Re: [Phys-l] drop a metal cylinder through a solenoid

["Marc \"Zeke\" Kossover" <zeke_kossover@yahoo.com>]

Having a little Phys-L/TAP-L overlap today. 

Anyway, I've made my own variable gap magnet using a pair of drilled and countersunk neodymium magnets and a Jorgensen wood clamp. 

I made a pendulum out of some plate aluminum and aluminum flat stock. I bought a small bearing to make it swing easily. I can quickly change the plate between thin (1/32 inch), thick (1/4 inch), punched with holes, and cut with slits. The whole thing works pretty well. 

You can see the directions here. http://kossover.squarespace.com/journal/2011/5/31/magnetic-clamp.html

Zeke Kossover



> ________________________________
> From: John Denker <jsd@av8n.com>
> To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> 
> Sent: Thursday, March 22, 2012 5:23 PM
> Subject: Re: [Phys-l] drop a metal cylinder through a solenoid
>
>
> On 03/22/2012 11:22 AM, Jeffrey Schnick wrote:
> > > He dropped an aluminum meter stick, oriented so that it extended
> > > vertically along its longest dimension, through the space between the
> > > poles.
>
> That's another good demo.  I recommend it for department "open house"
> day.
>
> This requires a big, open magnet, perhaps one that looks like this:
>   http://www.nanomagnetics.org/instrumentation_and_characterization/images/electromagnet_photo.jpg
>
> Such magnets cost a fortune, but they occasionally show up on surplus
> equipment lists.  Grab one if you can.  If you have one, on open-house 
> day it's worth disengaging it from whatever it had been doing, and
> setting up the demo as follows:
>
> Make a thing that looks sorta like a hammer, with a light but stiff
> plastic handle, and a symmetrical head made of thick-wall copper
> tubing, of a size that will fit between the poles of the magnet.
> The axis of the tube is perpendicular to the axis of the handle.
>
> A hollow tube is preferable to a solid chunk of metal, because the
> eddy current effects scale like the enclosed area, so metal near
> the axis contributes to the weight without contributing much to
> the desired effect.
>
> Use OFHC copper, i.e. oxygen-free high-conductivity copper.  Cool
> it by soaking it in liquid nitrogen.  This increases the conductivity
> by a factor of 4, which is quite noticeable.
>
> Observe the force required to move the copper from far away into the
> gap of the magnet.
>
> Observe the force (or lack thereof) required to wave the copper around
> within the region of uniform field.
>
> Observe the torque required to rapidly flip the copper end-over-end
> by twisting the handle.
>
> In particular, with the copper not in the field, demonstrate the desired
> rapid flipping motion, by spinning the thing around the axis of the
> handle.  Demonstrate putting the thing into the gap and waving it
> around without spinning.  It's easy.  Then hand it to a visitor.  Let
> them practice spinning it outside the magnet.  Then tell them to put
> it into the magnet and spin it like that.  The effect is huge.  It
> takes a huge torque to produce even a slow flip.
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l