## Electric Charge

• EM3.1 Electric Spark (Jacob's Ladder)

This is two metal poles that are next to each other, narrow at the bottom and farther apart at the top. (See picture) A spark appears at the base, travels up the length of the rods, and then dissipates. When it dissipates, another spark begins at the bottom. The spark is generated by a spark coil on the same shelf. The better the initial spark into the coil via the switch, the better the results. Performance varies with humidity.

• EM3.2 Electroscope Projection System and Oven

A Van de Graff generator is used to charge up the leaves of an electroscope. In order for the class to se this better, a light is placed behind the electroscope and projected towards a screen. A two lens system is used to enlarge the image and make it erect.

• EM3.3 Franklin's Gong

This demonstration has a silver-coated ping-pong ball hanging between the plates of a parallel plate capacitor. Using the Van de Graaff generator to put a charge onto the capacitor, the ball will become charged as well. It will be attracted to one side, hit the plate, gain that plates charge, and be repelled to the other plate, where the same thing happens. As it swings from plate to plate, it makes the sound of a gong.

• EM3.4 Induced Charge

A large glass plate is rubbed with a piece of wool or fur. A metal disk with a plastic handle (electrophorus) is touched to the plate, thereby inducing a charge on the metal disk. Charge can also be induced on plastic rods. Then proceed with qualitative demonstrations like picking up pieces of paper, etc.

• EM3.5 Lightning Rod Simulation

The Van de Graaff generator is used to charge two large horizontal metal plates. On the lower plate is placed a sphere on a stand. When the potential difference between the two plates is large enough, an arc of lightning is seen between the sphere and the upper plate. When a sharp point on a stand is placed next to the sphere, no arc is seen. This shows that charge continually builds on a sphere and discharges at a point. This is why lightning rods come to a point. Another apparatus is shaped like half a sphere attached to a cone. Sometimes this is hard to see, so dimming the room lights would help.

• EM3.6 Method of Images

This is a physical representation of the method of images. Using the Van de Graff, induce a charge on the witch's hair demonstration (see below). Then bring a sheet of metal close to their hair and the hair will realign itself as if there was a charge of opposite sign on the other side of the metal sheet and equal distance away.

• EM3.7 Van de Graff Generator

Using the generator alone, connecting it to itself charges the two spheres to opposite charges. Bending the smaller sphere towards the bigger one, an arc will be seen by the class when the spheres are close enough. There is a variety of apparati that can be connected to the generator.

WITCH'S HAIR - Witch's hair is a rod with hair on the top. As the generator puts a charge on the rod, the hair stands up (because it has the same charge as the rod and each other piece of hair). If some volunteer with long hair is willing, the same demonstration can be done by having that person stand on an isolation platform (E26).

FARADAY'S ICE PAIL - Another is a metal beaker with coated balls on the side. As the beaker gains charge, the balls rise. A test ball will show that charge stays on the surface of the pail, and not on the inside. Twm .. This twirl has sharp points which give off charge. This causes the twirl to spin. This is an electrostatic Hero's engine.

ELECTRIC WHIRL - The Electric Whirl is made of a number of brass wires, with rearward-facing sharp points, jointed at a hub. This is pivoted atop an insulated shaft. One terminal of an electrostatic machine is connected to the whirl and the other is grounded. As the charge builds up on the metallic parts of the whirl, the equipotential lines are bunched together at the sharp points, creating a large electric field there. Eventually the field becomes large enough to ionize the air molecules and create a space charge that is of the same sign as the point. The mutual repulsion between the space charge and the point causes the wheel to spin.

FLUORESCENT LIGHT BULB - Using the new Van de Graff generator and a small fluorescent tube, you can qualitatively demonstrate both teh effects of a potential difference and the 1/r dependence of voltage from a point source. If you make the room dark, turn on the VDG, hold the fluorescent tube close to the VDG and orient it radially, you can see the tube glow. It even glows more brightly at the nearer end, due to the 1/r dependence. Students should be impressed, as there are no wires attached to the tube! If you then turn the tube azimuthally so it is perpendicular to the VDG's field lines, the bulb goes dark, since the tube is now approximately oriented along an equipotential surface.