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

    S vs. P polarization (classical EM)

    Hello, I am having trouble wrapping my head around S vs P polarization of light. First, if linearly polarized light strikes an interface of two linear media, what determines whether it is S or P polarized? Also, why are these the only two options, i.e. why can't the polarization of the incident...
  2. R

    Allowed EM Waveforms: Which Ones?

    Homework Statement which of the following proposed space-time dependent electric fields in vacuum is/are allowed by the equations of EM theory? a) $$E_x=E_1\sin(kz-wt),E_y=E_2\sin(kz-wt),E_z=0$$ b) $$E_x=E_1\sin(kz-wt),E_y=2E_1\cos(kz-wt),E_z=0$$ c)...
  3. S

    I Searching Beyond EM Spectrum: Tech Needed to Discover New Wavelengths

    Hello PF members, I'm currently working on a project alongside others who are interested in the topic of searching for wavelengths beyond the EM spectrum. We've been through countless piles of literature and seem to have a mixed opinion. Our question is simple (ironically): What type of...
  4. T

    Want accurate cartoons of EM Waves

    If you were trying to explain to kids how EM Waves really look in nature, what would you show? None of these common graphics below are perfect. If you wanted to show comprehensive depiction of transverse EM Waves from the sun, from a laser pointer, from a radio / wifi transmission tower, how...
  5. J

    I How do we measure the EM field that a particle produces?

    [Moderator's note: posts from several threads on the same topic have been merged into this thread.] When we measure the EM field of a charged particle, what exactly are we measuring? Are we measuring the position of the EM field, the strength of it, what? Does a penning trap measure the EM...
  6. ThunderLight

    Orthogonal Polarisation in EM waves and Interference

    I've been trying to get my head around Polarisation and how it achieves orthogonality. I'm not sure if this should be in Physics or Electrical Engineering Section. (Mods can move this where appropriate) I know that 2 EM wave with linear polarisations where one wave is shifted by π, they would...
  7. D

    A Find EM Potential from EM Tensor | Math Solutions

    Hello, so my question is, if for some metric, we have found (somehow) Fμν, and we know that: Fμν=∂μAν-∂νAμ, how do we find Aν? I tried solving the differential system after imposing the Lorentz gauge ∂μAμ=0 but still, without some initial guess about which components of A are zero, the system...
  8. ThunderLight

    Why do EM waves of longer wavelengths spread out more?

    Why do longer wavelengths spread out more than shorter wavelengths? What is the physics principle/law which explains why radio waves spread out more than optic waves in free space?
  9. S

    A Change in the EM field energy due to dispersion of a wave packet

    Let’s suppose we have an electron with a Gaussian eigenstate, as the time runs, the wave spreads in space without changing its energy, however, the induced EM field caused by the particle decreases its energy. I assert this from the classical electromagnetism result in which the more...
  10. T

    Lift a weight up with an EM switch

    Hello, My question is simple. Let's suppose you have an electromagnetic coil with a magnetic weight in the center of it. This coil is energized, and the weight is lifted up vertically. Now, my weight is 30 kgs. I want to lift it to a height of 1 meter. How do I calculate how much power is...
  11. T

    EM Induction:current in one coil inducing current in another

    Homework Statement Two coils P and Q are placed close to one another, as shown in the first figure attached. The current in coil P is now varied as shown in the second figure. Show the variation with time of the reading of the voltmeter connected to coil Q for time t = 0 to time t = t 2...
  12. V

    I Derivation of phase change parameter in dispersive medium

    Hi, I'm trying to evaluate the derivates of first, second and third order of the phase change parameter in a dispersive medium. In such medium the refractive index is a function of the wavelength. In my case it depends on the wavelength in vacuum. \begin{equation*} n(\lambda_0 )\end{equation*}...
  13. Z

    A Description of imaging through lens using quantized EM field

    Usually the imaging through lens is described using geometrical optics or wave optics. However, I wish to study some imaging processes through lens where the light intensity is at the single photon level, under which condition I think the correct description should use a quantized...
  14. Arup Biswas

    Are light waves/ EM waves damped?

    We can't see objects from objects far away from us. Why? I think light waves damps! When it reaches our eyes it's amplitude is too small to be visualised! Is this true? If indeed EM waves are damped then why? If not please give a suitable definition for the mentioned phenomena too !
  15. M

    I Accelerate Charged Particle: Does Light Require Force?

    Does a charged particle need to experience a momentum changing force to emit light? Or is it sufficient for an observer to accelerate relative to the particle?
  16. D

    I Why do the extra terms cancel in the derivation of the EM field strength tensor?

    Hi I am trying to follow the derivation in some notes I have for the field strength tensor using covariant derivatives defined by Du = ∂u - iqAu . The field strength is the defined by [ Du , Dv ] = -iqFuv The given answer is Fuv = ∂uAv - ∂vAu .When I expand the commutator I get this...
  17. mktsgm

    EM wave penetration through walls....

