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Homework Help: Charge deflection in ink jet printer

  1. Jan 15, 2008 #1
    1. The problem statement, all variables and given/known data

    In an inkjet printer, letters are built up by squirting drops of ink at the paper from a rapidly moving nozzle. The pattern on the paper is controlled by an electrostatic valve that determines at each nozzle position whether ink is squirted onto the paper or not. The ink drops, 15 μm in radius, have a mass of 1.41 x 10-11 KG leave the nozzle and travel toward the paper at 20 m/s. The drops pass through a charging unit that gives each drop a positive charge q when the drop loses some electrons. The drops then pass between parallel deflecting plates 2.0 cm in length where there is a uniform vertical electric field with magnitude 9.1 x 104 N/C. If a drop is to be deflected 0.30 mm by the time it reaches the end of the deflection plate, what magnitude of charge must be given to the drop? (Answer in pC -- pico Coulombs.)

    2. Relevant equations

    F = (k*q1*q2)/r^2 , where k=8.99e9
    E(Electric field) = F/q

    3. The attempt at a solution

    I'm pretty stuck on this one and any help on how to do this or to get me started would be appreciated.
  2. jcsd
  3. Jan 16, 2008 #2


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    What is the force exerted on a charge in an electric field? Your second equation should give you the clue.
  4. Jan 17, 2008 #3
    how would this help you to find this force when we are missing two variables?
  5. Jan 17, 2008 #4
    first we found time t = d/v t=.001s

    X = Xo + Vot + .5at^2 to find our a

    F=ma=qE and solve for q, q=ma/E

    final answer came out to be 5.268 picoCoulombs, does that sound correct?
  6. Jan 17, 2008 #5
    hey did that answer work for you?
  7. Jan 18, 2008 #6


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    Sounds ok but I get a different answer. What did you get for a?
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