Crookes Radiometer, how it really works! - F-J's Physics - Video 148

[@AnthonyFrancisJones]

Hope you enjoyed the video. If you did please consider supporting my work by buying me a coffee at www.buymeacoffee.com/francisjonesa Very many thanks, F-J

[@peppermann]

My money was on momentum, absorption and reflection. I’ve learned something today! Great video as always 😎

[@AnthonyFrancisJones]

Thanks - most of what is out there is wrong! Not sure I did such a good job of explaining thermal transpiration but at least you know what does not cause it to turn! They are really interesting if you pour iced water over them and see the vanes go backwards. Thanks, as ever for watching, commenting and encouraging of course!

[@fractalnomics]

What interests me is air, alone, is a very poor conductor of heat, one of the poorest in nature (0.024 WmK) which implies (I presume) it has a poor ability to collide. 'Frozen air (aerogel) is industrially unique. But here, in this context, in a near vacuum, we are led to believe molecules collide so much it will stop the fins from moving (second explanation 9:27). I have heard the mean free path is around 1cm. I think it is number two. We don't need number three.

[@fractalnomics]

I found this in support. kzbin.info/www/bejne/bKCaZoWCoLtpeLM

[@AnthonyFrancisJones]

Interesting point Blair. Conduction is at a higher level a rather tricky concept. Number of colliding particles plays a part (one particle in a vacuum does not make for a perfect conductor!) but also there is a major role played by free electrons in some materials too. All a good talking point but I tried my best to get some misconceptions about this tricky device out for discussion! Thanks for watching again!

[@jonahansen]

The actual calculations that need be done to make a theoretical prediction for the kinetic recoil explanation are pretty complicated; well beyond just hand waving about the mean free path. For each temperature and pressure there is a distribution of particle velocities and the measured effect is a messy set of integrals over parameters. I take it on faith that people more skilled than me have ruled this out by doing the right calculations properly - I believe Einstein treated this device at one point. Maybe there is an experimental approach with shaped vanes that could shed some light on the matter...

[@AnthonyFrancisJones]

Indeed. You can do simple calculations using 3/2kT for the average translational kinetic energy but it is still way to low to impart enough momentum to the veins. I have no idea how to quantify the friction in the bearings of mine either! Yes it does need some detailed investigative work! Applied Science does a good job on his channel. kzbin.info/www/bejne/qGixdnyVeJ6MnpI

[@eugenioarpayoglou]

Why can I make it spin by swiping my fingers across the glass while holding the base with my other hand? It'll spin in either direction depending on the direction I swipe my fingers, and will spin quite fast.

[@AnthonyFrancisJones]

Interesting question. I am not totally sure but the heat from your hands may well be enough to get it going especially if you warm the glass. However, you would expect it to always spin in the same direction - silver sides leading (going away from you) when you view the vanes on the left hand side of the spindle from the front. It will rotate the other way if you cool it but I don't think this is happening in your case if the rotation direction is changing. Interesting and thanks for sharing your experiences of this simple but complex device!

[@eugenioarpayoglou]

@@AnthonyFrancisJones One other thing to note. It only seems to spin (in either direction) if I hold the base with my other hand, not if the radiometer is resting on a table.

[@AnthonyFrancisJones]

@@eugenioarpayoglou Interesting - it suggests some electrostatic effect but I am not sure that is really the case. I had a brief try with mine but it did not do quite what you said. I will try again as I did have it on the table.

[@Resonanttheme]

@@AnthonyFrancisJones I noticed if I walk fast it will spin up very fast and later learned just jiggling it for 3 seconds makes it go exceedingly fast. Perhaps it's like those gyro wrist powerballs how simple to and fro motion creates high speed spinning. Also the vibration of a stream of cold water about a foot below a faucet will make a vibrational energy that gets it going. If you have a base of some sort that vibrates, that too will spin it up.