    What are the factors that determine the property of wave penetration of bodies? For instance we can listen to fm radio from inside the walls of a room. But visible light is unable to penetrate the walls. Similarly some metals reflect some frequency but absorb some. How atomic orbitals play...
  18. J

    Explaining Light intensity with EM Field Oscillations?

    I have understood that the frequency of an EM wave is caused by the frequency by which a charged particle oscillates, which causes its electrical field to periodically change its strength with respect to a fixed location point at a distance from that particle. The more energy (heat) you add to...
  19. P

    EM Field Angular Momentum Emission Without Emitting Energy?

    To radiate energy, the Poynting vector must not drop faster than with the inverse square of the distance. Under what circumstances can EM angular momentum be emitted to the vacuum of space (i.e. without being recovered via inductive coupling) and yet not lead to energy losses through radiation...
  20. Z

    Prove that a capacitor driven by an AC voltage radiates EM

    Homework Statement When the capacitor driven by DC voltage ##V_0##,it has the electric field distribution ##f(x,y,z)## When ##V=V_0e^{iwt}##,how to show the EM travel in the space forever like the light? Homework Equations ##-{\nabla}^2E-u{\epsilon}\frac{{\partial}^2{E}}{{\partial}t^2}=0...
  21. R

    B EM Drive: Why Isn't Anyone Discussing It?

    I notice there has not been a post about the EM Drive on the website in quite some time! This surprises me considering it is such a widely discussed issue in the general public. With the recent peer-reviewed publication about the device, I thought there would be a lot more discussion aboutut it...
  22. Z

    Finding the magnetic field of a wave from the E field

    Homework Statement Many sources of electromagnetic waves (stars and light bulbs, for example) radiate in all directions. A simple example of the electric field for a monochromatic electromagnetic wave produced by a spherical source is $$E(r,\theta,\phi,t)=A\frac{\sin \theta}{r} \big(\cos...
  23. Z

    Magnetic field in an offset hole in a conductor cylinder

    Homework Statement A long (infinite) wire (cylindrical conductor of radius R, whose axis coincides with the z axis carries a uniformly distributed current I in the +z direction. A cylindrical hole is drilled out of the conductor, parallel to the z axis, (see figure above for geometry). The...
  24. Kaura

    The Controversy of the EmDrive: Legitimate or Feigned?

    There has been a lot of controversy surrounding the EmDrive and it supposedly being a reaction-less drive Many have pointed out that this goes against the conservation of momentum I have seen videos of a couple people dismissing the notion and outlining the apparent issues with the physics I am...
  25. yabb dabba do

    Induced potential - when do you have to consider EM waves?

    There's a long conductor carrying a 60 hz AC current. There's a second conductor parallel to the first current carrying conductor, and a hundred meters away from it. I want to know what the electric potential induced by the changing B field is in the second conductor. Theoretically I could...
  26. A

    Questions about batteries and magnets and EM induction

    Hello, I wanted to ask a few questions regarding magnets and batteries, and EM induction. First off, could I use batteries to replace a low voltage dc power supply, like say 2 V or so by connecting the ends of the battery in series to a circuit? As an extension, could I connect two batteries in...
  27. G

    Different formulations of the covariant EM Lagrangian

    Homework Statement I'm reading through A. Zee's "Quantum Field Theory in a nutshell" for personal learning and am a bit confused about a passage he goes through when discussing field theory for the electromagnetic field. I am well versed in non relativistic quantum mechanics but have no...
  28. G

    Intensity of EM wave independent of frequency?

    Hi. I'm a bit puzzled that the classical formula for the intensity of a monochromatic, linear EM wave $$I=\frac{1}{2}\cdot c\cdot \varepsilon_0\cdot E_0 ^2$$ seems to be independent of frequency whereas I find for the energy of a mechanical wave (e.g. on a string with total mass ##M##)...
  29. R

    B De Broglie Wavelength & Classical EM Wavelength Comparison

    Under what condition would particle de Broglie wavelength be equal to the classical EM wavelength?
  30. P

    Em waves and magnetic field lines

    So i know that magnetic field lines are closed, in an electromagnetc wave how can be these lines closed? i cannot picture this in my head
  31. Anypodetos

    I Interaction Term in EM Lagrangian - Explained

    The (classical, relativistic) Lagrangian for electrodynamics contains the field energy density -FμνFμν/4 and the interaction term -Aμjμ. I understand the maths of that - for one thing, the equations of motion turn out right if you plug this into the Euler Lagrange equantion. Now I recall having...
  32. Narayanan KR

    Can capacitors receive EM energy?

    Dynamos and transformers have inductor coils reacting with changing magnetic fields and importing energy into the circuit in form of induced current 1. What about the counterpart of above principle in case of capacitors? 2. Will capacitors interact with...
  33. Jackson Lee

    EM waves and traditional method of transmission

    Hey, guys. We all know power of AC currents is transferred via EM waves, but we seldom use that to calculate power in AC system. The reason for this is wavelength is very long. I want to know if there is really existed a transmission line which is 500km, just the length of EM wavelength, then...
  34. Jackson Lee

    Is AC energy really transferred as EM wave?

