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. H

    Classical How many problems to do from Jackson EM?

    Ideally one would want to do every problem in the book. However, the problems just take too much time and I was wondering if anyone had a recommended problem sets that tests the essentials of each chapter. I am self-studying from the book and feel some problems are overkill. Thanks
  2. J

    Negative Energy Density in EM Waves?

    Homework Statement The problem I have is that we are asked to show the complex relative permittivity of a good conductor is erc = 1 + i(sigma)/(omega*epsilon_0) where sigma is the conductivity and omega is the frequency of an electromagnetic wave in the medium. This is fine, I calculated it...
  3. S

    Intro Physics EM Theory Books: Find Textbooks to Enhance Learning

    Hello, Am starting to read Electromagnetic Theory. Can you suggest some good textbooks on the topic that will help clear the concepts. Thanks in advance
  4. E

    EM Wave creation in different frames of reference

    OK, I'm sure I'm just not thinking about this the right way, so please point out my simple mistake. Imagine a charged particle and 2 observers. Observer S is stationary relative to the charged particle, so sees no magnetic field from it. Observer A moves back and forth in front of the...
  5. T

    Relationship between an EM wave and its current

    Given the following: A directional radio wave transmitting antenna which creates a beam (diameter in mm), a current in volts, and a frequency (Hz), is it possible to calculate the em wave voltage (v/m or w/m2) immediately next to the antenna? and then at a distance of 10meters? What...
  6. S

    Does the ubiquitousness of EM fields explain single particle

    Does the (assumption of) ubiquitousness of EM fields explain single particle interference well/better? Below is a rough logic, however it maybe need modifications. The double slit or the mach-zehnder or any object changes the configuration of time-space and EM fields. The photon may not know...
  7. G

    Understanding Einstein's Derivation of EM Transformations

    I was reading Einstein's seminal work: http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_1905_relativity.pdfGo to this part: II. ELECTRODYNAMICAL PART § 6. Transformation of the Maxwell-Hertz Equations for Empty Space. On the Nature of the Electromotive Forces Occurring in a Magnetic...
  8. M

    Total angular momentum of EM fields

    Homework Statement The problem deals with a charged (Q) rotating sphere around its axis (Ω_0) z^^ (z hat) of radius a. I'm asked to find the total angular momentum of the EM fields. 2. The attempt at a solution There is a solution posted to this question and I was just wondering why my...
  9. S

    What is the trough in destructive interference of EM?

    My textbook says that the trough of the electromagnetic wave cancels out when it meets a crest. Okay, that makes sense, but if the crest is the actual photon/wave of energy, what is this trough that can cancel out the energy? It seems to mean that there are "negative energy photons" in between...
  10. pellman

    What is the concept of a pure state in the electromagnetic field?

    In Ballentine's Quantum Mechanics book, as part of a discussion of pure states vs non-pure (mixed) states, he says Polarized monochromatic light produced by a laser can approximate a pure state of the electromagnetic field. Unpolarized monochromatic radiation and black body radiation are...
  11. DoobleD

    Dipole EM wave field lines VS decreasing amplitude

    The E and B field lines of a dipole radiating EM waves looks like that (sorry for the poor quality) : What I wonder is how can we see that the amplitude of the fields decrease as 1 / r? When you look as such a picture it actually feels likes the amplitude should rises. The E field gets...
  12. S

    A question about the EM spectrum

    I was looking around for information on the electromagnetic spectrum, but I have found some sources saying there is a new part of it called "terahertz", which would be between infrared and microwave. I have found some sources, but I don't really believe them so. Here are some sources of what I...
  13. DoobleD

    EM wave near field propagating faster than light?

    Sources : paper here and http://www.quora.com/What-is-the-phase-of-the-EM-waves (fifth paragraph). This is beyond my knowledge so I am not looking for an explanation of the phenomenon. But I thought nothing could go faster than light so I am very surprised. Are there exceptions to the speed...
  14. R

    EM Wave Interference: What Happens to Energy?

    When EM waves interfere, what happens to the energy?
  15. DoobleD

    Struggling to understand self sustaining EM waves

    I've recently learned about EM waves. One thing I find hard to really get, is how the E and B fields are constantly "generating" each other. I think maybe the key for this is in the following equations, obtained from Maxwell’s equations in vacuum : (source...
  16. W

    Skin depth: Same for current as for incoming EM wave?

    For a good conductor, an incoming plane electromagnetic wave will be attenuated exponentially as it penetrates a distance ##z## into the conductor, ##|\vec{E}(z)| = |\vec{E_0}|e^{-z/ \delta}##. ##\delta## is called the "skin depth". The current generated by this incoming electromagnetic wave is...
  17. Finny

    Why is gravity but not EM 'ripples' in spacetime?

