Physics Lab: Measurement of an Electromagnetic Field
An iron bar wrapped with conducting wire is called an
iron core solenoid. When electric current flows through
the wire, it creates a magnetic field (whose strength is a
function of the number of turns of wire around the iron
core) which can be observed using a compass. The
compass’ needle is deflected when brought near the
electromagnet. The degree of deflection diminishes as the
compass is moved further away, reflecting the fact that
the strength of the field likewise diminishes with
increasing distance from the magnet.
Since the earth’s magnetic field is
aligned north and south, the addition of
another magnetic field oriented in some
other direction causes deflection of the
compass needle away from north.
Therefore, the degree of deflection is the net of
the two magnetic forces. If deflection
measurements are made in a magnetic field
whose orientation is perpendicular to the earth’s
field, the strength of the unknown field can be
estimated by the following equation:
B magnet = tan Ө B earth
Where: Ө = angle of deflection from north, and
Bearth = 5x10‒5 Teslas.
This principle is demonstrated as follows:
5.0 x 10‒5 T
Tan Ө =
compass
needle
Ө
B magnet
Ө
B magnet 5.0 x 10‒5 T
B magnet = (5.0 x 10‒5 T)(tan Ө)
Test Apparatus for
Measurement of an
electromagnetic Field
Electromagnet /
Iron Core Solenoid
Ruler
Power Supply
These are the
settings for the
power supply; do
not adjust.
Measure compass
needle deflection in
degrees from North at
a number of distances
from the magnet.
The readings here are:
800 at 4.0 cm.
Record your data in
this portion of the
worksheet and
perform the
calculations indicated.
Plot your data on the
graph: Distance in
meters on the x-axis
and magnetic field in
Teslas on the y-axis.
Connect the points
that you plot with as
smooth a line as you
can draw.
Use your TI calculator
to create a STAT table:
Enter the distance data
in L1, and the magnetic
field data in L2.
Plot these data in the graph
function. The data points
should outline a curve that
decreases exponentially.
Use the calculator to determine an exponential regression curve for the data points, and
write this equation in the appropriate area of the worksheet. Also determine the
correlation coefficient R for your plot. This value indicates how well your data points fit the
calculated curve.