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

    Are time-advanced EM fields necessary?

    It's widely known that the electromagnetic wave equation admits of time-advanced as well as time-retarded solutions, but the time-advanced solutions are often simply discarded as non-physical. This is reasonable enough in many contexts, but I am personally of an opinion that the time-advanced...
  2. P

    Helpppp EM Radiation doesn't make sense to me

    Helpppp please. EM Radiation doesn't make sense to me Hi. I don't see how you would get the answers to these questions. 1) A beam of light is shone on two sheets of paper, one of which is perfectly absorbing black and the other perfectly reflecting white. If the radiation pressure on the...
  3. J

    Comparing Radio and Light Waves: Is EM Radiation Misleading?

    As I've been studying up on EM radiation, I've come to the conclusion that radio and light waves are not the same thing. Radio seems to be nothing other than a continuously flipping magnetic field (source: http://www.pbs.org/wgbh/aso/tryit/radio/radiowaves.html). In this way, there do not seem...
  4. deccard

    Unraveling Temporal/Spatial Coherence of EM Radiation

    I'm bit confused about using terms temporal coherence and transverse/longitudinal spatial coherence when speaking about electro-magnetic radiation. I have understood that temporal coherence basically means how monochromatic light is. But I don't understand why temporal coherence is related...
  5. G

    How Do Stars Lose Energy Through Rotation and Magnetic Fields?

    I miss sometimes the most basic things in physics. (I think they are basic). For example, recent reading about stars in space made me wonder about few things... 1. stars seem to lose energy if they rotate, via magnetic field (eg. neutron stars) "The energy emitted by common pulsars has its...
  6. pellman

    What is the full EM + matter lagrangian?

    This is a followup to the old thread What are the FULL classical electrodynamic equations? which never really provided a satisfactory answer. I have decided to phrase it perhaps in a more straightforward manner. Given that we have the EM field, or the equivalent potential field, and charged...
  7. W

    Could Dark Matter be mass in EM fields?

    Didn't Einstein say that mass and energy were interchangeable quantities? Electromagnetic waves transport energy - Does this therefore mean they also transport mass? Could, therefore, all the radiation coming out of every decaying atom in the universe, every star and every accelerating...
  8. K

    Is Assistance Available for an Electromagnetic Theory Problem?

    can anyone help me with this problem? Thank you.
  9. nomadreid

    Maxwell's equations, Orthogonality, electric and magnetic fields in EM

    Maxwell's equations give that the electric and magnetic fields in E-M radiation are orthogonal. This is a classic equation, but can it be related to the orthogonality of, for example, the momentum and position operators which lead to non-commutivity?
  10. R

    Can Someone Help Me Better Understand the Relativistic Origin of Magnetism?

    I've been trying to better understand the relativistic origin of magnetism. I tried to do a slight variation on a common derivation and got an unexpected result. I'm hoping someone here can point to an error I made. The problem is to calculate the electric field a test charge would experience...
  11. P

    Creating Pressure Wave from EM Radiation: No Explosives

    Hello, is it possible to create a pressure wave from any form of electromagnetic radiation that travels inward to a central point emanating from a spherical surface. Also, the pressure wave must not be produced from explosives that are imploding.
  12. V

    Why can't time-varying EM fields exist in a perfect conductor?

    I understand that according to Maxwell's equations time-varying EM fields cannot exist in a perfect conductor (but static magnetic fields can). Also if you have a time-varying magnetic field you also have time-varying electric field and vice versa. And this knowledge is used to solve EM wave...
  13. S

    Light vs Non-EM Wave Velocity and Time Dilation

    hey..while looking into the consequences of the invariance of light...we see that time needs to be slowed down in order to incorporate it in all the appropriate condtns..normally every body( in most of the books) gives an example...if we are in a space ship(moving horizontally with a speed of v)...
  14. S

    Is there any truth to this? FTL Gravity and EM Waves

    http://www.youtube.com/watch?v=5wnk6FVXCmc&feature=related That is one of several parts, 4 or 5 I believe. So is there any real science or truth to what he is saying? If he is correct it could quite possibly be the most important discovery in human history. Mind you, I'm not talking so...
  15. J

    Can an antenna with a positive charge emit a radio wave, and why or why not?

    1. If you took a positively charged antenna and shook it up and down 500 million times a second, would it emit a radio wave. If so, why? If not, why not? I would say, the EM wave that it emit is heat due to friction with the air. anyone have any other suggestion. 2. Electric charges are...
  16. T

    EM Waves: How Do They Get Generated?

    I read that em waves get generated by oscillating and/or accelerating charges. I am confused on how this occurs. Doesn't this violate energy conservation? Say 2 charges( a +& a -) are separated by a certain distance. So they have potential energy. Then you let them go. The potential energy is...
  17. K

    Energy Distribution of EM Radiation ~ One Photon

    Hi All, I've been recently reading a thread, and a question keeps popping up there. Its not very difficult, it just requires a simple yes or no answer...in a single photon emission, is the energy of the photon distributed evenly on the sphere r=ct, or does this sphere represent the...
  18. B

    EM Waves: Understanding w/o Quantum Mechanics?

