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Table of ContentsRubber Rod and Wool (Negative Charge) -- classic method of producing a negative charge.
Glass Rod and Silk (Positive Charge) -- same as above, but with positive charge.
Acrylic Rod and Wool (Positive Charge) -- not as classic as glass and silk, but much more reliable and effective charge production.
Charged Rods on Turntable (Attraction/Repulsion) -- charged rods on a stand free to rotate are used to show attraction and repulsion between charged rods.
Attraction Between Rubber and Wool -- the rotation stand setup is used to show that the cloth used to charge a rod negatively becomes positively charged in the process and will attract the rubber rod.
Aluminized Ping-Pong Balls (Electrically Connected) -- two aluminized ping-pong balls hang from wires attached to a metal plate. Touch a charged rod to the plate and the balls separate.
Aluminized Ping-Pong Balls (Electrically Insulated) -- two ping-pong balls hang from a plastic rod so that they may be given opposite charges to demonstrate attraction. Ask about technique.
Three Ping-Pong Balls (Vector Forces) -- three ping-pong balls hang from a common point on thin wires so that they just touch. A bright light below the balls projects their shadows onto the ceiling, and the point in the middle of the three shadows is noted. The balls are now charged with a Wimshurst generator, causing them to fly apart by mutual repulsion. From the new positions of the shadows it is seen that the direction of the repulsive forces are from the center to each of the balls, indicating the vector nature of the forces.
Bouncing Ping-Pong Balls -- conductive ping-pong balls bounce up and down between two charged metal plates due to electrostatic forces on the balls. Also shows basic charge transfer.
Electroscope -- an electroscope with the pivoted detector shadow projected onto a translucent glass plate is used to show charge. One can charge the electroscope by direct conduction or by induction.
Electrostatic Deflection of Electron Beam (Oscilloscope) -- 45 V supplied by a battery to the plates of an oscilloscope deflect the electron beam. 90 V gives twice the deflection.
Faraday Cage -- a large-mesh grounded wire cage fits over the electroscope, shielding it from outside electrostatic effects (a high voltage source brought near the cage will not deflect the electroscope).
Faraday Ice Pail -- shows that charge resides only on the outside of a conductor. A metal trash can is charged using a Wimshurst machine, then the charge distribution on the can is investigated using a metal ball on an insulating rod. The ball is inserted into the can, touched to the inside, then brought out and touched to the electroscope, which does not deflect. If the outside of the can is touched, however, the ball collects charge which can be transferred to the electroscope, causing deflection. On a dry day this can be repeated many times.
Millikan Oil Drop -- a recreation of the classic experiment used to find the ratio of electron charge to mass. Must be viewed individually by the students during class changeover (qualitative only).
Thomson's E/M Experiment -- an electron beam in an open oscilloscope is deflected by an electric field, then by a transverse magnetic field which balances the electric field and reduces the deflection to zero. Ratio of electron charge to mass could then be calculated from the values for deflection and field intensities, but the demo is usually only done qualitatively.
Plastic and Aluminum Rods -- two rods form the arms of a “T” which is fitted into the electroscope - one arm is aluminum, the other plastic. Both arms have a metal ball on the end. By bringing a charged rod into contact with each ball in turn, it can be shown that the metal arm conducts the charge to the electroscope whereas the plastic arm does not.
Wire and String -- essentially the same as the Plastic and Aluminum Rods, but using wire and string as conductor and insulator.
Bouncing Ping-Pong Balls -- conductive ping-pong balls bounce up and down between two charged metal plates due to electrostatic forces on the balls. Also shows basic charge transfer.
Conductivity of Solutions -- a probe consisting of two metal prongs with 110 V between them is dipped into various liquids, solutions, etc. If the liquid conducts, current flows and lights a bulb.
Jacob's Ladder -- a classic electrical display often seen in the background of mad-scientist B movies. Two long vertical electrodes are close together at the bottom, but separate gradually towards the top. 15,000 Volts from a transformer starts an arc at the bottom. Since the voltage is AC, the arc breaks as the voltage goes back to zero; and the ionized air that was heated during the arc rises while the arc is off. When the AC voltage again becomes high enough to strike an arc, it goes through the ionized air that has risen above the point of the previous arc. The process continues until the arc reaches the top of the electrodes, where it breaks off and reforms at the bottom to begin the cycle again.
