Em Definition and 1000 Threads

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

    EM Field vs EM Wave: Understanding Distance & Strength Differences

    Why does the strength of an electrical field diminish with distance from its source whereas the amplitude and frequency of a beam of light do not vary, regardless of distance?
  2. S

    Understanding EM wave propagation

    I am generally an "artsy" person, though I am interested in science, too. I am curious about EM waves, particularly how they get started and what determines their amplitude and direction. I've read that an oscillating charge can generate a magnetic field, though I'm a bit confused about what...
  3. jaketodd

    Particle disturbance by passing EM wave reveals momentum of EM wave?

    Hey guys and gals, Here are two of a vast sea of sources that say a passing EM wave will disturb charged particles: http://books.google.com/books?id=jC4...age&q=&f=false "...a wave passing over a row of electrons; the arrows indicate the magnitude and direction of the electromagnetic...
  4. L

    EM field between 2 superimpose spheres

    Homework Statement Problem: 2 spheres each one of them with a radius R and uniformly charged with ro+ and ro-, are situated in a way they superimpose partially ( see figure). Let be "d" the vector from the positive center to the negative center...
  5. E

    Do only electrons emit a photon when accelerated in EM field?

    Do only electrons emit a photon when accelerated in EM field? Do protons, for example, radiate?
  6. M

    How can the spectral representation of a plane EM wave be found?

    Hi, Let E(r,t) = E(r)exp(-ikz)exp(iwt) be a plane wave in time domain, propagating along Z direction. I wonder how to find the spectral representation of it (i.e. E(r,w))?? I know, for a finite intensity field (i.e. |E(r,t)|^2 < infinity), we can give...
  7. B

    Question about example 3.9 in griffiths EM

    This is not actually a homework question, just something I am wondering about. A specified charge density dependent on /theta is glued over the surface of a spherical shell and you are asked to find the potential inside and outside the sphere, which is done in the example using separation of...
  8. B

    Does zero arc length mediation of EM cause the wavefunction

    The path taken by a ray of light, from an event E1 to event E2, follows a zero arc length curve such that E2 ∫ds = 0 1. E1 Where S is the interval along the null geodesic path between the...
  9. D

    Brief explanation what an EM mode is?

    Can anyone provide a brief explanation what an EM mode is? I've read some EM theory, but still have problems understanding what a mode really is! I know for some EM waves, the medium supports only certain modes, but is the 'mode' a property of a medium? Or is it something that should be...
  10. D

    How do EM waves transfer energy?

    I have some basic doubts regarding how the energy is transferred (e.g. transmission of radio waves) as EM waves. As I know, the wave is not anything that can be practically seen in the space, but it is a model that we use to describe how energy is transferred. But if we think practically, how...
  11. O

    What is the Impedance and Power Dissipation in an AC Circuit with a Generator?

    AC Circuits II: AC Power Generator Homework Statement A An AC generator supplies an rms voltage of 110 V at 60.0 Hz. It is connected in series with a 0.300 H inductor, a 5.80 μF capacitor and a 236 Ω resistor. What is the impedance of the circuit? B What is the rms current through the...
  12. Mentallic

    Why Does EM Radiation Penetrate Materials Differently?

    As grotesque as the title may appear to suggest, I basically want to understand just that. Why is it that EM radiation of each group of wavelengths (visible, IR, UV etc.) manages to penetrate materials with varying ease. From what I've seen so far - and I haven't studied this directly, I'm just...
  13. A

    Calculating Inductance Gradient for EM Launcher

    The equation widely used to calculate the force on a projectile in an electromagnetic launcher, more specifically a railgun, is: F = 0.5 * L' * I^2 where: --> F is the force in Newtons --> L-prime is the inductance gradient of the rails in henries/meter (H/m) --> and I is the current...
  14. E

    What is the unit of Magnetic charge?

    Can anyone please tell me that if for Electric charge we have its unit Coulomb what would be unit of Magnetic charge ? (if they exist)
  15. S

    Does light exhibit dispersion in a vacuum?

    Does light exhibit any type of dispersion in a vacume?
  16. X

    What textbook covers advanced topics in EM for photonics research?

    "Life" after Griffiths EM Hi, I need a textbook on electrodynamics. I have already finished Griffiths electrodynamics, a wonderful book but it does not cover the things I need now from an EM book (i.e. research on photonics). Therefore, I need a book which covers the mathematical methods...
  17. I

    EM Shielding Dilemma: Understanding Transverse Wavelengths

    In the classical picture of an electromagnetic wave, the wavelength is specified along the direction of travel. However, with EM shielding that is using a grid (microwave, chicken wire), I've heard that so long as the spaces are less than the wavelength, you will achieve electromagnetic...
  18. J

    Uncovering Heinrich Hertz: The First to Generate EM Waves in a Lab

    My notes say that Heinrich Hertz was the first to generate and detect EM waves in the lab. ... Uhhh, this might seem really basic to you guys, but what exactly are EM waves? Because I thought they were any type of wave that appears on the electromagnetic spectrum, including visible light...
  19. J

    How Does Temperature Affect Electromagnetic Waves?

