Recent content by dEdt

nsaspook, although the potential difference across a larger device is greater, so too is its resistance, so the current should be the same in a large device as in a small one.

Why would a smaller device have a smaller current?

According to an Oak Ridge National Laboratory technical report (http://web.ornl.gov/sci/ees/etsd/pes/pubs/ferc_Meta-R-320.pdf ), smaller electronic devices are better able to resist the damage caused by an EMP. I'm at a loss to see why this is true. According to Ohm's law, the current density...

Hey guys, I'm trying to understand the properties of normal modes of the electromagnetic field inside an arbitrary cavity, but I'm having some trouble. By definition, for a normal mode we have \mathbf{E}(\mathbf{x},t) = \mathbf{E}_0 (\mathbf{x}) e^{i \omega_1 t} and \mathbf{B}(\mathbf{x},t) =...

Well, let's imagine that you put a warm bottle of beer in a refrigerator to cool it down. At the surface of the bottle there is (at least initially) a temperature discontinuity, because the beer and the air in the fridge are at different temperatures. Newton's law of cooling has no trouble...

Fourier's law of thermal conduction states that \mathbf{j}=-k\nabla T, where \mathbf{j} is the heat flux. Integrating both sides of this equation over a closed surface gives the equation \frac{dQ}{dt}=-k\int \nabla T \cdot d\mathbf A. If there is a temperature discontinuity across this...

As Steven Weinberg put it, "the idea of quantum field theory is that quantum fields are the basic ingredients of the universe, and particles are just bundles of energy and momentum of the fields." At least, this is one way to look at QFT. The other approach is to imagine that these particles are...

Please watch this video (link: http://www.youtube.com/watch?v=JN-A3RryOC8) for reference. As you can see, the magnet does not slow down (at least, not appreciably) when falling through the copper coiled around a PVC pipe, and I can't figure out why.

Thanks for all the answers. Quick follow-up question: in neon lighting, does the neon become a plasma, or are the electrons in the neon atoms just bumped to a higher energy level?

According to Wikipedia, in fluorescent lamps "[a]n electric current in the gas excites mercury vapor which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the bulb to glow." Why don't manufacturers use a gas that emits visible light directly when...

I just checked Reif's book and found his derivation to be very clear. Thanks.

If a blackbody is in equilibrium with the surrounding electromagnetic field, the power emitted by the surface of the blackbody will be related to the energy density of the electromagnetic field by P=\frac{cu}{4}. Try as I might, I haven't found a good derivation for this equation (the...

The derivation of Planck's law in my textbook begins with the assumption that the energy of an oscillator with frequency ##\nu## is quantised in units of ##h\nu##. It follows that the average energy of such an oscillator (in equilibrium with a reservoir at temperature ##T##) will be...

Right, but I'm not thinking about a real solid but rather Planck's model of a blackbody, made up of a large number of quantized oscillators. (I shouldn't have used the word atom.)

It still seems like the photon's energy has to stay a multiple of ##h \nu##. If the photon's energy starts off as a multiple ##h \nu##, and if every atom's energy can only change by a multiple of ##h \nu##, then it seems like the photon's energy has to stay a multiple of ##h \nu##.