5.41 - Earth's Magnetic Field

Introduction:  Have you ever used a map application or played a game where your orientation of your view changes depending on the direction that you are holding your phone?  Have you ever used a star gazing application that shows you the position of planets and constellations in the sky?  How do you think an application knows the orientation of your phone relative to the Earth and the surrounding Universe?  Applications are enabled by the magnetometer sensor in your phone accurately measuring the 3-dimensional magnetic field of the Earth.  In this activity, you will characterize Earth’s magnetic field, which has been crucial for navigation for a thousand years.  It enables many modern technologies such as virtual reality and is essential for life to exist on Earth.

1. Open phyphox and select the magnetometer.  When the graph tab is selected, the phone will display a graph of the magnetic field for each of the three axes (x, y and z).  While holding your phone level in the x-y plane, collect data as you rotate in a circle two or three times.

   1. 1. Explain what you observe on the x and y axes and provide a hypothesis for your observation.

      2. Explain what you observe on the z-axis and provide a hypothesis for your observation.

      3. Based on the magnitudes of the magnetic fields, sketch a diagram showing the approximate direction of the total magnetic field vector relative to the Earth’s surface.

2. The Earth’s magnetic field bears a resemblance to what you should expect if a large dipole magnet was buried in the center of the Earth, with the south pole of the magnet pointing toward geographic north and the north pole of the magnet pointing toward the geographic south pole.  The resulting field lines are shown in the figure to the right. 

   1. Using the phyphox application, explore the x and y values of the magnetic field as you hold your phone with the y-axis pointing toward the geographic north.  Are the values for the x and y magnetic fields consistent with what you would expect from the figure?  Explain

   2. Explore other orientations of your phone and confirm that the measurement of the field lines is consistent with your expectations.  What is the approximate strength of the magnetic field parallel to the Earth’s axis (this is often called the horizontal intensity of the magnetic field)?  

1. Quantitatively measure the Earth’s magnetic field: Repeat the activity you performed in Part 1 of the Guiding Questions with particular focus on making the most accurate measurement possible.  Key experimental factors that will lead to a more accurate measurement include:

• Assure you are far away from any other magnets or ferromagnetic materials.  This could include electronics, structural supports in buildings or any ferromagnetic material.

• Carefully maintain motion in the x-y plane.  Consider various experimental approaches as it is difficult to accomplish this when holding the phone in your hand.

Consider experimental observations that might indicate that there might be local magnetic fields or out of plane motion influenced your measurements.  Make adjustments and repeat the measurement, as necessary.

1. Record values for the magnetic field in the table below.  You can use the “pick data” function withing phyphox or export your file to a spreadsheet for analysis.