Physics and Astronomy Demonstration Resources

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23. ELECTROSTATICS

23a. Electrical charge definitions:

Glass rubbed with silk takes on a positive charge.

Hard rubber rubbed with cat's fur takes on a negative charge.

Clear plastic pieces are plexiglass (charges + using either silk or cat's fur), reddish rods are amber (charges - using either silk or cat's fur) and black rods are hard rubber. Repulsion and attraction are shown using rods with a small hole at their center of mass. Charge the rod, slip it over a needle point support and bring up another charged object to show attraction or repulsion.

Can show that force of attraction exists when one object is uncharged and the other one is charged. This is due to induced charge redistribution on the uncharged object. A metal bar will show this effect very strongly. Thus, attractive force means either opposite charges on the two objects or charge redistribution by induction.

23b. Charged balloons offer a valid display of electrostatics. Two small balloons (10 cm diameter) hung from one meter threads using wood rod as insulator. Using measured balloon mass (5 or 6 grams) and distance between the balloon (2 m or so), you can calculate Q on each balloon after rubbing them both with cat's fur. Usual charge is about 1 microcoulomb. They also illustrate induced charge as your hand approaches one of the balloons.

23c. Coulomb's law may be roughly demonstrated using torsion balance. The device consists of a long torsion fiber attached to which is a 3 cm diameter metal sphere. Also attached to the torsion fiber is a mirror off which laser light is bounced onto the blackboard. A second three centimeter sphere is mounted on a movable shaft, attached to which is a scale. With the scale set at three centimeters, the spheres are brought into contact, and the position of the zero marked on the blackboard. The spheres are then charged using the electrophorous, the deviation in the spot of laser light being noted. As the movable sphere is progressively moved away from the sphere on the torsion balance, the laser deflection may be marked at regular intervals. From these data it is quite easy to interpret the nonlinearity of the force law and its inverse dependence on separation of the spheres. The dependence of the force on the quantity of charge may be demonstrated by touching the movable sphere with a third three centimeters sphere which is initially uncharged. The charge will be split equally between the two spheres and the deflection of the laser spot will be halved. (Note: when sharing charges to get Q/2, be sure third sphere is well away from the "sharing" spheres. This sharing may be repeated a number of times before the quantity of charge becomes too small to cause a significant deflection.

23d. Induced charges: Two five centimeters metal spheres are initially placed in contact. A hard rubber rod, which has been vigorously rubbed on a piece of cat's fur is brought into the vicinity of one of the spheres. Upon separating the two spheres with the rod left in place as they separate, it will be found that they have equal and opposite charges. This may be demonstrated by transferring a small quantity of the charge from one sphere to an electroscope and then transferring a small quantity of charge with the same instrument to the electroscope on the other sphere. Use same technique for transferring charges for both spheres. It will be found that the signs of the charges are opposite and the magnitudes are equal.

23e. Small pieces of paper are placed on the lecture bench and a vigorously rubbed rod brought into the vicinity of the paper. Two events will occur. The first, and most likely, is that the paper will be attracted to and stick to the charged rod. Under certain conditions of humidity, however, the pieces of paper will be attracted to, temporarily stick to, and can be repelled by the rod. Fine cork dust may be substituted for the paper, the motion of the dust being shown on the overhead projector. Polystyrene foam pieces also work well.

23f. Two pith balls are hung by fine threads from a common stand. When a charged rod is brought into the vicinity of the pith balls they will be attracted towards the charged rod. Immediately upon touching the rod, however, they will fly away from it and from each other.

23g. Volta's Hailstorm: A number of versions are available. The simplest consists of two parallel metal plates separated by a glass cylinder about 10 cm. long and 10 cm radius. Into the glass cylinder are placed a number of metalized pith balls or aluminum foil shapes. The plates are connected to the terminals of the Whimshurst machine. When the Whimshurst is started, the balls will be charged by contact from the bottom plate and will hence be attracted to the top plate. At the top plate, they will lose their charge and fall back to the lower plate. Due to the large forces involved, the pith balls move very rapidly making a rattling hailstorm like noise.

23h. Volta's pendulum is a small (.5 cm diameter) crumpled ball of aluminum foil which acts as the bob of a simple pendulum; the string should be a fine thread. The bob is hung between two vertical parallel plates. The bob can be projected on the screen. As the plates are charged (using either a charged rod or a high voltage power supply) the bob experiences no force. If the bob is now touched to one plate, however, it will begin to move back and forth in a manner similar to the hailstorm.

23i. Electroscopes are available to show general charge repulsion. Two main types are available: simple gold leaf electroscopes with one or two leaves and Braun electroscopes consisting of a fine metal bar carefully balanced on a pair of needle bearings. The Braun electroscope, while not as sensitive as the gold leaf type, is very much more visible and requires no additional projection devices in order to be seen by a large class.

