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.

View More On Wikipedia.org
Recent contents Top users
  1. K

    Questions about EM wave in material

    How can I find the induced current density by EM wave in a material? Should I have ma = Fbinding + Fdriving + Fdamping like treating it as a spring? Then the current density should be charge density of the material x velocity (ρv), isn't it? Is there any condition the EM wave cannot...
  2. P

    A 50cm wavelength EM wave in air has 50mW/m^2 of energy.

    Homework Statement A 50cm wavelength EM wave in air has 50mW/m^2 of energy. what is the frequency? what is the average E field? What is the B field strength? What would be the λ in water? Homework Equations I found the λ. How do I find it in water The Attempt at a Solution...
  3. B

    What is the formula for calculating average power of electromagnetic waves?

    Homework Statement Homework Equations Savg = Em * Bm / (2*μ0 * c) The Attempt at a Solution I solved for S avg without calculating inside the cos, but I got it right. The z and t is given, but is there the reason why you don't solve inside the cos, or is it the bug? it's...
  4. J

    Reissner-Nordström black hole: Spherical symmetry of EM field stregth tensor

    The setup: I am reading the review: arXiv:hep-th/0004098 (page 9-10). In Einstein-Maxwell theory, the gravitational field equations read: \begin{equation} R_{\mu \nu} - \frac{1}{2} g_{\mu \nu} R = \kappa^2 \left( F_{\mu \rho} F^{\rho}_{\;\;\nu} - \frac{1}{4} g_{\mu \nu} F_{\rho \sigma}...
  5. N

    EM: Linear combination of waves

    Homework Statement Hi In Griffiths (chapter 9) he states that any wave can be expressed as a linear combination of sinusoidal waves, f(z,t)=\int_{-\infty}^{\infty}{A(k)e^{i(kz-\omega t)}dk} = e^{-i\omega t}\int_{-\infty}^{\infty}{A(k)e^{ikz}dk} Is it correct to say that this in principle...
  6. D

    Hamiltonian describing energy transfer to bloch Electron from EM field

    Hi all, I've a hamiltonian that describes the coupling of electrons in a crystal (bloch electrons) to an EM field described by a vector potential A \begin{equation} \mathscr{H} = \frac{e}{mc}\left[\mathbf{p}(-\mathbf{k}) \cdot \mathbf{A}(\mathbf{k}, \omega)\right] \end{equation}...
  7. O

    Why can we explain gravity geometrically but not EM force?

    Why is a magnetic field "a field-of-force in space" while a gravitiational field is "not a field in space, but a curvature of space itself"? Why can't we describe and explain EM repulsion and attraction the way we explain gravitational attraction? Why don't we say that the presence of a charge...
  8. J

    Why does particle size affect EM radiation scattering

    It is my understanding that electromagnetic radiation is absorbed by an electron and then emitted at another angle. So why does particle size affect scattering when the phenomenon involves electrons? Also, why does the size of the wavelength of the light relative to the diameter of the particle...
  9. P

    How many dimensions are needed to fully describe the electromagnetic force?

    Maxwell's equations use four dimensions. But wouldn't we need more dimensions to completely describe the EM force. I'm thinking of polarization and twist. So how many mathematical dimensions are necessary for a complete description? That is, how big do the matrices need to be and how many...
  10. T

    Total energy per unit area of a EM pulse

    B=Boexp[-(kz-ωt)2)] i(hat) Calculate the electric field associated with the above magnetic field pulse. Calculate the Poynting vector for the EM field and the total energy per unit area. Use Maxwell's equations in vacuum and assume electric potential is zero. Homework Equations...
  11. S

    How to Adjust Frequency of EM Wave

    Alternating current produces EM Wave. What determines the frequency(or wave length) of an EM wave related to current or voltage?
  12. S

    What Determines the Polarization of EM Waves?

