Charging an Electroscope by Induction Using a Negatively-Charged Balloon

One common demonstration performed with the electroscope involves the induction process of charging. In the induction process of charging, a charged object is brought near to but not touching the electroscope. The presence of the charged object above the plate of the electroscope, induces electrons within the electroscope to move accordingly. With the charged object still held above the plate, the electroscope is touched. At this point electrons will flow between the electroscope and the ground, giving the electroscope an overall charge. When the charged object is pulled away, the needle of the electroscope deflects, thus indicating an overall charge on the electroscope. The process of charging an electroscope by induction using a negatively-charged balloon is depicted in the animation below.

As shown in the animation above, the presence of the negatively-charged balloon above the plate of the electroscope will induce the movement of electrons from the plate of the electroscope to the support and needle of the electroscope. This is explained by the "like charges repel" principle. The negatively-charged balloon repels the negatively-charged electrons, thus forcing them to move downwards. Once the electrons leave the plate and enter the needle, both plate and support/needle acquire an imbalance of charge. The plate acquires an excess of positive charge (since electrons have left this once neutral region) and the support/needle acquires an excess of negative charge (since electrons have entered this once neutral region).

When a positively-charged electroscope is touched, electrons enter the electroscope from the ground. Being positively charged, the electroscope attracts some electrons from the conducting material (in this case, a person). The negatively-charged electrons enter the electroscope and neutralize the positive charge. As the electroscope loses its charge, the needle relaxes back to its naturally position.

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