Em Definition and 1000 Threads

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. C

    EM waves in phase and E/B = c question

    My textbook (Serway and Jewett, Physics for Scientists and Engineers) says that Emax/Bmax = E/B = c. And that E and B are in phase. My question is, if they are in phase, they both reach zero at the same time. At that point, E/B = 0/0 and not c. I know I am missing something, but not sure what...
  2. K

    What charges generate EM waves in free space?

    as we know light travel in vacuum because of oscillation of electric and magnetic field and both are perpendicular to each other. But i don understand how these electric and magnetic fields get generated in vacuum. as electric filed can vary by oscillating charge and that generate varying...
  3. PumpkinCougar95

    Confusion about the Energy Density of EM waves

    I am a bit confused about the energy density in an EM wave. why do we take the Peak value of E vector while calculating the energy density? Like if the E field is ##E_0 Sin(kx-wt)## what is the energy density of the EM wave(Magnetic + Electric)? is it A) ##\frac {e_0E_0^2}{2} ## or B) ##...
  4. AdrianHudson

    I Particle Focusing using EM fields

    Good day all! I have a question that has been in my head bouncing around for a while and I figured the best resource would be here as I can't find anymore related information (Or I am just looking in the incorrect spot). My apologies if this is in the wrong section I wasn't too sure where to...
  5. Physics345

    Cell Phone EM Waves - Health Risks & Physics Explained

    Homework Statement A cell phone sends and receives electromagnetic waves in the microwave frequency range. a) Explain the physics of how an oscillator creates these waves. b) Research the possible side effects of using cell phones. citing at least three websites that you consider reliable...
  6. Gene Naden

    I Derivatives of EM Four-Potential: Euler-Lagrange to $\nabla \times B$

    So the Euler-Lagrange equations give ##\partial _\mu ( \partial ^\mu A^\nu - \partial ^\nu A^\mu ) = J^\nu## with ##B=\nabla \times A##. I want to convert this to ##\nabla \times B - \frac{\partial E}{\partial t} = \vec{j}##. I reckon I am supposed to use the Minkowski metric to raise or lower...
  7. Gene Naden

    A Euler-Lagrange Equations: EM Field Term

    This problem is about one small step in the derivation of Maxwell's equations in free space from the field Lagrangian. The Lagrangian contains a term proportional to ##\partial \mu A_\nu \partial^\mu A^\nu - \partial \nu A\mu \partial ^\mu A^\nu## where A is the four-vector potential. The...
  8. P

    I EM Radiation of "Permanent" Molecular Electric Dipoles

    Suppose I were to subject a polar molecule to a high-frequency electric field. The polar molecule responds to the high-frequency electric field and thus it has a time-varying electric dipole moment vector. If we treated this as a classical electric dipole, it would be expected to radiate some of...
  9. C

    Is it possible to make a passive element that doesn't reemit EM

    Is it possible (and if so how practical) to overdamp a conductor wire that is many times longer than the wavelength of the incident radiation so that it doesn't reradiate any detectable amount, turning practically all incident radiation into heat?
  10. I

    Two States of Polarization of EM Waves

    I am studying about the cavity radiation inside a metallic cube. In the textbook it states that there are two independent waves corresponding to the two possible states of polarization of electromagnetic waves. What does it mean by this? (My current assumption is the phase change of the waves)...
  11. P

    How Does Absorbing an Electric Field Affect the Magnetic Field Across a Surface?

