Day 3: Mapping a Dipole Magnetic Field
Magnetism is a non-contact force. The magnet can affect materials
across an intervening space. That is, we do not have to be at the location
of the source object to detect it.
The fact that the force acts across a space but can be modified
(strengthened, weakened, redirected) by intervening materials creates
an intellectual challenge. What is the important difference between
materials which do not effect magnetic fields, those that enhance them,
those that weaken fields, and those that redirect them? This may form an
excellent question to place on the blackboard and leave for a few days to
inspire investigation and thought.
Students will investigate the shape of the magnetic field of a bar magnet.
This will be done by placing the magnetometer at various locations and
recording the direction of alignment of the sensor magnet. Students will
look for the place where the magnetic field of the bar magnet can no longer
be seen. They will not find a discrete transition point but rather a region
of blending. The region may be fairly narrow as the field of a dipole falls
off as the cube of the distance to the center of the dipole. Note: the
magnetic field of the earth is always present but is overwhelmed by the
dipole field close to the dipole.
There is pedagogic value in motivating the students to think about the
contributions of the earth as well as the dipole magnet to their observations.
A great deal of science is about exploring methods of removing extraneous
"background" information from desired observations. Students to become
explicitly aware of the process of interpreting observations for new
information. The standard method of demonstrating a dipole field shape
using iron fillings clearly avoids the problem of interpreting raw observations
and gives the "right answer" efficiently. We wish to avoid unthinking acceptance
of the data.
Awareness of the impact of the background magnetic field from the earth will
be achieved by comparing the observed bar magnet field for different radial
distances from the dipole magnet and by comparing observations made at different
orientations relative to the magnetic field of the earth. The results will not
be absolutely precise or accurate. This is due to limits of the magnetometer
as an observation tool and it is due to interference from local producers of
magnetic field in the observation area. This is not a "cook book" lab activity
and is likely to produce some frustration on the part of the student-scientist.
A certain difficulty may arise with the bar magnets: sometimes a single bar
magnet will exhibit a field shaped like two dipoles placed end-to-end. My
experience is this happens with bar magnets that have been dropped or otherwise
violently disturbed. A fast check is to observe the direction of the field at
the midpoint of the long axis of the bar magnets to insure no defective magnets
are being mapped. A map of such a magnet will show field direction lines
perpendicular to the long axis of the bar magnet at the center. Such (defective)
magnets will be a source of confusion to students. Looking at the following web
pages may help students understand what is happening that makes a magnet
"defective" and what to do to repair it.
Students were asked in homework to answer the following questions:
- What is the magnet in the bottle sensing?
The magnet in the bottle is sensing the direction of the magnetic field
it finds itself in. It indicates this by lining up with its long axis
in the same direction as the field it is placed in.
- How long must you wait after placing magnetometer to get a reliable
Suggested Answer: The magnet in the magnetometer must come to rest
after being moved. This can take a few minutes. It is important not
to disturb the magnetometer or create mechanical vibrations in the system!
- How does rotating the bottle while making an observation affect
the reliability of the measurement?
Suggested Answer: Rotating the bottle slowly and carefully may not
appear to cause the orientation of the magnet to change much. That is,
it will still point almost at the same reference point. In general,
this reference point is called "magnetic north". Rotating the bottle
does cause rocking motion of the magnet and introduces a disturbance
that can only reduce the reliability of the observation. In general,
measurements ought to be taken when the bottle is at rest.
- How does your presence in the vicinity affect the measurement?
Suggested Answer: We hope not at all. But, if you have some metal
or other magnetic material on your person, that will affect the measurement.
You might demonstrate the lack of effect by placing a large volume of water
near the magnetometer and then repeating deflection observations.
- Can non-metal materials cause deflections?
Suggested Answer: Generally not unless they carry excess
electric charge and are moving.
- How far must one magnetometer be from another in order to keep them from interfering?
Suggested Answer: Assuming a weak magnet on the thread, several meters
is sufficient. The required distance increases with the strength of the
magnets used in the magnetometers. Also, we can not escape the earth's
field so using extremely weak magnets will lead to problems with the torsional
strength of the thread obscuring measurement.
Lesson Development/Writing: Ed Eckel
Web Design: Theresa Valentine
Last Updated: 8/24/2000