    Another thing that I've never really understood... As fas as I'm concerned, waves need a medium to travel through, but electromagnetic waves seem not to need one at all. I just can't visualise them as oscillations in anything. It sort of helps if I visualise EM waves as photons, because then...
  19. Labyrinth

    Exchange of photons/virtual photons and electrostatic or EM force

    How does the exchange of photons/virtual photons give rise to the electrostatic or electromagnetic force? Why do like charges repel and opposite charges attract?
  20. H

    Why do magnetic fields follow the right-hand rule instead of the left-hand rule?

    If a conductor connects your nose with the computerscreen with electricity going through it (allways in the same direction), the magnetic fieldlines will allways surround the conductor with one direction. Obviously nature prefers one direction more than the other. Is there some very important...
  21. K

    Are Photons and the EM field the same thing?

    I have it stuck in my head that a photon is the way the EM field mediates energy exchanges. With this in mind, I think that a photon always travels at the speed of light in a vacuum, c, regardless of the material it is in. It's the EM field of the material that changes the speed of light in the...
  22. P

    Sinusodial solution of EM equation

    what kind of sources of light generate sinusodially varying electric field that is solutions of the form E(x,t)=E_{o}(x,t) \sin(kx-\omega t)
  23. S

    What are the names of these formulas in EM

    # Hi # My instructor of EM course wrote two formulas on the blackboard and asked whether they are correct and what their names are. (He does not know the answers too) # These formulas give V and \vec{A}, given \vec{E} and \vec{B} V(\vec{r},t) = - \vec{r} \cdot \int_{0}^{1} d\lambda...
  24. S

    Medical EM - Diseases. Is there a link?

    Numerous BBC and Daily Mail articles ( yeah, trustworthy sources 8-| ) keep bringing this up, stating that there might indeed be a link between cellphone/radio waves and disease (usually cancer). My mother has informed me that at the latest Romanian Neurology Convention they stated that too...
  25. T

    Calculate Surface Charge Densities for Parallel Plates

    2 infinite parallel plates separated by a distance s are at the potentials zero and V0. find the surface charge densities on the plates. I got the answer that it first calculate E and then as 1. x=o, Ex = σ / ε, then could know σ 2. x=s, Ex = - σ / ε ---> here's what I don't know. why...
  26. T

    Some math met in EM hw solution

    I got this : 1 - z / (R^2 + Z^2) ^1/2 = 1- (1+ (R/Z)^2) ^-1/2 = 1 - 1 + (1/2) ( R/Z )^2 I'm confused why it got all these steps. it seems like taylor expansion ? or ? Thank you.
  27. marcus

    Hints of dispersion (both gravity and EM waves)

    This paper which just appeared on arxiv http://arxiv.org/abs/0709.2365 Loop quantum gravity corrections to gravitational wave dispersion Martin Bojowald, Golam Mortuza Hossain 27 pages (Submitted on 14 Sep 2007) "Cosmological tensor perturbations equations are derived for Hamiltonian cosmology...
  28. A

    IFRs, EM Waves & Moving at c Velocity: What's the Effect?

    Inertial frames of reference ( IFRs ) and Electromagnetic ( EM ) waves. What would happen if you move at c velocity ? ( at the same velocity of a EM wave ) Maxwell equations are valid ? No relativity, please.
  29. M

    Exploring the Mysteries of EM Waves

    Sorry if its a simple question but I was wondering what EM waves are composed of. Is it like an electron beam traveling through the air and the magnetic field a consequence of the moving electrons? Another thing that's been really confusing me and no matter how many radio circuit tutorials I...
  30. S

    Questions: Reg EM wave transmission

    Why microwave frequencies are more affected due to rain? How does water actually affect microwaves?And lower frequencies have more diffraction.Why is it so?
  31. K

    Amplitude of EM waves in vacuum ~

    Amplitude of EM waves in vacuum~~~ As we know, amplitude of waves decreases with distance due to energy loss. But what if EM waves in the space?? Since there's no matter around, I was wondering if there is any energy loss when EM waves travel through vacuum.
  32. F

    How cant EM wawe escape from a black hole?

    i need to have some scientific papers about this. I understand the massive ones but i can't understand it when it comes to a EM wawe. Can anyone advice me a book or a paper ?
  33. N

    Rectangular Optical Wavguide - EM Wave

    Hello All, this is my first post, and while I'm a newb to the forums I'm not a newb to Optics. However, I have a very light background in low level electromagnetics, I'm more of a network design person. I'm working on a very very long term personal project to understand optics better and I've...
  34. B

    Free 3-D EM Software for Modeling Transformers and Mapping Poynting Vectors

    Does anybody know of free 3-D EM software so that I can model a transformer? In particular, I'm interested in mapping Poynting vectors. Thanks.
  35. P

    Hertzian dipole generating EM Waves.

