Re: CBL/ Mag Field Sensor

[John Gastineau <john@GASTINEAU.ORG>]

> I would like to use the Vernier Magnetic Field Sensor with the CBL
> units in a high school physics course.
>
> What ideas would the list suggest for measurement?
> My current thoughts: strength of the earth's mag field, increasing mag.
> field strength with current (?is this possible with the probe)...

Try the magnetic field due to current loops, in a solenoid [use a metal slinky
to create a solenoid with easily variable coil spacing (which is, I suppose,
after another phys-l thread, a special form of density ;-))], or my favorite
is the magnetic field of a permanent magnet. A small magnet is a magnetic
dipole. The field intensity along the axis of a dipole follows an inverse-cube
function. You can use the parameter from a fit of the inverse cube function to
determine the magnetic moment of the magnet. Don't use a power law fit, but
instead a true inverse-cube fit.

For the current-based fields, it works best to use a current-controlled power
supply so you can just dial in the desired current. You don't have to worry
about the fact that you're placing a near-short across the power supply. A
current of 2 or 3 A works well.

The magnetic field sensor is a potentially confusing one for students, because
it senses a component of the magnetic field. Additionally, the field sensor
will surprise you by revealing that your classroom is FULL of stray magnetic
fields from magnetized iron in tables and chairs, currents flowing in computer
monitors, speakers... Add to this that the magnetic field from the Earth is
actually directed more down than north, and you have a great teaching
opportunity, as long as you are willing to track down sources.

It helps a great deal to never move the sensor during an experiment. Zero it
at the start, and then move the device casting the magnetic field, instead of
moving the sensor. This will help remove effects of background
fields--otherwise you'll need to re-zero it for every location.

At risk of being accused of shameless self-promotion, let me point out that
these experiments are described in detail in one or the other of my books
"Physics with CBL" (Vernier Software) and "College Physics with the CBL and
TI-86" (Texas Instruments).

 __________________________________

John E. Gastineau      john@gastineau.org               KC8IEW
900 B Ridgeway Ave.    http://gastineau.home.mindspring.com
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