An oil drop of mass m=3.5*10^-9 mg and density =0.950 gcm^-3hovers between the t

Question

An oil drop of mass m=3.5*10^-9 mg and density =0.950 gcm^-3hovers between the two plates of a capacitor separated by adistance of d=0.5 cm. A tension of V=214 V is applied.
a) How many elementary charges does the oil drop carry? (Hint:equate gravitational and electrostatic force)
b) At what speed v will the drop fall down in air once thepotential is removed (the drag force of a particle in air, i.e. theforce opposing gravity, is proportional to 6nrv. r=dropradius, n=viscosity of air (1.828*10-5 Nsm-2)).

Explanation / Answer

a) We know that           F = nqE           mg =nq(V/d)    the oil drop carries the charges n = mgd / qV Substitute values

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