physicsa.com
class code: SPS21 teacher: Mr. Elert
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Physics A: Problem Set 4: Conductors and insulators

recommended reading

High Marks: n/a
Barron's Let's Review: 8.3 Conductors and insulators
physics.info: Conductors
Wikipedia: Faraday cage, Corona discharge, Static discharger, Electrostatic discharge, Ground (electricity), Tesla coil, Wimshurst machine, Van de Graaff generator, Leyden jar, Capacitor, Lightning, Lightning rod, Kite experiment, Benjamin Franklin, Georg Wilhelm Richmann
YouTube: People struck by lightning who lived
Mr. Machado: 11 Thomson and Millikan Experiments

classwork

  1. Sketch the electric field around the following "large" charged objects.
    1. a positively charged conducting sphere
    2. a positively charged conducting "pointy sphere"
    3. two oppositely charged, parallel conducting plates

homework (hand it in)

  1. Read the following passages written by the American scientist Benjamin Franklin.

    In September 1752, I erected an Iron Rod to draw the Lightning down into my House, in order to make some Experiments on it, with two Bells to give Notice when the Rod should be electrify'd. A contrivance obvious to every Electrician.

    In Philadelphia I had such a rod fixed to the top of my chimney, and extending about nine feet above it. From the foot of this rod, a wire (the thickness of a goose-quill) came through a covered glass tube in the roof, and down through the well of the staircase; the lower end connected with the iron spear of a pump. On the staircase opposite too my chamber door, the wire was divided; the ends separated about six inches, a little bell on each end; and between the bells a little brass ball, suspended by a silk thread, to play between and strike the bells when clouds passed with electricity in them.

    Benjamin Franklin, 1753 & 1772

    Here are a few portraits of Franklin seated besides his device. If you look closely you can see the "little brass ball suspended by a silk thread" between the bells.

    Portrait of Benjamin Franklin
    Portrait of Benjamin Franklin. Mason Chamberlin. Oil on canvas (1762). From the Philadelphia Museum of Art.
    Benjamin Franklin of Philadelphia
    Benjamin Franklin of Philadelphia. Edward Fisher. Mezzotint (1763). From the Library of Congress.
    Engraving of Benjamin Franklin
    Engraving of Benjamin Franklin based on the previous two images. From a forgotten source.

    The video below the portraits shows a replica of Franklin's bells driven by a type of electrostatic device called a Wimshurst machine. (Ben Franklin never used a Wimshurst machine since it was invented 90 years after his death.)

    youtu.be/Z06vlX-2jDM

    1. When a storm cloud passes overhead, the bells become charged opposite each other. Explain why this happens.
    2. When the bells become charged, the brass ball will bounce back and forth between the two bells. Explain why this happens.
    3. What could this device be used for?
    4. Why don't you have one in your home today?