A spinning disc top speed

[alan2here]

Set up diamagnetic levitation like this. goo.gl/owIJo, goo.gl/1efGV. Above or below the magnet place a disc of materal, the materal must be thin and light, it will probbably be light enough if it can be made thin enough. Put fins along the top or bottom of the disc from the center to the edge that are black on one side and white on the other, they can't extend far from the disc. Put the experiment into a vacuum such as that found inside a light bulb and shine a laser onto the end of one of the fins.

 

Would this test the materal to destruction? How long would this be likely to take with for example steel?

[Spyman]

I think the interaction between the disc and the magnetic field will provide friction and resist the spinning.

[swansont]

Possible but not practical.

 

It sounds like you're basically describing a Crookes radiometer, except that the cheap ones rely on heating the air to move, rather than radiation pressure, which causes it to move in the opposite direction (it has to be in a decent vacuum to work this way). Radiation pressure is typically small. The force exerted on a surface is P/c, where P is the power of the light hitting the surface. (You gain up to a factor of 2 if the surface is reflective). IOW, a light source with 300 MW hitting the vane will exert 1-2 N of force on it. In principle, even a small force will result in large speeds (and I'm ignoring relativity here), so you could use other numbers and estimate the spin-up time.

[alan2here]

I think the interaction between the disc and the magnetic field will provide friction and resist the spinning.

At slow speeds at least if this occurs it is very low.

 

@swansont

Yes. The idea is influenced by the crookes radiometer, but in this case it dosn't have nearly so much friction. I recorn that it has less resistance in air than the crookes radiometer does in a vacume.

 

If I understand you correctly then for example a 200 Mw laser could cause about 1 newton of force. This is force required to accelerate 1KG by 1MPS? This sounds like a lot.

 

I presume a good way to measure the decay in speed is by how long it takes to half in speed. I can't test it now but lets presume it is 1 minute in air and an hour in a vacume.

 

The magnet weighs 0.21g for near optimum size. I don't know how much the disc and fins would weigh.

Edited January 12, 2011 by alan2here

[imatfaal]

Alan - are you confusing MW and mW? Unless I have the wrong end of the stick a 300 MEGAwatt laser is required to exert 1 newton (3*10^8 watts/3*10^8M/s = 1 newton) or 2 newtons with perfect reflection. The way you write makes me think you believe this is a normal laser that you can get on the internet - they would be 200 MILLIwatt a factor of 10^9 smaller. BTW I am not saying that 200mW lasers are weak - they can still blind and burn; whereas I think a 300 MW laser would go straight through you)

[swansont]

Alan - are you confusing MW and mW? Unless I have the wrong end of the stick a 300 MEGAwatt laser is required to exert 1 newton (3*10^8 watts/3*10^8M/s = 1 newton) or 2 newtons with perfect reflection. The way you write makes me think you believe this is a normal laser that you can get on the internet - they would be 200 MILLIwatt a factor of 10^9 smaller. BTW I am not saying that 200mW lasers are weak - they can still blind and burn; whereas I think a 300 MW laser would go straight through you)

 

Exactly; I used 300 MW because of the value of c, but this is not practical. You are talking about microNewtons of force from a laser you would be able to easily obtain, and perhaps milliNewtons if you had a good research budget.

[alan2here]

I presumed you meant milliwatts in the same way that if a ruler was 300mm long I would presume you meant millimeters not megameters.

 

Thanks for the correction.

 

I still can't do the rest of the maths. I also forgot to factor in the air resistance of the fins in my 1min to half speed in air estimate. In a vacume though the fins would make almost no difference.

Edited January 14, 2011 by alan2here

[imatfaal]

Alan - it's an annoying way of doing things but SI units use m (ie lower case m) for milli (10^-3) and capital M for mega (10^6) . There is a nice article on the SI units here on wikipedia - it's pretty essential that you can work with them and understand them.

 

Edited January 14, 2011 by imatfaal

[swansont]

I agree; it's an easy mistake to make if you aren't used to the protocol. I'm going to a workshop on jargon in science later today; I wonder if this will come up, or if I'll have a chance to bring it up.

[imatfaal]

My favourite target for rephrasing: hypo-/hyper- this tends to confuse many patients in doctor's surgeries and hospitals. I have met people with diabetes who never realised what the difference was - or more importantly that there were two different words hypoglycaemic and hyperglycaemic. Until you read these words there is no difference between them and many people just do not have the resources to research their illness and discover these two opposites that sound almost identical.

[alan2here]

There is a nice article on the SI units here on wikipedia

 

Thanks, so yes, mW and not MW. Also, it's interesting and pleasing that they are organized together by one group like that. I imagined it as being more organic.

 

Back on topic. I don't see where anyone has either tried this or why it would be impractical. It probbably would be with more conventional forms of magnetic levitation or anything with any sort of contact at all, even if only on a pinhead.

Edited January 14, 2011 by alan2here

[Spyman]

The Crookes radiometer, also known as the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.

http://en.wikipedia.org/wiki/Crookes_radiometer

[alan2here]

It's not exactly like that no, there are significant differences in my design that make this work and that (possibly including other designs that use magnetic levitation but differently) not work well at verry high speeds. It's a nice animation though.