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. P

    EM fields and Current between 2 charged cylinders

    The task is to find the magnetic field between the 2 long cylinders, which extend to infinity. Integration is involved to find the total current passing through the Amperian Loop shown below. What I do not understand is why only sides 1 and 3 contribute to that B ds part of Ampere's Law. Isn't...
  2. gepdiana

    Exploring the Propagation of Electromagnetic Waves: Visualizing the Concepts

    Hi everyone, I m not a physicist and I don't really speak english... please forgive me if I write any "rubbish". I'm quite curious, and I was wondering how do electromagnetic waves travels. I mean, from a "point" source, they propagate in every direction (I've been told) so I tend to imagine...
  3. A

    EM wave reflected or transmitted?

    As I learn in class, when EM wave goes from medium 1 to medium 2, there are 3 possibilities that can happen Totally transmitted (i.e when the angle of incident is 0 degree) Partially transmitted and reflected (i.e when the angle of incident is between 0 and critical angle) Total internal...
  4. E

    Average Energy density and the Poynting vector of an EM wave

    Hi, In Problem 9.12 of Griffiths Introduction to Electrodynamics, 4th edition (Problem 9.11 3rd edition), in the problem, he says that one can calculate the average energy density and Poynting vector as using the formula I don't really understand how to do...
  5. F

    I The Speed of Light: Comparing Photon and EM Wave Velocities

    Velocity of photon allways is c(photon is massless particle).While velocity of EM wave in medium < c.So does velocity of photon need not allways equal velocity of EM wave?
  6. F

    I Is there an uncertainty between amplitude and phase in an EM wave?

    Is there an uncertainty between amplitude and phase in classical quasi-monochromatic light?(E(t)=a(t)cos(phi(t)-omega_0*t))If it exist, what is the relation between classical and quantum uncertainty(delta I* delta phi>=1/2)?
  7. X

    Polarization of EM waves is preserved after reflection/refraction -- Why?

    Hello, here's my question: during the usual derivation of Fresnel's equations, it is assumed that an incident EM wave (plane monochromatic) is transverse electric or magnatic and that it keeps this nature after reflection and transmission. How can this be proven? Thank you!
  8. F

    I Can we quantize a static EM field?

    Can we have a quantization static EM field?If not, how can we interpret static EM field in stand point of QM?
  9. F

    I What is difference between EM wave and EM field?

    They say wave function is different to quantum field. Then what is the difference between EM wave and EM field?(By the way :Is that EM wave the wave function of photons?).It seem to me EM wave is the wave of EM field?
  10. AHSAN MUJTABA

    Lorentz and Gauge invariance of EM

    I have been reading the book of Chris Quigg, Gauge theories, Chapter 3, sec 3.3 in which he explains how local rotations transform wave function and variations in Schrodinger equation forces us to introduce the electromagnetic interaction between the particles. I need a bit deep concept of the...
  11. Dale

    I Number of photons in an arbitrary EM field

    Correct me if I am wrong. I understand that if ##\vec E## and ##\vec B## are solutions to Maxwell’s equations then ##\Psi= \vec E + i \vec B## is a solution to Schrodinger’s equation. Is there an easy way to calculate the statistical distribution of the number of photons, or at least the...
  12. T

    In what class does one typically learn that atoms emit EM radiation?

    I took Inorganic Chemistry I and Physics in high school. I took Inorganic Chemistry I and about half of Inorganic Chemistry II (dropped out mid-semster of Inorganic Chemistry II) in college. My Physics class in high school was just regular physics, not AP Physics, so I have never taken a...
  13. B

    Electromagnetism -- What do charges have to do to create EM waves?

    I am not sure, but I think the answer is between acceleration and velocity.
  14. H

    Exploring the Conditions for EM Radiation: Free Electrons vs Bound Electrons

    Suppose there are two free electrons. Their mutual repulsion causes each to accelerate. Is there EM radiation? Suppose those electrons are loosely bound to a nucleus, as in a metal. Is there EM radiation?
  15. Mr_Allod

    EM Wave Reflection and Transmission Between 3 Materials

    Hello there. I set up the problem like this, I have a wave incident from air on the anti-reflective coating consisting of: ##\tilde {\vec E_I} (z,t) = \tilde E_{0_I} e^{i(k_1z- \omega t)} \hat x## ##\tilde {\vec B_I} (z,t) = \frac 1 v \tilde E_{0_I} e^{i(k_1z- \omega t)} \hat y## This wave gets...
  16. david_nelson

    A EM Penetration Depth Into the Ground

    Hello, I'm wondering about the best EM to penetrate deep into ground and sense/detect what's there. The authors here (https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JE002528) are recommending P-band with 430 MHz to probe few meters into the ground of Mars; that's about 70-cm...
  17. DuckAmuck

    I EM Field Strength in Curved Spacetime: Is it Unchanged?

