The intrepid researchers in the demolab have discovered the secret of Blue Love. It's electricity. One side charges up positive, and one side negative. See for yourself.
The apparatus is shown below.
This is a very nice demonstration of Coulomb's Law with a fancy torsion balance and Laser spot readout. You can show that the electrostatic force varies as the square of the distance and is proportional to one of the charges. It is probably a good idea to practice a little with this one before hand.
This is a very impressive demonstration which will draw lots of comment! It can be used to emphasize the difference between conductors and insulators.
The dissectable Leyden jar is charged up with a Van de Graaff and then discharged by shorting the inner and outer can to show charge storage. Then the jar is recharged and disconnected from the Van de Graaff. The inner can is lifted out with an insulated tool, or, with care, by hand. At this point the parts of the jar are safe to handle, and the glass jar can be lifted out, the inner and outer cans touched to each other, or touched to the glass jar in any combination. You can even give the pieces to the students to handle.
When the Leyden jar is reassembled, the last step of inserting the inner can being done carefully, it will be found that the jar is still charged, as can be checked by shorting its terminals and drawing a spark.
The discovery of the Leyden jar as described in a letter written by Musschenbroeck to Reaumur in 1746.
"I wish to inform you of a new, but terrible experiment, which I advise you on no account personally to attempt. I am engaged in a research to determine the strength of electricity. With this object I had suspended by two blue silk threads, a gun barrel, which received electricity by communication from a glass globe which was turned rapidly on its axis by one operator, while another pressed his hands against it. From the opposite end of the gun barrel hung a brass wire, the end of which entered a glass jar, which was partly full of water. This jar I held in my right hand, while with my left I attempted to draw sparks from the gun barrel. Suddenly I received in my right hand a shock of such violence that my whole body was shaken as by a lightning stroke. The vessel, although of glass, was not broken, nor was the hand displaced by commotion: but the arm and body were affected in a manner more terrible than I can express. In a word, I believed that I was done for."
An E-field projection device with self-contained fluid is available. You can change the electrodes and show several different field patterns such as +/- point charges, parallel plates and others in a few minutes. This device is charged by a piezoelectric gun, not the Van de Graaff as shown in the animation below.
The equipotentials of a charged sphere are concentric spheres centered on the charged sphere. A small fluorescent tube is held on a plastic meter stick near the charged Van der Graaff sphere. When the tube is along a radial line of the sphere, the tube lights; the ends are at different potentials. But when the fluorescent tube is held tangent to a concentric sphere, the tube does not light; the ends are at the same potential.
To show that work is done in bringing a charge up close to another charge, alternately touch the grounded discharge rod to the Van der Graaff sphere and move it away. When the sphere is grounded the motor driving the belt runs faster - -more freely. No work is done to bring charge up. But when the grounding rod is moved away, the motor slows down, "lugging" away to carry charge up the belt close to the charge building up on the sphere. The charge carried up on the belt is delivered to the center of the sphere, whence it quickly moves to the outside of the sphere in obedience to Gauss's Law. As the charge on the sphere builds up, it begins to leak into the air. Equilibrium is reached when the charge leaks off the sphere as fast as the belt is able to bring new charge up.
An electrophorus consists of a flat insulator on which rests a removeable metal plate. The Lucite base of the electrophorus is charged positive by rubbing it with silk. When the metal plate is placed on it, the metal contacts the lucite in only a few spots and acquires very little positive charge. But if the metal plate is now grounded (by touching it with your finger), it acquires a large negative charge by induction.
The metal plate is now lifted off and its negative charge can be detected at some distance with an electroscope or with pith balls. The charged plate will cause a half-inch spark to jump to your knuckles (painless), or it will flash a neon or fluorescent tube briefly (best seen by turning off all lights).
The operation of charging the metal plate by induction can be repeated indefinitely, since essentially no positive charge is removed from the insulator base. You may wish to ask your class where the energy comes from, which could be used to flash a fluorescent tube indefinitely.
This demonstration uses a Van der Graaff generator, a 10 cm test sphere on an insulating handle, a can mounted on an electroscope (the "ice pail"), and a second electroscope to test for charge. The "medium" difficulty of the demonstration is in remembering to do all the steps in the correct order.
