Lab #2

Magnetostatics


This lab has three objectives: to use a Hall effect sensor to measure magnetic field, to observe the effects of the B-H hysteresis loop in an iron sample, and to test theoretical predictions of the magnetic field in a “magnetic circuit” - a toroidal iron core solenoid with a small air gap. 


Please read the lab write-up before coming to the lab.


Yes, I know that you may have forgotten a lot about magnetic fields. I will tell the TAs to take that into account.


Addendum


The TAs went through the lab, and gave me some feedback. This may help you in the lab:


Hello,

Ali and I did the lab this afternoon.  There are a couple of things
that could be clarified.

1) On page 2, the lab discusses the sensitivity of the Hall effect
sensor (nominally 25 V/T), and the output voltage for zero field
strength (nominally 2.5V).  However, the exact numbers can vary from
this, and the proper values should be taken from the calibration
sticker on the equipment.

2) Still on page 2, there is discussion about the screw and
micrometer.  Ideally, the micrometer should read 1000 for
(approximately) zero gap.  But, over the years, this seems to have
changed.  Some of the micrometers cannot reach 1000 (since the screw
will hit the Hall effect sensor), and some still have a sizable gap at
1000.  In some cases, a number other than 1000 should be used.  If
there is any doubt, it might be better to have a TA check their setup.

3) On page 3, under Experimental Set-Up, some of these steps are no
longer necessary (which is indicated in the footnote on page 3).  Last
year, some students did not realize this and inadvertently set the
voltage across the coil to be 11V.  A strong burning odor resulted.
So it might be a good idea to remove the following lines:

"Connect the Hall effect sensor to one of the DC voltage sources on
the lab power supply. Set the voltage to approximately 11 V. (The
exact value of supply voltage is not critical since a voltage
regulator has been built into the equipment to supply a bias of
exactly 5 V to the sensor)1."

As well, in the sentence after this, the common input of the
multimeter should be connected to the negative (-) output of the Hall
probe, instead of the power supply ground.

[Note: I edited the lab to reflect this. -Len.]

4) On page 4, in section 5.1, there are more references to the nominal
2.50V reading for zero field strength, although again, the students
would ideally refer to the calibration data on the equipment itself.

5) On page 5, regarding question c, which is asking about an
equivalent air-core solenoid: we are not sure if you cover solenoids
in class before this lab, but if not, the students may not know how to
figure out the current density.  Some of them may realize that
Equation (5) happens to give the right units (and the right answer),
but in that equation, the l_gap doesn't have the right significance.
The length is really the length of the solenoid in this case.  It may
be beneficial to add a note about air-core solenoids at the end of
section 4 (Theory).

I think these are all of the things that Ali and I noticed while going
through the lab.

Thanks,
Tyler