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Coulomb's Law: Electrostatic Force and Static Charges

Author: Jerry Silver

The Idea

According to Newton's law of universal gravitation, any mass exerts a force on any other mass. Electric charges work in a very similar way. The farther away you get, the weaker the force. Because the electric force is so much stronger than the gravitational force, it is much easier to measure. This experiment explores the nature of the electrostatic force and establishes the basis for Coulomb's law.

What You Need

  • 2 pith balls or conductively coated Styrofoam balls (conductively coated ping-pong balls are also an option)
  • 2 pieces of string about 16 inches in length
  • movable ring stand with a pendulum clamp (or other horizontal support)
  • small nonconductive post on a stand (the post should be a few inches in length and consist of a thin wooden dowel or a short glass or plastic rod)
  • ruler
  • rubber rod/wool pair (or equivalent) to apply a charge to the pith balls
  • optional: light source to project the image of the pith balls onto a screen (an overhead project or LCD projector can serve this purpose)

Method

  1. Attach one side of each of the two strings to the pith ball.
  2. Attach the other sides of the string to the pendulum clamp separated by a few inches, so the pith ball can swing in only one direction, as shown in Figure 96-1.
  3. Attach the other pith ball to the nonconductive stand.
  4. The swinging pith ball should be positioned so it can only swing closer to and further from the stationary ball.
  5. Draw a reference mark on the bottom of the ring stand to indicate the rest position of the swinging pith ball without being subjected to any force other than gravity.
  6. Vigorously rub the wool against the rubber rod to charge it up. Touch each of the pith balls to apply the same charge to them. Touching the two balls together will make the charges nearly equal, but it is not necessary to do this.
  7. Start with a distance between the pith balls that allows the swinging ball to hang vertically.
  8. Slowly bring the swinging pith ball closer until the repulsion between the two pith balls causes the swinging pith ball to move away from the stationary ball.
  9. Measure how far the swinging pith ball moves horizontally. You may do this by observing from above and measuring the distance the ball has moved from the reference point.
  10. Record the horizontal distance between the centers of each of the pith balls.
  11. Repeat this measurement a few times by moving the swinging pith ball in a little closer.
  12. The horizontal separation, x, between the unconstrained pith ball and its equilibrium position is a good indication of the force. (This can actually be worked out in terms of the force, but this is unnecessary to explore the key point of this experiment.) For small angles that the pith ball makes with the vertical, the electrostatic force is directly proportional to the separation from equilibrium.
  13. The separation between the stationary ball and the equilibrium positions is designated as d, as shown in Figure 96-2. The total distance between the two pith balls is given by d + x. Make a graph of the separation from equilibrium, x, and the distance, d + x, between the balls.

Static charges

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