23j. The Electrophorous as a charge producing device may be demonstrated. The plexiglass sheet is first vigorously rubbed with a piece of silk. The metal sheet is then placed on the plexiglass and the back surface of the metal sheet touched with the finger. With the finger removed, the metal sheet is removed by the insulating handle. It will be found to have a negative potential of many thousands of volts. The recharging of the metal plate can be repeated without rubbing the plexiglass sheet, depending upon the humidity. The Electrophorous is frequently used for charging capacitors and showing forces between charged conductors.

23k. The Whimhurst electrostatic generator may be shown as an elementary antique device for separating charges by doing work. It consists of two counter-rotating glass discs upon which are placed around the perimeter a large number of metal foils. At either side of the stand are metal combs that pick off charges from the metal foils, and at the top and bottom of each disc are metal grounding brushes. The device may be considered as four automatic electrophorouses. For a further description of the operation of the machine, you are referred to Sutton Demonstrations in Physics, 1937, which is available in the demonstration office.

23l. The induced forces produced when a charged object is brought close to an uncharged metal object may be shown in one of two ways. The simplest consists of a metal bar suspended at its center. When charged rods are brought into the vicinity of the metal, it will be found that metal is always attracted to the rod, whether the rod is + or - charged. A more quantitative version of this demonstration may be performed by using the torsion balance of 23c. A metal plate is set up in the vicinity of the sphere attached to the torsion fiber. The force may be measured between the three centimeter sphere and the metal plate for any sign or quantity of charge on the metal sphere.

23m. In preparation, the conservation of charge may be roughly demonstrated in the following manner. A long plexiglass tube has its entire outside covered with aluminum foil or pipe. The tube is mounted on a steep angle and a funnel attached to the upper end. At the lower end is placed an insulated metal can. The metal can and the aluminum foil are connected to two gold leaf electroscopes which are placed next to each other. When approximately 500 grams of lead shot is poured into the funnel at the top end and allowed to roll down the tube into the bottom metal cup, it will be found that the metal cup and the aluminum foil become charged oppositely. The sign of the charges produced may be tested using a charged rod and electroscope and observing the deflections as a small quantity of charge is brought from the two measuring electroscopes by a proof plane. That the quantities of charge are equal and of opposite sign may be shown by simultaneously touching a T-shaped metal bar with a long insulating handle to both electroscopes. Both will immediately deflect to zero.

23n. Ionic discharge of an electrometer can be displayed. A spiral electrode is placed in the top socket of a Braun electroscope. When a large quantity of charge is placed on the spiral electrode so as to produce a good deflection on the electroscope and a Bunsen burner brought into the region of the spiral electrode the needle on the electroscope will rapidly deflect to zero. A radioactive source will also work, if strong enough.

23o. Deflection of ions. The flame of a Bunsen burner which is fed with KNO3 solution by a wick, is placed between the plates of a large capacitor which is connected to the Wimshurst Electrostatic Generator. With the generator running it will be found that the flame splits into two halves, the positive ions being attracted to the negative plates and vice versa.

23p. Ionic speaker. Into the flame of a butane torch are placed two fine tungsten wires attached to the secondary of the transformer which steps the output of the audio amplifier up to approximately 1,000 volts. Ions are produced in the flame from a saturated KNO3 solution fed into its place from a wick. When music or any other audio input is played through the amplifier a distorted version of the input is heard from the flame. This is due to the motion of the ions in the electric field between the wires. If a photo cell is set up close to the flame, and the output amplified, it will be found that the light from the flame is also modulated. The quality of the sound may be improved by using a high voltage (500-1000v) DC supply to bias the flame.

23q. Induced student charges and repulsive/attractive forces can be shown using a student sitting on a swing. Use large diameter faintly risque plexiglass rods, rubbed with silk.

23r. (In preparation) Millikan's oil drop experiment can be performed in the lecture hall if a reasonable amount of time if allowed. The motion of the oil drop suspended from the electric field is viewed using the close-circuit television system directly attached to the microscope. Bright illumination is necessary so a carbon arc is used. Due to the nature of the optical system, much patience is required to get a single stable oil drop or latex sphere in the field of view of the TV camera.

23s. Forces on an electrophorous plate are shown. The plate is placed on its base and the small spring balance attached to the insulating handles. The deflection of the spring balance as the electrophorous plate is removed in its charged and uncharged state is noted. It will be found that when the electrophorous is charged, a much greater force in necessary to separate the two charges.

23t. Forces on charged plates can be shown by having Whimshurst generator running a rube goldburg machine as shown on "Mechanical Universe" video tape.

