A glass rod, rubbed against silk, is brought up to the ball of a charged electroscope, but does not touch it.

A glass rod, rubbed against silk, is brought up to the ball of a charged electroscope, but does not touch it. The electroscope leaves are coming closer. What is the sign of the electroscope charge?

The electrons released by rubbing the glass rod against the silk are concentrated on the surface of the silk, because it has a more powerful binding energy of electrons with the nuclei of atoms. They will collect on the surface of the fabric, charging it negatively. In glass, these bonds are weaker, which means that there will be a shortage of electrons on the glass, so it will charge positively. After that, the glass rod is brought to the ball of the charged electroscope, but does not touch it. The electric field of the rod promotes the influx of electrons to the ball, which will move away from the leaves of the electroscope. As the leaves of the electroscope come closer together, we can conclude that they are negatively charged.