    What exactly is tempertaure? As far as I know it represents the average vibration of atoms. How it relates to EM wave? because I have read in many Physics texts saying that 'the temperature of this wave is X Kelvin'. How do they find the temperature of individual EM wave? Correct me if I...
  20. S

    Does spacetime curvature of EM field affect zero-charge particles?

    I've read in a few places that the energy and momentum of the electromagnetic field will itself induce a curvature of spacetime, much like the presence of matter. I'm not very familiar with general relativity, but does this imply that particles with mass and zero charge will still be affected...
  21. maverick280857

    Variation in action for modified EM Field Action

    Hi everyone I am teaching myself QFT, and am currently learning Lagrangian Field Theory. Here is a question I am trying to solve, and I am not absolutely sure if my solution is correct because I am new to this notation and material. I would be grateful if someone could go over it and let me...
  22. D

    Proof of EM Wave Orthogonality: Ideas Needed

    So I am trying to work through the proof why why the direction of proporgation, the E field and B field are all orthogonal to one another. What i have is... E=E_{0}e^{i(k\ \bullet \ r-\omega t)} B=B_{0}e^{i(k\ \bullet \ r-\omega t)} \nabla \times E= -\frac{dB}{dt} \Rightarrow k...
  23. M

    EM Wave Reflection at Dielectric Boundaries: Exploring Normal Incidence

    I have a simple question about reflecting EM waves at dielectric boundaries. To best illustrate my question, consider normal incidence. The incident wave has the wavevector k positive, and the reflected has k negative. Since B = k x E , and k has changed sign, B must also change sign. This is my...
  24. I

    Frequency of EM Waves in an LC Circuit

    Will the frequency of EM waves produced by oscillating current in an LC circuit be the same as the frequency of the current? Thanks!
  25. T

    Understanding Reflection and Ion Movement in EM Waves

    1. Since skin depth is the attenuation of the wave in a conductor, this means the reflection isen't the incident wave... so what is the reflection? 2. Water doesn't stop light since it's frequency is too high, i saw on forum it's because the big ions can't move fast enought. How can that be...
  26. V

    Help with EM Fields and Dirac Delta Needed

    Hi guys. I play now a bit with EM fields and I have encountered some problems connected with Dirac delta. By coincidence I visited this forum and I thought I could find some help in here. The problem is that in order to get a potential in some point from a single charge you need to just...
  27. M

    Understanding Energy Transfer and Heating in Materials Exposed to EM Radiation

    Hello everyone! Let me first say how cool it is that this forum exists, I wish I'd thought of coming here years ago. I am trying to understand how EM radiation works. From the high school physics that I've learned I have been under the impression that reflection was due to electrical...
  28. W

    Reflection and Transmission of EM waves

    Hi all, I was reading "Geometrical Optics Reflected Fields" chapter from "Introduction to Uniform Geometrical Theory of Diffraction" textbook. The author assumed that the surface is perfectly conducting and therefore he only considered the reflected waves. He derived a formula for the...
  29. R

    "Intensity of Incoherent EM Waves

    Homework Statement Two incoherent EM waves of intensities 7I0 and 13I0 travel in the same direction in the same region of space. What is the intensity of EM radiation in this region? Homework Equations The Attempt at a Solution I have tried using I = I1 + I2 + ( 2* square root of...
  30. V

    Intensity of EM radiation for two incoherent waves?

    Homework Statement Two incoherent EM waves of intensities 10I0 and 14I0 travel in the same direction in the same region of space. What is the intensity of EM radiation in this region? Homework Equations I = I1+I2 + 2√(I1I2) or I = I1+I2 - 2√(I1I2) The Attempt at a Solution I...
  31. S

    Produce an EM wave through interference

    I am trying to understand why it is not possible to produce an EM wave through interference that is completely different from the source waves. For example why can't 2 or more EM waves outside the visible spectrum add up to produce light? I haven't had any physics since high school and I don't...
  32. K

    Transimission Of EM waves in conductors at an angle of incidence

    We know that in electrodynamic when an incident Electromagnetic wave strikes a charged barrier it can either be reflected or transmits. And both of the incident, reflected and transmitted waves are in the same plane called plane of incidence. Now, i met a problem in which they state that the...
  33. X

    Energy per unit area of EM wave

    Homework Statement The E field in an EM wave has a peak of 22.8 mV/m. What is the average rate at which this wave carries energy across unit area per unit time? Homework Equations S = e_0*c*E^2 The Attempt at a Solution where e_0 = 8.85*10^-12 c = 3*10^8 E = 0.0228 V/m S =...
  34. M

    Understanding the Time Offset in EM Plane Waves

    This isn't a homework question so much as a general question about the equation I see used in the textbook: The E field of an EM wave is defined as E(x, y, z, t) = E0sin(kx - wt)y_hat and the B field of an EM wave is defined as B(x, y, z, t) = B0sin(kx - wt)z_hat. Why is there a minus sign...
  35. S

    How does polarisation of EM wave work?

