Physics 問題

(a) When two things(solids) are rubbed together, some of the charges of one of the things are transferred to the another thing, so one becomes positively charged and one becomes negatively charged. If the things are not rubbed together again, they will soon carry no charges as the water droplets in air will discharge them.
Whether a rubbed thing carry positive charges or negative charges depends on the material of the thing.
(b)( i ) As the paint droplets are charged positively, their same charges make the droplets repel each other and so spread out more widely than the uncharged paint droplets.
( ii ) The paint droplets are charged positively while the car door is charged negatively and the two things attract each other and helps the paint to stick to the car door more firmly, thus improves the paint spraying process.