Van de Graaff with Streamers -- a Van de Graaff generator with long paper streamers attached to the dome is used to show the radial shape of the electric field produced by the machine; one can also show the distortion of the field due to the introduction of a grounded metal rod.
Grass Seed Electric Field -- variously shaped electrodes are placed in a transparent basin filled with mineral oil and a small amount of grass seed. When a Wimshurst machine is connected to the electrodes and cranked, the shape of the resultant electric field is shown (on the overhead projector) by the orientation of the grass seed along the electric field lines. This is a finicky demo, so check on technique before using.
Software:
EFIELD -- display of electric field lines and equipotentials for a group of point charges. Standard charge configurations can be used (single charge, dipole, etc.), or select your own setup of charges.
EM FIELD -- A good program for both electric fields and magnetic fields. Drag and drop charges or currents, then compute field strength or draw field lines around them.
POLARI -- shows the distortion of a uniform electric field when an infinitely long dielectric is introduced.
SPARKS -- same as above, but with a conducting cylinder.
Faraday Cage -- a large-mesh grounded wire cage fits over the electroscope, shielding it from outside electrostatic effects (a high voltage source brought near the cage will not deflect the electroscope).
Faraday Ice Pail -- shows that charge resides only on the outside of a conductor. A metal trash can is charged using a Wimshurst machine, then the charge distribution on the can is investigated using a metal ball on an insulating rod. The ball is inserted into the can, touched to the inside, then brought out and touched to the electroscope, which does not deflect. If the outside of the can is touched, however, the ball collects charge which can be transferred to the electroscope, causing deflection. On a dry day this can be repeated many times.
Electrostatic Voltmeter -- a voltmeter which reads the potential near charged conductors without affecting the charge.
Leyden Jar with Ground -- a charged Leyden jar may have both of its plates grounded alternately without seriously affecting the stored charge. Shows that it is potential difference which matters, not “absolute” potential.
Egg-Shaped Conductor -- show and tell item used to discuss charge density as a function of radius of curvature.
Lightning Rod -- a model house with a conductor in the chimney is placed on the Toepler-Holtz machine. One electrode of the machine connects to the chimney, the other to a “cloud” suspended directly above the chimney. When the machine is cranked, impressive sparks between cloud and chimney simulate lightning bolts striking the house. A sharp-pointed lightning rod (which is electrically connected to the chimney) is then pushed up out of the top of the house, and the resulting corona discharge stops the “lightning” immediately.
Pinwheel -- a pinwheel with sharp points at the ends of the arms is mounted so as to spin horizontally on one of the electrodes of the Toepler-Holtz machine. When the machine is cranked, the corona discharge from the points causes an “ion motor” reaction that sets the pinwheel spinning.
Point and Candle -- a burning candle brought near a sharp point attached to the Toepler-Holtz machine is nearly blown out due to the electrostatic repulsion on the ions in the flame and the coronal wind from the point. By comparison, holding the candle near the large ball electrode to which the point is attached produces a much smaller effect.
Van De Graaff and Wand -- a grounded wand with a round bulb on one end and a sharp point on the other is brought near a charged Van de Graaff - the bulb end draws out impressive sparks, while the pointed end produces only corona discharge.
Wimshurst Machine -- small hand-cranked generators produce sparks in the hundred-kilovolt range.
Toepler-Holtz Machine -- large 200 - 300 kV discharges from this antique generator are very impressive. Caution: Crank slowly.
Van de Graaff -- commercially made Van de Graaff machines; used for a number of demonstrations, including the classic stand-your-hair-on-end.
Electrophorus -- an aluminum disc with an insulating handle is set atop a plastic plate which has been charged by rubbing with a cloth. A finger or grounded wire touched to the top of the plate removes the induced charge that is the same polarity as the charge on the plate, then the disc is lifted off the plate, leaving the disc charged to a high voltage. This may be repeated many times without re-charging the plastic.
Kelvin Water Dropper -- an unusual induction machine in which dripping water acts as the carrier for charge buildup in two metal cans. When a sufficiently high voltage is reached, the cans discharge through a small neon lamp.
Electroscope -- bring a charged rod close to the plate of the electroscope, then touch the plate with your finger. This draws off the free charge. Remove your finger and the rod, and the electroscope is left with an unbalanced charge opposite that of the rod.
