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. Y

    How do you relay EM wave parameters to Transmission line parameters?

    I am studying EM wave and transmission lines. I see both derive equations for propagation constant \gamma: Plane wave velocity is 1/\sqrt{\mu\epsilon} and \eta = \sqrt{\mu/\epsilon} Transmission line velocity is 1/\sqrt{LC} and Z0=\sqrt{L/C}. From that the book just to say the velocity...
  2. T

    Momentum of a coil during EM induction

    I have heard many things about electromagnetic induction, but I have got still more questions than answers. For example if a magnet is closing fast to a coil, a current will be induced in a coil. This also means, that part of the momentum will be given to those charged particles. If we count...
  3. M

    Comparing Photons & EM Waves in Physics

    I am trying to review some basic physics, purely for my own interest, and have been looking at particle physics in the form of the Bohr model and EM theory in the form of Maxwell’s equation. While I still have a way to go on these topics, I was wondering if anybody is in a position to help me...
  4. N

    Proving EM Waves Equations: E = Emsin(kx-ωt) and B = Bmsin(kx-ωt)

    Show that the electric field: E = Emsin(kx-ωt) and magnetic field: B=Bmsin(kx-ωt) satisfy the following equations: -\frac{\partial B}{\partial x} = \mu_{0}\epsilon_{0}\frac{\partial E}{\partial t} and \frac{\partial E}{\partial x} = -\frac{\partial B}{\partial t} I have no idea...
  5. T

    Solve Calc 2 Integral 0 to h: 1/((h-r)^2+r^2))

    Homework Statement this integral owned me, can anyone tell me what to do to solve it/ how to solve it or a step by step solution. from 0 to h, 1/((h-r)^2+r^2))dr Homework Equations The Attempt at a Solution Im thinking partial factions or trig sub...
  6. C

    Detecting Virtual Photons & EM Force: A Particle Physics Quandary

    Hey all, I am an undergraduate taking a particle physics class and we just learned that the EM force was caused by charges "spitting" virtual photons back a forth. In like second grade we learned that the EM force was dependent on the two charges involved (keeping this simple) and the distance...
  7. Cyrus

    News Should CEO Salaries be Public Information?

    Round em up, send em to GITMO! I'm watching CNN and YET AGAIN another CEO is spending wild sums of money (1.3 million) to redecorate his office. He spent 14k on a waste basket. Over and over, all these CEOS are scum bags. I mean jesus christ, how disgraceful.
  8. L

    EM radiation cross over materials

    Anybody knows if there is a rule for EM radiation cross over materials ? The wavelenght in comparison the size (or structure) of molecule is determinant ? Ex. why infrared pass through the concrete but not metalics ? Thank you.
  9. E

    EM Wave Amplitude: Questions & Answers

    Hi I hope this is the right place to post. I have some questions regarding EM wave amplitudes. There are some other posts in the past regarding this and some people replied by saying that amplitude of a light wave is equal/close to wavelength of the wave. This is deduced from the fact that...
  10. K

    Solution to EM Wave: Questions & Answers

    I know that in free space, the general solution of the wave equation about electric field is of sine and cosine form. One can also write it in complex form as E = E_0 \exp(i\vec{k}\cdot\vec{r} - i\omega t) I have two queations about this solution 1) If consider the polarization, how...
  11. T

    Exploring Warp Drive Possibilities with EM Waves

    I love Star trek and I was thinking, how to create a warp drive. I have read a discusion about EM waves on this forum, there was written that they do not need any medium for propagation and : “In a simplified sense the oscillating E field sustains the oscillating B field which sustains the...
  12. K

    Boundary condition of EM field

    On the boundary (surface) of two regions, the tangential components of electric fields on above and below surface are continuous. I wonder if it is also true for displacement \vec{D} and polarization \vec{P}? That is, can I say: the tangential component of \vec{D} or \vec{P} on above and below...
  13. Y

    What cause a phase shift between Ex and Ey of a polarized EM wave?

    In EM wave polarization. Where is linear polarization where \gamma =0 or \pi . \gamma = +/- 45 deg and amplitude of Ex and Ey are equal to give circular polization etc. My question is why there are different angle of \gamma on the first place? If you set the reference at x...
  14. Y

    Speed of EM in good conductors

    Speed of EM travel through the good conductor is \omega/\beta I know the speed of signal travel in stripline is c/\sqrt{}\epsilon_{}r_{} My symbols don't look very good but I think you get what I mean. Obvious they are different. I am confuse because in both case EM wave travel in good...
  15. K

    Change EM Wavelength: Is There a Substance?

    Alright first time poster here with a question that has been eating at me for a while. I am curious to know if there is a substance that when one part of the EM spectrum is passed through it, it would come out as another. I know that if you are to shine a lightbulb on some black cloth you would...
  16. TheFerruccio

    Deriving EM Wave Equations from Faraday's & Ampere-Maxwell's Laws

    I am trying to derive the electromagnetic wave equations from Faraday's law of Induction, and the Ampere-Maxwell law. But, I am having a problem with the 1/c^2 disappearing. This is what I am using: \nabla\times\vec{B}=\mu\vec{J}+\mu\epsilon\stackrel{\partial\vec{E}}{\partial t}...
  17. L

    How Do You Calculate the Flux of a Vector Field Through a Parametric Surface?

