Orange Coast College
Physics 280
Experiment #6
Magnetic Field of a Bar Magnet
Goal: To measure the strength of the magnetic field of a bar magnet and to determine the
dependence of the magnetic field strength on the distance from the center of the bar magnet.
Part 1. Mapping the Magnetic Field ! of a Bar Magnet
(1) Lay a bar magnet on a sheet of paper. Use a small compass to map out the magnetic field
on the sheet of paper at various locations around the bar magnet.
Part 2. Finding the Strength of the Magnetic Field !
At our latitude the Earth generates a magnetic field with an approximate magnitude of 0.5 Gauss
that is directed toward the south magnetic pole of the Earth. We will calculate the strength
(magnitude) of the magnetic field generated by a bar magnet for cases where the bar magnet’s
magnetic field points perpendicularly to the Earth’s magnetic field. We will achieve this task by
measuring the angle θ that the Earth’s magnetic field makes with the net magnetic field of the
Earth and bar magnet. One can easily show that for this case the magnitude of the bar magnet’s
magnetic field is given by the formula ! = 0.5 !"# !
(1) Move all magnets far away from your table and tape a sheet of paper on your table. Use
your compass and protractor to draw reference lines going east-west and north-south on
your sheet of paper.
(2) Set the meter stick on the table and use the reference lines so the meter stick points eastwest.
(3) Place your compass on top of the meter stick so the center of the compass is at the 10 cm
mark.
(4) Rotate the compass until the angle reads 0o.
(5) Place the magnet on top of the meter stick pointing east-west as shown in the figure
below.
(6) Move the bar magnet toward the compass, recording the distance from the center of the
bar magnet to the center of the compass, AND record the value of the angle in the table
below labeled “polar”. Calculate the strength of the magnetic field generated by the bar
magnet for each position of the bar magnet using ! = 0.5 !"# ! .
1
Repeat the procedure above, but now with the meter stick pointing north-south along the
reference lines on the sheet of paper. The magnet should again be oriented east-west.
Analysis:
Graph B versus d for both orientations. Do a power fit and
find the exponent for each case.
2