COLLEGE PHYSICS LABORATORY EXPERIMENT
MAGNETIC FIELD FROM A BAR MAGNET
THEORY
The spatial dependence of the electric field produced by an electric dipole composed
of charges q and − q placed on the z-axis can be investigated as follows. The Figures
below show the spatial dependences of the parallel electric field (Ez ) and the perpendicular
electric field (Ex ) as a probe might be displaced on the z-axis or x-axis, respectively. The
plots shown here have actually been generated by plotting the theoretical electric-field
components associated with an electric dipole with positive and negative charges located
at z = +1 and z = − 1, respectively. Each plot shows a graph of log y versus log x, from
which we can determine the power law y = a xb , where the y-intercept on the log-log plot
is log a while the slope is the dimensionless power b.
In the first Figure, we plotted the logarithm of Ez versus the logarithm of distance
(z − 1) from the positive charge. Note that in the vicinity of the positive charge, i.e., for
log(z − 1) < 0, the slope of the plot log Ez versus log(z − 1) is very close to − 2, as expected
from the electric field due to a single charge, and, thus, the near-field spatial dependence
of the electric-dipole field is approximated by an inverse-square law. As we move the probe
farther out along the z-axis, we enter into the far-field range of the electric-dipole field,
whose power law is represented by an inverse-cube law.
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In the second Figure, we plotted the logarithm of Ex versus the logarithm of distance R
from the center of the electric dipole. Note that in the vicinity of the center of the electric
dipole, the electric-dipole field does not change rapidly. As we move the probe farther out
along the x-axis, however, we enter into the far-field range of the electric-dipole field, whose
power law is once again represented by an inverse-cube law.
In this experiment, we investigate the spatial dependence of the magnetic field produced
by a bar magnet by following a procedure similar to the procedure outlined for the electric
dipole (see Figure below).
Hence, by moving a magnetic probe either along the long symmetry axis of the bar magnet
(which crosses the bar the long way through its center) or the short symmetry axis of the
bar magnet (which crosses the bar the short way through its center), we may investigate
the spatial dependence of the parallel magnetic field and perpendicular magnetic field,
respectively, of the bar magnet. In particular, we are interested in the far-field spatial
dependences of each component through the establishment of far-field power laws.
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MATERIAL & PROCEDURE
Material: Magnetic probe (Hall Effect transducer) and Universal Lab Interface (ULI)
Connections, LoggerPro software, Bar Magnet, and 2 meter sticks.
Procedure
• A brief demo will be presented at the beginning of the experiment on the
use of the probe and LoggerPro software.
• Prepare experimental set-up for measurement of parallel magnetic field Bk
and measure background magnetic field Bk∞ .
• Measure parallel magnetic field Bk (z) from near-field to far-field region as a
function of parallel distance z from one of the magnetic poles.
(How would you define these regions?)
• Prepare experimental set-up for measurement of perpendicular magnetic
field B⊥ and measure background magnetic field B⊥∞ .
• Measure parallel magnetic field B⊥ (r) from near-field to far-field region as a
function of perpendicular distance r from the center of the bar magnet.
(How would you define these regions?)
DATA ANALYSIS
Using Excel, plot log[Bk(z) − Bk∞ ] versus log z and log[B⊥ (r) − B⊥∞ ] versus log r for the
parallel and perpendicular magnetic field, respectively, and determine the near-field and
far-field power laws.
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