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. jegues

    EM Problem: Floating Conducting Cylinder

    Homework Statement See figure attached. Homework Equations The Attempt at a Solution Originally I have solved this problem using Gauss's law by defining a gaussian surface in the form of a cylinder around the floating cylinder to get the electric field distribution underneath...
  2. S

    Is it accurate to refer to invisible radiation as light?

    I am a trainee science (specialising in physics) teacher. I upset my mentor a little bit by saying to a year 10 science class that there is "light" that people cannot see... by which I was referring to wavelengths of EM radiation that lie outside of the 400-700nm range. After the lesson, my...
  3. 1

    Inductive coupling and EM Power Chargers

    Recently saw the invention on PhysOrg for a device the recharges a AA battery using "waste" EM radiation from the ambient environment. In the photos, the author was placing an array of them near a power sub-station. This brings up two questions: 1) Does this create an inductive load on the...
  4. shounakbhatta

    Pulsars - Rotating neutron star produces EM radiation?

    Pulsars -- Rotating neutron star produces EM radiation? Hello, If a neutron star is composed of neutrons, which do not carry any electric charge then how it's rotation produces pulsars which are electromagnetic radiation? Thanks.
  5. Y

    Question on characterization of elliptical polarization of EM wave.

    I am using "Advanced Engineering Electromagnetics" 2nd edition by Balanis AND "Antenna Theory" 3rd edition also by Balanis. I found an inconsistency in how to characterize RHC (CW) and LHC ( CCW) elliptical polarization. 1) In Advanced EE Page 159, for \vec E(0,t)=Re[\hat x...
  6. C

    Electro-Magnetic Force: Photon Carriers Explained

    I got confused now while thinking of electro-magnetic force. Magnetic force is caused by electron spins. Electric current is a directed flow of electrons. The carriers of the electro-magnetic force are photons, but they don't cause any of the forces I wrote upwards. Why are they the carriers...
  7. Y

    Characterize EM wave propagate in +z and -z

    For RHC EM wave travel in +z direction, the unit vector is \hat {E}=\frac{\hat {x}+\hat{y}j}{\sqrt{2}} ignoring the ωt-kz. What if the RHC EM wave travels in -z direction? The unit vector should be \hat {E}=\frac{\hat {x}-\hat{y}j}{\sqrt{2}} ignoring the ωt+kz. Am I correct?
  8. Q

    Detecting visible light EM radiation with an antenna

    If visible light is part of the EM spectrum just like radio waves, is it possible to construct a radio like antenna (metal pronged structure) to detect them?
  9. A

    Propagating a Gaussian-Profile EM Wave in a Medium

    Homework Statement Hi everyone, As part of my final year project I am modelling the propagation of an electromagnetic wave through a medium of refractive index, n. I begin at the boundary between vacuum and the medium, x = 0 and propagate forward to some distance x. I have initially...
  10. Y

    Please verify my derivation on elliptical polarization of EM wave

    This is not a home work, it is part of the textbook on elliptical polarization. Attached is a page in Kraus Antenna book, I cannot verify the equation on the last line. Here is my work E_y=E_2(\sin{\omega} t \cos \delta \;+\; \cos \omega {t} \sin \delta) , \sin\omega {t} =\frac {E_x}{E_1}\;,\...
  11. Y

    Determine the polarity of the reflected EM wave.

    Attached are the drawings. Does the polarity Eof depend on \Gamma =\frac {\eta_2-\eta_1}{\eta_2+\eta_1} So if \eta_2<\eta_1, the Γ is negative and the polarity of the reflected wave is opposite polarity? I understand \theta_i=\theta_r and all that. As shown in the drawing, if you look at...
  12. L

    Are Poisson Brackets Consistent in Electromagnetic Field Theories?

