tag:techtv.mit.edu,:Array/184592500 2008-06-26 13:51:08 -0400 MIT Department of Physics Technical Services Group <p>Two flexible wires are suspended vertically. The wires are conected in series or parallel to a 12V storage battery. When the wires are connected in series and power is applied they will repel each other; when they are connected in parallel they weill attract one another.</p><p>This effect is due to the magnetic fields created by the charge flowing through the wires. When the wires are in parallel, the currents in each are going in the same direction and thus attract. In series the currents are going in opposite directions and repel. </p> 50 tag:techtv.mit.edu,:Array/184564900 2008-06-25 16:28:50 -0400 MIT Department of Physics Technical Services Group A magnet with a very strong magnetic field is held in place on an aluminum disk. The disk is attached to a motor powered by variable AC current. When the disk rotates, the magnet will levitate above it due to <a href="http://en.wikipedia.org/wiki/Eddy_currents">eddy currents</a> generated in the disk. With the disk spinning, these eddy currents form to oppose the magnetic field of the magnet, making it levitate. When the motor is turned off, the magnet falls back to the disk. 48 tag:techtv.mit.edu,:Array/184531920 2009-01-14 12:37:06 -0500 MIT Department of Physics Technical Services Group A <a href="http://en.wikipedia.org/wiki/Wimshurst_machine">Wimshurst electrostatic generator</a>, working on the principle of induction, generates high voltage differences and sparks between two movable electrodes. By increasing the distance between the electrodes, higher potential differences can be built up. <br><br> <a href="http://en.wikipedia.org/wiki/Electrostatic_induction">Electrostatic induction</a> refers to the principle that charges in an object (especially a conductor) redistribute themselves in the presence of nearby charges. Opposite charges are attracted to each other, while similar charges are repelled. <br><br> Larger charges can be stored by connecting the knobs to Leyden jars which are component parts of the machine. 151 tag:techtv.mit.edu,:Array/184511320 2009-01-16 16:40:13 -0500 MIT Department of Physics Technical Services Group First, a dipole consisting of two conducting balls connected by an insulating rod swings at the end of a long plexiglass rod. The two balls are charged oppositely by touching it to a conducting rod in the field of a Van de Graaff generator. The generator is negatively charged, which induces a positive charge on the closer silver ball, and a negative charge on the further gray ball. Going around the generator, the dipole always aligns itself with the electric field, i.e. the silver ball always stays closest to the generator. <br><br> Next, the Van de Graaff generator induces a positive charge on the closer side of a helium-filled conducting balloon, attracting it towards the generator. When it touches the generator, it picks up a net negative charge and is repelled away. When it hits the demonstrator (who can be thought of as "ground"), the net negative charge is removed and it is once again attracted to the generator. <br><br> The last clip combines these two ideas. The two outer bells are connected by a conducting rod and form a dipole in the field of the Van de Graaff generator, while the inner bell is grounded. The small balls are attracted to the outer bells the same way that the balloon was attracted to the generator. Once the balls hit the outer bells, they pick up a net charge and are repelled away. When the balls hit the grounded bell they lose their net charge and are attracted back to the outer bells. As you can see, the chimes do not work when the Van de Graaff generator is grounded. <br><br> A Van de Graaff generator is a machine that produces a lot of charge (in our case negative) on the outside of a conducting sphere. It was developed by <a href="http://en.wikipedia.org/wiki/Van_de_Graaff_generator">Robert J. Van de Graaff</a>, an MIT professor. 172 tag:techtv.mit.edu,:Array/184482680 2009-01-22 15:13:38 -0500 MIT Department of Physics Technical Services Group <a href="http://en.wikipedia.org/wiki/Leyden_jar">Leyden Jars</a>, an early style of capacitor, can be connected to the two electrodes of the Wimshurst Machine. They will store charge as it is generated, giving a much stronger spark across the gap. <br><br> Next we have a three-piece dissectible Leyden Jar consisting of two metal cups separated by a glass cup. When charged with the Wimshurst machine, we see by touching it with the shorting rod that it holds a large amount of charge. However, when disassembled, the metal cups can be brought into contact with each other and no spark will be generated. When the jar is reassembled it can then be discharged. This demonstrates that, in this situation, the charge actually resides on the surface of the glass (a <a href="http://en.wikipedia.org/wiki/Dielectric">dielectric</a>), not on the metal. 175