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.

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  1. D

    The doppler effect and EM waves

    Homework Statement Suppose that a police car on the highway is moving to the right at 28 m/s, while a speeder is coming up from almost directly behind at a speed of 34 m/s, both speeds being with respect to the ground. The police officer aims a radar gun at the speeder. Assume that the...
  2. A

    Lagrangian density of the EM field

    Hello, I took an Electrodynamics course this semester, where we derived Maxwell's equations from the field's Lagrangian density. As a motivation, we "looked" for a scalar (in the relativistic sense) having something to do with EM fields - and had we found one we would have declared it a...
  3. Mentz114

    What Group Preserves the Invariance of E^2 - B^2 in Electromagnetic Fields?

    I understand that E^2 - B^2 is invariant under various transformations. If we consider the vector ( E, B ) as a column, then E^2 - B^2 is preserved after mutiplication by a matrix - | cosh( v) i.sinh(v) | | i.sinh(v) cosh(v) | I think this transformation belongs to a group...
  4. C

    Are EM radiation frequencies quantified?

    Maybe the wrong word but can you get electromagnetic radiation in any frequency or is there a "quantum" frequency for which all other frequencies are some integer multiple? Related question - is there an absolute low and high frequency?
  5. Mentz114

    Invariant quantities in the EM field

    I understand that the quantities E^2 - B^2 \vec{E} \cdot \vec{B} (the dot is vector inner product). where E and B are the electric and magnetic components of an EM wave, are invariant under Lorentz/Poincare transformations. Can someone explain the physical significance of this ? Is...
  6. S

    Solving Easy EM Problem Homework Statement

    Homework Statement For a time independant or static situation -\int_{a}^{b} \vec{E} \bullet d\vec{l} = \Phi(b) - \Phi(a) a) Show that for any closed path \oint \vec{E} \bullet d\vec{l} = 0 b)Using Stokes Theorem show that \oint \vec{E} \bullet d\vec{l} = 0 implies \vec{\nabla}...
  7. M

    Lagrangian density for the EM field in a dielectric medium

    Why does the Lagrangian density for the EM field in a dielectric medium take the form d^3 \bf x \left[ \epsilon \bf E^2 - \bf B^2 \right]? I can see that the expression for Lagrangian density has units of energy per unit volume as you would expect but that's about it. Much appreciated.
  8. M

    The action integral for the EM field in a dielectric medium

    Show that the action integral for the electromagnetic field in a matter with dielectric constant \epsilon is given by S_m=\frac{1}{8 \pi} \int dt d^3 \bf x \left[ \epsilon \bf E^2 - \bf B^2 \right] where the standard expressions for the E field and B field E=-\frac{1}{c}\partial_t \bf A -...
  9. R

    How does an em wave propagate at the speed of light?

    How an em wave propogates?? i understand that an em wave can be produced due to an oscillating electric field or oscillating magnetic field... but how does this wave move forward at the speed of light??
  10. Hans de Vries

    Detecting and analyzing higher dimensions via the EM radiation field.

    This should be possible with table top experiments rather than LHC scale experiments:abstract: Electromagnetic radiation decays with 1/r in three dimensional space, while the non radiating Coulomb field decays faster with 1/r^2. The general expressions for any dimension are 1/r^{(d-1)/2} for...
  11. D

    EM Spectrum Continuous or Discrete?

    I have a question about the EM Spectrum. Is it existing in strips or is it completely continuos? Can a photon have a frequency of any value or does it have to have specific wavelengths according to the space between 2 electron orbits? I understand photons are generated when an electron drops...
  12. F

    EM Troubles - Conducting body / potential

    So I have an upcoming project that I started working on. I thought I had the foundation to actually tackle the task at hand, but now that I've started it, I realized I'm kind of weak with some fundementals. If someone would be so kind to read what I am going to type, and add to it, or tell me...
  13. A

    EM Wave in Vacuum: How Does It Travel?

    How does Electromagnetic waves travel in vacuum ?
  14. S

    EM Fields, Space-Time, Gravity: Unifying Theory?

    Instead of calculating relative mass and density of an object to quantify gravity and gravitational forces, could Em field density be inserted into these calcs? why the shift from EM to Gr, relative to size? I know string theory is all the rage for unification, but why complicate an already...
  15. S

    Yup another oh crap I don't have the book issue. EM Fields, Wangsness

    Yup another "oh crap I don't have the book" issue. EM Fields, Wangsness my elements of solid state physics book finally came in the mail but now I need my EM book If you have it I just need the questions, the book got returned to sender so I need to wait longer for it than expected(I'm not...
  16. T

    Polarization Vector for Quantum EM Field

    I'm doing some calculations and I've run into something rather strange. I need to evaluate the following dot products \vec{\epsilon}_{k,\,s}\cdot\vec{\epsilon}_{-k,\,s'} = ? \vec{\epsilon}_{k,\,s}^*\cdot\vec{\epsilon}_{-k,\,s'}^* = ? where \vec\epsilon_{k,\,s} is the normalized...
  17. T

    Integrating EM Energy Density: Is E=hf?

    when you integrate the energy density (from electromagnetic field) times the differiential volume of the whole 3D space for a photon...would you get the energy of it? E=hf ? one more question... if there is a positive charge in 3d space... when i integrate the electromagnetic energy density...
  18. shintashi

    What is the Relationship Between Electromagnetic Amplitude and Power?

