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

Physics A: Problem Set 1: Electric Charge

recommended reading

Barron's Let's Review: 8.1–8.5 What is Electricity?
physics.info: Electric Charge
Wikipedia: Electric charge, Static electricity, Triboelectric effect, Electrostatic induction

drawing

  1. With the aid of diagrams, show the distribution of charges on two identical uncharged metal spheres in contact during the following experiment.
    1. Bring a positively charged rod near one of the spheres on the side opposite the point of contact.
    2. Separate the two spheres and remove the rod. What charge does each sphere have?
    3. Touch the two spheres together. What charge does each have?
    4. Repeat the procedure described above, but replace the positively charged rod with one that is negatively charged.

    The solutions as annotated diagrams

  2. electroscope.pdf
    With the aid of diagrams, show the distribution of charges on an electroscope and the position of its leaves for each of the following scenarios.
    Eight experiments with an electroscope
      initial charge
    of electroscope
    charge on rod position of rod
    relative to terminal
    a. uncharged positive in contact, then removed
    b. uncharged negative in contact, then removed
    c. uncharged positive near, but not in contact
    d. uncharged negative near, but not in contact
    e. positive positive near, but not in contact
    f. positive negative near, but not in contact
    g. negative positive near, but not in contact
    h. negative negative near, but not in contact
  3. electroscope-hand.pdf
    With the aid of diagrams, show the distribution of charges on an electroscope and the position of its leaves during the following experiment.
    1. Take an uncharged electroscope and place your finger on the terminal.
    2. Bring a negatively charged rod near the terminal.
    3. Move your finger and the charged rod away from the terminal at the same time. What is the charge on the electroscope?
    4. Repeat the procedure described above, but replace the negatively charged rod with one that is positively charged.

writing

  1. Explain how an object that is electrically charged (like a balloon after it has been rubbed on hair) can be attracted to an object that is electrically neutral (like a wall in your home or classroom).

    Let's say that after a party ballon has been rubbed on hair, the surface of the balloon acquires a positive charge. A neutral wall has positive and negative charges in equal amounts within it. When the charged side of the ballon is brought near the wall, negative charges in the wall move closer to the balloon and positive charges are driven away. The negative charges in the wall are closer to the balloon than the positive charges are, which means the attractive force between the balloon and the wall is stronger than the repulsive force.

    Cartoon of a balloon stuck to a wall

  2. Describe a procedure that could be used to determine the sign of the charge on an object.

    The only way to determine the sign of the charge on an object is to test it next to a pair of objects with known charge — one positive one negative. The one that is repelled by the unknown object is the one with the same charge as the unknown object. If the unknown object is repelled by a positive object it is positive. If the unknown object is repelled by the negative object it is negative.