[@AnthonyFrancisJones]

@@Resonanttheme I think what is happening here is something I noticed with mine (even when filming) that unless the light is really bright it does not get going. Static friction being larger than dynamic friction, if you give it a tap or a shake it overcomes the static friction and goes really well for a while! So sounds like you shaking it does this. Also it is really sensitive to heat once turning so any heat from your hand, even nearby, will cause it to work and possibly even faster. They are interesting things! Thanks for watching and making a really interesting observation.

[@jonahansen]

You know, there's a ton of videos on the Crookes' Radiometer on KZbin (as well as elsewhere), but I have yet to see one where someone goes in and experimentally tries to rule out various explanations. For example, if the edge effect of Reynolds was contributing, one would expect that vanes that had more "edge" would go faster; maybe vanes with lots of slots or perforations could be tried. If it went slower (there's less active absorbing area), that would be evidence against it. I think it's a heck of a lot easier to make a video and put it on KZbin than to actually do any serious experimental work...

[@AnthonyFrancisJones]

Yes, you are completely right! Experimental work on this device is really difficult. I almost have the kit I need and what you suggested is exactly what would be good to do. I thought I would introduce those that were perhaps not so familiar with the workings of this device to its oddities (cooling it is really interesting too!). A basic start but I hope I made a reasonable job of it. Now, Applied Science goes much further than me and closer to what you were suggesting but you may have already seen this. kzbin.info/www/bejne/qGixdnyVeJ6MnpI Should you do any experimentation on this yourself do comment back as it would be interesting to find out how you got on. Thanks for watching!

[@pianplays3659]

Cool video I’ve been trying to research how laser nudging works and came across Crookes radiometer experiments. Is there any fully proven explanation or just speculation around the science community ❤

[@AnthonyFrancisJones]

Good question! There is much argument (as you can see here) about the mechanism but from even simple school level calculations it is clear that it is not just the kinetic energy imparted by photons hitting the vanes. I think most 'academics' look to an explanation that might involve a number of mechanisms with thermal transpiration being the main one. It is similar to my video on the syphon - still no real clear an agreed upon mechanism. Hope that helps and good luck with your research and thanks for watching and taking the time to comment.

[@reze3416]

Not related to the radiometer at all but may I inquire where you got that cute dog stuffed animal? 🐶

[@AnthonyFrancisJones]

Good question, it's Laika from The Science Museum's (London) shop which you can get online too! shop.sciencemuseum.org.uk/products/laika-cuddly-toy-dog?srsltid=AfmBOootiLFsnLuk57pRA1U5rBHxRD7rEGBGvNkVcLTE00ku92TUjnWo

[@ppryorjr]

Reynolds was wrong. You can prove it by placing the radiometer in dim light and shining a laser on the center of a light colored vane. The center not the edge will heat, and the vane will move away. NO "transpiration". Thanks for the video.

[@AnthonyFrancisJones]

Peter, firstly thank you so much for your support and kindly 'buying me a coffee'. I am very grateful. Secondly, what you say is really interesting and I have not tried that. Do you have a mechanism that explains this. Could there be a number of processes that enable this apparatus to work? Be interested in your thoughts.

[@h7opolo]

Yet another youtuber failing to properly explain a phenomenon beyond their academics.

[@AnthonyFrancisJones]

Sorry the video did not meet your expectations. Yes, it is a very tricky device to try to explain. I could put in some more maths and explain thermal transpiration in more detail if you think that is what it needs to make things clearer. What do you feel the video is missing? If you have made a film on this too I would be very pleased to see it and learn further from that too. Hope future videos are more to your liking but thanks for watching regardless.

[@h7opolo]

​@@AnthonyFrancisJonesI'm being harsh. Your video was enlightening. More math is, as you postulated, what is lacking, IMO.

[@AnthonyFrancisJones]

@@h7opolo Thanks, yes for many a more mathematical approach would be the answer (if I have any answers!). I have tried to make the videos a bit 'wider appeal' so try to avoid too much maths if I can, which may dilute explanations. In the end I am just a humble school teacher trying to enthuse students and others to look into things, as you have done, more deeply.