# Electric charges.

#### j88k

1. The problem statement, all variables and given/known data

In an inkjet printer, letters and images are created by squirting drops of ink horizontally at a sheet of 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.

http://session.masteringphysics.com/problemAsset/1011260/12/153611.jpg

The ink drops have a mass = 1.00×10−11 kg each and leave the nozzle and travel horizontally toward the paper at velocity = 19.0 m/s. The drops pass through a charging unit that gives each drop a positive charge q by causing it to lose some electrons. The drops then pass between parallel deflecting plates of length D0 = 1.60 cm, where there is a uniform vertical electric field with magnitude E = 8.15×104 N/C.

If a drop is to be deflected a distance d = 0.320 mm by the time it reaches the end of the deflection plate, what magnitude of charge q must be given to the drop? Assume that the density of the ink drop is 1000 kg/m^3, and ignore the effects of gravity.

Also see attached screenshot.

2. Relevant equations

1) t = D0 / v

2) d = ( a * t^2 ) / 2

3) F = m * a

4) F = E * Q

3. The attempt at a solution

1) t = (0.016)/(19.0) = 8.42 * 10^-4 s

2) (0.32 * 10^-3) = (a * (8.42 * 10^-4)^2)/2 -> a = 9.03 * 10^2 m/s^2

3) F = (1.0 * 10^-11)(9.03 * 10^2) = 9.03 * 10^-9 N

4) (9.03 * 10^-9) = (8.15 *10^4) * Q -> Q = 1.11 * 10^-13 C

**Not sure if I have the right answer there.

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anyone?

Homework Helper
Looks OK to me.

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