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Map the field of a Bar Magnet

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

Many of us know that bar magnets have a North and South pole, but haven't had a chance to put a visual image on the magnetic field. Even if we've seen the arrows, what do they really mean? barmagnetFor a rectangular magnet, the strongest parts of the field are at the ends. What does the field look like when it transitions from North to South Poles?

2 Dimensional Mapping
In this exercise students will place a bar magnet on a piece of paper and draw the direction of the magnetic field as they move a small button compass around the sides. Students draw arrows in the direction of the North arrow on the compass at each location. They will observe that at one end of the magnet the north arrow points away, and on the other side it points toward the magnet.

3D Dimensional Image of the Magnetic Field

In this exercise, a cow magnet is placed inside of a test tube and suspended inside a clear container, which holds iron filings. When the container is shaken, the iron filings will stick to the magnet and orient themselves in line with the field, illustrating the direction of the magnetic field at all points around the magnet. Students will be able to identify the location of poles.

Results
These two maps of are of different kinds of magnets. The tops bar magnet has a North and a South pole, as indicated by the arrows.

1) 2-Dimensional Map

result1

2) 3-Dimensional Map with Iron Filings

The cow magnet has 3 poles, North at both ends and South in the center. The filings are perpendicular to the magnet at the poles but parallel near the transitions between the poles. The end of the upper North pole is inside the cylinder of plastic tube, so the filings don't have access to it to align with the magnetic field.

filingscow arrow

Materials (for 1 station of each activity)

  • 1 rectangular bar magnet
  • 1 cow magnet (ordered on Amazon, $5 per magnet)
  • paper
  • button compass (ordered on Amazon, 40 for ~$6)
  • colored pen
  • clear jar with lid (used a brown rice container)
  • nail or drill
  • plastic test tube that fits the cow magnet (used a 15 ml centrifuge tube)
  • iron filings (KLM - 3124-01, 500 g for $8.50)
  • decorative duct tape

Procedure

2-Dimensional Map

  1. Place the bar magnet down on the center of the paper and trace it.
  2. Place the button compass at one end. Draw the direction of the North arrow at that location of the bar magnet.
  3. Move the compass and repeat until you've draw arrows all the way around the bar magnet.

3-Dimensional Map

  1. Tap the bottle on a flat surface, so that all the iron filings have fallen to the bottom.
  2. Shake and rotate the bottle until the filings come into contact with the magnet.
  3. Note where the filings stick and how they are oriented.

Do the iron filings have a pattern similar to the one you drew on the paper in 2-Dimensions? What new information can you see? What information about the magnetic field is not illustrated?

 

Preparation

There is virtually no preparation for the 2-Dimensional mapping other than acquiring the supplies. For the 3-Dimensional illustration there is a little preparation of the apparatus.

Test Tube Prep
I had difficulty finding a test tube that would precisely fit the cow magnet, so I used the 15 ml centrifuge tube with screw cap that is standard in many biochemistry labs. I used a hack saw to remove the triangular tip so that the cow magnet would hang just past the end of the tube. It's important to have the magnet fit closely inside the tube and to hold it in place.


Iron Filings Shaker
jar

  1. Put a hole in the lid that the test tube can fit snugly into it. I flame heated a nail and melted it through the lid and then eventually gently forced the test tube through while the plastic was still a bit soft.
  2. Place the tube in the lid with the cow magnet inside.
  3. Stuff paper or some other object so that the bar magnet doesn't move around when shaking. If the magnet moves, the filings will too and you'll lose the illustration of the field.
  4. Tape or caulk around the tube so that filings won't spill through the holes around the lid.
  5. Make absolutely sure the jar is totally dry and put in some filings. I probably have about 1/4 cup.
  6. Screw the lid onto the jar.
  7. Decorate the jar if desired.top jar
  • Initially I used a stir bar that fit a plastic test tube measuring 15 mm x 100 mm and was purchased at KLM Bioscientific.
  • It's important that the test tube fit the magnet pretty closely or else the magnetic field won't be strong enough to hold the filings in place.

Download Files and References

The 3 dimensional mapping is slight adaptation to an original activity described at the Exploratorium website. I had difficulty finding a test tube - bottle combination that fit snugly, so I decided to create a hole in a plastic lid.