Induction Spheres -- two identical metal spheres on insulating stands can be charged by touching them together, holding a charged rod near one, then separating them. Induction pulls the charge from one sphere onto the other, and when they are separated they have equal and opposite charges; this can be shown using the electroscope.
Metal Rod -- a metal rod on a rotating stand will be attracted to a charged rod of either sign due to induced charge in the metal rod.
Wooden “Needle” -- a wooden 1x2 is placed on a rotating stand, and will be attracted by either a positively or negatively charged rod due to induction and polarization of the charge within the wood.
Electrophorus -- an aluminum disc with an insulating handle is set atop a plastic plate which has been charged by rubbing with a cloth. A finger or grounded wire touched to the top of the plate removes the induced charge that is the same polarity as the charge on the plate, then the disc is lifted off the plate, leaving the disc charged to a high voltage. This may be repeated many times without re-charging the plastic.
Wimshurst Machine -- the Wimshurst machine is in principle a continuously operating electrophorus (see above).
Leyden Jars on Toepler-Holtz -- basic demonstration of the ability of capacitors to hold charge. The Toepler-Holtz machine is first run without the Leyden jars, and frequent but weak sparks are observed. The Leyden jars are then hooked to the electrodes of the machine, and the discharges become less frequent but much more powerful.
Parallel Plate Capacitor (Variable Separation) -- two parallel circular metal plates which form a capacitor are supported so that the distance between them can be varied. The plates are connected to an electroscope and charged, and the changing voltage between the plates as the separation is changed is reflected in the divergence of the electroscope. This can also be done with an electrostatic voltmeter in place of the electroscope.
Parallel Plate Capacitor with Dielectric -- uses the same setup as above, but instead of varying the separation a large plastic or cardboard sheet is inserted between the plates while they are charged, and the changing voltage is noted.
Tuning Capacitor (Variable Area) -- similar to the Parallel Plate Capacitor demo, but this is a large version of the tuning capacitor used in AM radios, in which the area between the plates can be changed.
Charge vs. Voltage -- a small capacitor is charged at 1.5V, then discharged through a projection ballistic galvanometer, the amount of deflection of the galvanometer giving an indication of the charge the capacitor held. The same capacitor is then charged to 3V, and the deflection of the galvanometer upon discharge is approximately doubled.
Series and Parallel Capacitors -- two identical capacitors mounted on a vertical board which may be used singly or hooked together in series or parallel with copper strips are charged with a battery. After a combination has been charged (to 1.5 V) it can be discharged through a projection ballistic galvanometer, and the amount of charge the combination held is reflected in the reading from the galvanometer.
Dissectable Leyden Jar -- A Leyden jar with removeable inner and outer conductors is charged with a Wimshurst machine, then discharged through an insulated metal rod - a large spark is observed. The Leyden jar is charged again, then disassembled by removing the inner and outer conductors. These can be touched together, grounded, etc., and no sparks are seen. However, if the Leyden jar is now reassembled and discharged with the metal rod, a discharge occurs which is almost as large as the first - showing that the charge has been hiding on the glass jar.
Force on Dielectric -- a circular plastic disk on the end of a seesaw arm is balanced between and slightly above the plates of a large parallel plate capacitor. As the capacitor is charged, the force on the dielectric pulls it down between the plates.
Exploding Capacitor -- three 1500 mF capacitors connected in parallel are charged to 400 volts. The capacitors look innocent enough after being charged, but lay a discharge strip across the terminals and the resulting BANG! will wake up that guy in the back row, and teach the students that capacitors are a potential danger ( sorry).
Human Capacitor -- instructor takes a charge from a Van de Graaff or other source, then walks around the room. If his shoes are good insulators, he may wait for several minutes before finally demonstrating his stored charge by depositing it on the electroscope.
Sample Capacitors -- show and tell capacitors of many types and sizes.
Battery and Separable Capacitor -- a parallel plate capacitor with its plates separated by a thin mica sheet is hooked to the electroscope and charged with a 90 V. battery. The plates are pulled apart, and the decrease in capacitance raises the voltage enough to deflect the electroscope, showing that battery electricity and static electricity are one and the same.
Electrostatic Telegraph -- a long wire is hooked to the electroscope at one end and a metal sphere at the other. A charged rod brought near the bulb will deflect the electroscope - a minute current has been drawn along the wire by the charged rod.
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