    Homework Statement An electromagnetic wave is incident on a surface which absorbs all the electric field. Use Maxwell’s equations to determine the magnetic field on the other side of the surface. Homework Equations The Attempt at a Solution My initial thought was that ##B=0## as a varying B...
  12. J

    Phase Change and Reflection of Electromagnetic Waves

    Homework Statement Note : There are no minus signs in first two options . Homework EquationsThe Attempt at a Solution The wave is propagating in +z direction whereas the electric field is varying in x direction . On reflection , there is a phase change of π . Also wave starts traveling in...
  13. Pushoam

    Relation between E and B of em radiation

    Homework Statement Homework EquationsThe Attempt at a Solution From the equation it is clear that the wave is propagating in y direction. So, the magnetic field is along z- axis. ……….(1) Dimensionally B = E/v……….(2) According to the above two conditions , the correct option is (c). Is this...
  14. Pushoam

    Wavelength of em radiation of H - atom

    Homework Statement Homework EquationsThe Attempt at a Solution E = (13.6 – 3.4 = 10.2)eV = ## \frac {hc } { \lambda } ## =1.22 * 10^{-5} cm So, the correct option is (a). Is this correct?
  15. S

    Graphics illustrating vector potential for plane EM wave

    I'm looking for a diagram or animation that shows the vector potential A (in the form of arrows or whatever) superimposed on the E and B fields of a plane EM wave. Since A is not unique, maybe two or three versions of the diagram (including one with Coulomb guage). An animation with a slider to...
  16. H

    A Supersymmetry and the EM Force

    (This is easy for you folks that know SUSY.) I know in SUSY the agent of the EM force is the photino (a spin 1/2 fermion). The EM force that we all learned about in JD Jackson (and the QED version in Sakurai) has the photon (a boson) as the agent of force. Is the SUSY version of E&M, and the QED...
  17. D

    EM Wave Propagation Homework.Incident/Transmitted Power Density

    Homework Statement An E field with f = 2.45*10^9 Hz passes through a material with the following properties e_r = 10 u_r = 1 sigma = 1 (S/m) The Incident E field has peak magnitude of 300 V/m at the air to surface boundary. (a) *solved* Find the incident power density at the material...
  18. Z

    Relation of EM radiation equation to radioactive decay

    Hello, I am a Mechanical Engineering student but I am a TA for an electricity and magnetism course, and I had a student ask a question that's a little bit outside my understanding. The question was related to the equation for a radiating electric field from an accelerating charged particle...
  19. DoobleD

    EM wave generation using a single charge in a vaccum?

    I wonder if it is possible to generate a propagating EM wave by making a single charge oscillate in vacuum ? In practice, when we create EM waves, we (always ?) have opposite charges oscillating, like with a dipole antenna : In the dipole antenna case, I can see how the fields detach...
  20. L

    Electric dipole EM field using Lorentz Transformation

    Homework Statement An electric dipole instantaneously at rest at the origin in the frame K' has potentials \Phi'=\mathbf{p}\cdot\mathbf{r}'/r'^3 and \mathbf{A}'=0 (and thus only an electric field). The frame K' moves with uniform velocity \mathbf{v}=\vec{\beta }c in the frame K. Show that in...
  21. BuCDr3H4

    Energy of beamed EM field that can heat human tissue

    I only have a background in mathematics, and I lack knowledge on this matter. I want to get quantitive value about that problem. What would be the kind of wave or EM field that could heat human tissu of 1°C in (say a body part like the thigh), say in one hour, at a distance of 2m ? We could...
  22. Kara386

    I Define inner product of vector fields EM

    I'm reading a textbook on electromagnetism. It says that for two vector fields ##\textbf{F}(\textbf{r})## and ##\textbf{G}(\textbf{r})## their inner product is defined as ##(\textbf{F},\textbf{G}) = \int \textbf{F}^{*}\cdot \textbf{G} \thinspace d^3\textbf{r}## And that if ##\textbf{F}## is...
  23. Borek

    I Neutron star merge why didn't all EM radiation came at once?

    Bear with me, I am just a chemist. Observations took several days (up to two weeks if memory serves me well). What I wonder is - why had different types of the EM radiation came at different times? Gamma burst was observed at almost exactly the same time gravitational waves were detected, but...
  24. Ontophobe

    I EM Force vs. Gravity: Modeling Spatio-Temporal Curvature

    Why is it that the EM force can't be modeled on spatio-temporal curvature the way gravity can be?
  25. T

    B Why is a wire necessary for tasers to deliver an electrical shock to the target?

