Physics by the Fridge: Zip Magnets
Almost as confusing as an on and off relationship, these flat refrigerator magnets alternate between attraction and repulsion, but only when pulled in the right direction.
What you Need
- Two flat refrigerator magnets (the kind you might get from your dentist as appointment reminders).
What to Do
- Flip over one magnet so that you can only see the black back side. Then place the other magnet on top of the back (as shown in the picture above).
- Slide the two magnets in opposite directions along the long side of the magnets. How do the magnets slide? What do they feel like?
- Repeat step 2, but this time slide the magnets apart along the short side of the magnet. How do the magnets slide? What do they feel like?
What’s Going On?
Flat refrigerator magnets are special. Unlike regular magnets which have a north end and a south pole, refrigerator magnets are composed of strips of magnets that alternate between north and south. Picture a bunch of horse-shoe magnets (which look like a “U”) lined up next to each other, making alternating stripes of north and south poles, like the figure below:
These stripes are typically one to two millimeters apart and if you look at the back of the magnet, you may even be able to see them. One way to make them even more visible is to put a sheet of paper over the back of the magnet and then sprinkle some iron filings over the paper. You should get a pattern that looks like the image below.
When the magnets are slid across each other lengthwise (step 2) the stripes of the south poles line up with the stripes of the north poles from the other magnet, which allow the magnets to slide easily and without disruption. However, when the magnets slide across each other widthwise (step 3), you can feel a disruption and may hear a zipping or chattering sound. This is caused by the stripes alternating between being attracted and repelled. They are attracted when a north pole stripe from one magnet lines up with a south pole stripe of the other magnet because opposite poles attract each other. They are repelled when a north pole stripe from one magnet lines up with a north pole stripe from the other magnet and also when two south pole stripes line up because poles that are the alike (i.e. north and north or south and south) repel each other.
- Try repeating the experiment with other magnets. Do you get the same effect?
- Try putting the magnets together back-to-back. Can you get them to line up?
- Try cutting up the magnets into strips and then putting the magnet back together. What happens?