Question

Normal seeds can be separated from discoloured ones and from foreign objects by means of an electrostatic seed-sorting machine that operates as follows. The seeds are observed by a pair of photocells as they fall one by one inside a tube. If the colour is not right, a voltage is applied to a needle that deposits a charge on the seed. The seeds then fall between a pair of electrically charged plates that deflects the undesired ones into a separate bin. One such machine can sort dry seeds at the rate of 100 per second, or about 2000 kg per 24 hour day.

i) over what distance must they fall if they must be separated by 20 mm when they pass between the photo-cells? Neglect air resistance.

ii) Assuming that the seeds acquire a charge of 1.5 10-9 C, that the deflecting plates (large enough to consider negligible the side distortions of the field) are parallel and 50 mm apart, and that the potential difference between them is 25 kV, how far should the plates extend below the charging needle if the charged seeds must be deflected by 40 mm on leaving the plates? Assume that the charging needle and the top of the deflecting plates are close to the photocell.

Answer 1