    Hi I don't understand the polarization of EM waves. I have same questions about that. 1-) As you know, EM wave is consist of both electric and magnetic field/wave. And there is 90 degree between them. Is that possible to separate these two fields or waves? I mean is that rule: If there is...
  13. A

    Momentum density vector of a plane em wave

    Homework Statement Show that in vacuum the magnitude of the momentum density vector of a plane electromagnetic wave is equal to the energy density divided by the speed of light, c. Homework Equations \vec{S} = \vec{E}x\vec{H} The Attempt at a Solution This doesn't seem too hard if you know...
  14. C

    2 Questions about EM wave propagation

    I was just reading about EM wave propagation and had two questions I would appreciate an answer to. 1-I read how the electric and magnetic fields of an EM wave oscillate sinusoidally and perpendicular to each other and the direction the wave is traveling. Is there any significance or special...
  15. sweetser

    Gauge symmetry in EM by inspection

    Hello: I was under the impression that gauge symmetry was a property of the Lagrange density. Here is the Lagrangian for EM written out in its components: \begin{align*} \mathcal{L}_{EM} &= J\cdot A +\frac{1}{2}\left(B^2-E^2\right) \quad eq.~1\\ &=\rho \phi - Jx Ax - Jy Ay - Jz Az \\...
  16. jaketodd

    Materials that radio frequency EM waves can penetrate

    Can radio waves penetrate steel? What about titanium? Maybe a characterization of what they can't penetrate would be more informative? ...Or a way to figure out what materials they can penetrate. Thanks, Jake
  17. Z

    Does This Answer Make Sense (basic EM)

    Homework Statement The plastic rod of the length L=1 m has a non-uniform charge density λ=cx where positive constant c =2x10-6 [some unit]. What unit does c have? Find the electric potential at the point on the x-axis 1 m to the left from the left end of the rod. Find the electric...
  18. N

    Fourier Transform Homework: Solving P(t) with E(t_1) & E(t_2)

    Homework Statement Hi I wish to Fourier transform the following expression P(t) = \int\limits_{ - \infty }^\infty {dt_1 dt_2 \chi (t - t_1 ,t - t_2 )E(t_1 )E(t_2 )} What I do is the following \int\limits_{ - \infty }^\infty {P(\omega )e^{ - i\omega t} } = \int\limits_{ -...
  19. L

    EM wave propagation direction.

    Homework Statement Given the following EM wave propagating in vacuum, find the direction along which the electric field oscillates and the direction of propagation of the wave: \vec{E} = (-3\hat{i} + 3\sqrt{3}\hat{j}) 10^4 e^{i[\frac{\pi}{3} (\sqrt{5}x + \sqrt{5/3} y10^7 - 8.1246 *10^{15} t]}...
  20. C

    Understanding the Nature of the Electromagnetic Field: One or Many?

    When I studied EM we always treated EM fields as being "individual" autonomous fields generated either from the separation of charges or induced by other phenomena. However, I'm reading that modern physics is treating the zero-point energy as fluctuations in THE EM field. Susskind clearly states...
  21. O

    EM reflection coefficient off a conducting plane

    Let's say there is a conducting plane of conductivity \sigma. Boundary conditions for above and below such plane are: E_1^t = E_2^t (D_2 - D_1 ) \cdot n = \rho i.e. tangential components of E field are continuous and perpendicular components of D are discontinuous because of surface charge...
  22. S

    What *would* the night sky look like if we could see the entire EM spectrum?

    Assuming, of course, that we simply rescaled our color perspectives so that longest wavelengths = red, shortest wavelengths = violet. Are there any such pictures?
  23. A

    How do I get E field?, etc.[Griffiths EM example 7.2, p 287.]

