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Electrostatics

  • J2-03 VAN DE GRAAFF GENERATOR WITH GROUND SPHERE

    J2-03
    Demonstrates the operation of a Van de Graff generator and illustrates electrostatic concepts
    The ground sphere is positioned a few inches away from the Van de Graaff dome and grounded to the base of the Van de Graaff. When the machine is turned on, the dome becomes charged negative and the ground sphere becomes charged positive by induction. The ground sphere is attracted to the dome, as can be easily seen. After the spark, the two spheres lose their charges, and the ground sphere relaxes to its original position, whereupon the cycle repeats. See below for a paper by Dr. R. Berg on the fabrication and maintenance of the belt.
    J2a

  • J2-11: FRANKLIN'S WHEEL

    J2-11
    Demonstrate an electrostatic rotator.
    The Franklin's wheel is placed on the Van de Graaff dome or held by a person in contact with the Van de Graaff dome. When the Van de Graaff is turned on, the device rotates with the points in the direction opposite the rotation. It has sometimes been suggested erroneously that this device works sort of like a rocket: that the electrons leave the pointed ends, and the reaction force on the rotator causes it to rotate in the direction opposite that taken by the electrons. Calculation of the momentum of the electrons quickly dispels this theory. In reality, when the electrons leave the points they become attached to air molecules adjacent to the point. The electrostatic force between the negatively charged tips and the negatively charged air adjacent to the tips pushes the rotator.
    J2a

    j2-11a

  • J2-12: FRANKLIN'S WHEEL - AC

    J2-12
    Demonstrate Franklin's wheel in a disturbing way.
    This is an AC version of the famous Franklin's wheel which is generally powered by a high-voltage electrostatic generator. The Franklin's wheel is mounted on a 5000 Volt 60 Hz AC transformer. Electrostatic forces between the tip of the rotator and the charges cloud of gas adjacent to the rotator tip provide the driving force. The direction of the current inverts, but the direction of the force does not.
    J2b

    j2-12a

  • J2-13 PLASMA MACHINE - EYE OF THE STORM

    J2-13
    Demonstrates electrostatic discharge
    This device is a commercial apparatus often used in magic shows or to enhance the look of a laboratory in a science fiction movie. When the machine is turned on a discharge occurs between the inner electrode and the outer glass; placing your hand on the glass draws the discharge but does not create a shock. The spark can also be controlled by ambient sounds. Prof. Dennis Papadopoulos has calculated that the operating voltage of this device is approximately 6 kV in the range of 20-40 kHz.
    J2b

  • J2-14 LIGHTNING ROD SIMULATOR

    J2-14
    Demonstrates how lightning rods really work
    A ground sphere is positioned adjacent to the Van de Graaff generator so that the ground sphere is charged by induction and creates large sparks. While this system is working, a grounded point is aimed at the Van de Graaff dome from a distance of several times the distance between the dome and the ground sphere. The grounded point discharges the dome at a much lower potential, preventing buildup of charge on the ground sphere and the concomitant spark discharge.
    J2a

  • J2-16: ELECTRODELESS DISCHARGE

    J2-16
    Demonstrate electrostatic discharge.
    A small fluorescent tube is held near the high-voltage electrode of the Tesla coil, lighting the tube.
    OS1

  • J2-17: ELECTRIC WIND

    J2-17
    Demonstrate the "electric wind" phenomenon.
    A lighted candle is positioned between a discharge point and a flat plate, which are in turn connecteed to a Van de Graaff generator as shown in the photograph. When the Van de Graaff is turned on, the "electric wind" blows the candle flame, as can be seen in the picture at the bottom.
    J2a

    j2-17a

  • J2-31: JACOB'S LADDER

    J2-31
    Shows electrical discharge
    The Jacob's ladder is positioned on top of the Tesla coil. The spark starts at the bottom of the ladder, where the small spacing encourages electrical breakdown. The discharge heats the air, which begins to rise. Because the discharge is a region of least electrical resistance, the spark continues along that path, which continues to rise. At the top of the ladder the discharge path rises until it becomes so long that its resistance is greater than the resistance of the shorter path at the bottom of the ladder. At that point the discharge ceases and starts again from the bottom.
    OS1

  • J2-32: JACOB'S LADDER - PORTABLE

    J2-32
    Demonstrate Jacob's ladder.
    This version of Jacob's ladder works using a spark coil. The discharge travels up a tube, creating an interesting sound which you can ask your students to explain using their vast knowledge of acoustics.
    J2b

  • J3-04: ELECTRIC FIELD LINES - SOAP BUBBLES

    J3-04
    Show the shape of electric field lines for a large dipole by observing soap bubbles move along the lines.
    A Wimshurst machine produces a high potential difference, one side of which is connected to a conducting sphere and the other side connected to a person blowing bubbles, forming an electric dipole. The bubbles attain the charge of the person blowing them and follow electric lines of force to the sphere, which has the opposite charge. A few bubbles will actually touch the sphere, pick up the charge on the sphere and return to the bubble blower. Because of the volume and high ceiling required to do this experiment, it works best in the lecture halls.
    J1a

    j3-04a

  • J3-05: VAN DE GRAAFF - INDUCTION WITH SPHERES AND NEON BULB

    J3-05
    Demonstrate the existence of electric fields and to identify the polarity of the charge on a sphere.
    A neon lamp is mounted on a long plastic pole with one side of the lamp grounded. The neon lamp lights when held in the vicinity of the Van de Graaff dome, even without direct contact, indicating the existence of an electric field. The side of the neon bulb which lights is the negative side, so the direction of the electric field can be determined by noting which side of the neon bulb is lighted, if you know how the internal connections of the neon bulb are made. The magnitude of the glow on the neon bulb is proportional to the strength of the field.
    J3a

