Em Definition and 1000 Threads

Bare with me, I haven't gone to 3rd level yet. Hopefully, if I get enough points, I will be at University next year. I am just wondering, what is the difference between energy traveling as a wave, like electro magnetic radiation, and the wave state of a particle? Are these the same waves...

The following wikipedia article derives the energy stored in an E-field (under "Energy stored in an electric field"): http://en.wikipedia.org/wiki/Electrical_energy I don't quite get how the following term goes to zero in the article? \frac{\epsilon_o}{2}\int V\mathbf{E}\cdot dA

Hi, I'm rather new here. I've got a question that's bothering me, and therefore decided to ask here. Why is it that in a lift/elevator, radio signal usually gets weakened? And, as far as i know, FM radio operates on the radio wave band, as well as cell phones. But why is it that mobile...

In my Book (Becker, Becker, Schwarz) it is stated (eq 3.23) that the holomorphic component of the EM tensor is given by T_X(z)=T_{zz}=-2 : \partial _z X \cdot \partial _z X : Now why is the expression for the (holonorphic, zz, component of the) energy momentum tensor in complex coordinates?

In my ST book the world sheet EM tensor for the Polyakov action is given by T_{\alpha \beta} = -\frac{2}{T} \frac{1}{\sqrt{-h}} \frac{\delta S_{\sigma}}{\delta h^{\alpha \beta}} Why is this? (Searching through the internet I found clues to what is called Hilbert's prescription, namely...

I was wondering, when an EM wave goes from one medium to another why is it that the frequency remains constant?

suppose that I want to take a graduate level QM course, what kind of math do I need? would a solid knowledge on linear algebra and differential equations suffice? what about a graduate level EM course?

I can't understand how the D'Alembertian x 4-vector Vector potential = mu x 4-vector J Have \nambla^2\A-1\c^2\ frac{ \partial^{2}A}{\partial {t}^{2} } } = \-mu\J

I looked everywhere to find a certain Tshirt and no where are they made so.. I went to a local shop. The poor guy had NO idea what it meant. History My son's High School scolastic team was holding an NAQT tourney at his school and since the kids were working so hard I figured that something...

Homework Statement An important news announcement is transmitted by radio waves to people who are 81 km away, sitting next to their radios, and by sound waves to people sitting across the newsroom, 2.3 m from the newscaster. Take the speed of sound in air to be 343 m/s. What is the...

Hi, The energy of the quantum mechanical harmonic oscillator is proved to be quantized after solving the Schrodingers equation which leads to Hermite equation and discovering that normalizable solutions of the wavefunction exist only for a discrete spectrum of energy. When the...

Homework Statement energy density in a laser beam is 9.5x10^-8 J/m cubed. the laser beam is cylindrical, and it delivers a power of 15W to a certain area. What is the radius of this circular area? Homework Equations Intensity : S=cu and S=Power/Area Area of a circle (or possibly the...

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...

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...

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...

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?

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...

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}...

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.

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 -...

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??

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...

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...

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...

How does Electromagnetic waves travel in vacuum ?

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...

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...

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...

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...

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?

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...

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...

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

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)...

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...

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...

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...

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...

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...

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...

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

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...

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...

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...

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

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...

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...

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...

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?

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...