Equipment

• Spin electroscope
• Polyethylene rods (2)
• Nylon cloth

Demo

Ground the electroscope by touching the top gently with your hand. Induce a negative charge in one of the rods by rubbing it with the nylon cloth for several seconds. Now, charge (via conduction) the electroscope by touching the rod to the electroscope in several places, thus inducing a negative charge on the electroscope. Once again rub the nylon cloth on a polyethylene rod and place one end of the charged rod near the electroscope, being careful not to let the rod touch the electroscope. At a distance, there is repulsion between the rod and the electroscope, but as the rod is brought closer to the electroscope, the electroscope is attracted to  the rod.

Now rub both polyethylene rods with the nylon cloth. While the electroscope is negatively charged, place both rods at opposite ends of the electroscope. The electroscope is now repelled by the rods.

Explanation

By rubbing the polyethylene rods with the nylon cloth, they become negatively charged. When the charged rod is brought into contact with the electroscope, the electroscope becomes negatively charged. When a charged rod is again placed near the electroscope, the electrons in the electroscope move away from the rod, since the like charges repel. Thus, the portion of the electroscope that remains near the rod acquires a positive charge. Hence why the electroscope will rotate towards the rod.

When two negatively charge rods are placed near opposite ends of a negatively charged electroscope, the electrons in the electroscope can no longer move away from the negatively charged rods. Thus, the electroscope is forced to spin away from each rod.

Written by Madison Harris