class code: SPS22 teacher: Mr. Elert
classroom: A314 office: A214
test day: Friday phone: (718) 724–8500 ext. 2141

Physics A: Problem Set 4: Conductors and Insulators

recommended reading

Barron's Let's Review: 8.3 Conductors and Insulators 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


  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

    Electric field diagram

    Electric field diagram

    Electric field diagram

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.

    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 130 years after he built his device.)

    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?
    1. The sky and the ground become charged during severe weather. The bell attached to the ground becomes charged by conduction. The bell attached to the rod pointing up becomes charged by induction. The two bells wind up with opposite charges.
    2. The brass ball will be attracted to one of the bells by induction. When the ball touches a bell, it acquires the charge of that bell by conduction. The ball is now repelled by the bell it just touched and is attracted to the other bell, which has the opposite charge. It touches that bell and flies off in the opposite direction. The process repeats until the charge on both bells is dissipated.
    3. Franklin used this apparatus to detect approaching severe weather.
    4. An iron rod in your home that projects up and out of your house is likely to be struck by lightning. Inviting lightning into your home is not recommended.