Message-Id: <v01540b11af2ac038f5d8@[192.174.2.173]>
Date: Fri, 14 Feb 1997 17:22:19 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: re electrostatics
Hi Doug
You can read in most textbooks the incorrect information that the force law
between two magnets decreases as the inverse square.
However the real rule is that the force between two magnetic monopoles
decreases as the inverse square while magnets tend to be dipoles (i.e. with
a north and south pole close together.) The force between dipoles (electric
or magnetic is inverse square, see a good calculus physics text for the
derivation for electric dipoles.)
I've never had problems with the scale readings due to magnetic fields on
any of the many scales I have used.
I have students plot the data on log log paper and get a slope of -4 or I
have them use regular graph paper and then plot the log of the force versus
the log of the distance.
Maurice uses corrugated cardboard:
The thickness gives a slightly better dipole field pattern and it is
stickier (more friction) than the plastic petri dish. You sprinkle the iron
filings then tap the cardboard to allow them to move into position.
Perhaps you can line the dish with cardboard? Unless you want to project
the patterns.
We also use out of date photo paper, make the patterns, and then burn them
into the paper with sunlight. (use no developer solution just wash the
resulting print in fixer)
You can also use wax paper and melt the iron filings into the wax with a lamp.
Some photocopy developers are magnetic others are not. However I suspect
that fine grain powders will not make as good field displayers as larger
more rod shaped iron filings. (for example those produced from steel wool
shavings.)
The styrofoam sounds good.
I don't know a good source of piezoelectric igniters.
Thanks for the comments and questions.
Paul D