Force between two charged pith balls: Application Experiment


To decide whether the force between the two charged spheres in the video depends on the distance between the spheres the same way as it does for point-like charged objects.

Prior Knowledge

  1. Knowledge of how the force between point charges depends on the distance between the charges
  2. Vectors, trignometry
  3. Newton's second law
  4. Graphing and data analysis skills.

Description of the Experiment

In the video below you will see two pithballs being charged with similar charge. One ball is suspended from a long string, the other is attached to wooden block. The charged balls are then placed on top of the overhead projector so that their shadows are projected onto the chalkboard. Observe what happens to the pithball on the string as the pithball attached to the block is brought closer and closer. Try and think of how you can use the data presented to figure out an answer to the original question: Namely, do two charged spheres (of finite radius) behave the same way as point charges? I.E.: Does the force between them depend on the distance between them in the same way as for point charges?

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  • Describe in words what you observed during the experiment.
  • Describe what you observe during the experiment with pictures. Be sure to draw the string holding the ball on the left (the string is not shown during the major part of the video). Draw pictures of the experimental set-up for different distances between the two balls, when only the hanging ball is present (call thisthe initial position), when the ball on the stick is brought near it, when the ball on the stick is brought closer.
  • What important physical quantities change during the experiment? What quantities can be measured from the video? To help you to answer the question for this experiment, using the data collected, we suggest the sequence of steps below.
    1. Draw a free body diagram for the hanging ball, and draw the axes. Use the free body diagram to write Newton’s second law in component form for horizontal and vertical axes.
    2. Use your knowledge of trigonometry to find how the distance that the hanging ball moves from its initial position is related to the electric force exerted on the hanging ball by the ball on the stick. (Is the actual value of the length of the string important or not? Why?)
    3. Record relevant data about the separation between the pithballs and the force between them.
  • Use the data you have collected to decide whether the force between two charged spheres in the video depends on the distance between the charged objects the same way as it does for point-like charged objects. Construct graphs to help you answer the question, if necessary.
  • List assumptions that you made analyzing the data. How do these assumptions affect the outcome?