In a particular Millikan oil-drop experiment used to measure the elementary char

Question

In a particular Millikan oil-drop experiment used to measure the elementary charge, two horizontal parallel plates are 1.0cm apart. With a 180V potential difference between the plates, a charged droplet of mass 8.8x10^-16 kg is suspended motionless.

a.) If the upper plate is at a higher potential, is the charge on the droplet positive or negative?

b.) What's the electric field between the plates under these conditions?

c.) Compute the charge on the droplet. To how many elementary charges (e) does this correspond?

Explanation / Answer

a)

as force due to gravity downward so, force due to plates must be upward i.e. towards high potential

it is ossible only when charge is negative

negative

b) E = V/d
d = 1 cm = 0.01 m
so,
E = 180/0.01 = 18000 V/m

c) qE = mg
=> q*18000 = 8.8*10^-16*9.8

=> q = 4.8*10^-19 C

3 elementary charges

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