2. Electret filters are key components in medical face masks that exhibit enhanced ability to capture airborneparticulate matters (PM). Normally, electret filters are made by corona-charging or the so-called melt-blowntechnology to load charges onto nonwoven fabrics. Suppose here, a high energy electron beam with currentI=0.5pA is used to bombard the polypropylene fabric (with relative permittivity εr=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.