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Activity: Slinkies and Magnetic Fields

(Activity courtesy of Teachengineering.org and VaNTH-ERC, Vanderbilt University). Level: Grades 10-12. Time required: 50 minutes. Cost: Durable goods purchased include: Magnetic Field Sensor, Physics with Vernier Lab Book, DC Power Supply, Ammeter and connecting wires, $250.00.

Overview

In this activity for grades 10-12, students use an old-fashioned children’s toy, a metal slinky, to mimic and understand the magnetic field generated in an Magnetic Resonance Imaging (MRI) machine. The metal slinky mimics the magnetic field of a solenoid, which forms the basis for the magnet of the MRI machine. Students run current through the slinky and use computer and calculator software to explore the magnetic field created by the slinky. Exploring the properties of this solenoid helps students understand the MRI machine.

This is one of a series of activities and lessons in a curriculum for accelerated high-school physics students entitled MRI Safety Grand Challenge. The curriculum is intended to teach electricity and magnetism topics including the magnetic force, magnetic moments and torque, the Biot-Savart law, Ampere’s Law, and Faraday’s Law. For more on the curriculum, click here.

Learning Objectives

After this activity, students should be able to:

  • Determine the relationship between magnetic field and the number of turns per meter in a solenoid.
  • Explain how the field varies inside and outside a solenoid.
  • Design an experiment that measures the value of μ0, the permeability constant.

Standards

National Science Education Standards Science. Grades 9 – 12, 1995.

Results of scientific inquiry — new knowledge and methods — emerge from different types of investigations and public communication among scientists. In communicating and defending the results of scientific inquiry, arguments must be logical and demonstrate connections between natural phenomena, investigations, and the historical body of scientific knowledge. In addition, the methods and procedures that scientists used to obtain evidence must be clearly reported to enhance opportunities for further investigation. (Grades 9 – 12) [1995]

See teachengineering to check alignment with other state standards.

Materials List

Slinky Lab Handout

Each group needs:

  • Magnetic field sensor
  • Physics with Vernier Lab Book
  • Metal Slinky
  • Switch
  • Meter stick
  • DC power supply
  • Ammeter
  • Connecting wires
  • Non-conducting Tape such as masking tape

The cost of durable goods that may have to be purchased for this lab includes Magnetic Field Sensor, Physics with Vernier Lab Book, DC Power Supply, Ammeter and connecting wires, totaling approximately $250.00.

Note: The lab description describes the lab using Vernier magnetic field sensors and equipment, but the lab can be adapted to any sensor and calculator or computer. Vernier sensors can be ordered from www.vernier.com. Other companies include Pasco (www.pasco.com), and Texas Instruments (www.ti.com)

Courtesy of Helen Smith, Australian National University

Magnetic fields due to a solenoid. Courtesy of Helen Smith, Australian National University

Background

An MRI machine uses a large solenoid to create its magnetic field. By exploring the properties of a small solenoid (a slinky), students can predict the properties of the MRI magnet. It is important to note where the solenoid’s magnetic field is strongest, and ways of making the magnetic field of a solenoid stronger in order to understand MRI Safety.

Solenoids are used in electronic circuits or as electromagnets, and are made by taking a tube and wrapping it with many turns of wire. A metal Slinky is the same shape and will serve as the solenoid. When a current passes through the wire, a magnetic field is present inside the solenoid.

In this lab, students explore factors that affect the magnetic field inside the solenoid and study how the field varies in different parts of the solenoid. By inserting a Magnetic Field Sensor between the coils of the Slinky, they can measure the magnetic field inside the coil. They will also measure μ0, the permeability constant, a fundamental constant of physics.

Before beginning this activity, teachers may want to watch this video of an MIT Physics lecture on electromagnetic fields.

Procedure

The following procedure has been adapted from Vernier’s online lab. The original lab may be found in pdf form, or online.

Have students follow the initial setup indicated on the handout then design an experiment to answer the later questions in the handout. Students should be able to design and describe their own procedure. During the experiment, walk around the groups of students and answer questions as needed.

mri thingy

Initial Setup (student handout)

1. Connect the Vernier Magnetic Field Sensor to Channel 1 of the interface. Set the switch on the sensor to High (0.3 mT).