    Why do we say gravity [GR] is a theory about ‘spacetime curvature’ and gravitational waves are ‘ripples’ but nobody uses such a description for electromagnetic fields? Don't EM waves 'ripple' spacetime? For example, one might imagine different types of spacetime curvature associated with each...
  18. K

    Question about light as an EM wave

    I'm so puzzled right now (sorry if I'm being stupid or something) , we can clearly see that EM waves oscilate the electrons in a conductor,they start vibrating and create EM waves again and these EM waves oscilate some other electrons and again they create another EM wave. All EM waves are...
  19. D

    EM: B field at boundary with different permeabilities

    Hey this isn't so much a homework problem but one I have just had an exam over. I have absolutely no idea how to calculate it and in all past papers/tutorial questions and the notes, makes no mention of the sort of problem. I'm not bothered over the exact answer, just how you go about it...
  20. BWV

    EM vs Neutrino interactions w matter

    basic question- neutrinos pass through matter because they are only subject to the weak interaction whereas photons interact with matter because they are subject to EM? Does the small mass of the neutrino contribute to this - if there was a neutrino with the mass of a proton how differently...
  21. kelvin490

    Question about intensity of EM waves

    For electromagnetic wave if it's reflected from a perfect conductor standing wave can be form. I wonder why Poynting vector can be used to describe the intensity of standing EM wave. (see p.19 of http://web.mit.edu/viz/EM/visualizations/coursenotes/modules/guide13.pdf ). From some textbooks...
  22. A

    What determines the penetration of UV rays through the ozone layer?

    According to Max planks equation, E=h*f, the more is the frequency the more is the energy of the EM waves. So why UVC (shortest uv ray or high frequency uv ray ) is absorbed by the ozone and UVA and UVB rays ( longer uv rays than UVC penetrate the ozone and reach the Earth's surface?
  23. hideelo

    What does the EM gauge freedom have to do with U(1)

    I know that we have a free parameter in the relativistic potential for electromagnetism. I also know that we can introduce this as a scalar field ψ which gives this free parameter. I understand that this can be related to U(1) by mapping ψ: ---> e^iψ, which is the U(1) group. It just seems a...
  24. Abhirikshma

    Energy dissipation in a travelling EM wave

    How is energy dissipated in a traveling electromagnetic wave ? Will there be any dissipation if it were to travel through vacuum ?
  25. B

    Is there measureable spacetime distortion from EM fields

    Please forgive me if I am posting in the wrong forum. Also the board limits the length of the title. I wanted to ask about Effect of high intensity electric and magnetic fields on spacetime. To begin, 15 years ago this was published...
  26. fluidistic

    Motion of a particle in an EM field

    Homework Statement There's a uniform EM field given by ##\vec E=E\hat y##, ##\vec B=B\hat z## with respect to an inertial reference frame K. A charged particle with rest mass m and charge q>0 moves in the field with an initial velocity orthogonal to ##\vec B##. 1)Write down the equation of...
  27. fluidistic

    Polarization of an EM wave passing through another medium

    Homework Statement 2 media whose refractive indices are 1 and n respectively are separated by a flat interface. An EM plane wave goes from medium 1 to medium 2 with a polarization vector making an angle of 45° with the plane of incidence. Determine the incidence angle for which the reflected...
  28. S

    Photon Displacement in EM Waves (Amplitude)

    I am curious on what is meant by amplitude in an EM wave. From all the searching I have done the amplitude has something to do with the density of photons in the EM wave, and the frequency with energy of the photons. The part I am confused about is the fact that the wave is oscillating. I...
  29. rumborak

    Arbitrarily changing EM fields and their photons

    This started with me trying to read up how static electric/magnetic fields are described with photons, but it quickly evolved into the realization that I don't really know how the photonic viewpoint describes *any* changing EM field that isn't a neat monochromatic wave. Some sites mention...
  30. P

    EM Waves & Conservation of Energy

    Hello folks. This is Sandeep. I have many lingering doubts in physics that I am here to get cleared. This has become more of an issue ever since I decided to teach physics to my young nephew. My first question pertains to electromagnetic waves and the conservation of energy. Consider an AC...
  31. rumborak

    Regarding the "acceleration" of the EM field

    In the thread about EM waves, the EM wave equation \left(c^2\nabla^2 - \frac{\partial^2}{\partial t^2} \right) E = 0 got me pondering. The term \frac{\partial^2}{\partial t^2} E is the second derivative of the field. In a solid-body wave equation, that same term (not with E, but the...
  32. G

    What pushes the EM waves in forward direction?

    nothing in the universe moves without force, so what does makes em waves move through space?
  33. M

    EM: Vector potential vs. Field tensor: Which is fundamental?

    In my lecture we were discussing the Lagrangian construction of Electromagnetism. We built it from the vector potential ##A^\mu##. We introduced the field tensor ##F^{\mu \nu}##. We could write the Langrangian in a very short fashion as ##-\frac{1}{4}F_{\mu \nu}F^{\mu \nu}## In the end we...
  34. R

    Do All EM Waves Have a Sinusoidal Shape?

