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

    Explanation on how to arrive at equation for EM wave?

    It is known that the electric field component of the EM wave is given by E=E_{0}cos(kx-\omega t) How do I arrive at such a form? It is quite different from the standard sinusoidal equation of y=Acos(\omega t+\varphi) Any guidance? What does the kx-\omega t describe physically?
  2. D

    Reason behind speed of em waves being constant

    Do we have any understanding as to why em waves have same speed in all inertial frames?
  3. R

    Can microwave em radiation interferes with radiowave em radiation?

    Just a thought because of the anomalous Planck satellite cmb data, and the second largest continuous structure in our solar system that almost fried the pioneer probe... Jupiter's magnetosphere which is known emitting radiowaves.
  4. P

    Energy Momentum Tensor - EM - Derivative

    I have a short question about the derivative of a given EM-Tensor. ##\rho## = mass density ##U^\mu## = 4 velocity ##T^{\mu\nu} = \rho U^\mu U^\nu## Now I do ##\partial_\mu## Should I get a) ##\partial_\mu T^{\mu\nu} = (\partial_\mu \rho) (U^\mu U^\nu) + \rho (\partial_\mu U^\mu)...
  5. Astrum

    Coupled ODEs in Electromagnetism

    Homework Statement Solve the equations of motion ##\ddot{y}= \omega \dot{z}## and ##\ddot{z}= \omega (\frac{E}{B}-\dot{y})## Homework Equations The Attempt at a Solution Integrate the first equation to get ##\dot{y}=\omega z + c_1## and plug into equation 2: ##\ddot{z}=\omega...
  6. I

    What is an EM Wave? Exploring the Nature of Radiation

    i think I've asked this a number of times on different posts stemming from various discussions but i don't think this question was in the wrong thread. so I've decided to make a new thread for this. so what 'IS' an EM wave? i understand that the acceleration of charges could produce a wave...
  7. I

    What is the amplitude of an EM wave?

    i always hear about the frequency/wavelength, and the speed of the wave, but i never heard about the actual amplitude of the wave itself? how would one derive this? I'm not referring to intensity... intensity deals with the number of photons being captured by a detector... I'm referring to the...
  8. I

    Polarizing EM radiation, how does it work?

    for RF, how does this mechanism work? http://i.imgur.com/Crx4Amx.png how does the conductor do this? and why does the unpolarized RF not seep through the spaces between the conductive bars?
  9. T

    Variation of EM radiation(Sun) in different latitudes

    So we know that there is variation of EM radiation in different latitudes we receive from Sun. My question is, it same through all EM radiation like uv rays, visible spectrum , IR spectrum or specific to only to one type of radiation like only uv or IR rays? i.e. higher latitude, there is less...
  10. A

    Momentum per unit area per unit time in EM

    Homework Statement In fact it is problem 8.5 in Griffiths 3rd ed p357 Consider an infinite parallel plate capacitor with the lower plate carrying the charger density - \sigma , and the upper plate carrying the charge density + \sigma . (a) Determine all nine elements of the stress...
  11. Barnak

    Angular momentum loss from quadrupole EM radiation

    I'm looking for the general formula for Angular Momentum radiated away from quadrupolar electromagnetic radiation. I searched the usual books (Jackson, Landau-Lifchitz, ...) and just found the usual dipolar contributions. Using dimensional analysis and energy radiated away, I found this...
  12. N

    Power of EM Wave Absorbed by Circular Disk of Radius 2m

    Homework Statement A plane polarized electromagnetic wave propagates with Erms = 30 V/m. What is the power transmitted to a circular disk of radius r = 2m, if all of the light is absorbed by the disk and S is perpendicular to the disk? Homework Equations There is a passage that goes...
  13. G

    EM fields from an accelerated charge: something doesn't add up?

    I have seen on several books that the expression for the E field generated by an accelerated charge, at enough distance and in the non-relativistic aproximation, is something like that (taken from Jackson): where "β with the dot above" is the acceleration divided by c, n is a vector...
  14. C

    Help with EM waves travelling through a plasma

    I've been given a practice question for an exam and I can't quite get it started. The question is: Intelligent beings on a distant planet try and communicate with Earth by sending powerful radio waves swept in frequency from 10 to 50MHz per minute. The linearly polarized emissions must pass...
  15. TheFerruccio

    I need some intuition for various EM phenomena

    I am using a computational electromagnetics software that solves for the resultant electric and magnetic fields around materials represented as lattices of discrete dipoles. Some of the results are the extinction, absorption, and scattering efficiency factors of EM waves, represented as a graph...
  16. dextercioby

    Geometric Description of Free EM Field

    I'm not a geometer, so I beg for indulgence on the below: In a modern geometrical description of electromagnetism (either in flat or in curved space-time*), I see at least 3 (or 4) (fiber) bundles over the 4D space-time taken to be the base space: * 1 the cotangent bundle and the bundle of...
  17. Q

    Lorentz transformation matrix applied to EM field tensor

    In a recent course on special relativity the lecturer derives the Lorentz transformation matrix for the four vector of position and time. Then, apparently without proof, the same matrix is used to transform the EM field tensor to the tensor for the new inertial frame. I am unclear whether it...
  18. T

    What are some advanced resources for applied electromagnetics in engineering?

