Normal seeds can be separated from discoloured ones and fromforeign objects by means of an electrostatic seed-sorting machinethat operates as follows. The seeds are observed by a pair ofphotocells as they fall one by one inside a tube. If the colour isnot right, a voltage is applied to a needle that deposits a chargeon the seed. The seeds then fall between a pair of electricallycharged plates that deflects the undesired ones into a separatebin. One such machine can sort dry seeds at the rate of 100 persecond, or about 2000 kg per 24 hour day.
i) over what distance must they fall if they must be separatedby 20 mm when they pass between the photo-cells? Neglect airresistance.
ii) Assuming that the seeds acquire a charge of 1.5 10-9 C, thatthe deflecting plates (large enough to consider negligible the sidedistortions of the field) are parallel and 50 mm apart, and thatthe potential difference between them is 25 kV, how far should theplates extend below the charging needle if the charged seeds mustbe deflected by 40 mm on leaving the plates? Assume that thecharging needle and the top of the deflecting plates are close tothe photocell.
