The Case of the curiously ringing cymbal

Originally posted by CircleOfDust
reply to post by alfa1

 

are you sure they don't work in a complete vacuum?

If you mean the Crooke's radiometer pictured at that link posted by alfa1, that type of unit doesn't use a complete vacuum. As you make the pressure lower and lower, it's going to get harder and harder to turn. So whether it works or not as the pressure gets lower would depend on factors such as the intensity of the light source, the amount of friction at the pivot point, etc.

reply to post by Arbitrageur


It'll still work in a complete vacuum however.

Originally posted by CircleOfDust
reply to post by EasyPleaseMe

 

You certainly do have your work cut out ahead of you, for you first will have to convince CERN of your conclusions instead. Do that, and then we'll talk.

And why would I have to do that? What could I possibly learn from talking to you?

Originally posted by CircleOfDust
reply to post by Arbitrageur

 

It'll still work in a complete vacuum however.

What makes you think that?
If by work you mean spin, no it won't spin in a complete vacuum according to this:

Crooke's Radiometer

The effect begins to be observed at partial vacuum pressures of a few torr (several hundred pascals), reaches a peak at around 10−2 torr (1 pascal) and has disappeared by the time the vacuum reaches 10^−6 torr (10^−4 pascal) (see explanations note 1). At these very high vacuums the effect of photon radiation pressure on the vanes can be observed in very sensitive apparatus (see Nichols radiometer) but this is insufficient to cause rotation.

You need something more sensitive than a Crooke's radiometer in a vacuum.

Originally posted by alfa1

Originally posted by XL5
Try repeating the experiment but this time, cover the flash with black paper. In my opinion, its the magnetic flux that is making it ring and not the light.

Could be, but it may also just be the hot air in front of the flash, or indeed the heating of that part of the cymbal. Whatever it is, it sure isnt any "momentum" imparted by the flash's photons, as the text seems to imply.

I'd really like to see what effect a flash bulb on a Crookes radiometer would have.

reply to post by stormcell


Hey, we'll have no free-thinkers here

a reply to: CircleOfDust

anybody who knows anything about relativity should know that photons have momentum. Only for low velocities can you write mv for momentum. And, a massless particle (photon) has momentum hf/c

I am the guy who does AskthePhysicist.com quoted in this thread about radiation pressure. The expression there was ok only for a single wavelength which the flash is not. Rather, it is better described as a blackbody spectrum which includes all frequencies. If you try to just think about the visible light, you get a really tiny radiation force (on the order of 10^-13 N). However if you use the Stefan-Boltzmann law for radiation flux of a black body, flux=sigma*temperature^4 which includes all wavelengths of the spectrum, you would get something on the order of a micro-N. You can see more detail at askthephysicist.com...&a.html#radiationpressure (don't know why, but this whole link address does not seem to be clickable, maybe there is a maximum length to links?)

By the way, Crooke's radiometer does not work because of radiation pressure.

originally posted by: Arbitrageur
If it's not momentum, it's probably something like the rapid heating of the air for 1/2000 second next to the surface creating a shock wave that moves the surface (of the cymbal or shoebox), or it could even be some combination of something like that and momentum.

Yes, or more likely just plain very rapid differential heating of the cymbal exciting acoustic modes. Cymbal of course has very low acoustic dissipation so you can hear it even though the initiation was very short.

I also doubt the direct momentum transfer from photons.

Another possibility is the rapid electrical discharge creates a transient magnetic field which results in currents in the cymbal which results in a mutual force. This is very temporary and short-lived and so also induces an acoustic oscillation. The system is an inductor. Inductors are known to make noise from current transients.

The cymbal clearly has high Q resonant acoustic modes and you excite it with broadband force.

Presumably these two could be distinguished if you could create a similar current discharge nearby but don't transmit the light, and vice versa if you had a light pipe and the discharge current happened much further away.