# John Proud's Physlab Page

 My home school is in Honolulu at punahou school. Punahou is a K-12 independent school with 3700 students. The school was founded by missionary families in 1841, and is the oldest independent school west of the Mississippi River.

# 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.

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?

 DATA TABLES Part 1 Part 2 Current in Wire Magnetic Field Distance from Wire Magnetic Field
 (A) (mT) (m) (mT) 4.0 1.0 3.5 2.0 3.0 3.0 2.5 4.0 2.0 5.0 1.5 6.0