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Balloon Electroscope

When you rub a balloon with a wool cloth, the balloon captures electrons from the wool, leaving the balloon with a negative charge and the wool with a positive charge. If both balloons are rubbed with wool, they will both have same charge (negative) and therefore will repel each other. When you spray water near the balloons, the water droplets carry the charges away from the balloons, leaving them uncharged. As the balloons become neutral, they will fall back together.

Another way to neutralise the charges on the balloons is to touch them with a damp or moist hand.

Doing static electricity experiments in damp or humid weather can sometimes have confusing or unexpected results! Moisture in the air acts just like the spray of water in this demonstration and carries charges away from objects. This makes it harder to build up charges in your experiments.

Objectives

  • Explain how static charge causes materials to attract or repel each other.

Materials

  • Per Student:
    2 identically shaped balloons
    2 lengths of thread (approx. 40 cm each)
    wool cloth
    a spray bottle filled with water

Key Questions

  • Why do the balloons repel each other after rubbing?
  • Why do the balloons fall back against each other after spraying?
  • How else could we get the balloons to fall back against each other?
  • During what type of weather would it be best to do experiments on static electricity?
  • What does damp weather do to the electric charges?

What To Do

  1. Blow up 2 balloons to about the same size and tie a thread to each of them.
  2. Hold the two threads together. The balloons will hang against each other.
  3. Now let a student hold the threads while you rub the balloons with the wool cloth.
  4. Let the balloons hang back again. The balloons will not touch.
  5. Spray a mist of water at the balloons (they will fall back against each other).

Extensions

  • Students can make their own balloon electroscope to experiment with.
  • Rub the balloons with different types of cloth (e.g. nylon, silk, plastic, paper) and see how this affects the experiment.
  • What happens if you bring your wool cloth near the charged electroscope?
  • What happens if you bring a third charged balloon near the charged electroscope?