7 Lab 2: Magnetic Fields

Acknowledgment

This project is made possible with funding by the Government of Ontario and through eCampusOntario’s support of the Virtual Learning Strategy. To learn more about the Virtual Learning Strategy visit: https://vls.ecampusontario.ca.

PHY137 Stay-At-home Experiment 2

Magnetic Fields

What you will do

Use the built-in magnetometer of your cell phone to explore the magnetic fields of the Earth, the neodymium magnet, and a solenoid.

What you will need

• A cell phone that has a built-in magnetometer
• The Phyphox app installed on the cell phone
• Copper wire (more than 1 foot in length)
• 1 AA battery
• 1 neodymium magnet
• A small non-metal object that is about 3~5 cm thick (such as an eraser, a pen cap, or the prism in the lab kit with the paper packaging on)

Demonstration Video: https://youtu.be/rElg7NlKAGo

Procedures

Part I – the magnetic field of the Earth

1. Make sure your phone is at least 1 meter away from any wall, metal, or magnetic objects at home. Make sure you are not carrying any metal or magnetic items with you.
2. Open the magnetometer sensor in the Phyphox app. (If this sensor is grayed out, it means your phone is not equipped with a magnetometer. Please contact us if it is your case.)
3. Start a magnetometer measurement. Turn and tilt your phone until the Magnetometer y component reaches the maximum positive value. (When the y component reaches maximum, the x and z components should be close to zero.) (If the “accuracy” showing on the top of the screen is not “high”, you can calibrate the magnetometer by moving the phone along a figure “8”, as described here: https://calibratecompass.com/)
4. Keep the phone stationary at the location and orientation of Step 3, stop the magnetometer measurement. Export the data as a csv file. (You may find it helpful to rest your hand on your body to reduce inadvertent hand motion.)
5. Still keeping the phone stationary, quit to the main menu and start an inclination measurement. Stop the measurement in 3~5 seconds. Export the data as a csv (comma, decimal point) file.
6. Repeat Steps 1-5 above for a total of 3 different locations. If you do not have enough space indoors, feel free to go outdoors.
7. Briefly describe the surroundings of your 3 locations (how much empty space do you have, what are the main obstacles nearby. You may attach photos if you do not mind.) You may attach photos if you do not mind.
8. In the data exported for each Magnetometer measurement, find the magnetic flux density (B-field) of the geomagnetic field. Attach the magnetic field vs. time curve, and indicate which segment was used for calculating the average value and the uncertainties.
9. In the data exported for each inclination measurement, take the average inclination angle (‘Flat.csv’, column ‘tilt up/down’) during the entire measurement time as the inclination angle, and the standard deviation as the uncertainty in the average. (If the numbers are shown as #####, increase the width of the column.)
10. Compare the results from the 3 locations with the theoretical value (You may use  this online geomagnetic field strength and inclination calculator: https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml?#igrfwmm). Are your results consistent with the theoretical values? Discuss about what might have affected the accuracy of your measurements.

Part II – the magnetic field of the neodymium magnet

1. Open the magnetometer sensor in the Phyphox app.
2. Use a small non-metal object that is about 3~5 cm thick to space the neodymium magnet from the surface of the phone. (If you have the experimental kit, you may use the packaging of the prism without the prism inside, then simply stack the magnet on the top of the packaging.)
3. Start a magnetometer measurement of the absolute magnetic field. Keep the phone fixed. Move the magnet with the spacer along the surface of the phone until the absolute magnetic field reaches maximum. (If the reading suddenly drops, your magnet might be too close to the phone. In which case you need to increase spacing.) (If )
4. The location where the phone reads the maximum magnetic field is the location of the magnetometer in your phone. Find a photo of your phone (either online or use another camera) and indicate the location of the magnetometer on the photo.

Part III – the magnetic field of a solenoid

1. Wrap the copper wire 5~10 times around a battery to form a solenoid. Make sure to leave some space between loops so that the loops do not short-circuit.
2. Take a side view photo of your solenoid. Assume that electric current flows in from one end and exits from another, predict the direction of the magnetic field that is generated in the solenoid.
3. Use alligator wires to connect the solenoid to a single AA battery. Leave the last alligator clip disconnected so that the circuit is temporarily open. Place the solenoid vertically at the location of the magnetometer on your phone. Keep the solenoid on the surface and avoid moving the solenoid.
4. Start a magnetometer measurement. Then briefly close the circuit by tapping the alligator wire to the end of the battery briefly. You should see a spike of magnetic field on the z component of the magnetometer. The spikes will be in the positive direction if the magnetic field of the solenoid is pointing out of the screen.
5. Repeatedly connect and disconnect the battery for a total of 5 times. Stop the measurement and export the data as a csv file. (Do NOT leave the circuit closed for too long, otherwise the battery will get hot and run out faster.)
6. Take necessary size measurements of your solenoid. Use the measured magnetic flux intensity to estimate the current that was flowing through the solenoid when the battery was connected.

Tips for data analysis:

• Complete all calculations that are required in the procedures above. Show sample calculations in the appendix.
• Answer all questions in the procedures section above.
• Attach graphs or tables when necessary. (Do NOT attach the raw data!)
• If you write a program for calculations, attach the code in the appendix.

Suggested topics for discussion:

• Find the uncertainty values for each calculation in the results section. Show a sample calculation of your error propagation in the appendix.
• Identify the sources of error in your measurements.
• Feel free to do more tests to verify your claims if possible.
• Make a reasonable conclusion based on your results.
• Compare your results with literature values (and add citations). You may reference the textbook.
• What other quantities can be determined with the same experimental design?
• What can be done to reduce the uncertainties at no cost while staying at home?

We value your feedback!

Did you enjoy this experiment? What are the aspects that you dislike the most about this experiment? – Let me know in the “Student Feedback Survey for Lab 2” on the discussion board on Quercus! We may award the most helpful inputs with +1 bonus mark for the lab report!

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