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curator:Tim Erickson
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 The Eddy Tube Drop

The curator dropped small, strong magnets though an "eddy tube," a thick copper pipe. The moving magnetic field sets up electrical currents in the pipe, which in turn set up a magnetic field, which -- doesn't it just figure? -- opposes the field of the permanent magnets, making them slow down. Anyway, it's cool how slow they go.

And how slow is that?

As usual, it depends, on many things: the electrical conductivity of the pipe, the strength of the permanent magnets, the geometry of the everything, the mass of the falling thing, and so forth. In this case, since we had two magnets, we decided to stick pennies between them, thereby letting us vary the mass. Would more pennies make the assembly fall more quickly?

It turned out that the times were short enough that stopwatch measurement was impractical. (It might be with a longer, more expensive pipe.) So we set up Vernier photogates at the top and bottom of the tube, and did our best to drop the magnet sandwich from just above the upper beam.

Pennies is the number of pennies between the magnets.
T1 is the time when the sandwich went through the top photogate, in seconds.
T2 is the time when it went through the bottom photogate.

How does the time it took compare with what it would have taken in free fall?

What seems to be the relationship between time and number of pennies?

Clearly, forces are at work making the magnet sandwich fall so slowly. How big are they? What do they depend on? Can you come up with any models -- and distinguish between them using the data?

More information you might want: the two magnets had a combined mass of 17.7g. And the average mass of the pennies we used was 2.66 grams (we had a few old ones in the set and were not careful about which ones we used in any given drop).

(data by Tim Erickson, March 2004)

You can find some theory here. Don't look until you have a theory of your own!

PenniesT1T2
24.562525.55262
23.503564.52572
24.029024.99922
05.360526.56555
04.088535.44156
03.756524.83712
42.405763.16043
43.289134.10074
61.971032.63842
61.904162.61972
82.564523.17232
31.315122.12974
31.594822.47584
51.454362.18416
51.909942.67892
11.552522.59096
12.062923.12896
02.091223.31156
11.427522.46752

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 ©2003 eeps media 866.341.3377 or Last updated February 14, 2007 supported by NSF award DMI-0216656