2. Stretch the Slinky until it is about 1 m long. The distance between the coils should be about 1 cm. Use a non-conducting material (tape, cardboard, etc.) to hold the Slinky at this length.

3. Set up the circuit and equipment as shown in Figure 1. Wires with clips on the end should be used to connect to the Slinky. If your power supply has an accurate internal ammeter you do not need an additional external ammeter.

4. Turn on the power supply and adjust it so that the ammeter reads 2.0 A when the switch is held closed. Note: This lab requires fairly large currents to flow through the wires and Slinky. Only close the switch so the current flows when you are taking a measurement. The Slinky, wires, and possibly the power supply may get hot if left on continuously.

5. Open the file “Magnetic Field in Slinky” in the Physics with Vernier folder. A graph will appear on the screen. The meter displays magnetic field in millitesla, mT. The meter is a live display of the magnetic field intensity.

Experiments:

1. Hold the switch closed. The current should be 2.0 A. Place the Magnetic Field Sensor between the turns of the Slinky near its center. Rotate the sensor and determine which direction gives the largest magnetic field reading. What direction is the white dot on the sensor pointing?

2. What happens if you rotate the white dot to point the opposite way? What happens if you rotate the white dot so it points perpendicular to the axis of the solenoid?

3. Stick the Magnetic Field Sensor through different locations along the Slinky to explore how the field varies along the length. Always orient the sensor to read the maximum magnetic field at that point along the Slinky. How does the magnetic field inside the solenoid seem to vary along its length?

4. Check the magnetic field intensity just outside the solenoid.

Hall Probe and Solenoid Usage:

1. Adjust the power supply so that the current will be 1.5 A when the switch is closed.

2. With the Magnetic Field Sensor in position, but no current flowing, click to zero the sensor and remove readings due to the Earth’s magnetic field and any magnetism in the metal of the Slinky. Since the Slinky is made of an iron alloy, it can be magnetized itself. Moving the Slinky around can cause a change in the field, even if no current is flowing. This means you will need to zero the reading each time you move or adjust the Slinky.

3. Click the “collect” button to begin data collection. Close and hold the switch for about 10 seconds during the data collection. As before, leave the switch closed only during actual data collection.

4. View the field vs. time graph and determine where the current was flowing in the wire. Select this region on the graph by dragging over it. Find the average field while the current was on by clicking on the Statistics button. Count the number of turns of the Slinky and measure its length. If you have any unstretched part of the Slinky at the ends, do not count it for either the turns or the length. Record the length of the Slinky and the average field in the data table.

5. Repeat Steps 1 – 4 after changing the length of the Slinky various different lengths. Each time, zero the Magnetic Field Sensor with the current off. Make sure that the current remains at 1.5 A each time you turn it on.

Your Job:

Your lab group must design and execute an experiment to answer the following questions. Your experiment should record the field produced by the current for various lengths of the slinky. You should be careful to remember to use your knowledge of error analysis and reporting.

1. What is the shape of the graph relating turns per unit length of the solenoid (slinky) to the magnetic field produced by the current? Find an equation that fits the data for this graph. Should the equation produce a curve that passes through the origin? Explain.

2. What is the physical meaning of the coefficient(s) in your model? Do they make sense? Why?

3. What is the value of μ0 (the permeability of free space) according to your experiment?

4. Does the current through the slinky have anything to do with the slope of your graph? For example, if you increased the current to 3 Amps, how would your expect the new data to compare to your original graph?

5. How is this related to an MRI machine?

6. What have you learned during this experiment about creating a safe environment around an MRI machine?

The Report:

Your report should begin with a clearly stated procedure. A reader should be able to recreate your experiment by following the steps listed. You do not need to spell out how to use the computer or hall probe. Turn in all your graphs and data by pasting directly from Excel into Word. Be sure to answer all of the questions in paragraph form.

Assessment

Students should create a lab report in which they design experimental procedures to answer a number of questions from the handout. These questions also ask students to apply what they have learned about solenoids to creating a safe MRI machine.

References: Physics with Vernier, Vernier Software and Technology. Copyright © 2006 by VaNTH ERC, Eric Appelt, Primary Author

Image credit: of the University of Wisconsin

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