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What happens to the sound of the radio after the wire mesh is in place.

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What happens to the sound of the radio after the wire mesh is in place.

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  1. Shown below in the photograph at the left and in the detail at the right is a device called a Faraday cage. In the detail photograph three suspensions of aluminum foil chunks on pieces of string can be seen. Each of the suspensions include a string passing over the top of the cylindrical screen with small rectangular chunks of aluminum foil, a few centimeters in size, attached to the ends of the string. Note that one end of each string is outside the cylinder and one end of the string is inside the cylinder. What do the foils do when the van de Graaff generator is turned on, putting large negative charge onto the cylindrical screen and perhaps by conduction onto the foils. In particular, when the cylinder is charged, which, if any, of the aluminum foils will move away from the cylinder, either inward or outward. When the cylinder is charged: (1) only the inner foils will move away from the cylinder. (2) only the outer foils will move away from the cylinder. (3) both the inner and the outer foils will move away from the cylinder. (4) neither the inner nor the outer foils will move away from the cylinder.

  2. Shown below is an unusual Faraday cage constructed from two sieves. An electroscope is attached to the sieve at the rear, as seen in the photographs at the center and the right. These photographs were taken with no charge on the system. Now the van de Graaff generator is turned on for a few seconds, charging the Faraday cage system. The electroscope continues to indicate no charge - it remains in its upright position, as seen in the center photograph. When the sieves are separated, as seen in the photograph at the right, but with the system charged, what will happen? (1) The electroscope will indicate charge. (2) The electroscope will stay as it is. (3) Something else will happen (specify what).

  3. A radio is tuned to Washington, DC, SportsTalk 980 when it is suddenly and for no apparent reason covered by the wire mesh seen in the photograph below. What happens to the sound of the radio after the wire mesh is in place. After placing the wire mesh on the radio, the radio will: (1) be louder. (2) be softer. (3) remain at about the same volume.

  4. Millikan Oil Drop Experiment • Robert A. Millikan used the apparatus below to experimentally determine the elementary charge on an electron. He won the Nobel Prize in physics (1923) for his work on this as well as his work on the photoelectric effect. • The principle of the experiment was very simple. He used a pair of oppositely charged plates to generate an electric field. A charged oil drop was released into this electric field. The potential between the plates was adjusted until the oil drop floated its weight being cancelled by the electric force on the charge. • From this data he determined that the charge on the drop was an integer multiple of the elementary charge (e = 1.6 x 10-19 C) Pictures courtesy of Clarkson University

  5. Photo Courtesy of American Museum of Science and Energy Photo Courtesy of Jefferson Lab Applications • Van de Graaff Generator • Uses motion of a belt to build up charge that is transferred to the conducting shell on top. • Potential at all points on the shell is the same, but high above zero potential of the ground (typically 100 kV – 200 kV, but can be as high as 20,000 kV). • If you touch the conducting shell charge is transferred to your body via conduction

  6. Applications • Electrostatic Precipitator • Used to clean exhaust from manufacturing plants. • High electric potential is used to ionize air. The ionized air collides with other particulates transferring charge. • The charged particulates are separated and through periodic agitation are dropped into a collection bin for disposal.

  7. Applications • Photocopier and Laser Printer • Uses a photoconductive material that is a good conductor in light but not dark. • Light from the image is projected onto the drum made up of this material. Wherever the light hits the drum, it becomes charged. Dark areas remain uncharged. • Oppositely charged toner particles attach to the charged areas of the drum. • The drum rolls over paper that has the same sign charge as the drum. The toner particles adhere to the paper. The paper is then heated melting the toner into a permanent image. • Photocopier – focused light • Laser printer – laser beams

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