Version 1: The induced charge equals the inducing charge
Charge the small sphere with the Van der Graaff. Ground the Van der Graaff sphere and the pail to make sure there is no extra charge around. Show that the small sphere is still charged by bringing it near the second electroscope.
Lower the small sphere into the pail without touching it. The electroscope deflects. Remove the sphere, and the deflection disappears.
Lower the sphere into the pail again without touching it. The electroscope deflects. Ground the pail by touching it. The deflection disappears. Remove the sphere. The electroscope deflects again. You have induced a charge, opposite in sign to that on the sphere, onto the pail from the ground. We will now prove that the magnitude of this induced charge on the pail is equal to the magnitude of the charge on the sphere.
Lower the sphere slowly into the pail. The deflection goes to zero. Lower the sphere down until it touches the bottom of the pail. If you listen carefully, you can hear the sound of the spark jumping. The deflection remains zero. Now take the sphere out. The deflection remains zero, and you can test the there is no charge on the sphere by bring it to the second electroscope. All charge has been neutralized.
Suggested by Bill Layton
Version 2: Gauss's Law
Start as above. Charge the small sphere and lower it into the pail without touching it. The electroscope deflects. Remove sphere. Deflection disappears. Put sphere back in. Deflection returns. Lower the sphere so it touches the bottom of the pail, and you can hear a spark jump. Deflection remains unchanged. Remove the sphere. Deflection remains unchanged. Finally, bring the sphere to the second electroscope. There is no deflection. The sphere is completely discharged. All of the original charge on the sphere went over to the pail.
When a charged cloud went overhead the bells rang to alert Franklin so that he could do his experiments. A pair of pith balls allowed him to determine which charge (positive or negative) the cloud carried. We have a set of Franklin's bells that can be charged with the Van De Graaff Generator.
That charge is only on the outside surface of a conductor is shown by several demonstrations.
Cold, dry days are best for these demos.
Note: All electrostatic demos work better on cold, dry days.
a. Charge flows to the points and sprays off. In this classic demonstration, the professor or a student volunteer stands on the insulated base and places his/her hand on the sphere of the generator. An assistant turns the generator on, and the demonstrator's hair stands on end. The demonstrator should have a key or other pointed object concealed on his person to hold up and spray off the excess charge when the demonstration is over.
A similar effect can by demonstrated by placing a wig on the sphere, or by connecting the sphere to a paper plume. The electric flier shown below will spin by spraying off charge when connected to the sphere. |
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b. Action of a lightning rod. Two Van de Graaffs are provided, one of which charges its sphere positive, and the other negative. When both are turned on, they will spark to each other over 8 -12" distance. However, if a small point is placed on one sphere, aimed in any direction, even at the other sphere, no sparks will jump, because the point dissipates the charge into the air preventing the potential from building up. | |
c. Charge density is Greatest at the areas of highest curvature. When the pear-shaped metal sphere is charged by touching it to the Van de Graaff, a larger charge can be removed from the narrow end than from the fat end. The amount of charge is tested by the deflection of an electroscope. To produce a noticeable effect this demonstration must be done carefully. | |
d. Gauss' Law --charge is on the outside of a conductor. Several demonstrations of these effects are described here [1]. | |
e. Conductors and non-conductors. A string connected between an electrostatic generator and an electroscope will not conduct charge, but a metal wire will. | |
f. Smoke precipitator. Smoke blown into a tube (from a cigarette) rapidly disappears when the electrodes on the ends of the tube are connected to the generator. | |
g. "Shot from guns" A paper cup full of puffed wheat or small Styrofoam chips placed on top of the generator produces a spectacular effect. Bring your own puffed wheat. |
The Wimshurst Static Machine generates large sparks for entertainment or for use in various electrostatic experiments. The associated Leyden jars can be connected in or out of the circuit to illustrate the function of a capacitor -- in circuit, the sparks are much fatter and louder (Q larger), but of the same length (same maximum V), and much less frequent (longer time to build up the large Q).
Links:
[1] https://demoweb.physics.ucla.edu/node/219