    Hi, I have a question about electromagnetic amplitudes. we can easily find the wavelength of any EM, but how do we determine its amplitude? in terms of joules vs. length vs. frequency?
  19. S

    EM Spectrum Limits: Frequency, Temperature & Planck Time

    I was wondering if there is upper and lower limit (absolute) of frequencies for electromagnetic waves? My reasoning was as follows: the lower limit is bound by close to 0 Kelvin temperature of emitter and the upper limit of possible frequency for EM wave is by "planck's time" by relationship...
  20. P

    In class final for a graduate EM class?

    I question the educational value of such an exam. We've been working out of Jackson this year, and based upon my experience the typical jackson problem (just one) takes at least two hours, even if you know what you're doing and make minimal mistakes (ie dropping signs, forgetting terms in your...
  21. R

    Transparency, Reflection, & EM Rays: Exploring Physical Properties

    What is the factor that determines whether an object will be transparent, translucent, or opaque? Also, what makes reflecting surfaces reflect EM rays?
  22. P

    Intensity of EM waves in a conducting medium

    I'm attempting to calculate the above, but I'm stuck. What I have thus far: intensity is the time average of the poynting vector, where the poynting vector is the cross product of the E and B fields. Using the given expressions, I get: \frac{\sqrt{a^2 + b^2}}{\mu \omega} E_0^2 \exp(-2bz)...
  23. H

    Tesla's anti-gravitation engine (EM)

    Hello, Here on this movie: http://video.google.com/videoplay?docid=2256433507046558974&q=tesla&pl=true at 19:35 There is shown Tesla's engine, which balances EM forces, that are able to raise a piece of metal into the air which then keeps on staying there. If anybody is familiar...
  24. K

    What electronic comonents damaged in an EM Pulse

    I was debating on another forum about EM Pulse (yes, I just watched War of the World :-p ). Anyway, does anyone know exactly what electrical components will be damaged by an EMP? I know silicon chips would be dead, but what about capactors, diodes, resistors, etc. Also, if a cars...
  25. L

    EM: accelerated charge and E field

    An electron is initially at rest. At a time t1 = 0 it is accerated upward with an acceleration of 10^18 m/s^2 for a very short time (this large acceleration is possible because the electron has a very small mass). We make observations at a point A, which is 15 meters to the right of the...
  26. E

    EM Wave in Plasma: Reflection, Attenuation & Radiated Power

    Can a monochromatic plane EM wave exists in a plasma when \omega is less than \omega_p (the plasma frequency)? If so, is it attenuated? I read in a text that a wave incident on a plasma with \omega < \omega_p is completely reflected, but does this mean such a wave can't exist in a plasma? If...
  27. T

    Calculating Induced EMF from AM Radio Station at 4 Miles Away

    Here is the question i am working on: An AM radio station broadcasts with average power 4000 W in all directions. A dipole receiving antenna 65 cm long is at a location 4 miles from the transmitter. Compute the amplitude of the emp that is induced be this signal between the ends of the...
  28. C

    Energy in EM Waves problem (finding sun power output from EM wave intensity)

    Hi, if someone could help me figure this out I'd really appreciate it. Estimate the average power output of the Sun, given that about 1350 W/m^2 reaches the upper atmosphere of the Earth. I know the answer is 3.8 X 10^26 W but I don't know how to get there. I've tried different equations...
  29. S

    What Happens When You Connect Two Objects by a Wire?

    I'm a little confused about what happens when we connect two objects by a wire. Does this just equal the potentials, or does it equal the charges? Also, does this depend on the nature of the objects (is it different for e.g conductors or capacitors?) thanks for your help
  30. E

    How Does the Lorentz Force Fit into Classical Electromagnetism?

    I thought in classical physics, only four forces account for all mechanics: 1 gravitational attraction 2 electromagnetic force 3 nuclear strong force 4 nuclear weak force There is also a Lorentz force on a charged particle moving through a magnetic field. How does classical EM...
  31. D

    Calculating Rate of Change of Electric Field in a Circular Capacitor

    A circular parallel plate capacitor has plates of radius 0.1m; the plates are separated by 0.10 mm. A current is supplied to charge the capacitor. While charging, the potential difference across the capacitor increases by 10 V/μs. What is the magnitude of the rate of change of the electric...
  32. Gamma

    EM theory and therom dynamic books

    Hello, I need to purchase two books, for EM theory and Thermodynamics. I have briefly used couple of books years back and I can't seem to remember their titles or author. I slightly remember that the EM book was a blue color with a lots of information for first year - 4th year...
  33. D

    L's Contradiction Method: Proving EM Laws Independently of Frame of Reference

    Whats the simplest proof that the Electromagnetic laws are independent of frame of reference? .What I know is contradiction method which einstein also used to conclude the postulate of SR. BJ
  34. S

