Lab 3 -- Magnetic Field Lines
Physics 242 Online Class
Note, there is an accompanying video on the Canvas which explains this lab. To
prepare for this lab you should read about the magnetic field (chapter 27),
especially about how to draw field lines and field vectors.
I. Concept of magnetic field lines and field vectors
In this lab you will use a compass to map out the magnetic field of a small magnet. In the
figures below I show the concept for a bar magnet. Your magnet is a little different and I don’t
want to give away the result, but these pictures help to illustrate what you will be doing in the
lab. In the first picture I have a bar magnet with a north (N) and south (S) pole. All permanent
magnets (which are made of magnetic materials) have an N and an S pole. The magnetic fields
are probed by the compass. The red needle of the compass indicates the direction of the Bfield vector. Also, in this picture there are iron filings that have been sprinkled on the page.
Notice how the filings line up and give literally a picture of the magnetic field.
It is important to distinguish between field vector and field line. Actually they are related and
contain the same information. The B-field vector is a vector which points in the direction of the
B-field at that location in space. The vector is what the compass needles detect. The B-field
line is a continuous curve that goes from a N pole to an S pole, see the figure on the right
below. The relationship between vectors and lines is (1) the vectors are tangential to the lines;
(2) the length of the vectors indicates the strength of the B-field at that location, while the
density of the lines also indicate the strength. Notice in the diagram that where the field is
strongest (near the poles) the field lines are densely packed and the vector (arrows) are longer,
whereas farther away the line density is smaller and the vectors are smaller. With the compass
it is difficult to detect the field strength, so we won’t try. We will only determine the directions
of the field vector at various locations.
Procedure
You will use the magnets and compass, both provided in kit2234 if you buy that, to map out the
magnetic field of a simple magnet. If you buy a bar magnet and a compass on your own, it’s
fine, provided the magnet is powerful enough to move the compass needle. I suggest you
make the magnets (those in the kit2234) stick together; this will reinforce the magnetic field.
Lay a piece of paper on a table and indicate where the magnet is. Move the compass to several
locations and note which way it points, then draw a vector there which will indicate the B field
vector. You may try to follow the curves that are expected (see diagrams above); these are the
B field lines. The B field vectors are everywhere tangential to the B field lines. You won’t be
able to measure the magnitude of the B field vectors, but you can measure their directions
from which way the compass points.
Draw several vectors spanning the region around the magnet where the field is big enough to
influence the compass. About 30 or 40 vectors is good enough.
On your drawing connect the field vectors to make a field line, thus making a sketch of the field
lines which should look roughly like the curves shown in the figures above, except your magnet
may be different.
In the next step, you will try to use the iron filings (they are in kit2234) to map out the B field.
Try to make a flat surface out of paper just above the magnet. Construction paper or cardboard
might work; I used a set of small bowls (see video), the main point is that the paper does not
sag and lies flat. The paper should be as close as possible to the magnet, and just above it. I
think it’s not such a great idea to let the iron filings come into contact with the magnet because
it will just stick and make a mess. Now, sprinkle the iron filings and let them settle into a
pattern. You might need a few trials to get a good pattern. Take a picture of this and embed
the picture in your report. If you can’t take a picture, make a sketch, but I prefer a picture.
Your lab report should consist of
1. Introduction (~3/4 to 1 page on the theory involved, without plagiarizing)
2. The sketch of the B field lines from the mapping procedure described above
3. The picture or sketch of the iron filings pattern
4. A short discussion (1/2 page) of your findings.