    Homework Statement two long cylinders (radii a and b) are separated by material of conductivity (sigma). if they are maintained at a potential difference V, what current flows from one to the other, in a length L? Homework Equations Please see the attachment The Attempt at a Solution...
  24. C

    EM Destructive interference of two coherent beams

    Suppose you have two coherent microwave beams, one beam at 45 degrees to a reflecting wall and the other one at 90 degrees, with the two beams pointing to the same point on the wall. If the two beams are 180 degrees out of phase and destructively interfere at the point on the wall, will the...
  25. R

    EM Radiation oscillating charged mass

    Homework Statement A particle of mass m and charge q is attached to a spring with force constant k, hanging from the ceiling. Its equilibrium position is a distance h above the floor. It is pulled down a distance d below equilibrium and released, at time t = 0; Under the usual assumptions...
  26. C

    EM Waves in Conductors: Why Does B Field Lag E Field?

    When an EM wave goes in a conductor it says that the B field component lags the E field component, What causes this? I looked in Griffiths and I couldn't find the answer. Does it have something to do with the fact that when the EM wave enters the conductor it is moving the free electrons in the...
  27. C

    I was always told that EM radiation is a far field effect. Does this

    I was always told that EM radiation is a far field effect. Does this mean that the light emitted from the accelerating electron is not right next to the electron but a little further out. And also how do you calculate the frequency of the light coming off. When I looked through Griffiths...
  28. C

    Continuous EM fields vs. fixed freq photons

    I understand the classical view of EM fields as being (theoretically) continuous. What I don't quite get is how this can be reconciled with the QM view of photons coming only in fixed frequencies (The electromagnetic field may be thought of in a more 'coarse' way.). Is the number of possible...
  29. G

    Where Does the Energy Go in an EM Field When a Charged Particle Stops?

    When an accelerated charged particle is shot into an electromagnetic field such that the accelerated particle comes to a complete stop in the field, we naturally have loss of energy from the charged particle. But what is this energy converted to? And where? Does the particle release radiation...
  30. D

    Can diff. EM frequencies create diff. magnetic fields with diff. properties ?

    ¿ Can diff. EM frequencies create diff. magnetic fields with diff. properties ? I understand that a magnetic field is generated via moving charged particles . Electricity and magnetism are linked through electromagnetism . Electromagnetism can be in different wavelengths and frequencies but...
  31. M

    Heat or Absorbed Energy and EM Radiation / Conductivity

    I would like to know whether Kelvin or KJs matter when it comes down to a material radiating and / or conducting its heat.
  32. P

    Why do electromagnetic waves use complex numbers?

    why electromagnetic waves are represented by complex numbers?
  33. G

    EM field of cell, microtubules, quantum behavior in biological systems

    The cytoskeletal structures were considered responsible for the elementary cellular intelligence in the past. (For example, see the articles from Albrecht Buehler. http://www.basic.northwestern.edu/g-buehler/FRAME.HTM.) Microtubules are pipe-like structures made of two kinds of proteins...
  34. Q

    Questions about EM properties of ferrous liquids in spherical form

    I am an ameteur physicist (i actually have my degree in meteorology), and i have some questions about the EM properties of liquid metals or ferrous liquids when in spherical form. I understand if you are too busy or if i sound off, but if you do have the time to answer a few questions, it would...
  35. K

    EM field of dipole derivation from Green's function

    Hi all, I know that the electric field generated by a dipole is given by \mathbf{E}= [1-i(\omega/c) r]\frac{3 (\mathbf{p}\cdot\mathbf{r})\mathbf{r}-\mathbf{p} }{r^3}+(\omega/c)^2\frac{\mathbf{p}-(\mathbf{p}\cdot\mathbf{r})\mathbf{r}}{r} e^{i(\omega/c)r} where \mathbf{p} is the dipole's...
  36. A

    Show EM Wave equation invariant under a Lorentz Transformation

    Homework Statement Show that the electromagnetic wave equation \frac{\partial^{2}\phi}{\partial x^{2}} + \frac{\partial^{2}\phi}{\partial y^{2}} + \frac{\partial^{2}\phi}{\partial z^{2}} - \frac{1}{c^2}\frac{\partial^{2} \phi}{\partial t^2} is invariant under a Lorentz transformation...
  37. A

    What is the energy density and total energy of an electric field?