    It seems a gravitational field does not alter the electromagnetic field strength. Is this correct? My reasoning: With no gravity, field strength is: F_{\mu\nu} = \partial_\mu A_\nu - \partial_\nu A_\mu Introduce gravity: \partial_\mu A_\nu \rightarrow \nabla_\mu A_\nu = \partial_\mu A_\nu +...
  18. A

    EM radiation temperature vs particle temperature

    I just realized I'm having a problem in understanding this. So let's take an example the CMB is around 160 Ghz and the blackbody temperature within this frequency range is 2.7K which is rather cold as it is close to absolute zero. Then let's take another example, Iter plasma will achieve about...
  19. Quarinteen

    EM fields and magnetic induction from an experiment

    If you take copper wire and wrap it tightly around a bolt will that alone create and hold a magnetic field? I add an image for clarification
  20. berkeman

    This is why EM interference can be an issue....

    I've been on these "fox hunts" before, where you are trying to track down an interfering signal. We do them frequently for practice (as HAM radio operators), and have used the skills a few times for real. I haven't done it in the rain yet, though... :smile...
  21. C

    Frequency of EM waves produced by linearly accelerating charges

    I was wondering about EM waves produced by linearly accelerating charges, as opposed to oscillating charges. With oscillating charges, the frequency of the wave depends on the frequency of the oscillation of the charge. But what determines the frequency of the wave produced by a linearly...
  22. C

    How Is the Energy Density of EM Waves Related to Capacitors and Inductors?

    The energy density of an EM wave is given as (1/2) ϵ E^2 + (1/(2μ)) B^2. This is derived from the energy density of the electric and magnetic fields of capacitors and inductors, respectively. But why should the energy density of the fields of capacitors and inductors be the same as that of...
  23. anuttarasammyak

    Energy transfer and conservation cases for pendulum motion and EM wave

    Let me ask a very primitive question. To and fro motion of pendulum under gravity tells us potential energy + kinetic energy = const. At the top points potential energy: max kinetic energy :0 At the bottom point potential energy: 0 kinetic energy :max EM wave is usually illustrated as...
  24. A

    I How to measure a 21 cm EM wave if it is Doppler shifted?

    we know that all emission from asctrophysical context is doppler shifted. So, how to make sure the doppler shifted 21 cm not contaminated by some other emission?
  25. J

    I Falling EM system contradicts the equivalence principle?

    The following is an improved version of my previous post https://www.physicsforums.com/threads/falling-electric-dipole-contradicts-the-equivalence-principle.964594/ Consider the following system comprising a particle on the left with charge ##+q## that is a large distance ##d## away from two...
  26. bhobba

    Exploring the Evidence for Visible Light as EM Radiation

    Hi All In discussing another issue it occurred to me the only experiential evidence I know that normal visible light is EM radiation is Faraday Rotation. I strongly suspect we have a lot more these days. Amusing story. Fermi's wife had a degree in general science that did not cover...
  27. phys194

    EM: Separation of variables to find magnetic induction

    Consider the static field configuration shown in the image. There are three layers: 0 = vacuum, 1 = magneto-optic fluid and 2 = covering shell. Each of these layers have their own permittivity and permeability (ε_i,μ_i) (isotrope). A uniform electric field H_0 = H_0/sqrt(2) * (e_x + e_y) is...
  28. PainterGuy

    How is the wavelength of emitted EM radiation measured?

    Hi, Source: https://en.wikipedia.org/wiki/Second#"Atomic"_second How were such huge number of periods per second measured originally? Was an instrument such as Michelson interferometer used? I think it was Albert Michelson who invented the interferometer to make calculations of wavelength and...
  29. nomadreid

    NASA NASA connection to EM drive research

    The following is a five-year-old post on one of NASA's websites. https://www.nasaspaceflight.com/2015/04/evaluating-nasas-futuristic-em-drive/?fbclid=IwAR02ifoAVrPGa0StAxsTjIuS3b08fUOyMcPh1-nBjeiwgwWkI3H7jnA0Has It exposes the holes in the state of affairs at that moment -- first, the article...
  30. A

    EM waves, longitudinal EM propagation?

    Hey, after doing some reading I stumbled across a few fundamental questions.1) Do all EM waves across the EM spectrum , if they travel through space have their E field and B field amplitudes exactly equal and in phase and shifted 90 degrees from one another? If the answer is yes then... 2) In...
  31. T

    How can I solve my ADS EM simulation problem?

  32. W

    The every day use of the magnetic part in EM radiation

    As I understand it, it is the magnetic part of electromagnetic radiation which creates a current (AC) in a radio receiver (i.e in an antenna) which can be translated into sound/graphics and so on. Is this correct? I mean in the sense that it is the magnetic field in the incoming radiation which...
  33. B

    Frequency of TV signal from EM waves

    Having some trouble with this question I believe phenomenon behind this one is that the student is passing between nodes (minimum displacement) to antinodes (maximum displacement) which explains why the signal weakens and strengthens continuously. Hence the ans to this is option C? For the...
  34. mgkii

    EM Wave - basic question on energy conservation in a wave

    I've searched threads and can't find easy explanation - sorry if I'm missing something basic / have a basic understanding error! In the classic picture of an EM wave with the Electric and Magnetic components oscillating at 90 degrees to each other, both components cross the middle axis at the...
  35. vis viva

    I Personal COMSOL challenge (EM)

    Hi Y'all For the purpose of exploring COMSOL, I challenged my self to plot the E/M-fields of a piece of current carrying wire in 3D. It's quite a simple task to plot the fields inside the wire, but I fail when plotting the fields outside the wire. For plotting the outside fields I have...
  36. U

    When is the propagation of an EM wave not reversible?

