electrostatic

charge plastic ruler 用sharing, inducting都應該唔得
因為plastic 係insulator, 如果佢可以charged by sharing/induction,,
咁當d metal ball 有charge 時下面gei stand都應該可以被charge

至於要charge insulator,,
因為一個insulator 要being charged 要有energy provided to the electrons
令佢地甩出黎,,
所以我諗除左rubbing,, 應該冇方法可以charge 到佢
[just i think]