    Since I couldn't find any reference on the subject of Poisson bracket formalism of classical field theory, I'm posting a few question here: A) What are the Poisson brackets of the source-less EM field? B) Does the law that the Poisson brackets between a dynamical variable and its conjugate...
  13. A

    Conceptual difficulties with the generation and description of EM waves

    Hello all, I had a question about Electromagnetic waves. Although it isn’t homework (just to settle something in my own mind) I thought this would be the best place to put it. Perhaps it would be best if I could explain a situation I could understand and then hopefully someone could bridge...
  14. B

    EM waves: multiple planar interfaces (PEC-backed lossy dielectric)

    Homework Statement Our prof has told us we can get help from wherever/whoever we want as long as it isn't classmates. This is a take-home test. The relevant question: You have a three-layer dielectric. | Layer 1 | Layer 2 | Layer 3 | Layers 1 and 3 are semi-infinite. Layer 1 is air (εr =...
  15. M

    Is Computing Using Electromagnetic Waves Possible?

    Hi all, I was just wondering if there has been research into computing using electromagnetic waves. It seems to me like a sealed off box with internal EM pluses could be used to mould waveforms through interference, which would act as logic gates to provide useful computational results. Would...
  16. T

    Gauge invariance of stress-energy tensor for EM field

    For free EM field: L=-\frac{1}{4}FabFab Then the stress-energy tensor is given by: Tmn=-Fml∂vAl+\frac{1}{4}gmnFabFab The author then redefines Tmn - he adds ∂lΩlmn to it, where Ωlmn=-Ωmln. The redefined tensor is: Tmn=-FmlFvl+gmv\frac{1}{4}FabFab It is gauge invariant and still satisfies...
  17. X

    Pick the easiest module from QM, EM or Relativity.

    The situation is messy so I'll avoid details. I absolutely must do one module this February that is listed as 'level 3' at the Open University without formally taking the prerequisites. I have to teach myself the required mathematics, in two months, to give myself a decent chance of passing. If...
  18. C

    Did bad in undergrad EM. retake or directly go to grad EM?

    I did pretty bad in undergrad EM and remember nothing. However, my advisor says that since I'm doing well in my mathematical physics class, and if I get an A, I should directly take the grad EM course to not waste time. However, he acknowledges that I had difficulties with EM in the past, so...
  19. C

    Why do metal boxes block EM waves?

    A metal box will block most EM waves. Is this because the oscillating E field will be canceled when in contact with the conductor. And because the oscillating E field is the source of the changing B field does this then kill the B field.
  20. A

    EM waves and blackbody radiation question

    Homework Statement A rectangular metal plate measures 0.20 m long and 0.3 m wide. The plate is heated to a temperature of 1,433 K by passing a current through it. Assuming that it behaves like a blackbody, how much power does the plate radiate under these conditions? Homework Equations...
  21. H

    Visible light wavelength discrepancy on the EM spectrum?

    Hello all, I'm not all too familiar with the electromagnetic spectrum, nor generally with physics but there is something that keeps me up. I learned, and also read now everywhere that visible light (to humans) lies in the wavelength range of about 400-700 nm. No I have a science book here...
  22. N

    A Unit Amplitude Polarized EM Wave Solution?

    A "unit magnitude" EM Wave? Homework Statement Write down the solution of Maxwell's equations corresponding to a plane polarised EM wave of unit amplitude and wavelength λ=2∏m, polarised in the zy plane and traveling in vacuo in the minus y direction.Homework Equations ∇.E = 0 ∇.B = 0 ∇xE =...
  23. mishima

    EM field and wave interactions of a point charge

    I've been thinking of 2 point charges separated by some distance in static equilibrium. When one charge is moved from rest, the EM field would change the way it looks at the location of the other point charge. This "changing in the looks" of the EM field as I understand propagates from the...
  24. R

    Complex Representation of Moving EM Waves

    Homework Statement Moving in negative x-axis: E1 = E sin(wt + kx) Moving in right axis: E2 = Esin(wt-kx-θ) moving to right write complex representation The Attempt at a Solution E1= E Im(exp[i*(wt+kx)]) E2= E Im(exp[i*(wt-kx-θ)]) are these correct or do I have to take into...
  25. M

