Los Angeles
Exhibit Set A
Feb 4, 2004
Hands-On
Explorations of Magnetism
Exhibits
Black Sand exploration, black sand can be collected at a beach using magnets, it is made of the magnetizable mineral magnetite, Fe3O4.
Magneflux is a very fine grain magnetizable powder. It is used to find cracks in engine blocks, when an engine block is magnetized the magnetic field stays inside the iron of the block except where there is a crack, there some field lines leave the metal and attract the magneflux particles. Magneflux can be suspended in oil to create an inexpensive magnetic liquid similar to ferrofluid. (Ferrofluid contains particles so tiny that they are held in suspension, they will not settle out like the magneflux powder.)
Color TV and Magnetism. A cathode ray tube color television set shoots a beam of electrons from the rear of the set toward the display screen. These electrons collide with phosphors. Phosphors give out light when struck by high energy electrons. The single beam of electrons is swept left to right across the screen and also top to bottom using magnetic coils inside the television. You can deflect the electron beam by bringing a magnet close to the screen. Caution, there is a metal screen made of nickel just inside the color TV screen, this nickel screen is called a shadow mask and it is magnetizable. Use only junk color televisions for this demonstration, once the shadow mask is magnetized it will distort the colors of the television image.
Eddy Currents. When electric charges such as electrons are moved across a magnetic field they experience forces which may cause them to move as an electric current if they are in a conducting material. The resulting electric currents swirl in circles and are called eddy currents. Electric currents that move in circles create electromagnets, the resulting electromagnet interacts with the magnet that created the eddy currents in the first place to create drag forces which oppose the motion of the metal plate. Eddy currents are used to stop some freefall rides at amusement parks. Large neodymium magnets are placed on the back of a car. The magnets surround a rail. Part way down the fall the rail is made of copper. Eddy currents in the copper slow the car to a safe stop.
Falling Magnets. When magnets fall next to an electrically conducting aluminum plate eddy currents in the plate exert forces on the falling magnets which slow their fall. When multiple magnets are used they can be arranged with all of their north poles pointing in the same direction so that the magnets repel each other, this creates interesting patterns of falling magnets.
Magnetic Guiderail. When a magnet moves near a conductor it creates eddy currents in the conductor. These eddy currents make magnetic fields that push on, and oppose the motion of, the moving magnet. If the magnet is disk shaped and is rolling near the edge of the conductor then eddy currents will be greater on the side of the magnet nearest the center of the rail. The larger drag force on the side of the magnet nearer the center of the rail will steer the magnet back toward the center of the track.
Floating
on Copper. Place the flying magnet between
the slabs of copper.
Raise and lower the lifting magnet to fly the flying magnet between
the slabs of copper. If you completely remove
the lifting magnet then raise the flying magnet and drop it you will
see that it falls slowly. Much slower than freefall under
gravity. When a magnet moves near a conductor
of electricity electric currents called eddy currents are induced to
flow in the conductor. These eddy currents make magnetic fields which
oppose the motion of the magnet. These eddy
currents slow the fall and the rise of the flying magnet. They slow
the motion of the flying magnet enough that the human eye-brain-hand
feedback loop has enough time to adjust the position of the lifting
magnet to keep the flying magnet in the air between the slabs of
copper.
Pendulum Snake Notice that the pendulums move together at first, then some begin to change position relative to their neighbors, creating the eponymous snake pattern! Eventually, all apparent pattern is lost until suddenly, every other pendulum is moving opposite its neighbors. Then chaos returns again. Eventually, all the pendulums return and move together in one line.
Take it from the Top. A stack of wood can be piled up until the top board is not over the bottom board at all.
Turbulent Orb. A sphere full of flow visualization solution shows patterns of flow when it is rotated.
Strange Attractor. A magnetic pendulum oscillates in a chaotic manner.
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Scientific Explorations with Paul Doherty |
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2 February 2004 |