sdfs_logoem_logo

Compasses: A tool for navigating using Earth's Magnetic Field

  • Introduction
  • Results & Video
  • Materials
  • Procedure
  • Preparation & References
  • Downloads

earthmagnetEarth's Magnetic Field.

Did you know that Earth's geographic North Pole is closest to its Magnetic South Pole? It wasn't always this way, scientists estimate that 780,000 years ago, the Earth's poles flipped (ref). This event is termed a Polarity Reversal. In addition to Polarity Reversal, Earth's magnetic shifts. The magnetic South Pole, which resides about 1200 miles from the geographic North Pole moves approximately 7 miles per year. The magnetic North Pole moves as well. Earth's magnetic field is weak, approximately 0.5 gauss in field strength but can be detected thousands of miles into space. The magnetic field is caused by Earth's rotation of its molten core. The molten core supports electrical currents. Rotation of an electric current induces a magnetic field, much like current running through a wire. If Earth rotated faster, it's magnetic field would be stronger (ref). In fact, Jupiter rotates faster than Earth and has a stronger magnetic field. Venus rotates more slowly than Earth and has a weaker magnetic field. Question: If there were 30 hours in a day, would Earth's magnetic field be stronger or weaker than it is?

The Compasscompass_small

The needle of a compass aligns with Earth's magnetic field. The north end of the compass needle points toward the south pole of bar magnets, and likewise toward the magnetic South pole of Earth. Students demonstrate this phenomenon in the Mapping Magnetic Field exercise. The direction that the compass arrow points is termed, magnetic north, which is close to but not the same as true north because the magnetic pole is located 1200 miles from the North Pole. For truly accurate navigation with a compass, it's necessary to take into consideration the angle of declination from true north. The angle changes depending where on Earth the compass is located. For this activity small groups of students will use a compass (neglecting the angle of declination) to navigate their way through the school campus to other activities. Each time they complete an activity, they will receive a piece of the DC motor that they will build at the last station on the map.

Video & Results

The kids will need to rotate the globe near the compass to see what parts of the Earth the North needle points. If they try to move the compass along the surface of the model Earth, it will not stay horizontal and then the compass does not work properly. Near the equator, true north, as shown by the longitudinal lines, and magnetic north, as indicated by the compass, are very close. However, as the compass gets closer to magnetic north, the angle of declination gets larger.

The compass is pointing to magnetic north (white pin), and the longitudinal lines on the globe run true north to the geographic north pole.

 

 

Materials

Model of the Earth's magnetic field (see Preparation for Instructions)

  • 10 inch styrofoam sphere (purchased from Micheal's, but it's not online)
  • 12 inch inflatable globe beach ball
  • stack of neodymium magnets (10 -15)
  • packing tape
  • saw with fine teeth

Navigation

  • 1 compass (orderd on Amazon, 12 compasses for $10) for each small group of students. We tried "button" compasses but found them to be too small and unreliable. These are a compromise between price and quality.
  • Printed instructions for navigation to several stations.
  • Chalk to mark North on the ground

Procedure

I. True North and magnetic North

  1. Carefully hold the globe and locate North America, South America, Europe, Asia & Australia.
  2. Identify the geographic North and South Poles.
  3. Hold a compass horizontally next to the globe and move it along the surface. The compass arrow will point toward the magnetic north pole of the globe, (which is actually the south pole of the magnetic field).
  4. The lines that run up and down the globe are longitudinal lines. These lines run north and south along the earth. When the compass is pointing to true north, the compass arrow will be parallel with the longitudinal line. Does this happen everywhere along the globe?
  5. Place the compass on the North geographic pole. Where is it pointing?

Notice: There are places along the globe where the compass arrow is not parallel with the longitudinal lines. In these locations the angle toward true north and magnetic north are substantially different. See if you can find the south pole of earth's magnetic field.

II. Using a compasscompass

  1. Verify that your compass is working correctly by checking the north arrow is pointing north.

  2. Hold the compass in front of you so that it is level and steady. Rotate the compass, and yourself, until the arrow pointing north is pointing straight in front of you. Imagine that this is 12 on a clock.

  3. South will be the direction behind you (like 6 o'clock)

  4. East will be to your right 1/4 of a turn (like 3 o'clock).

  5. West will be to your left 1/4 turn (like 9 o'clock).

  6. Next, locate NW, SW, SE and NE directions which are at angles between the 4 primary directions.
    NW = between north and west
    SW = between south and west
    SE = between south and east
    NE = between north and east

  7. Once you can identify these directions, follow the instructions given to you to take steps in a described direction. If you follow the directions correctly, you will find the next station.

Preparation

Model Globe with Magnetic Field

We built a model globe with an inserted collection of neodymium magnets. We used this model so that kids can learn about the difference between magnetic North and geographic North. globe

  1. Measure the globe from end to end and mark the midpoint in several places. Use the saw to cut the styrofoam sphere in half. I rested it on the top of a wicker garbage can to hold it steady and get a straight line.
  2. Using a pencil, screwdriver or small knife, cut a space out of the center that will snugly fit the neodymium magnets. Press the magnets into place.
  3. Temporarily tape the sphere together and verify that the magnetic field is strong enough for the compass to feel the magnetic field on the surface. The compass will need to be held horizontally to function properly.
  4. Once enough magnets are secured to the sphere, you can tape it shut using packing tape.
  5. magnetic_north
    Location of magnetic north (white pin)
  6. Take the globe beach ball and cut a slice somewhere along near the international dateline. I sliced longitudinally.
  7. Gently insert the styrofoam sphere into the sliced beach ball.
  8. Position the South pole of the magnetic field near Baffin Island (northeast of Hudson Bay).
  9. Secure the position of the mapped ball and the styrofoam relative to each other using pins. I used sewing pins.
  10. Tape the sliced part of the beach ball back together.

 

References

Lisa Ziga (2009) Physics.org Reversal of Earth's Magnetic Field Explained by Small core Fluctuations.

Sten Odenwald (2003) Nasa Website An Introduction to GeoMagnetism An introductory explanation of the origins of Earth's magnetic fields and some accompanying descriptions of hands-on activities.

Interactive map that points a compass to magnetic north. Fun for kids to find the magnetic pole.

Download Files