Equipment

• Silk braid electroscopes (2)
• Plexiglass rod
• Polyethylene rod
• Silk cloth
• Nylon cloth

Demo

Begin by making sure the silk braid electroscopes are grounded by touching the metal on top of the electroscope. Then, rub the plexiglass rod with the silk cloth. Touch the rod to the top of one of the electroscopes. The braids of the electroscope will diverge from the base. Charge the other electroscope the same way. The braids on each electroscope will remain diverged until brought into contact with other charges. Move the electroscopes close together and the braids on each electroscope will repel each other (Figure 1).

Figure 1: Silk braid electroscopes charged with like charges.

You can repeat this process, instead charging one of the electroscopes by rubbing the  polyethylene rod, and the braids on each electroscope will attract one another.

Figure 2: Silk braid electroscopes charged with opposite charges.

Explanation 

By rubbing the plexiglass rod with silk, the rod becomes positively charged because it loses electrons. Then, when brought into contact with the silk braid electroscopes, each electroscope acquires a positive charge, while the rod neutralizes. Thus, since each electroscope carries the same charge, when they are moved towards each other, they repel due to the fact that they are of like charge.

Similarly, when the polyethylene rod is rubbed with the silk cloth, electrons from the cloth are transferred to the rod, giving it a negative charge. When the rod is touched to the electroscope, the electroscope becomes negatively charged. Thus, when the electroscopes are brought near each other (one charged with the plexiglass rod and one charged with the polyethylene rod) the attract since they have opposite charge.

Notes

• This demo may be used by charging the electroscopes via induction rather than  conduction.
• There are several combinations of materials available that will produce different charges. Electroscopes may be used to determine whether different combinations produce like or opposite charges (see TriboElectric series)
• Be sure to rub the polyethylene rod with the nylon cloth vigorously for several seconds to accumulate charge

Written by Madison Harris