    Em wave incident on a surface current

    Consider an infinite sheet of surface current described by the surface current density , K=dI/dl, where dl is a length element perpendicular to the current. For this sheet, the magnetic field B=μK/2, above and below the sheet, independent of the distance from the sheet, where μ is for free...
  26. N

    EM calorimeter vs hadronic calorimeters

    What is the difference between the two, in that EM calorimeter will stop photons and electrons effectively wheras hadronic calorimeters stop hadrons effectively (but both do not seem to affect much muons)? What materials are used in each type? One usually hears that the EM calorimeters...
  27. J

    What is the recommended textbook for upper division EM at community college?

    My community college physics instructor thought a certain textbook would be a good supplement to Lorrain and Corson's Fundamentals of Electromagnetic Phenomena. I was looking at something at the time and didn't pay enough attention, so now I don't remember exactly any of the authors, although I...
  28. D

    How Do You Solve EM Wave Transmission Through Multiple Media?

    Hi there, I'm stuck with this problem, I've already worked out some algebra but I can't seem to get what I am asked, when trying to solve the equations I simply can't get the answer, so I was hoping you could help me. Here we go Homework Statement Calculate the transmission coefficient T...
  29. S

    Finding the minimas from EM radiation.

    Homework Statement 6. S1 and S2 in the diagram are effective point sources of radiation, excited by the same oscillator. They are coherent and in phase with each other. Placed 4.0 y m apart, they emit equal amounts of power in the form of 1.0 m wavelength electromagnetic waves. (a) Find...
  30. F

    Quantum EM field hamiltonian

    Homework Statement Consider a charged particle of charge e traveling in the electromagnetic potentials \mathbf{A}(\mathbf{r},t) = -\mathbf{\nabla}\lambda(\mathbf{r},t)\\ \phi(\mathbf{r},t) = \frac{1}{c} \frac{\partial \lambda(\mathbf{r},t)}{\partial t} where \lambda(\mathbf{r},t) is...
  31. J

    So if FM radio signals modulate the frequency of the EM wave

    then how can you tune into, say, 101.1 when its not at 101.1 MHz all the time given the frequency change?
  32. J

    How are soundwaves sent via em waves?

    When the mic hears the noise, it creates an alternating current electrical signal right? So how does that translate to em waves? Is it just that a higher frequency radio wave length would resemble a higher pitch? If so, is it impossible to hear the pitch "produced" by, say, visible light?
  33. D

    General physics (or EM) book using vector forms for EM

    So far the best general physics book for EM I've found has been Alonso and Finn. The problem is that I just spent too much time trying to understand electric displacement using the hand-wave magic mathematical definitions they give. The rest of the book seems fine (it gives vector forms for...
  34. Drakkith

    EM Wave Phase - Fewer Photons at 0 Degrees?

    Let's say we have a radio wave coming into a receiver. When the phase is near 0, does this mean that fewer photons are being absorbed by the receiver compared to when the phase is near 90? (Hope I'm using the right terms here)
  35. B

    What is the effect of distance on the force created by a laser in outer space?

    Homework Statement Two astronauts are at rest in outer space, one 15.3 m from the Space Shuttle and the other 30.6 m from the Shuttle. Using a 123.0-W laser, the astronaut located 30.6 m away from the Shuttle decides to propel the other astronaut toward the Space Shuttle. He focuses the laser...
  36. snoopies622

    Limit to SHO / EM field analogy

    For a simple harmonic oscillator, the creation and annihilation operators can be expressed as linear combinations of the position and momentum operators, \hat {a} = \sqrt { \frac {m \omega} {2 \hbar} } ( \hat {x} + \frac {i \hat {p} } { m \omega } ) \hat {a} ^{\dagger} = \sqrt...
  37. J

    If EM waves travel forever, why do things get dimmer as they get further away?

