Physics Lab Activity Answers

Electrostatics 1

Charging by Friction & Contact

Part 1 - A Single Pith Ball

  1. When the charged strip is brought near but not touching the pith ball, the pith ball was attracted to the strip.
  2. (a) After the pith ball was allowed to touch the strip, the pith ball was repelled by the strip.
    (b) This happens because when the charged strip touches the pith ball, some of its electric charge is transferred to the pith ball. Then, both the strip and the pith ball have the same charge, and like charges repel.
  3. (a) After I touched to charged pith ball with my hand, it behaved just like it did in step 3 - that is, like a neutral pith ball.
    (b) Touching the pith ball "grounds" it - that is, it provides a path for electrons to move freely between the Earth and the pith ball. If the charged pith ball had extra electrons, they can escape to the Earth. If the charged pith ball needed electrons, it can get them from the Earth.
  4. A method:
    (1) Secure an object of known charge. (A rubber rod rubbed with fur will have a negative charge, and a glass rod rubbed with silk will have a positive charge.)
    (2) Charge the pith ball with your charged rod.
    (3) Bring the known charge near the pith ball. If it is repelled, it has the same charge as your known charge, and if it is attracted, it has the opposite charge. The charged rod will have the same charge as the pith ball.
  5. You can tell if another combination of strip and cloth has an opposite charge by charging the pith ball with one of the strips. If the pith ball is attracted to the other charged strip, the charged strips have opposite charges.

Part 2 - Two Pith Balls


  1. When the two pith balls are charged, they repel each other, since like charges repel.
  2. If you bring the charged strips close to the charged pith balls, the pith balls are repelled by both the strip and each other. Like charges repel.
  3. If you bring an oppositely-charged strip close to the two charged pith balls, the pith balls are attracted to the strip. Unlike charges attract.

last update February 16, 2001 by JL Stanbrough