    Homework Statement 1. The expression F = [x,y,z] defines a vector field. Given the parametric representation of a surface S:[u cos v, u sin v, u^2] = r (u,v), where the parameters cover the ranges 0 ≤ u ≤ 2 and 0 ≤ v ≤ 2π, calculate the flux F through the surface S.Homework Equations How do i...
  18. K

    What is the Relationship Between EM Wave Velocity and Electric Field in Space?

    Homework Statement Find the velocity of EM waves as a function of \epsilon_{0} and \mu_{0} 2. The attempt at a solution E = E_{0}cos(kx-\omega t) Using v= \frac{\omega}{k}
  19. G

    What to learn to link EM wave to photon picture?

    I know undergrad QM fairly well. There they use the EM potential to introduce EM fields into the momentum. What should I study to understand the connection between EM fields and actual photon particles?
  20. S

    Contact force, EM force and the loop

    The maximum I have found about contact force is very roughly this: it is nothing but electromagnetic force, meaning that, if two bodies collide, the electrons of the atoms of the outer shell of each body repel each other, those atoms displace towards their neighbours and repel them in turn, thus...
  21. H

    Sketching EM Waves with imaginary amplitudes

    Homework Statement Sketch in 3D, the following waves. (both E fields are in x direction E_1 = 3*exp(-j*8*Pi*z) E_2 = 4j*exp(-j*8*Pi*z) (where j=sqrt(-1)The Attempt at a Solution I know how to sketch E_1, but my question is how to treat the imaginary amplitude, 4j in E_2. here is my stab at...
  22. A

    Voltage in coil due to moving in EM field

    Hi, i am working in a motion capture project one of its parts is a sensor i need a formula of the voltage generated in the coil due to moving in electromagnetic Field from an inducer i think it depends on ( B , cosθ , µ , N , L , r ) i post this thread in another forum but no one answer...
  23. F

    EM field and its sources

    EM field and its sources... hello forum, a simple question about EM field and its sources. In a time-varying situation, maxwell equations tell that the E field has the time-changing charge density rho as a source, while the B field has the time-changing current density J. But both...
  24. P

    Solving EM Wave Emission from Cavity at T: Power per Unit Area

    Homework Statement Basically, the problem states that a cavity at temperature T is emitting EM waves isotropically in all directions (with frequency distribution of Planck's Law). If the time averaged density is <e>, find the value of d<S>/dw where w is the solid angle and the quantity is the...
  25. S

    Examining EM Radiation and Charge: The Truth About Emission and Movement

    Does a stationary charge emits electromagnetic radiation? Does a moving charge in magnetic field emits electromagnetic radiation?
  26. M

    Difference between EM wave & a Matter wave ?

    Homework Statement Difference between EM wave & a Matter wave ?
  27. P

    What is Modeling EM Transmission at 100 kHz with COMSOL and RF Module?

    Hi all, I have brave hopes of modeling the following problem: Imagine an antenna transmitting an ~50V 100kHz sine wave. Imagine a receiving antenna placed three or four feet apart, with an amplification and detection circuit. First, measure the amplitude of the received wave. Next...
  28. T

    Understanding EM Stress Energy Tensor: Sources and Current Density

    I've been trying to obtain the EM stress energy tensor... but I'm not sure if what I got is correct. In all the websites I googled, I found T^{\mu\nu} =- \frac{1}{4}g^{\mu\nu} F^{\alpha\beta}F_{\alpha\beta} + F^{\mu}\,\!_{\lambda}F^{\nu\lambda} However, is this with the sources or without...
  29. A

    Relativistic-Strength EM Field

    Homework Statement Demonstrate (qualitatively) that an electron in a linearly polarized EM beam that is driven transversely at \omega will also oscillate longitudinally at 2*\omega (due to the B field, apparently). Homework Equations I want to use a phasor form for the fields...
  30. G

    Criteria for propagation of EM wave

    Homework Statement When an EM wave propagates, the E and M field at a point are perpendicular. If E and M at a point are NOT perpendicular to each other, can it be that an EM wave is passing through that point?Homework Equations The Attempt at a Solution Consider a linearly polarized light...
  31. A

    Empirical evidence for gravitational field of EM radiation

    Hello, Is there any empirical evidence that electromagnetic radiation produces gravitational fields? I understand that as a form of energy it is expected to induce a spacetime curvature, but has this prediction of GR actually been tested? Is it at all subject to any controversy? Thanks, Armin
  32. R

    Generating EM Radiation: Natural and Artificial Methods Explained"

    What are the ways (both natural and artificial) in which em radiation is generated? Here is one of the methods: We take a point charge (or more practically a charged sphere with uniform charge distribution) such that we can voluntarily fluctuate the amount of charge on it. We fluctuate the...
  33. S

    Effects em fields have on em waves

    Ok my question is pretty simple and is in two parts. 1. say you created an em field what would have to an em wave the is propogating towards it. 2. what would would to an em wave if it went through an em coil. If you could would you be able to point me in the direction a...
  34. S

    A question on wave nature of EM radiation

    There is a EM radiation of frequency suppose x Hz.Let it has to travel a distance of y m.Now what's the time required for the radiation to travel the distance?The question seems to be very easy, but my confusion is in how can the frequency,distance,speed and time can be related? Can someone give...
  35. A

    Idea that EM radiation propagates through space in straight lines

    OK so I'm familiar with the idea that EM radiation propagates through space in straight lines at a uniform speed, that of light. The idea that light moves in waves, that each wavelength carries a uniform energy and that the wavelength differs yet because the same speed is maintained each...
  36. C

    The 'hazing' that is Jackson: Is it necessary to understand EM?

