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Magnetic Fields Lab Report, Lab Reports of Physics

Willamette UniversityPhysics

When current passes through a wire when current flows through these wire

Typology: Lab Reports

2020/2021

Uploaded on 05/11/2021

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jugnu900 🇺🇸

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Chapter 7
Magnetic Fields
7.1 Purpose
Magnetic fields are intrinsically connected to electric currents. Whenever a current flows
through a wire, a magnetic field is produced in the region around the wire. The purpose
of this lab is to investigate magnetic fields around simple geometric configurations of wires
carrying current.
7.2 Introduction
Note: For this experiment, you will write a complete (formal) lab report and
hand it in at the next meeting of your lab section. This lab can not be your
dropped grade for the semester.
Magnetic fields are vector fields. A vector (direction and magnitude) describing the
magnetic field can be associated with each point in space.
The magnitude of the magnetic field from a long straight wire is:
B=µ0I
2⇡r(7.1)
where B is the magnetic field in Tesla, µ0is the permeability of free space (4⇡x107T·m/A),
Iisthecurrentandristheperpendiculardistancefromthewiretothepointwherethe
magnetic field is being measured. The direction of the field is given by the right hand rule.
(Refer to your text book for a description and a derivation of the formula from the Biot-
Savart Law or Ampere’s Law and Figure 7.1). Also note that a Tesla is a very large unit
of magnetic field strength. Magnetic fields are also measured in units of ’Gauss’ which are
equal to 104Tesl a. The ap para t us us e d in thi s exper i men t disp lay s magn e tic fi e lds in Te sla.
The magnetic field inside of a solenoid is given by:
B=µonI (7.2)
41
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Chapter 7

Magnetic Fields

7.1 Purpose

Magnetic fields are intrinsically connected to electric currents. Whenever a current flows through a wire, a magnetic field is produced in the region around the wire. The purpose of this lab is to investigate magnetic fields around simple geometric configurations of wires carrying current.

7.2 Introduction

Note: For this experiment, you will write a complete (formal) lab report and hand it in at the next meeting of your lab section. This lab can not be your dropped grade for the semester. Magnetic fields are vector fields. A vector (direction and magnitude) describing the magnetic field can be associated with each point in space.

The magnitude of the magnetic field from a long straight wire is:

B =

μ 0 I 2 ⇡r

where B is the magnetic field in Tesla, μ 0 is the permeability of free space (4⇡x10 ^7 T · m/A), I is the current and r is the perpendicular distance from the wire to the point where the magnetic field is being measured. The direction of the field is given by the right hand rule. (Refer to your text book for a description and a derivation of the formula from the Biot- Savart Law or Ampere’s Law and Figure 7.1). Also note that a Tesla is a very large unit of magnetic field strength. Magnetic fields are also measured in units of ’Gauss’ which are equal to 10 ^4 Tesla. The apparatus used in this experiment displays magnetic fields in Tesla.

The magnetic field inside of a solenoid is given by:

B = μ (^) o nI (7.2)

Figure 7.1: Right hand rule.

Figure 7.2: Closeup of the Hall probes at the end of the sensor.

where n is the number of turns (coils) per unit length and I is the current. The direction of the field is given by the right hand rule. A device which measures magnetic fields is called a magnetometer. One common type of magnetometer is a Hall probe. The senor used in this experiment has two Hall probe devices mounted perpendicularly to one another at the end of the clear plastic probe end of the sensor. The position of each Hall probe sensor is indicated by a white dot at the end of the clear plastic probe body. See Figure 7.2. A switch on the body of the sensor indicates which Hall probe is used. There is also a switch for the range and a tare (zero) button. The Hall probe used in this experiment displays readings in units of Tesla.

7.3 Procedure

7.3.1 Long Straight Wire

Special Cautions:

  • Do not exceed 5 amps on the long straight wire or 4 amps on the solenoid.
  • Do not disconnect or break the long straight wire.
  • Turn o↵ the current and move the Hall probe to 1.5 cm (0.015 m) from the wire. Zero the probe using the ’TARE’ button. Increase the current back to 5 Amperes. Record the current, the magnetic field including the sign, and the distance of the Hall probe from the wire as before.
  • Repeat the procedure for distance values of 2.0 cm, 2.5 cm, 3.0 cm and 3.5 cm with the current at 5 Amperes.
  • Plot the data with magnetic field on the y-axis and position from the wire on the x- axis. Plot the expected values for the magnetic field at the appropriate distances from the wire from Equation 7.1. How do the calculated values compare to the measured values? Calculate the percentage di↵erence for the magnetic field for each distance from the wire.
  • Using the right hand rule, explain why the sign of the field is correct.
  • The magnetic field of the earth is about the same magnitude as the field measured near the long straight wire. Why could we ignore this field in our measurement?

7.3.2 Solenoid

Special Cautions:

  • Do not bend or distort the coils of the slinky.
  • Do not exceed 4 Amperes through the coil.

A metal stretched-out slinky will provide a reasonable helical coil of wire (solenoid) to generate a solenoidal magnetic field. The field is constant inside the coil away from the ends.

  • Remove the power supply connections to the long straight wire.
  • The same file (’magnetic field) will be used for this part of the experiment. The Hall probe selection and range select switches should remain the same as in the first part of the experiment. Remove the banana wires from the long straight wire board.
  • Stretch the slinky 60 cm to 70 cm (0.6 - 0.7 m) on the meter stick. See Figure 7.4. Make sure the coils of the slinky are uniformly distributed over the central region where the measurements will be made.
  • Using the alligator clips at each end of the slinky, connect the banana leads from the power supply through the ammeter to each end of the slinky. Note and record the polarity of the connection to the solenoid.
  • Place the meter stick and solenoid (slinky) so the compass needle points perpendic- ularly to the solenoid. In the middle region of the slinky where the probe will be inserted, count the number of complete turns (coils) in .25 m (25 cm). Calculate the number of turns per meter and record the value.

Figure 7.4: The solenoid (slinky) stretched along the meter stick used in the second part of the experiment.

  • With the current in the coil at zero Amperes, insert the Hall probe into the center of the middle region of the solenoid through the side of the slinky. (The body of the probe should be perpendicular to the axis of the solenoid.) Zero (tare) the sensor.
  • Turn on the power supply and set the current to 4 amperes. Do not exceed 4 Amperes. Measure the magnetic field at the center of the solenoid. Record the current and magnetic field. Is the magnetic field reasonably constant over the interior cross-section of the solenoid?
  • Repeat the above procedure with currents of 3.0, 2.0 and 1.0 Amperes in the coil.
  • Make a plot with the magnetic field on the vertical axis and the current on the hori- zontal axis. Does the magnetic field increase linearly with current?
  • Do the magnitude and direction of the magnetic field agree with the value calculated from Equation 7.2? Calculate the percentage di↵erence between the calculated values of the magnetic field and the measured values for each value of the current. Why is the field larger with the solenoid than with the straight wire?