Millikan's oil drop experiment-terminal velocity.

[Jenkz]

Homework Statement

Find the terminal velocity of an oil drop with radius 1x10^-6 m at 20 degress under zero field conditions. How long would it take to fall 5mm (range of microscope graticule)?

Homework Equations

Terminal velocity; Weight = Drag
Weight = mg

Assuming Stokes law holds; Drag = 6(pi)rηv
v - terminal velocity
r - radius of droplet
η -viscosity of air, 1.832×10–5 Pa s (at 20 degrees)

Mass, m = (4/3) pi (r^3) $$\rho$$
$$\rho$$ = Density of oil, 874 kgm^-3 (at 20 degrees)

time= distance/ velocity

The Attempt at a Solution

Mass = 3.66 x 10^-15 Kg
v = (mg) / (6(pi)rη) = 1.04 x10^-4 ms^-1

time = 48s

The velocity seems too small, and the time taken is definitely too long. Please help?

 

[Quinzio]

I'd say the result is theorically true.
Small particles tends to "fly" in air rather than drop down like stones.

In real world the particle will actually fly away following some slow air flow generated eg. from one near hot lamp, or by the body of someone near.

 

[Jenkz]

Really? I'm still skeptical about my answers... Thanks though.