Static electricity builds up in Sally when she rubs her wool socks on the nylon carpet because the two materials have different levels of electron affinity. Wool typically has a higher electron affinity, meaning it holds on to electrons more strongly than nylon does. When Sally rubs the wool socks against the nylon carpet, electrons are transferred from the wool to the carpet. This leaves the wool positively charged and the carpet negatively charged, creating an imbalance of charges that results in static electricity buildup.