  • J3-06 ELLIPSOIDAL CONDUCTOR

    J3-06
    Demonstrates that charge distribution on a conductor depends on the surface curvature
    The ellipsoidal conductor is charged, either positive or negative, using a charged rod from J1-01. Simultaneously touch proof planes to the larger and the smaller ends of the figure, and then to the corresponding electroscope. A few transfers show that there is more charge on the smaller end than on the larger end.
    J3b, J1b

  • J3-07: VAN DE GRAAFF - DISCHARGE TO VARIOUS RADII

    J3-07
    Demonstrate that for a charged conductor a smaller radius produces a higher electric field.
    The Van de Graaff discharges to the ground sphere at the left in the absence of the smaller elliptical conductor (from J3-06) at the right. When the grounded elliptical conductor is positioned near the Van de Graaff dome, the end with the smaller radius of curvature discharges the dome with less of a spark, indicating a smaller electric field. Discharge to the larger end is more similar to the ground sphere.
    J3a, J3b

  • J3-08 VAN DE GRAAFFS - INTERACTING FIELDS

    J3-08
    Shows field lines for two identical charges
    The two Van de Graaffs are turned on briefly to charge them to the same potential. (They may be touched together to assure that the potential is the same magnitude.) The tissue streamers indicate the paths of electric field lines, which are radially outward for isolated charges. When the two domes are brought together, the inner field lines are pushed away from each other
    J3a

  • J3-14 FLUX MODEL - ELECTROSTATICS

    J3-14
    Aid in visualizing flux at an angle through a surface
    Shows red flux lines and blue plane, oblique to the flux lines, represented by the blue vector area
    J3b

  • J3-21 FARADAY CAGE

    J3-21
    Demonstrates that the electric field within a closed conducting surface is zero
    When the cage is charged, the aluminum foil "pith balls" move away from the outside of the cage, indicating the presence of an electric field. However, on the inside of the cage the pith balls do not move away from the wire cage, indicating that there is no charge and therefore no electric field on the inside of the conducting surface.

    Note that this is most easily charged with the charging materials from J1-01.
    J3, J1

  • J3-22: FARADAY CAGE - ELECTROSCOPE

    J3-22
    Demonstrate that the electric field within a closed surface is zero.
    Charge the two wire strainers with triboelectric materials (J1-01), which are connected electrically to the electroscope indicator. With the two meshes separated, the electroscope deflects. When they are placed together, forming a closed sphere, the electroscope deflection is zero, indicating zero electric field within the closed cage. The charge that was originally on the electroscope has flowed to the outside of the conductor. Note that if you then re-open the two hemispheres, the electroscope again deflects as the charge flows back into it. An alternative way to use this demo is to bring the wire strainers near to a charged van de Graaff, first open, then closed. The difference in this case is that the electric field being shielded is fully external to the device, in contrast to the first method where it was an internally generated field.

    This is best used with a light source to project the shadow of the electroscope needle through the mesh onto the wall.
    J3, J1

    j3-22a

  • J3-24 HOLLOW CONDUCTING SPHERE

    J3-24
    Demonstrates that charge resides on the outside surface of a conductor
    Charge the sphere several times by scraping charge off a rod, either positive or negative. Touch the proof plane several times to the outside of the sphere and then to the electroscope. The electroscope charges, indicating that there is charge on the outside of the sphere. Touch the proof plane several times to the inside of the sphere and then to the electroscope. The electroscope does not charge, indicating that there is no charge on the inside of the sphere. Repeat for the outside to demonstrate that the sphere is still charged.
    J3

  • J4-12: ELECTROSTATIC FORCE - MOVING LUMBER

    J4-12
    Demonstrate polarization of water molecules.
    An eight-foot long pine 2x4 is balanced on a relatively friction-free support, so that it can rotate about the balance point. A rod charged by "friction" (either positive or negative) is held alongside either end of the 2x4. A force is exerted on the 2x4 and it rotates toward the rod, so that you can pull the 2x4 around with the rod. Changing to the other polarity rod creates the same force, and again the 2x4 can be pulled around by the rod. The non-uniform field of the rod lines up the polar water molecules in the wood and exerts an attractive force on them.
    OS0, J4

  • J4-13: MATCHSTICK ON NICKLE UNDER GLASS

    J4-13
    A simple trick easily solved with the application of electrostatic force and the polar nature of water molecules.
    A nickle is balanced on its edge in the center of a piece of paper, and a wooden match is balanced on the nickle. A plastic cup is placed upside down over the nickle and match, and a second nickle is balanced on bottom of the upside-down cup. The question is: how to remove the matchstick without disturbing either of the nickles? The answer lies in the polar nature of the water in the matchstick, which is affected by the presence of a nearby charged rod.
    J4