    Hi, in my notes for the Hertzian Dipole I have a derivation of the vector potential A, and the scalar potential (phi). However, I'm missing the derivation of the E and B fields from these potentials. It seems that only the theta component of the E field exists, and I have ... well, I can't write...
  36. J

    Deriving EM energy and momentum with noether's theorem

    I think I know how to derive conserved energy and momentum currents of a free EM field. Lagrangian is \mathcal{L}=-\frac{1}{4}F_{\mu\nu}F^{\mu\nu} I then substitute x^\mu\mapsto x^\mu + \lambda u^\mu, and take the derivative in respect to lambda. With some trickery I've got \partial_\mu...
  37. B

    Am I missing something obvious here? - EM induction

    Homework Statement A square coil of wire has sides of length 10mm, consists of 2000 turns and has a resistance of 250\omega. The coil is placed in a magnetic field such that the normal to the plane of the coil is parallel to the direction of the field. When the coil is rotated through 180...
  38. J

    Similarities/differences between EM waves and particle waves

    Bare with me, I haven't gone to 3rd level yet. Hopefully, if I get enough points, I will be at University next year. I am just wondering, what is the difference between energy traveling as a wave, like electro magnetic radiation, and the wave state of a particle? Are these the same waves...
  39. S

    Deriving EM Energy: Calculation & Explanation of E-Field Energy Storage

    The following wikipedia article derives the energy stored in an E-field (under "Energy stored in an electric field"): http://en.wikipedia.org/wiki/Electrical_energy I don't quite get how the following term goes to zero in the article? \frac{\epsilon_o}{2}\int V\mathbf{E}\cdot dA
  40. S

    How Does EM Wave Penetrate in an Elevator/Lift?

    Hi, I'm rather new here. I've got a question that's bothering me, and therefore decided to ask here. Why is it that in a lift/elevator, radio signal usually gets weakened? And, as far as i know, FM radio operates on the radio wave band, as well as cell phones. But why is it that mobile...
  41. D

    World sheet EM tensor in complex coordinates

    In my Book (Becker, Becker, Schwarz) it is stated (eq 3.23) that the holomorphic component of the EM tensor is given by T_X(z)=T_{zz}=-2 : \partial _z X \cdot \partial _z X : Now why is the expression for the (holonorphic, zz, component of the) energy momentum tensor in complex coordinates?
  42. D

    World sheet EM tensor (Hilbert principle?)

    In my ST book the world sheet EM tensor for the Polyakov action is given by T_{\alpha \beta} = -\frac{2}{T} \frac{1}{\sqrt{-h}} \frac{\delta S_{\sigma}}{\delta h^{\alpha \beta}} Why is this? (Searching through the internet I found clues to what is called Hilbert's prescription, namely...
  43. M

    Need Jump Start On Two EM Questions

    1) I am asked to find the rate of radiation emitted from a charged particle with constant velocity moving through a magnetic field at an angle and then with an added parallel electric field. I know how to do it without the fields and just the point charge, I just don't know how to factor in the...
  44. S

    Exploring the Constant Frequency of EM Waves

    I was wondering, when an EM wave goes from one medium to another why is it that the frequency remains constant?
  45. T

    Questions about graduate level QM and EM

    suppose that I want to take a graduate level QM course, what kind of math do I need? would a solid knowledge on linear algebra and differential equations suffice? what about a graduate level EM course?
  46. N

    Confused by D'alembertian operator in EM

    I can't understand how the D'Alembertian x 4-vector Vector potential = mu x 4-vector J Have \nambla^2\A-1\c^2\ frac{ \partial^{2}A}{\partial {t}^{2} } } = \-mu\J
  47. V

    Couldn't find em, so I had them made

    I looked everywhere to find a certain Tshirt and no where are they made so.. I went to a local shop. The poor guy had NO idea what it meant. History My son's High School scolastic team was holding an NAQT tourney at his school and since the kids were working so hard I figured that something...
  48. D

    Solve EM Waves Problem: Time Difference of Important News Transmission

    Homework Statement An important news announcement is transmitted by radio waves to people who are 81 km away, sitting next to their radios, and by sound waves to people sitting across the newsroom, 2.3 m from the newscaster. Take the speed of sound in air to be 343 m/s. What is the...
  49. A

    What is the theoretical proof of the quantization of the energy of the EM field?

    Hi, The energy of the quantum mechanical harmonic oscillator is proved to be quantized after solving the Schrodingers equation which leads to Hermite equation and discovering that normalizable solutions of the wavefunction exist only for a discrete spectrum of energy. When the...
  50. D

    Calculating Radius of Circular Area from Laser Power and Energy Density

    Homework Statement energy density in a laser beam is 9.5x10^-8 J/m cubed. the laser beam is cylindrical, and it delivers a power of 15W to a certain area. What is the radius of this circular area? Homework Equations Intensity : S=cu and S=Power/Area Area of a circle (or possibly the...
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