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).