2. Electret filters are key components in medical face masks that exhibit enhanced ability to capture airborne particulate matters (PM). Normally, electret filters are made by corona-charging or the so-called melt-blown technology to load charges onto nonwoven fabrics. Suppose here, a high energy electron beam with current I=0.5pA is used to bombard the polypropylene fabric (with relative permittivity ε_r=2.5) over an area A= 5x5 cm² for 1 sec. Assume that the all the electrons are uniformly trapped in a very thin layer.