Iron filings reveal the pattern of a magnet’s invisible force field.Saturday, March 20th, was not only Vernal Equinox, but the annual Sun-Earth Day: a NASA-promoted effort around the country to focus attention on the special connections between the Sun and the Earth. This year’s theme: Magnetic Storms!
That gave me a lot to work with—Sun-Earth Day usually does, but the more opportunity to create hands-on experiences for our visitors, the better, and when it comes to curious natural phenomena, magnetism is a fertile subject for all sorts of seemingly magical fun.
So, I turned Chabot’s Chemistry/Physics classroom into a public magnetism laboratory, giving visitors a chance to learn, or relearn, some of the basics of magnets, as well as to connect the tabletop experiments to phenomena that take place on enormous scales on the Sun and the Earth.
First was magnetic polarity: playing with a set of magnets, visitors got a feel for the behavior of magnetic poles—N and S—and how opposite poles attract and like poles repel. (It’s always fun to feel the pull of attraction between two magnets, but there’s something extraordinary about feeling the push of repulsion—your mind just expects to see little bumpers on the magnets, but there’s seemingly nothing there!)
The Earth itself is a giant magnet, as most of us know—but what many of the adults found surprising and intriguing is the polarity of Earth’s magnetic field. Using small magnetic compasses, we sought out the Earth’s magnetic poles: north and south. By taking careful notice of which type of magnetic pole the compass needle ends pointed to, the fact that the magnetic pole of the Earth up near the geographic north pole is a south—or ‘S’—magnetic pole was revealed! This is why in physics we are often careful to refer to magnetic poles as ‘S’ and ‘N’, not south and north, to avoid confusion.
At another station, visitors made their own compasses by magnetizing an iron nail stuck through a Styrofoam packing peanut and floating it in a bowl of water. Darned if that floating nail didn’t stubbornly turn to point in the same direction, no matter what direction we tried to turn it!
Station 3 was about mapping the invisible magnetic force field surrounding various magnets. Human eyes cannot see magnetic fields—but they are there and have an influence. I had constructed magnetic field mapping devices for this purpose: used CD jewel cases, with paper labeling removed, filled with a sprinkling of iron filings. When shaken gently back and forth—as if panning for gold—the iron filings align and connect in gritty little strings and conform to the pattern of the magnetic field. The strong field converging at the two poles of a magnet were boldly evident, but also to be seen were the more tenuous curls of field lines arcing through the space around the magnet.
The patterns formed by the filings were very similar to the patterns seen in images of sunspots we compared them to. On the Sun, it is not iron filings that trace the invisible magnetic fields for us to see, but hot, electrically charged gas, or plasma (mostly hydrogen and helium, but also traces of calcium, iron, and other elements). Electric charges (electrons and ionized atomic nuclei) are strongly affected by magnetic fields when they move through them. Numerous ultraviolet images of the Sun were available on computer screens around the lab for visitors to compare the magnetic patterns and shapes to.
We had more: building an electromagnet from wire, an iron nail, and a battery. This demonstrates how magnetic fields are created by moving electric charge—in the electrically conductive wire of the electromagnet, in the circulation of electrical current inside the Earth’s iron core, and in the motions of plasma on the Sun. It’s all moving electricity, friend.
We also conducted “Magnetic Yacht Races”: pushing, via the repulsion of like poles, a floating, magnetized ‘yacht’ across a pond of water. The challenge of steering and propelling the yachts led to some interesting yacht designs; certain configurations of packing peanuts and iron nails proved easier to maneuver and accelerate than others.
Happy Sun Earth Day 2010! I wonder what we’ll be doing next year….