24. ELECTRIC FIELDS

24a. Electric field patterns may be demonstrated for a number of configurations of charges. The lines of force are made visible by using small plastic hairs suspended in a mineral oil. Stir up the black fibers well, apply the field, wait a bit. Charges are maintained on the electrode configuration using a 6,000 V power supply. The electric field pattern is projected by using an overhead projector. Single charges, dipoles, two like charges, capacitor plates and a point charge near a plane are five of the many configurations possible. Practice this before lecture. Keep one hand in your pocket! The power supply is potent!

24b. A large Gaussian surface made from chicken wire is available. Inside the Gaussian surface are two model charges, one positive (red), one negative (green), and a further charge is placed outside the Gaussian surface. An element of the surface is marked with vectors representing the electric field and its normal projection. A large model of this surface element is available.

24c. An electron beam is produced by a simple electron gun apparatus and projected at grazing incidence across a phosphor screen, which is marked off in cm. Above and below the screen are placed metal plates connected to a variable high voltage power supply. The trajectory of the electrons as they traverse the phosphor in the presence of an electrical field is made clearly visible by using a close-circuit television camera with a close-up lens. Meters are available for measuring the acceleration potential of the electron gun and the deflection potential.

24d. Faradays' Ice Pail experiment may be performed very successfully. The device consists of a pail approximately 8 cm in diameter on an insulating stand. The outside of the pail is connected to a gold leaf electroscope placed adjacent to a second electroscope used to test the presence of charge on a proof plane. A sphere is charged from the electrophorous and placed inside the ice pail but not touching the wall. The deflection of the electroscope is noted. The sphere is then removed and the electroscope seen to deflect to zero. The sphere is then placed inside the pail and touched to the inside surface. Upon removing the sphere it will be found when touching it against the second electroscope that all charge has been removed from the sphere. It can be shown that there is no charge inside the pail by touching the sphere a number of times to the inside and then to the second electroscope. The electroscope will not deflect; however, if the sphere is touched to the outside of the ice pail and then touched to the second electroscope, a deflection will be seen immediately. The magnitude of the deflection may be increased by transferring charge from the outside to the electroscope a number of times. If a small deflection is seen when charge is attempted to be transferred from the inside of the pail, it will be in all probability due to the fact that the diameter of the sphere is quite large compared to the diameter of the ice pail and that the ice pail is not a completely closed volume.

24e. Biot's hemispheres are available for demonstrating the presence of charge on the outside of a conductor. The device consists of a sphere of metal on an insulating stand and two hemispherical shells which fit tightly around it. With the hemispherical shells removed, charge is placed on the sphere. The hemispheres are then attached to the main sphere and then removed. It will be found that all charge has disappeared from the sphere but that each of the hemispheres is strongly charged.

24f. A charged cylinder on an insulating stand has suspended inside it two small pith balls; also, two further pith balls are attached to the outside of the cylinder. When a charge is placed on the cylinder from the electrophorous, the pith balls on the outside will rapidly deflect away from one another and the cylinder; however, the pith balls inside the conductor will remain stationary.

24g. The force on an electric dipole in an inhomogeneous electric field is directed towards the center of the field. This may be demonstrated by using as a dipole water molecules in a stream of water that is made to traverse the lecture bench. The stream of water is illuminated using a carbon arc. When a highly charged rod such as plexiglass is brought into the region of the jet of water the entire jet will be deflected through a large angle and sometimes it will be seen to break into small particles.

25. ELECTRIC POTENTIAL

25a. 2-D equipotential lines can be traced using various pairs of electrodes with a voltage between them. Can use voltmeter (DC) to trace out V=constant surfaces of known voltage. Paper is conducting so marks can be made directly on paper. From equipotentials electric field lines can be inferred. Need TV camera and projector. Instead of paper and TV camera can use electrolyte tank and a grease pencil for marking, AC voltmeter needed; use overhead projector.

25b. A small Van de Graaff generator is available. It is an induction-charged device capable of producing between 100 and 200 kV with great reliability, the terminal being either positive or negative with respect to ground. The short circuit current produced by the generator is about 20 micro amps.

25c. Two electric wind demonstrations are available. The first consists of a rotor shaped in the form of an S the ends of which are sharpened. The device is placed on the terminal of the Van der Graaff upon a needle bearing. When the Van der Graaff generator is started, the rotor will start to rotate. The rotation may be attributed to Coulombic repulsion between the charged point and the ionized air. A hand placed in the vicinity of the rotor will feel a breeze as the rotor passes. The second demonstration consists of attaching to the terminal of the Van der Graaff a sharp point, such as a needle. A candle placed near the end of the point will be seen to flicker and on occasion to be blown out, when the Van der Graaff generator is started.

25d. The action of a lightning rod can be shown with the Van der Graaff generator. Arrange to have sparks (1 or 2 inches) jumping between the top terminal and the grounding sphere. Show the sparks, then turn off the generator. Ground the terminal, place the sharp needle in the top terminal, remove ground, then start the generator. No sparks will result because charge is leaking off the needle point.