    As the title says, I've worked through all of Griffiths in my EM classes. It's a great book, but I plan to continue in the field of applied EM so I now need something more advanced. I'm more interested in the engineering/applied side of things, so I'm interested in stuff like materials...
  19. M

    Learning curve of EM? (Not homework)

    I'm taking a first year physics course and have been having a little trouble with the basics of Newtons laws and forces and whatnot, though nothing that can't be fixed with a bit more hard work. I'm looking ahead now and seeing a lot of EM material, and after kind of taking a brief look at...
  20. Z

    Phase velocity of EM radiation in free space

    Homework Statement (i) What is the phase velocity of EM radiation in free space? (ii) What is its group velocity? Homework Equations The Attempt at a Solution Using the definition as per this page: http://physicsdaily.com/physics/Phase_velocity (i) Vp = (C^2)/Vg (ii)...
  21. H

    Understanding Mechanical Energy in the EM System: A Griffith Perspective

    In griffith it is mentioned that the sum of the mechanical energy and electromagnetic field energy are conserved in a system, how is the mechanical energy defined in this case since a potential energy cannot be defined? Is it just the kinetic term by itself?
  22. mesa

    EM Waves, how are they created?

    A moving charge creates a Bf so is an EM wave created (for example) by an electron oscillating in a conductor producing a changing Bf which then induces an Ef with both perpendicular to the direction of propagation? If so I am presuming the point where the Bf crosses the axis represents the...
  23. J

    Is a static EM field made of photons? Do they move at c?

    So i am taking the MOOC of Galitski et al on Exploring Quantum Physics. I like it. But i keep running into some difficulty with my understanding of what it means to quantize a static EM (or E or M) field: Is the field made up of photons? If so, are they somehow moving at the speed of...
  24. M

    EM fields in unpolarized light

    How is it that the electric and magnetic fields in unpolarized light are not canceled out? The simplest example would be two photons of the same energy traveling coherently in time along exactly the same path in space, differing only by a rotation of 180 degrees around the axis of travel...
  25. Y

    What happens to EM waves outside the universe?

    If the universe is expanding at less than the speed of light, what happens to EM radiation that is emitted from stars at the edge of the universe?
  26. cryora

    EM waves and the direction of E-fields & B-Fields

    What I'm wondering is whether or not the E-fields and B-fields making up an EM wave always point outwards. Would it be possible to have an EM wave where either the E-fields or B-fields, or both, point inwards towards the path of propagation? The only diagrams I see in books and the internet...
  27. H

    Cherenkov radiation Why the EM waves are constructive

    I was reading an article about Cherenkov radiation in wikipedia. If the electron moves slower than light the EM waves in the medium are destructive and if they travel faster than light waves are constructive .Why so?When the electron travels faster than light the radiations cannot interfere...
  28. T

    Find Magnetic Field Amplitude from EM Wave Amplitude

    Homework Statement A plane electromagnetic wave propagating in free space has an amplitude (\sqrt{3}/2)a_{y}-(1/2)a_{z} V/m. Find the amplitude of the accompanying magnetic field assuiming the frequency of the electric field is 1 GHz. Homework Equations f_{E} = f_{H}...
  29. R

    Attenuation of em waves- unknown relationship help.

    Sorry if this is in the wrong place. Was not sure what it is categorised as. I have come across the following:e^{-\frac{\omega}{c} nkz}, referring to attenuation of a wave. It was found here...
  30. G

    How Is Energy Flux Calculated in an Electromagnetic Wave?

    Hello, The energy density of an electromagnetic wave is ε_{0}E^{2}. To calculate the energy flux, at least in the derivation's I've seen, people just multiply by the speed of the wave, i.e., c. But doesn't this assume that the energy density is constant at all points?; but E changes...
  31. V

    Couple of Questions Regarding EM Waves

    I'm a little unsure of my own answers to these questions. Hopefully I'll clear up my misconceptions. 1. If I measure the E-field and B-field at a point in space where there is an EM wave, would I be able to determine the direction it propagates from? My Ans: I don't think so, since when...
  32. Roodles01

    Polarised (polarized) light; EM explanation.

    Facets of a rock specimen sparkle brightly In terms of electromagnetism how can I show that a linearly polarised light source can be used to determine that the facets are dielectric rather than metallic? Should have to do with scattering, but not sure whether it could be explained with a...
  33. M

    How photons and static em feilds work

    The photon as I understand it is a massless particle that "must" travel at the speed of light. However I do not understand the idea that a photon is also the carrier of the em force. If there is a simple ball of static charge and a test charge comes near do photons exchange between the two...
  34. R

    How Do You Calculate the Intensity of an Electromagnetic Wave?