    In short: Is a class based on the Jackson text necessary to truly understand electromagnetism? In length: I have heard many people (some on this forum) state that the Jackson text is more of an exercise in mathematical physics and/or is the “hazing” that one must endure to be a PhD in...
  37. N

    EM waves - wave equation derivation

    Hi, Something has been bothering me about deriving the wave equation for a plane EM wave. We were showed this derivation in class and had to reproduce it but something is not making sense to me... The derivation is as follows: Suppose you have a plane EM wave (in a vaccuum) traveling in the...
  38. J

    Why do different frequency EM Waves behave different ?

    Hi, i was wondering Sorry if i don't have the right words, i study in France Why are Microwaves better for heating up food ? I mean since E=hv, these should contain less energy Right now, i know vaguely, that atoms absorb photons where the energy can make it go from one of its characteristic...
  39. J

    How can an antenna capture + receive EM Waves ?

    Hi I was wondering how can antennas capture + receive Electromagnetic waves I was thinking, in particular of FM Transmitters Antennas, and Fm receiver Antennas. And also, how come metals act as a shield to EM Waves I know that antennas have a resistance, and that they consume Energy by...
  40. GRB 080319B

    The Physics of Spacetime & EM: Answers to Questions

    In GR, gravity is seen as property of the geometry of spacetime (curvature) as opposed to a force field. Does this theory extend to electromagnetics ,i.e. could EM be described as a property of spacetime, or is EM strictly dealing with fields? Do light waves and gravitational waves travel in the...
  41. H

    Exploring the Interaction of EM Waves & Molecular Vibrations

    Quote from Organic Chemistry, L.G. Wade, Jr.: "Not all molecular vibrations absorb IR radiation." "One of the component of an electromagnetic wave is E field. This field alternately stretches and compresses a polar bond." "If this alternate stretching compressing of the bond occurs at the...
  42. W

    EM Field Generation and Energy: A Simple Explanation

    Hi, I understand that EM fields have energy and they travel at c. Light is an EM field so it travels at c as well. So say you run a current through a wire - an EM field is generated, propogating outwards at c. Since an EM field is generated, and EM fields have energy, that energy has to come...
  43. R

    Quantisation of EM field

    Particle energy and momentum are obtained from the wave function as eigenfunctions of the spatial and time derivative operators. Is this true of ElectroMagnetic fields? In other words are E and B eigenfunctions of a differenial operator? I can see that E and B could be interpreted as a...
  44. P

    Q values for strong weak and EM interactions?

    I have just read, in general, weak interactions, have the lowest Q values; meaning they have the smallest cross sections, which means they control processes in stars at some time in the early universe. What would be the next relative strength? Cheers
  45. T

    Showing the components of a plane EM wave are perpendicular

    Homework Statement I've derived the EM wave equations from Maxwell's equations, and I now need to show that the E and B components are both perpendicular to each other and to the direction of propagation. The textbook I've been using attempts to show why this is, but it isn't particularly...
  46. M

    Optimizing Poker Strategies: The Impact of Cooperation in Hold'em Poker

    When playing poker at a 10 person table, each player obviously has a 10% chance of winning. This means that, when a person is the first player to act at the table, he should only play hole cards that are in the top 10%, in terms of non-losing percentage (ties count as wins here). [In any...
  47. U

    How Does Acceleration Affect EM Wave Wavelength in Charged Particles?

    So, accelerating charges produce EM waves, and I understand that the greater the acceleration, the shorter the wavelength of the EM waves. It is also my understanding that when one charge is attracted to another, EM waves are somehow exchanged between the two. Now, let's say you have a unit...
  48. U

    EM wave field strength and energy

    Another noobish question: Let's say there is an electromagnetic wave of constant wavelength and constant peak amplitude that travels a known distance. Let's say that we also know the total energy of this EM wave. Is there some formula that can tell us what the peak value of the electric field is...
  49. S

    EM: Magnetic induction at a point due to a line of current, or square coil.

    Hi everyone, (nb: I posted this in the introductory physics section, but maybe it should be here? I'm not exactly sure how to divide physics into introductory and advanced. I hope this isn't against the rules - it's only my second post!) I am trying to understand my EM course again, and I...
  50. T

    Deriving Poynting Vector from EM Waves

    can anyone help me derive the poynting vector from energy density of electromagnetic waves? and how we obtain average value <S> thanks
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