Mapping a Magnetic Field. Evaluation copy. Figure 1: Detecting the magnetic field around a bar magnet

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1 Mapping a Magnetic Field Experiment 16 The region around a magnet where magnetic forces can be detected is called a magnetic field. All magnets, no matter what their shape, have two poles labeled north and south. When two like poles are brought near each other (i.e. north and north or south and south), they repel each other. When two unlike poles are brought together, they are attracted. The forces of repulsion and attraction are present because of the magnetic field that completely surrounds the magnet. The magnetic field lines around a bar magnet form a similar pattern to the magnetic field lines around the Earth. They extend out from the North Pole, and then curve back into the South Pole. The field lines are more concentrated near the poles of the magnet so the magnetic field is said to be stronger near the poles. In this activity, you will use a Magnetic Field Sensor to map the magnetic field along a bar magnet. You will use a simple robot to do the actual data collection. OBJECTIVES Figure 1: Detecting the magnetic field around a bar magnet In this experiment, you will Use a Magnetic Field Sensor and NXT to map the magnetic field along a bar magnet. Build a robotic device to collect data. Make conclusions about the magnetic field at various points on a bar magnet. Evaluation copy MATERIALS computer LEGO NXT Intelligent Brick MINDSTORMS Edu NXT 2 software Vernier NXT Sensor Adapter NXT cable Vernier Magnetic Field Sensor NXT Four-Wheel Cart bar magnet STEM 2 with Vernier and LEGO MINDSTORMS NXT 16-1

2 Experiment 16 PRE-LAB PROCEDURE 1. To get a smooth graph for this experiment, we recommend you build a motorized cart capable of moving in a straight line at uniform speed. Any simple cart that moves in a straight line can be used. All it needs to do is hold the Magnetic Field Sensor as it moves slowly along a bar magnet. One possibility is the basic Four-Wheel Cart described in Appendix E of this book. The Magnetic Field Sensor Side Holder described in Project 19 can be used to hold the sensor. 2. Prepare the NXT for data collection. a. Set the switch on the Magnetic Field Sensor to 6.4 mt (low amplification). b. Connect the Magnetic Field Sensor to the Vernier NXT Sensor Adapter. c. Connect the Adapter to Port 1 on the NXT using a LEGO NXT cable. d. Make sure the NXT is connected to the computer (USB or Bluetooth) and turned on. 3. Launch NXT 2 Programming. 4. Open the NXT program 16 Mag Field.rbt. This program creates a data file as the robot moves along a bar magnet while holding a Magnetic Field Sensor. If you wish to write the program yourself, the steps are listed below. a. Type the name 16 Mag Field under Start New Program and click the Go button. b. Place a Wait block on the programming rail (see Figure 2). Adjust the Control setting to Time, and configure it to wait for 0.5 seconds. c. Place a Start Datalog block on the programming rail after the Wait block. Set the Duration to 5 seconds. Set the Rate to 10 Samples per Second. Set the Port 1 sensor to Vernier Mag. Field 6.4 mt. d. Place a Move block on the programming rail after the Start Datalog block. Set the Power to 35. Set the Duration to 5 seconds. Make sure both the B and C motors are set to move forward. e. Place a Stop Datalog block on the programming rail after the Move block. Set the Name to mydata. Figure 2: Data-collection program for magnetic field Choosing a Data-Collection Method Method 1 requires that the NXT be connected to the computer while collecting data. This method should be used if you plan to keep the robot and magnet on the table top. Method 2 allows you to disconnect the NXT from the computer for data collection. This method should be used if you plan to run the robot on the floor STEM 2 with Vernier and LEGO MINDSTORMS NXT

3 Mapping a Magnetic Field PROCEDURE Method 1: NXT Remains Attached to a Computer 1. Set the NXT robot on the table. Place the north end of a bar magnet about 10 cm in front of the robot with the long edge of the magnet parallel to the front and back wheel. The goal is to have the robot move so that the tip of the Magnetic Field Sensor passes along the magnet from north to south (see Figure 1). The white dot on the sensor should be facing downward. Note: The robot will move for 5 seconds. You can alter the Duration in the program if you want the robot to move for less time (shorter distance). 2. Click the Download and Run button to begin data collection. When the NXT beeps, notice that the computer switches to the Data Logging screen. The data will be graphed as the robot moves along the bar magnet. 3. Analyze the data set. a. Click the Analysis Tools button, and select Point Analysis. b. Drag the yellow dotted line to the locations shown in the data table. Record the value of the Magnetic Field at each location. Method 2: Using the NXT Remotely 1. Click the Download button. Note: This does not run the experiment, but stores the program in the memory of the NXT. 2. After the download is complete, disconnect the NXT from the computer. 3. Set the NXT robot on the floor. Place the north end of a bar magnet about 10 cm in front of the robot with the long edge of the magnet parallel to the front and back wheel. The goal is to have the robot move so that the tip of the Magnetic Field Sensor passes along the magnet from north to south (see Figure 1). The white dot on the sensor should be facing downward. 4. Press the Enter button on the NXT to run the 16 Mag Field program. When the NXT beeps, the robot will move along the bar magnet for 5 seconds. Note: You can rerun the program collecting multiple datasets before reconnecting the NXT to the computer. 5. After the data have been collected, upload the magnetic field data to the computer. a. Reconnect the NXT to the computer (USB or Bluetooth). Make sure the NXT is turned on. b. Launch NXT 2 Data Logging. c. Choose Open from the File menu. d. Highlight your NXT in the Browse Devices window. Be patient. It may take a few seconds for your NXT to appear. e. Highlight the Log File named 16 Mag Field.log A graph will appear in the Log Preview window. Note: If you took more than one run with your NXT, you will see other files listed with the same name and a numeric extension. f. Click the Open button. (Click OK if you get an Overwrite request.) Notice your dataset is stored in the table under the name of the sensor used to collect the data. The extension _p1_1 is automatically added. 6. Analyze the data set. STEM 2 with Vernier and LEGO MINDSTORMS NXT 16-3

4 Experiment 16 a. Click the Analysis Tools button, and select Point Analysis. b. Drag the yellow dotted line to the locations shown in the data table. Record the value of the Magnetic Field at each location. DATA Location Magnetic field (mt) At start (10 cm from N pole) Near the N pole Middle of the magnet Near the S pole At the end (away from the S end of the magnet) PROCESSING THE DATA 1. Where along the bar magnet was the largest positive magnetic field reading observed? 2. Where along the bar magnet was the most negative magnetic field reading observed? 3. Where along the magnet was a zero magnetic field reading observed (you may need to refer back to your graph)? 4. Why does the graph have both positive and negative magnetic field values? EXTENSION 1. Test the strengths of different magnet types at the same distance from the sensor. Which magnet types are strongest? Weakest? 16-4 STEM 2 with Vernier and LEGO MINDSTORMS NXT

5 Vernier Lab Safety Instructions Disclaimer THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB. This copy does not include: Safety information Essential instructor background information Directions for preparing solutions Important tips for successfully doing these labs The complete STEM 2 with Vernier and LEGO MINDSTORMS NXT lab manual includes 12 lab activities and eight projects as well as essential teacher information. The full lab book is available for purchase at: Vernier Software & Technology S.W. Millikan Way Beaverton, OR Toll Free (888) (503) FAX (503) info@vernier.com