25e. Electrostatic voltmeters are connected to two spheres of dissimilar radii and the potentials so produced are measured. To transfer "equal" amounts of charge, a very small sphere is used to move charge from the terminal of the Van der Graaff. Note these voltmeters measure the magnitude of the voltage regardless of polarity.

25f. Van der Graaff potentials as a function of distance from the sphere may be shown; as a sphere, the Van der Graafrf generator is used. With the belt running slowly, the potential of the Van der Graaff is maintained at a reasonably constant value. To measure the potential, an electrostatic voltmeter is used. One terminal is connected to ground, the other is connected through a length of highly insulated cable to a spiral electrode placed above a candle flame. The candle flame and the electrodes are attached to a long plexiglass rod and placed on a moveable stand. With the Van der Graaff running, the candle and the electrode are moved towards or away from the terminal of the Van der Graaff and the voltage as a function of distance from the Van der Graaff is measured. The candle potential detector functions by the ions (produced by the candle flame) being moved by the potential difference between the flame and the electrode. When the electrode reaches the potential of that point in space, ions cease to be attracted to the electrode.

25g. The Corona Discharge from a small sharp point placed in contact with the terminal of the Van der Graaff generator can be seen if the lecture hall is blacked out as completely as possible. In conditions of high humidity it may be necessary to use the Whimhurst generator instead of the Van der Graaff, a sharp point connected to one terminal being placed in a region of a large metal plate connected to the other terminal.

25h. Several gassy electron beam tubes are available, but fussy to get working. Use 0 to 6 kV power supply, keep one hand in your pocket! Focused beam from parabolic electrode produces very high glowing temperature of the target. A paddle wheel will rotate as electrons transfer their momentum; some suggest that it is the false radiometer effect due to temperature difference between two sides of the target.

25i. Fields and Curvature: An egg shaped conductor has attached at its small end a sharp point. When the conductor is connected to ground and brought in to the region of the terminal of the Van der Graaff which is running at full potential, it will be found that large sparks can be made to pass between the large rounded end of the conductor and the terminals of the Van der Graaff. However, it will be impossible to produce sparks between the sharp point and the terminal. Experiment 25a can be used to justify statement that "Electric field is greatest where curvature of surface is smallest."

26. CAPACITORS

26a. Spacing dependence of capacitance: The large parallel plate capacitor is connected by two insulated leads to the 4 Kv high impedence volt meter. A quantity of charge is placed on the capacitor from the electrophorous and the variation of voltage as the separation of the plates of the capacitor is varied is demonstrated.

26b. Charge dependence on V and spacing: The large parallel plate capacitor is set up and charged from a power supply set at approximately 2 Kv. The power supply is then disconnected and the quantity of charge on the capacitor measured using a charge meter. The variation of the quantity of charge is a function of capacitor plates separation may be graphed. The parallel plate capacitor is set up the plates being separated by approximately 1 cm. The quantity of charge is measured using the charge meter as a function of voltage betweens the two plates of the capacitor is demonstrated.

26c. Capacitor dielectric: The apparatus is set up as for 26a with the plates separated by approximately 1 cm. Large sheets of particle board or wood are inserted between the plates of the parallel plate capacitor and the change in voltage noted. Materials of different dielectric constants may be inserted also. It is necessary to run the dielectric into the gap touching the grounded plate of the capacitor to prevent discharge. The change in voltage as a function of ratio of dielectric to air dielectric may be demonstrated. Use of plexiglass sheet is complex as it picks up and retains surface and volume charges.

26d. Parallel capacitors: A number of .01 micro farad capacitors are connected in parallel. They are charged to the same potential and the charge residing on the capacitor is measured using the charge meter. This quantity of charge may be compared with the quantity of charge residing in a single capacitor charged to the same potential.

26e. Series capacitors: A number of capacitors are connected in series and charged to some potential. The quantity of charge for each capacitor may be measured using the charge meter and the value may be compared with that for a single capacitor.

26f. Energy stored in a capacitor may be demonstrated in a number of ways. The simplest and probably most dramatic consists of charging a two microfarad 3,000 V capacitor to its full potential. A pair of discharge tongs is then rapidly placed between the two terminals of the capacitor and the resulting spark and loud explosion are used as an indicator of the energy stored. Alternatively a 110,000 microfarad capacitor (charged to 10 or 20 volts, see rated potential) can be used to run a motor to lift a weight, or run a light bulb for a short period of time.

26g. A disectable Leyden jar is used to show the energy of a capacitor is stored in its dielectric. The jar is first charged to a large potential using the electrophorous. Then, the inner and outer metal coatings are removed using a pair of insulating tongs. It will be found that little charge resides on the metal; however, when the capacitor is reassembled, the full energy is available for making a spark. Needs practice before lecture; not easy.