    Why does a wire is needed for carrying the electrical current down the target in tasers? Perhaps without the wire the elecrical current needs to be much more strong? Why don't construct some kind of wireless taser that uses ultra-super-strong-electromagnetic pulse (which would make it as...
  26. F

    Is the polarization of electromagnetic waves definite or in superposition?

    Hi, In Classical electromagnetic wave.. does it have definite polarization when the EM wave leaves the sun for example? Or is it in superposition and the polarization only exist after measurement just like in QM? I don't understand the Maxwell Equation. Does Superposition in Maxwell Equation...
  27. RJLiberator

    EM: Electric field, Two thin rods uniform line charge

    Homework Statement Two identical thin rods of length L carry the same uniform line charge distribution (charge per unit length) of . If the two rods are collinear (aligned along the same line), with a distance of d between their nearest ends, calculate the Coulomb force (magnitude and...
  28. L

    How to determine the direction of the E-field of an EM wave

    Homework Statement Consider a wave vector which hits a plain boundary between water and air. The wave vector hits the boundary with an angle α1 measured from the vertical axis. The magnetic field amplitude has a y-component only. Also, notice: The z-axis is the horizontal axis, the x-axis is...
  29. Daniel Petka

    How is the orientation of EM waves related to the wavelength and antenna height?

    A while ago, I saw an explanation of radio waves. Overall, it's a decent vid. I bumped into some nasty problems. Basically, the image shows that the wavelength of a EM-wave corresponds to the height of the antenna. The visualisation however can't be right since the wave cannot go upwards- the...
  30. B

    I Absorption rate on a quantised EM field

    I have this problem: Consider the next state for the EM field: ##|A\rangle=\frac{1}{\sqrt{N!}}\left ( \int e^{-k^2} \boldsymbol{a}^\dagger_{k,+} \right )^N |0\rangle##. There is a particle in the unique bound state for spherical potential ##V(r)=-\delta ^\prime(r)/r##. for absorb a photon if...
  31. D

    Other Books for Geometry, Real Analysis and EM

    Hi, all. I would like to read books about the topics - Geometry, Real Analysis and Electricity and Magnetism. And I find the followings. Are they decent and rigorous? Geometry The Real Numbers and Real Analysis Introduction to Electrodynamics Classical Electricity and Magnetism Electricity...
  32. C

    I How do we interpret an EM wave using Quantum Mechanics?

    I know that if the intensity of a light beam with of a certain frequency varies it means the number of photons the light beam is composed of varies and not the individual photons energy. That would mean the E and B field amplitudes vary. This would mean that that the amplitudes of the E and B...
  33. V

    Experiment to measure EM waves

    Okay, let me prelude this by saying I only have an -EXTREMELY- limited understanding of classical physics, and zero knowledge of quantum mechanics. This is really just my asking a few questions in regards to an idea I had for an experiment, and what would be a good and accurate way to...
  34. A

    How can two equal and opposite forces produce motion? [EM]

    I am having hard time understanding the definition of work done to move a charge in an electrostatic field book says that if charge A and B sitting at some position and now if I move the charge say A towards B from arbitrary direction that force required is exactly equal and opposite to the the...
  35. S

    B Local/Proper Time of EM Waves: Explained

    This may be a dumb question, but some of these other thread got me wondering: is there a concept of local/proper time for electromagnetic waves? I imagine the only 'clock' that could measure time (ticks) at the speed of light would be the field oscillations.
  36. X

    I EM Waves in a Rotating Frame: Questions & Answers

    Hello there, I have a question (two very similar questions) about the time and phase delay between rotating objects. I want to describe two extreme cases here: I would appreciate any helps. Case 1 Imagine two observers (people with telescopes maybe) in space that are standing thousands of...
  37. C

    Apparent violation of momentum conservation in EM

    We have two cylindrical shells (with radii a and b and charges +Q and -Q) along the z-axis. There is a uniform magnetic field (in the positive z-direction) out to radius r from the z-axis. Because of the symmetry, the total field momentum is zero. Suppose we now connect the shells using a wire...
  38. Pushoam

    Variance of the EM wave equation under Galilean transformation

    For using Galilean transformation, I have to assume that speed of light w.r.t. ether frame is c. W.r.t. ether frame, E = E0 eik(x-ct) W.r.t. S' frame which is moving with speed v along the direction of propagation of light, E' = E0 eik(x'-c't') Under Galilean transformation, x' = x-vt, t' = t...
  39. Pushoam

    Do photons emitted by charged particles in a uniform loop get canceled out?

