Magnetic Field Strength vs. Distance

Materials

• Computer with Internet access
• Color Printer
• Compass (the kind that indicates direction)
• Ruler
• Adhesive tape
• 3 identical bar magnets
• Various other magnets (optional)

Procedure

1. Place the compass on a flat surface so that the needle is facing to your right. Turn the compass so the needle lines up with the mark labeled ‘0.’
2. Place a ruler perpendicular to your compass arrow so that the largest number on the ruler in centimeters touches the edge of the compass, like this:
3. Use tape to stick the ruler and the compass to the table top so they won’t move. 
4. Place one bar magnet alongside your ruler and slowly nudge it towards your compass. Make sure that the needle gets pulled towards the magnet as opposed to being repelled. If the needle points away, flip the magnet over or turn the magnet around.
5. With your magnet facing the same direction, place it next to your ruler so that the tip lines up with the ‘0’ mark.
6. Move the magnet towards the compass one centimeter at a time. When you see the compass’ needle move, record both the point along the ruler the magnet was and the direction the compass’ needle pointed to.
7. Continue moving the magnet towards the compass one centimeter at a time, recording the same data in step 5 for every centimeter.
8. Repeat the experiment with two joined magnets, then three. Make sure to record your data in the same manner.
9. Make a line graph of your data. What do you notice?

Extra: Repeat the experiment using magnets of different shapes and sizes. You might try a horseshoe magnet, which is stronger than other magnets because the poles (opposite ends) are bent closer together.  You can also try comparing the strength of permanent magnets (materials that are always magnetized) with ferromagnets (materials that can become magnetized, like paper clips, for instance). 

Results

So how can we describe the relationship of magnetic field strength vs. distance? Your data should have suggested that magnetic field strength decreases as distance increases. In fact, a magnet’s power can all but disappear when it’s moved even a slight distance away from the compass. However, stronger magnets can continue to influence the direction of the compass’ needle from greater distances away.

Why?

Your bar magnets were competing with the earth’s own magnetic field, which pulls the tip of the compass needle north. As you scooted each bar magnet progressively closer to the compass, the magnet’s field started to take over, pulling the needle slightly away from north.

When you joined multiple bar magnets together, you created one large super magnet that has the power of all of the smaller magnets’ magnetic fields combined. However, your magnet still isn’t strong enough to have the same reach as the center of the earth’s own magnetic field, which can move your compass’ needle from 4000 miles away. Now that’s a strong magnet!