    Howdy everyone. 1. Ok, so we have and electromagnetic wave propagating through a vacuum. We know wavelength, and the magnitudes of B in the x and y directions. Also, B=[Bx(i)+By(j)]cos(kz+ωt). That's all the prereq stuff. 2. So Intensity is: <P>/A & (1/2)Bmax^2/Zo I'm def feeling...
  35. Roodles01

    Lorentz force - e- in EM field

    Homework Statement An electron in an electric field E = 3.0x107 ex NC-1 and a magnetic field B = 3.0 ez T has velocity v = 1.0x107 (2ex - ey) ms-1. Calculate electromagnetic force on the electron. Homework Equations F = q (E + v X B) where q = e- = -1.6x10-19 C The Attempt at a...
  36. S

    Angular momentum of the EM field of rotating sphere

    The angular momentum of the electromagnetic field is defined as, $$ \vec{L_{em}} = \int \vec{l_{em}} d^3r. $$ To solve this for a rotating sphere I must consider the cases where r < R and r > R. When I did this problem I thought that there would be two solutions, one for both cases; however...
  37. S

    Degrees of freedom of an EM field?

    Homework Statement Can anyone point me in the right direction (i.e. an explanation or resource) that shows why an EM field has two degrees of freedom (attributable to the KE and PE, due to the harmonic oscillator description of the field)? The matter is mentioned in passing in a chapter...
  38. J

    Why are our eyes tuned to a specific band of the EM spectrum?

    I mean why is it that the eye evolved to detect "visible" light instead of, say, microwaves. What is it about the visible light spectrum?
  39. N

    What's the source of EM radiation

    An electron and a positron annihilate and energy is released in the form of quantized EM (gamma etc.) rays: Now some childish yet inquisitive questions; 1. Where were there quanta prior to annihilation, inside the electron or positron (or both)? 2. Were these quanta really present...
  40. A

    Tuning an LC circuit to a particular EM wave.

    Hello Everyone, I am making an LRC circuit by having a Coil attached to a Capacitor attached to a Light-bulb which is attached to the Coil. Provided that the radio signal (of 2000 Mhz) I'm trying to "collect" is strong enough and the coil/capacitor is properly tuned I should see the...
  41. J

    EM field due to a long wire submitted to an AC current

    Hello there, I am having a real stupid moment. In the space XYZ, a long wire is along the Y axis and is submitted to an alternating current I_{0}\sin \omega t. I am trying to establish the equations for the B(t) and E(t) fields along the X axis. I assume that the distance to the cable...
  42. quasar987

    Understanding EM in GR: Maxwell's Equations and Particle Motion Explained

    So apparently (wiki), the manisfestly covariant form of Maxwell's equations is dF=0 and d*F=µJ for F the Faraday 2-form and J the current 3-form. My question will probably seem silly to you but I am simply wondering how does this field affect the motion of a particle of charge q in GR? Is it...
  43. E

    Interaction of EM waves with matter

    The interaction of light with matter, or EM waves in general, falls into 3 categories: transparent where they pass through, opaque where they are scattered, and shiny where they are reflected. What on the quantum mechanical level about the atoms electrons determines those properties? I think the...
  44. A

    EM plane wave incident upon infinite sheet of current

    Homework Statement What are the electric and magnetic fields due to an infinite sheet of surface current \vec{K} = \hat{j}Ko in the plane x=0? The plane is electrically neutral and Ko is a constant. Plane electromagnetic waves are APPROACHING the plane x =0 from either side, incident normally...
  45. J

    Calculating EM field using only cross-section of past light cone?

    If you want to know the value of the electromagnetic field at some point in space P at time t1, I assume that since EM is a relativistic theory, it should be possible to derive it using only the value of the field (along with charges, but let's say we are dealing with fields in free space) at an...
  46. H

    Is my understanding of EM field quantization correct?

    Essentially how I understand it is, (this is for the quantization of an EM field in vaccum) -Fields become operators, a specific E operator will contain the number and multiple of rising and lowering operators needed to raise a vacuum state into the desired quantum state -The electromagnetic...
  47. dbmorpher

    What is the Height of EM Radiation

    Hello, I have always wondered, what the relation of wavelength and frequency to the amplitude is. Is there an equation? Does it vary with different types of waves? Can it change?
  48. BruceW

    EM Tensor Invariants - Are There More?

    Hey everyone I was looking up the EM tensor on wikipedia, and they mention two invariants. There is the obvious one - The Lorentz invariant B^2- \frac{E^2}{c^2} And there is also the product of the EM tensor with its dual, which they call the pseudoscalar invariant: \frac{1}{2}...
  49. M

    EM algorithm convergence KF log likelihood decrease

    Hi everyone, Im running the KF to learn parameters of a model, the log likelihood of the p(Y_{k}|Y_{k-1}), however decreases. Can anyone advise, does this mean my implementation is wrong or can this just be the case. Advice appreciated Thanks
  50. T

    EM Field: Angle between H and x-axis; frequency

    Homework Statement Hello all, could someone please assist me with the following problem? Homework Equations f = ω/2π λ = (2π)/βThe Attempt at a Solution I've already made an attempt to part (b), (c), & (d) of this problem. Below is my work. I'm having trouble with part (a). I drew the...
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