The Magnetic Field around
a Long Straight Wire
When an electric current flows
through a wire, a magnetic field is produced around the
wire. The magnitude and direction of the field depends on
the shape of the wire and the direction and magnitude of the
current through the wire.
In this lab, you will examine how
the magnetic field is related to both the distance from the
wire and the current through the wire. A Magnetic Field
Sensor will be used to detect the field in the space
surrounding the wire. A complication that must be considered
is that the sensor may also detect the Earth's field and any
local fields due to electric currents or some metals in the
vicinity of the sensor.
OBJECTIVES:
1. Use a Magnetic Field Sensor to
measure the field around a current carrying
wire..
2. Determine the relationship
between magnetic field strength and the distance from the
wire.
3. Determine the relationship
between magnetic field strength and the current in the
wire.
Figure
1
MATERIALS
Power Macintosh or Windows PC Polar
coordinate graph paper
Universal Lab Interface
Ammeter
Logger Pro Rheostat and Knife
Switch
Vernier Magnetic Field Sensor
Magnetic Compass
Adjustable Power Supply Long
Insulated Wire (at least 2 m)
Graphical Analysis or Graph Paper
Connector Wires, Clamps and Rods
INITIAL SETUP
1.Connect the Vernier Magnetic
Field Sensor to the Universal Lab Interface. Set the switch
on the sensor to High.
2. Assemble the setup shown in
figure 1 so that the long insulated wire is stretched
vertically next to the working table.
3. Tape a 1/2 sheet of the polar
coordinate paper to the table so that the center of the
paper is adjacent to the vertical wire.
4. Connect the wire, rheostat,
knife switch, ammeter, and power supply, as shown in Figure
1.
5. Prepare the computer for data
collection by opening Logger Pro. A graph will appear on the
screen. The vertical axis has magnetic field scaled from
&endash;0.10 to +0.10 millitesla. The horizontal axis has
distance scaled from 0 to 5 cm. The Meter window displays
magnetic field in gauss.
PRELIMINARY QUESTIONS AND
ADDITIONAL SETUP
1. Close the knife switch and set
the power supply so that the ammeter reads a steady 4 amp
current. Hold the plastic rod containing the Magnetic Field
Sensor vertically and place it on the table next to the
vertical wire, approximately 3 cm from the wire. Click to
begin data collection. Observe the meter reading as you
rotate the sensor, continuing to hold it in the same
vertical orientation. Determine the orientation of the dot
on the sensor when the magnetic field is at a maximum, and
compare the direction that the dot on the sensor is pointing
with the pattern of the graph paper. Move the sensor to
other points around the wire at 3 cm. What did you discover
about the direction of the magnetic field? Warning This lab
requires fairly large currents to flow through the wires. Do
not leave the switch on except when taking measurements. The
wire and possibly the power supply may get hot if you leave
current flowing continuously.
2. Orient the dot on the sensor
tube so that it is aligned perpendicular to the circular
lines radiating from the center of the graph paper. Click to
begin data collection. Observe the meter reading as you move
the sensor away from the wire, continuing to hold it in the
same vertical orientation. What did you discover about the
strength of the field as you move the sensor away from the
wire?
PROCEDURE
Part I: How Is The Magnetic
Field Around a Long Straight Wire Related To The
Current?
For the first part of the
experiment you will determine the relationship between the
magnetic field strength and the current in the wire. As
before, leave the current off except when making a
measurement
1. We will first zero the sensor
when no current is flowing; that is, we will remove the
effect of the Earth's magnetic field and any local
magnetism. With the switch open, click .
2. While keeping the distance at a
constant 3 cm, place the Magnetic Field Sensor in a vertical
position. With the switch closed, rotate the sensor about a
vertical axis and observe the magnetic field values in the
Meter window. Find the position that indicates a maximum
positive magnetic field. The flat end of the sensor should
be perpendicular to the circular graph lines. Keep the
sensor in the same position for the remainder of the
experiment. The sensor will provide a magnetic field value
to the computer when you enter a current value. Beginning
with a current of 1 amp, obtain field strength values for
every increase of 0.5 amps in current. Do not exceed 4
amps.
3. Save the graph of your data to
determine a functional relationship between the current in
the wire and the strength of the magnetic field at a fixed
distance.
Part II: How Is The Magnetic
Field StrengthAround a Long Straight Wire related to the
Distance from the Wire
For the second part of the
experiment you will determine the relationship between the
magnetic field strength and the distance away from the wire.
The Magnetic Field Sensor should be oriented as before. Use
a current of 3.0 A for all of part II. Leave the current off
except when making a measurement.
1. We will first zero the sensor
when no current is flowing; that is, we will remove the
effect of the Earth's magnetic field and any local
magnetism. With the switch open, click .
2. While keeping the current at a
constant 3 amps, place the Magnetic Field Sensor in a
vertical position. With the switch closed, place the sensor
at a distance of 1 cm from the wire. Rotate the sensor about
a vertical axis and observe the magnetic field values in the
Meter window, and find the position that indicates a maximum
positive magnetic field. The flat end of the sensor should
be perpendicular to the circular graph lines. Keep the
sensor in the same orientation for the remainder of the
experiment. The sensor will provide a magnetic field value
to the computer when you enter a distance value. Beginning
with a distance of 1 cm, obtain field strength values for
every increase of 1.0 cm in distance. Do not exceed 6
cms.
4. Save the graph of your data to
determine a functional relationship between the current in
the wire and the strength of the magnetic field at a fixed
distance.
ANALYSIS
Part I
1. Plot a graph of magnetic field
vs. current Use either Graphical Analysis or graph paper.
2. What is the relationship between
the current in a long straight wire and the resulting
magnetic field strength?
3. Determine the equation of the
best-fit line through the data points. Explain the
significance of the constants in your equation. What are the
units of the constants?
Part II
4. Plot a graph of magnetic field
vs. the distance from the center of the wire.. Use Graphical
Analysis or graph paper.
5. How is magnetic field strength
related to the distance?
6. Either using the linear
regression tool in Graphical Analysis or by hand, determine
the best fit line through the data points. Explain the
significance of the constants in your equation. What are the
units of the constants?
7. Remember that you zeroed the
sensor before taking data in this lab. Should the line you
fit in Step 6 go through the origin?
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