    I understand that sinusoidal EM waves result from charged particles in harmonic motion, e.g., up and down an antenna. But what if the charge is undergoing some more complicated periodic motion? Wouldn't the EM waves be non-sinusoidal? I saw in a textbook a hypothetical EM wave with infinite wave...
  35. O

    Complex reflection and transmission coefficient of EM waves

    I have a question about complex reflection and transmission coefficients. For example, I am modeling a wave in air (medium 1) ## \varepsilon = \varepsilon_0 ## reflecting on, and transmitted to, a medium 2 with ## \varepsilon = \varepsilon' -j \varepsilon'' ## If the wave would have traveled...
  36. A

    Splitting and combining EM waves & amplitude/intensity

    I am reading through my course guide and problems worked out and something bothers me. I think at this point we are working with classical waves, moving towards QM eventually. The idealization we work with is this. We have a light source creating a wave ψ(x,t)=Acos(ωt+kx) . The light beam...
  37. ohwilleke

    Does Chiral EM Asymmetry Imply Matter-Antimatter Asymmetry?

    A Science Daily report discusses observations of cosmic gamma rays that suggest that magnetic fields in the universe disproportionately have left handed rather than right handed helicity. It goes on to suggest that under a 2001 theory of another physicist that this could explain...
  38. morrobay

    In Mechanical Wave v = w/k. EM wave w/k = c. How Equated ?

    With ω/k = 2π/T / 2π/λ = velocity for both transverse mechanical waves and EM waves. I can understand velocity as distance over time in mechanical wave. But how is the ratio Em/Bm = ω/k = c. That is the maximum amplitudes of the E and B fields in the y and z planes corresponding to c in...
  39. fluidistic

    Maxwell stress tensor to calculate force (EM)

    Homework Statement A sphere with dielectric constant ##\varepsilon## and radius R is placed inside a homogenous external electric field ##\vec E_0##. The sphere is divided in 2 hemispheres such that their common interface is orthogonal to the external field. Using the energy-momentum tensor...
  40. R

    Charged massive particle reacting to EM wave

    Homework Statement Consider a particle of charge q and mass m, free to move in the xy plane in response to an electromagnetic wave propagating in the z direction (might as well set δ to zero) a) Ignoring the magnetic force, find the velocity of the particle, as a function of time. (Assume the...
  41. U

    Cross Section for e+e- in EM interaction - Is it the same?

    I was studying my notes and specifically for the ##e^+e^- \rightarrow \mu^+ \mu^-## process, cross section is given by \sigma = \frac{4\pi}{3} \left( \frac{\alpha \hbar c}{W} \right)^2 where ##\alpha = \frac{g_{EM}^2}{4\pi}## and ##W## is the centre of mass energy. Is this the same for...
  42. itssilva

    When are quantum corrections significant for EM?

    As a rule of thumb, we might say that quantum theory becomes essential when we're analyzing systems at small distances (of the order of atomic sizes or less) and few enough particles (suppose particle number is conserved, as in QM); however, the world as a whole is quantum, and even a system...
  43. S

    Help needed with concept of electricity, magnetism and EM

    Hi, so I've just started learning about physics and I get the gist of most of it. I understand motion, torque, energy, light, radioactivity. But there is one topic that absolutely stumps me. Electricity. Can someone explain to an absolute moron like me what it is? It seems like everyone around...
  44. blue_leaf77

    Hamiltonian of an electron under EM radiation

    I might have learned what I am going to ask during my electrodynamics class long time ago but just that do not remember it now. I always wonder why does an electron moving in space with EM radiation have Hamiltonian of the form ## H = \left( \mathbf{p}-e\mathbf{A}/c \right)^2/2m +e\phi## where...
  45. H

    EM wave from electrons V protons

    this is theoretical (and possibly stupid) question; accelerating protons would create an EM wave equivalent to an EM wave generated by electrons at same frequency but the amplitude would be opposite, is that possible. has anyone made a communication circuit from accelerating protons? are...
  46. L

    Which properties of waves are influenced by the medium?

    Which of the following of a wave can be affected by the medium it travels through? (i) wave crest velocity, (ii) wave frequency, (iii) wavelength
  47. M

    EM wave interacting with refelcting surface

    When trying to explain reflection through the EM treatment of light waves, how do we account for the fact that the electric/magnetic field of the incoming light would penetrate into the medium from which it is reflecting off of? Diagrams like these: show the 'reflection point' on the axis of...
  48. T

    Confusion about EM Transmission coefficient - critical angle

    Hi, The reflection coefficient (R) of an EM wave is, as far as I know, 1 if there is total reflection and -1 if there is total reflection AND the phase changes by 180º. However, we also know that the transmission coefficient is given by T = R + 1 (or by its own formula, which should give the...
  49. teroenza

    Four Tensor Derivatives -- EM Field Lagrangian Density

    Homework Statement Given the Lagrangian density \Lambda = -\frac{1}{c}j^lA_l - \frac{1}{16 \pi} F^{lm}F_{lm} and the Euler-Lagrange equation for it \frac{\partial }{\partial x^k}\left ( \frac{\partial \Lambda}{\partial A_{i,k}} \right )- \frac{\partial \Lambda}{\partial A_{i}} =0...
  50. A

    What were the clues that the weak and EM force are the same

    Hello, I've been reading a book on particle physics for the general audience, and as you might expect, I was left with a slew of nagging unanswered questions. There was a chapter on gauge symmetry where the author described that the weak and the EM forces are the same force at some energy...
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