    Hey, guys. There is an interesting question about EM waves. I know electricity is transferred on transmission line as electromagnetic wave. But electromagnetic wave is one type of transverse wave, then how is AC electricity like? I feel it hard to imagine it. Besides, conductors could be used...
  35. Mayan Fung

    Can we define a circular loop with moving electrons as magnetostatic?

    When I learned magnetostatics. My teacher and book said that it is the case of steady current. However, if I consider a circular loop, the electrons are in fact moving in uniform circular motion. That means they are accelerating. How come we can still define it to be a magnetostatic situation
  36. H

    How could 2 crossed fields polarize and deflect EM radiation

    In the 1953 science fiction novel Childhood’s End by Arthur C. Clarke, characters use two crossed fields in outer space to block some of the solar radiation traveling towards earth: “Somehow, out in space, the light of the Sun had been polarized by two crossed fields so that no radiation could...
  37. R

    EM Drive Question: Coupled Photons & Thrust Change

    Why is an interferometer needed to prove the new Finnish researchers hypothesis about coupled photons and not simply an EM drive, which exhaust angles can be changed in order to measure a corresponding thrust change?
  38. A

    Clarification "Force Carrier" and "Radiation" Gravity and EM

    I need clarification on the difference between a “force carrier” and “radiation.” Imagine two electric charges separated by a distance “d” but not moving. They exchange “force carrier” photons which tell the electric charges to attract/repel. But you can’t see these “force carrier” photons...
  39. C

    B Classical EM Wave: Can Model be Proved Wrong?

    There is also a classical model of a photon available, which to me explains a particular behavior of it. Can this model be proved to wrong ?
  40. M

    What causes oscillations in EM wave fields?

    I've been trying to understand what an electromagnetic wave is, and have spent quite a while now reading around and piecing different bits of information together to try and get an answer. I haven't yet found an answer to my title question. It might just be because I have a lack of...
  41. S

    Metals touching, produce EM waves

    Can you help me on this question I had for years? Every time I touch two metals together (holding them with my fingers) and place a shortwave SSB radio nearby, I hear clicks and noises on the radio. Why is that happenning? Has anyone observed this phenomenon before?
  42. D

    Charge at constant velocity emitting EM waves?

    So far I have came to know that when a charge is accelerated the electric field magnitude around the charge changes and the effect is not felt instantaneously. The change in magnitudes of electric and magnetic field travels outwards at speed of light creating the so called EM wave. So the EM...
  43. Mr-R

    A Canonical quantisation of the EM field

    I have just gone through chapter 14 on the QFT for the gifted amateur by Lancaster and Blundell. Quantising the electromagnetic field results in the Hamiltonian: $$\hat{H}=\int d^3p \sum^{2}_{\lambda=1} E_p \hat{a}^\dagger_{p\lambda} \hat{a}_{p\lambda}$$ with ##E_p=|p|##. In this post ##p##...
  44. E

    I At what point does classical EM become relativistic EM?

    By classical E&M I mean the general basics. I understand that even classical EM has clear relativistic undertones, but is anything taught classically actually wrong under the context of relativistic EM? For example, does Faraday's law continue to hold etc. I know this is kind of a broad...
  45. S

    Any sunlight band pass filter for 400-700nm of EM Spectrum?

    This 400-700nm slice is the Photosynthetically Active Radiation (PAR) required of plants and, for many applications, there is much unwanted UV and IR heat outside of it. Is there any known translucent material or film that will pass a high % of that 400-700nm wavelength from sunlight while also...
  46. P

    I Deriving EM Energy Conservation from Lagrangian

    I'm trying to derive the conservaton of energy for electromagnetic fields with currents from the action principle, but I have some trouble understanding how the interaction term in the Lagrangian fits into this. The approach I have seen so far has been to express the Lagrangian density as...
  47. carllacan

    I Why can't the real scalar field and the EM be coupled?

    According to David Tong's notes the real scalar field can't be coupled to the electromagnetic field because it doesn't have any "suitable" conserved currents. What does "suitable" mean? The real field does have conserved currents, why aren't those suitable?
  48. LarryS

    I Is Canonical Momentum conserved?

    Given a system of charged particles interacting with an EM field. Is the canonical momentum always conserved? If so, what is the associated symmetry? Thanks in advance.
  49. P

    How exactly are high energy EM waves harmful for us?

    I've always read in my Physics textbooks that high energy EM waves like x-rays and gamma rays, if our body is exposed to them for a long time, can damage the skin significantly. However, how does that happen at an atomic level? As far as I'm concerned, the thing that differentiates a high...
  50. D

    How EM Waves Move: Exploring Electric & Magnetic Fields

    An EM wave is nothing but just magnetic and electric fields regenerating each other. Now if a charge oscillates and it produces sinusoidally varying magnetic field which induces an electric field perpendicular to it at the same place.This induced electric field even varies sinusoidally thus...
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