The Physics
Opus in profectus


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  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
  2. Two metal spheres are electrically charged. One of them carries +6 μC and the other −12 μC. The two spheres are carefully brought in contact and then separated. What is the new charge on each sphere if…
    1. the spheres are the same size
    2. the positive sphere has twice the radius of the negative sphere
  3. Write something.
  4. Write something completely different.


  1. Why are plastic computer cases lined with metal foil or thin metal plating?
  2. When a student places her hand on an operating Van de Graaff generator the hair on her head stands up. Why does this happen?
  3. Why are lightning rods pointed?
  4. In a region where the electrical field is zero, as it is inside a charged conducting sphere, is the voltage also zero? Explain your answer.
  5. 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 (source: Philadelphia Museum of Art). 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?