    A clump of charged particles moving in circular motion emits em radiation. But if I go on increasing no. of charged particles till the time the system becomes current in circular path, then since each particle is having same acceleration and velocity, the current will be uniform. So, now the...
  40. Pushoam

    Explaining transverse theory of light using EM theory

    just below eqn 3.25, it is said: At any given time, Ex is constant for all values of x, but of course, this possibility cannot therefore correspond to a traveling wave advancing in the positive x-direction. Why can't Ex be a constant?
  41. Arman777

    How to Calculate Energy in EM Waves?

    Homework Statement Homework EquationsThe Attempt at a Solution I didnt understand much the question. Should I use, ##υ=εE^2## ? Then I ll take rms value of that E.But I am not sure that is that the max value.Is it max value ? I didnt understand part (B)
  42. Pushoam

    Variation of EM radiation with frequency

    Homework Statement X-ray pulses, visible-light pulses, and radio pulses (the latter corrected for dispersion in the interstellar plasma) emitted by an astronomical object called a “pulsar” are all observed to arrive simultaneously at the Earth — with an uncertainty of only 200 microseconds. The...
  43. B

    I Why is the EM spectrum contiguous instead of gapped

    I understand the EM spectrum is made up of photons at different energy levels (wavelengths); where, the higher the energy level the shorter the wavelength. What I do not understand is how collectively photons are created at a range of energy levels that produce a contiguous EM spectrum. (When I...
  44. M

    The EM Drive: Exploring the Possibilities of Laser Propulsion

    I just had this thought while watching a video about the EM drive...basically using 2 lasers to bounce them off prisms inside an object to push the craft through space at a small but constant acceleration...could someone explain why this would not work?...please see attached image :)
  45. Vaibhav Sahu

    Propagation of EM Waves in metamaterials

    We have materials that have negative effective permittivity and permeability. In such materials, when the product of permittivity and permeability is negative, solving the wave equation yields a wave with a purely imaginary wave number. Does this mean complete attenuation of the wave ?
  46. 2

    How does an index of refraction affect the EM field?

    Hello. Say I have some refraction index n in a homogeneous material. Say I also have equations for the EM field (E and B vectors). Is it true to say that all that changes is the wavelength \lambda \to \frac{\lambda_0}{n} and consequently the wave vector k \to k_0 n ? Is it enough to account...
  47. pchu

    Casimir force experiments under environmental EM noises?

    I've always been curious about how rigorous are the Casimir force measurements carried out, because a couple of years ago I read some news about people inventing perpetual motion machine which turned out to be draining power from environmental EM noises, say, radio station, wifi, 3G/4G that keep...
  48. B

    B Is Classical EM Field the Same as Photon Wave Function?

    Quantum mechanics is non-local EM or electromagnetism obeys QM. But why is EM not non-local? How to make EM non-local?
  49. jlmccart03

    EM Waves - Would the bulb glow?

    Homework Statement A long, thin steel wire is cut in half, and each half is connected to a different terminal of a light bulb. An electromagnetic plane wave with E and B moves past the wire as shown. Part B: Suppose the wire were oriented parallel to the y-axis, as shown above. WOuld the bulb...
  50. M

    A Can classical EM be derived from QFT?

    In QFT, one can derive the equations for particles interacting electromagnetically by demanding phase invariance for the field when writing down the free field lagrangian for the klein-gordon or dirac equation. Question: Does classical EM follow from this method also? (At least theoretically...
Back
Top