Question

5. A seed-sorting machine Figure on the right shows a sketch of an electrostatic seed-sorting apparatus which separates normal seeds from discolored ones. The seeds are observed by a pair of photocells as they fall one by one. If the color is not right, 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 (i.e. extra-charged) ones into a separate bin. (By the way, the figure does not exactly depict the set-up described here.) One such machine can sort peas at the rate of 100/s, or about 2 tons per 24-hour day. Negative Ground Electrode Charged Electrode REPELLED NON-REPELLED GARNET Assume that each wrong-colored seed acquires a charge of 1010el and falls right at the center of the gap between the two plates, where the deflecting plates are parallel and 5.0 cm apart and the potential difference between the plates are 25 kV. How long should the plates extend below the charging needle if the charged seeds must be deflected by 2.0 cm on leaving the plates? Assume that the charging needle and the top of the deflecting plates are close to the photocell. (Hint: The magnitude of E field inside a pair of parallel plates is, roughly (10 pts.)

Answer 1

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