    In another recent thread it was being discussed whether there is energy in the field of an electron according to U = \frac{1}{2}\epsilon_0 E^2. I began by saying "yes", then changed to "no" because of the way the equation was derived. (It was derived by bringing together N=2 or more charges...
  38. I

    Do Accelerating Charges Always Emit Radiation?

    Why does a moving particle particle emit EM-radiation?
  39. P

    Can I Make a 5-Lambda EM Wave?

    is it possible to make am em wave of length 5(lamda)...lamda=wavelength of em wave...
  40. Y

    EM wave right above and right below the conducting surface.

    Assume a infinite depth good conductor block with width in y and length in x direction. Boundary is at z=0 and air is at z=-ve and conductor at z=+ve. Let a voltage apply across the length of the block in x direction so a current density established in +ve x direction. We want to look at the EM...
  41. Y

    Question on EM wave in space and matters.

    My understanding from the book about EM wave that E and B is perpendicular to the direction of propagation only in medium that is charge free so \nabla \cdot \vec E = 0. What cases when E and B not perpendicular to the direction of propagation? Thanks Alan
  42. J

    I just noticed that we tend to call EM wavelengths shorter than

    I just noticed that we tend to call EM wavelengths shorter than visible light "rays" while we call we call those that are longer "waves". Why is that? Did we ever think that "rays" and "waves" were physically different?
  43. J

    Can EM Radiation Frequency Be Changed and Who Carries Different Frequencies?

    If photons "carry" visible light, and light is just a specific frequency of EM radiation... who carries radio waves? [PLAIN]http://www.antonine-education.co.uk/physics_gcse/Unit_1/Topic_5/em_spectrum.jpg And can an EM radiation at specific frequency be changed to a different frequency? I...
  44. K

    Sources of EM radiation in organisms?

    Organisms radiation EM energy. I am aware of the brain and nervous system as a source (because of electrical currents, etc) and that we have a lot of IR radiation. Is the majority of the energy due to blackbody radiation, a by-product of chemical reactions, or something else?
  45. ?

    EM attraction between a solenoid and interstellar hydrogen

    Okay I have a really odd question that might be difficult to answer. I have searched for an answer in several physics books and websites and don't think I have come across a specific answer for this question. Suppose I were to send a negative electromagnetic charge through a solenoid and want...
  46. I

    Magnetic Fields and EM Radiation

    Since a changing current creates a magnetic field, and if there's a wire near by, the magnetic field will induce a current in that wire. What is needed / what does it take to turn that magnetic field into EM radiation such as a radio wave? Or is it purely just the frequency of the current, so...
  47. P

    Light as an EM wave to explain photoelectric effect?

    OK, so recently I have learned that in the early 20th century, while Max Planck was attempting to explain the quantum nature of light, two men named Philip Lenard and Heinrich Hertz discovered the photoelectric effect. They found that an electron must absorb a specific amount of energy...
  48. S

    Construction Cranes EM Induction

    Can anyone explain why cranes in construction sometimes experience induced currents from antennas radiating from distances of up to several Kilometers. I have looked into the phenomenon several times, and have not been able to find a study or baselines to follow. I have a basic understanding...
  49. D

    Lorentz Transformation Applicability Re: EM & Casuality

    Hi, From what I've learned so far, Lorentz transformation meets certain criteria, such as the constancy of EM wave propagation speed in vacuum, &/ casuality, among others. My question is, why would it/would it not be applicable to phenomena that have nothing to do with EM interaction? In...
  50. S

    Calculating Field Strengths of EM & Gravitational Fields

    Hi there PF How does on calculate the field strenght of the electromagnetic field (from Maxwells equations) and of the gravitational field (from general relativity). I want to relate this to particle physics, where the relative strenghts of the fundamental interactions are calculated here...
Back
Top