In an inkjet printer, letters are built up by squirting drops of ink at the pape

Question

In an inkjet printer, letters are built up by squirting drops of ink at the paper from a rapidly moving nozzle. The ink drops, which have a mass of 1.4 x 10-8 g each, leave the nozzle and travel toward the paper at 20 m/s, passing through a charging unit that gives each drop a positive charge q by removing some electrons from it. The drops then pass between parallel deflecting plates 2.0 cm long where there is a uniform vertical electric field with magnitude 8.0 x 104 N/C. If a drop is to be deflected 0.30 mm by the time it reaches the end of the deflection plates, what magnitude must be given to the drop?

Explanation / Answer

There is no force parallel to plates. so speed paralel to plates remains constant.
Time to cross plates = length of plates / speed = 0.02 / 20 = 10-3 sec
Force in vertcal direction = qE = q 8x104
accelartion in vertical direction  = F/m = q 5.7x1012
Deflection in time t = 1/2 at2 = q 2.86 x 106 = 0.3 x 10-3
q = 1.04 x 10-10 C

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