    Does it removes either the electric wave of magnetic wave component of the EM wave? And if so, won't the wave exiting the polariser not a EM wave anymore? More like an E wave or M wave. If the above argument is correct, won't the speed of light become\frac{1}{\sqrt{\epsilon_{0}}} or...
  36. P

    EM Field strength and plane waves

    Hi ! I've a question. Where is the connection between the (kinetic) Lagrangian - \dfrac{1}{4} F_{\mu \nu} F^{\mu \nu} and a plane wave of the form \vec{\varepsilon} exp(i \vec{k} \cdot \vec{x}) / \sqrt{V} (the epsilon is a polarization vector) confined in a box with a finite volume V ? I...
  37. B

    How do you reconcile EM fields with frequency of light?

    I know that a charged particle emits photons which are the "carriers" of the electric and magnetic fields, and that these photons, interacting with another charged particle, cause an attractive force or a repulsive force. I also know that examining the amount of energy in the photons - which...
  38. T

    P and T transformations of EM vector potential

    Help! What are the P and T transformation laws for the electromagnetic vector potential, A_\mu? and how are these consistent with the transformation laws of the electric and magnetic vectors that I am familiar with? under P: E is odd, B is even under T: E is even, B is odd When I try to...
  39. H

    EM Radiation from Power Transformers

    Hello, The city added a new electric transformation station near my house and has recently closed down a kindergarten adjacent to it due to 'radiation safety' reasons. My house is two doors down and my mother is concerned that radiation from the station might be hazardous to her health. She...
  40. snoopies622

    Calculating the EM Field around a Moving Charge

    I want to know how to find the electromagnetic field at a given point around a moving charge and this has led me to a few questions. My first one is, do both the \mu _0 \bf {J} term and the \mu_0 \epsilon _0 \frac {\partial}{\partial t} \bf E term contribute to the...
  41. R

    EM Laplace Equation Homework: Solving for Potential in Gap

    Homework Statement Hi, I've attached the question as I don't know how to write equations on here without them looking awful.Homework Equations Laplace -> del^2 V=0The Attempt at a Solution I've done the first bit (expression for Q0). For the next bit I tried to solve laplace's equation to find...
  42. A

    Can the Human Body Create EM Fields to Affect Electronics?

    To star this post off, I would like to say that I am in not an expert in any field of physics but I do have an interest. So, my question is... Can the human body produce an EM field that is strong enough to affect electronic devices? Examples. computers- I'm pretty competent with them...
  43. P

    Calculating EM Radiation from a Star 13 Million Light-Years Away

    Homework Statement A certain star is 13 million light-years from Earth. The intensity of the light that reaches Earth from the star is 6 10-21 W/m2. At what rate does the star radiate EM energy? Homework Equations i know you have to convert lightyears to m. (1.23e17m) multiply this...
  44. Q

    How much energy is transported by an EM wave with a given E field strength?

    Homework Statement How much energy is transported across a 1.15 cm^2 area per hour by an EM wave whose E field has an rms strength of 36.5 mV/m? Homework Equations E=hv? The big problem I'm having is that I don't know what formulae to use. The Attempt at a Solution I know what V/m...
  45. S

    Flavor conservation in EM interactions

    Hi Friends, I am an undergrad currently taking my first course in particle physics. In thinking about various possible reactions, I have run across what appears to be a rather basic question: Why are reactions of the form e^- + e^+ \rightarrow \gamma \rightarrow q + q^' where q and q' are...
  46. rjbeery

    EM Waves Vs Photons: Can Physical Explanations Explain All?

    Are there any EM wave behaviors that cannot be explained in a purely physical, "photonic" manner? I'm speaking specifically of the photon's path from emitter to absorber, irrespective of "why" that path was taken (in other words, the double-slit experiment does not qualify in the context of my...
  47. C

    Is the Uniqueness Theorem in EM Always Proven by Contradiction?

    isnt the field in any given volume always uniquely determined to prove by contradiction assume E1 and E2 div(E1)=charge density/epsilon 0 =div(E 2) by fundamental theorem of divergences integral of E1.da = integral of E2.da over entire surface thus E1=E2 then why does griffith in his...
  48. P

    Single Particle Dynamics in EM Fields.

    Hello, I am having trouble seeing how to derive the energy equation for a charged particle from the equation of motion. The equation of motion is m\frac{d\bar{v}}{dt}=q(\bar{E}+\bar{v} \times \bar{B}) Then in the notes I have it says "projecting the eq. of motion onto the particles...
  49. B

    Energy in an Electric Field of an EM Wave

    I have been desperately trying to find an equation (or a set of equations where I can derive an equation) that shows a relationship between the energy in an electric field and the wavelength or frequency of an electromagnetic wave. I am trying to show a relationship between Wavelength and...
  50. B

    Measuring the frequency of the e.m. oscillations in the em. wave

    Consider please that an observer is located in a plane sinusoidal and polarized e.m. wave. He erects a metalic antenna normal to the direction of propagation. A sinusoidal potential difference appears between the two ends of the antenna having the same frequency as the e.m. has. Is that a...
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