy...
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