    By reversibility, if we turn the direction of the light propagation by 180 degrees, then the new propagation path follows the old propagation path. I suspect that when there is diffraction, the light propagation is not reversible?
  37. LarryS

    Classical EM vs Orbital Angular Momentum

    Consider the following experiment from the point-of-view of classical mechanics and classical electromagnetism: An originally free electron then passes through a magnetic field that is oriented so that it causes the electron to turn to, say, the right. During the “turning” of the electron (a...
  38. D

    I Energy Momentum Tensor: Real Physics & Cosmology

    The energy momentum tensor and its correlation to reimannian curvature is fascinating to think about. How much are the components and which of the components are taken into consideration when doing real physics. I suppose astrophysicists and cosmologists would be the main group of scientists...
  39. fisher garry

    Calculating the energy in an EM wave

    You don't have to read all this theory to answer my question. I added it just in case. Above they use the definition work energy theorem in vacuum to get to (8.12). Since it is in vacuum I would guess that one could use the equations for B and E field from EM-waves so that magnitude...
  40. D

    Poynting vector -- Calculate the EM power transmitted down a coax cable

    I don't know which part gets wrong
  41. W

    I How to derive the currents of Z boson and the EM current?

    I am a student minor in physics and I am taking this course of particle physics. And I have been lost since the Non-Abelian Gauge Theory which a few lectures before Electroweak unified Theory lecture. i am completely confused and overwhelmed by the math since the mid-term exam, since I was not...
  42. SherlockHolmes

    A Classical fields: EM fields vs. Gravitational fields

    I'm reading on Wikipedia about quantum field theory and read this: "Quantum field theory naturally began with the study of electromagnetic interactions, as the electromagnetic field was the only known classical field as of the 1920s". Why wasn't Newtonian gravitation regarded as a classical...
  43. C

    I Relationship between EM and matter fields in QED

    It seems that QED treats the matter and EM fields as independent yet coupled fields. On the other hand the EM field equations emerge immediately under local change of the phase of the matter field, exactly as required to reestablish local (gauge) invariance. From that perspective it almost seems...
  44. e101101

    How do I modify plane waves to represent the EM field of a lightbulb?

    I don't know where to start for part (c), I've managed to get (a) and (b). Can someone simply guide me, I think I'm having trouble understanding what the teacher means by just having an x dependance... Does this mean I only work with the x components of the magnetic field and the electric...
  45. N

    Vector and scalar potentials for an EM plane wave in a vacuum

    Lorentz gauge: ∇⋅A = -μ0ε0∂V/∂t Gauss's law: -∇2V + μ0ε0∂2V/∂t2 = ρ/ε0 Ampere-Maxwell equation: -∇2A + μ0ε0∂2A/∂t2 = μ0J I started with the hint, E = -∇V - ∂A/∂t and set V = 0, and ended up with E0 ei(kz-ωt) x_hat = - ∂A/∂t mult. both sides by ∂t then integrate to get A = -i(E0/ω)ei(kz-ωt)...
  46. Creedence

    I Eigenvectors of the EM stress-energy tensor

    My question is that what is the physical meaning of the EM stress-energy tensor's eigenvectors? Thanks for the answers - Robert
  47. jdou86

    A Understanding time invariant of EM

    on the conquering the physics gre book it says e.g. for time invariant "if you can see someones eyes in a mirror, they can see yours as well" so what the hell does that mean? isnt person A sending photons to person B and person B sending different sets of photons to person A? how does that...
  48. engnrshyckh

    Understanding EM Wave Reflection: Wavelength Changes Explained

    Problem Statement: 1) The wavelength of an electromagnetic wave after reflection at angle on a surface [A]. remains same as the wavelength perpendicular to the surface . remains same in the free space @ [C]. increases in actual direction of propagation [D]. decreases in actual direction of...
  49. U

    Exploring the Electromagnetic Parameters of Carbon Fiber for CST Simulations

    I would like to know parameters of this material, such as: ##\epsilon##, ##\mu## and ##\sigma##, for EM simulations with CST. I can't find anything similar to carbon fibers or general plastic in the CST material library. Google is not helping me this time. Does anyone know something more on this...
  50. Creedence

    I Equilibrium state of sourceless EM field

    Given a box made of perfectly conducting material. At some part of it at t=0 there is a localized magnetic field. It's sourceless and there aren't any dissipation. After t=0 it starts to spread and fill the box. What is the equilibrium state? Thanks for the answer(s), Robert
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