    Other than nearby light pollution. Or is that the only reason?
  38. J

    Can Light Be Made Invisible Using Metamaterials and Graded Refractive Index?

    Recently a device invisible to a static magnetic field was described. The device features a cylinder with two concentric layers. While the inner layer consists of a superconducting material that repels magnetic fields, the outer layer is a ferromagnetic material that attracts them. Placed in a...
  39. N

    Advanced EM Radiation Books: Info on Velocity EM Field

    Hi ,does anyone know a good book in advanced EM radiation and it contains more information on velocity EM field , I have already finished Griffith's book
  40. B

    EM Waveguide Modes: What are they and how are they used

    Title pretty much says it all. In EM waveguides (rectangular, circular, what have you), I understand there are various "modes." I'm gathering that these represent specific, discrete solutions to the Hemholtz equation, and therefore there are only specific frequencies that will propagate? What...
  41. A

    EM Wave Creation and Physical Characteristics

    My book seems to not go into a lot of details as to how EM-waves are created. More or less they just state that they are created by accelerated charges. I guess that somehow makes sense since an accelerated charge is clearly altering the electric field everywhere in space and that is as far as I...
  42. B

    Voltage, capacitance and EM energy of a parallel plate capacitor

    Homework Statement This is an EM waves class, but we are given the following problem, which seems deceptively simple. I'd like some feedback to see if this is done correctly. Homework Equations Consider a parallel plate capacitor connected with a resistor with resistance R, as shown in the...
  43. X

    Photon Emission from electrons and the EM field

    Ok, I've been reading up on the EM field and how it exerts force on charged particles. By exerting this force it creates 'ripples' in the EM field and this is felt by other charged particles as a force (either of attraction or repulsion). We say that the two particles exchanged a virtual photon...
  44. U

    Experimental proof of retardation in EM fields

    In the light that QED suggests the speed of interaction is infinite, are there any experiments which makes it clear that mediating fields are retarded in classical electrodynamics ? Whereas, on the theoretical grounds the causality requires time assymetric interaction, but do we have any...
  45. M

    Exploring Possibility of Splitting EM Wave Components

    Hi eveyone, I am new to this forum and I hope that I can gain lots from this forum. Recently, one question comes into my mind, that is about the possibility of splitting the electric component and magnetic component of EM wave. This two components seem to always stick together. However, if...
  46. D

    Know of a field that combines EM, fluid mech and stochastic processes?

    Touch of background, about half way through a mechatronics engineering degree, I found that I love fluid mechanics and from previous studies I know I enjoy EM. To throw a curve ball in I also am fascinated by the concept of randomness and the associated mathematics. Now for the question, could...
  47. J

    Emission of EM Wave: Why Does it Happen?

    In wikipedia's description of transmitter, if an alternating current is given to an antenna, then the antenna radiates off em wave. But why this happens so?
  48. Roodles01

    How are modes classified in rectangular waveguides?

    Homework Statement How do I classify modes? If there is a rectangular waveguide how can I show it is operating in TE10, TE20 or TE11 mode. Homework Equations Could someone help clarify how the 10, 11 or 20 comes about? Is it due to the dimensions of the guide or something else?
  49. A

    Relationship between EM Radiation Energy and Electric/Magnetic Field Magnitude

    the energy of em radiation (or photon) is proportional to the frequency of the radiation. the em radiation (or photon) is composed of oscillating Electric and Magnetic fields. as such this energy must be stored in the oscillating electric and magnetic fields constituting the radiation...
  50. N

    Something to be clarified about EM Wave: TE, TM and TEM

    I have been reading different articles about EM wave modes, it is said that in TE waves the E_z is zero, similarly for TM H_z is zero, and TEM both E_z and H_z are zero. How about the E_x and E_y in TE wave? Someone said E_y is also zero and only E_x exists in TE...is this true? Is it...
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