    EM Waves: Source & Coupling - My Little Doubt

    I have a little doubt. EM waves are coupled electric and magentic oscillations. But, to produce an electric field you need a charge and for a magnetic field, you need a moving charge ,but,electric field doesnot exist with a moving charge. So, what is the source of Em waves and moreover how...
  35. T

    Calculating Amplitude of Elliptically Polarized EM Wave

    Suppose I have an eliptically polarizied EM wave, whose electric field can be written as \vec{E}=A\cos(kz-\omega t)\vec{e}_x+B\sin(kz-\omega t)\vec{e}_y, where the constants A and B are amplitudes of the x and y components of the beam, respectively. How would I report the amplitude of the...
  36. E

    Magnetic Force from an EM Wave

    I'm not sure if this belongs here or in the relativity forum but associated with an electromagnetic wave is a magnetic field B and the force of this field on a charge q with velocity v is qv x B. Since any charge will be moving with speed c relative to the wave, will the magnetic force on a...
  37. E

    EM wave - nodes at metal surface

    I've read in a book, that if you had a standing electromagnetic wave between two metal plates, only nodes would be at the metal plates. This is due to the infinite high conduction of metal, the book says. Why is that so?
  38. M

    Is Einstein's Approach Key to Unifying Gravity and Electromagnetism?

    Einstein had it. He just looked to far: Light Falls. And Light gravitates. That's all there is to it.
  39. S

    Are Electromagnetic Waves Constantly Changing in Amplitude?

    argh hello guys, I've been trying to find out a simple answer to this question but all the resources I've looked at seem to skim over it or contradict each other, but simply: Do electromagnetic waves have a variable amplitude? "Intensity" is not necisarilly the same, if you're using the...
  40. F

    Simple EM Problem (not to me for some reason)

    I'm unsure what I'm doing wrong here... it has to be something. So let me explain the problem: I'm supposed to rank the net magnetfic field strength for various arrangements. In all of the arrangements the magnitude of the current is the same. The distance between each of the charges is the...
  41. C

    Medical EM fields: a plausible correlate of consciousness?

    http://www.surrey.ac.uk/qe/pdfs/cemi_theory_paper.pdf I approached this initially with extreme skepticism, especially since it seemed impossible considering the myriad em fields we're immersed in without any perceptible effect on our consciousness, but I ultimately walked away viewing this...
  42. B

    Why is the Lorenz gauge chosen for causality in EM potentials?

    Choosing the Lorenz gauge implies that sources of the EM potentials at a given point are the charge density (for scalar potential) and current density (for vector potential) that cross a collecting sphere converging at the speed of light toward that point. It is often said that the retarded...
  43. M

    The (E) theory: A new attempt to unify EM and gravity

    This paper (broken into three peaces; see attachments below) introduces a re-discussion of the Lagrange density. The tool to get this result is a new mathematical operation whose existence can be guessed from some considerations concerning the calculation of the variations along the time of the...
  44. J

    How are electromagnetic waves created?

    Hi, My question: Explain how an electromagnetic wave is produced? My Answer A charged particle produces an electric field. the electric field exerts a force charged particles. Positive charges accelerate in the direction of the field and negative charges accelerate in the direction...
  45. N

    Exploring the Basics of Electromagnetic Waves and Their Applications

    just want to ask what are electromagnetic waves and how are they produced? also, do you where i can get a simple application of EM waves? just really need it... thanks!
  46. A

    5-suit Texas Hold 'Em: ace high beats one pair

    This isn't really a brain teaser as much as a game. I have a Five Crowns deck at home. This consists of a deck of 55 cards (well, two decks of cards) with five suits. 3 is lowest and King is highest. You can't do 3-King in a straight. There are jokers, but I'm not using them --...
  47. D

    Motor em interference and tapping into sensor

    I have a photointerrupter sensor that is near an electric motor in a scooter and I'm tyring to tap into it. When I tapped into the sensor with a wire, the scooter stops running. Initially I thought it was my speedo circuit but then i found out that if I tap into the sensor with just a plain...
  48. H

    Poisson brackets and EM Hamiltonian

    Consider the following general Hamiltonian for the electromagnetic field: H = \int dx^3 \frac{1}{2} E_i E_i + \frac{1}{4}F_{ij}F_{ij} + E_i \partial_i A_0 + \lambda E_0 where \lambda is a free parameter and E_0 is the canonical momentum associated to A_0, which defines a constraint (E_0 =...
  49. M

    EM: Net Force on Test Charge Q in 12-Sided Polygon

    ok i have been thinking about this problem for a lil bit now and i think i have it correct but i would just like to double check: (a) twelve equal charges, q, are situated at the corners of a regular 12-sided polygon (for instance, one on each numeral of a clock face). What is the net force...
  50. sweetser

    Unifying Gravity and EM

    Hello: I will try to meet the terms of the 8 guidelines. 1. The behavior of light is explained with a rank 1 field theory, the Maxwell equations. Gravity is explained with a rank 2 field theory, general relativity. The two can be combined in one Lagrange density, but they are not in...
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