#electrodynamics

#PhysicsFactlet:
Signals (e.g. light) move at a finite speed, so there is a time lag between when they are emitted and when they are detected. If the source is moving, the detector will "see" the signal that was emitted at a previous time, not the signal that is being emitted right now, and this time lag can change with time in a complicated way.
(Notice that, as the source is always moving slower than the signal, the detector sees the signals in the same order they were emitted.)
#Physics #ITeachPhysics #Electrodynamics #Optics #Relativity

As someone working in #Optics, it will never cease to amaze me how easy it is for people to misunderstand each other when the word "mode" enters the discussion.
(To be fair, the proper definition of "mode" in #electrodynamics is far from obvious and/or trivial.)

Wilhelm Eduard Weber died #OTD in 1891.

He was a German physicist and, together with Carl Friedrich Gauss, inventor of the first electromagnetic telegraph, which connected the observatory with the institute for physics in Göttingen.

The first usage of the letter "c" to denote the speed of light was in an 1856 paper by Kohlrausch and Weber (Elektrodynamische Maassbestimmungen). The SI unit of magnetic flux, the weber (symbol: Wb) is named after him. via @Wikipedia

#science #electrodynamics

Short story time:
When I was doing my PhD, we had in the lab an old Argon laser (which we used to pump a Ti:Sapphire, for those familiar with lasers). If you have never seen one, Argon lasers are massive, can output a ton of power, and eat a crazy amount of current, so much that the laser had its own dedicated industrial pentaphase plug.
I don't remember how many Amperes of current flew in those cables. What I remember is that, when you turned on the switch in the morning, the change in current (from zero to whatever the steady state value was) was enough to make the cable shake.
This happens because the electromagnetic field inside and around the cable stores momentum, and so it kicked the cable when building up.
I am not sure that laser still exists, and I have never been able to find a video of a cable shaking when the current is switched on, but it would be great to have such a video when teaching electrodynamics (and in particular how momentum and angular momentum can be stored in an electromagnetic field).
#ITeachPhysics #Physics #Electrodynamics #Laser

#PhysicsFactlet
If you put a dipole close to a mirror it is easy to see that its emission pattern is modified due to interference. A bit less easy to see (and thus less known) is the fact that also the total emitted power depends on the distance from the mirror, as at certain distances a significant fraction of the reflected wave goes back on the dipole in antiphase with the emission, thus reducing the emitted power.
#Physics #ElectroDynamics #Optics #LDOS #Visualization