Science Fair Board: Does the Charge on Both Objects Matter?

Project Board: Pushing Without Touching

1Eighth graders Tariq Lindholm and Esperanza Mwale started their science fair project with a surprise. While cleaning up after a class party, Tariq rubbed a balloon on a wool scarf and hung it from a doorway. When Esperanza rubbed a plastic pipe on the same scarf and brought it near, the balloon swung away from the pipe before the two touched. Their teacher, Mr. Ignatius Ferreira, explained that rubbing with wool leaves an electric charge on both objects, and objects with the same kind of charge push each other apart. He asked the pair to turn their surprise into questions they could test.

2Their main question was: what affects how strongly a charged pipe pushes on a charged balloon? They hung a balloon from a 1 m thread in front of a meter stick taped to the wall. Each time, they rubbed the balloon and the pipe on the wool scarf a set number of times, using strokes of the same length. Then Esperanza held the pipe level with the balloon at a set starting distance, and Tariq read how far sideways the balloon swung away. A bigger swing meant a stronger push. They ran each setting three times and recorded the averages. Between runs, they wiped both objects with a damp paper towel and let them dry, so no charge was left over from the run before.

3In Test 1, they changed the number of rubs on the pipe. In Test 2, they changed the number of rubs on the balloon. In Test 3, they rubbed both objects 10 times and changed the starting distance between them. The air in the classroom was dry on every test day.

4A judge at the school fair asked whether magnetic forces depend on materials too. So the pair added Test 4. They wound a coil of 40 turns of wire around an empty paper tube, connected it to the same battery pack each time, and slid different rods into the tube. Each rod was the same size. They counted how many steel paper clips the coil could lift, switching it on for only a few seconds so the wire stayed cool.