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**Millikan Oil drop experiment.**

For my current lab, we are recreating the milian oil drop experiment to measure the charge of an electron. However, we are using 1-micron diameter latex spheres in place of oil drops.

**Problem:**

I am having difficulty deriving an equation for the speed of the drop. Only the linear part of air resistance is taken into account. Without an electric field the particle takes about 15 seconds to fall a distance of 15mm. With the current applied, different spheres travel at different velocities dependent on their charge. And the same sphere moves faster when the field is applied in the direction of gravity.

**Attempt at solution:**

v1 is rising against gravity

v1 = [qE - mg] / (6*pi*eta*r)

where eta is viscosity of air

and

v2 is when field is reversed and aids gravity

v2 = [qE + mg] / (6*pi*eta*r)

---> How do I find qE if I know the voltage difference between the two plates is 50V. What about 100V or 150V?

---> How do I measure the viscosity of air to use in the equation for v1 and v2?

---> Does the charge “q” on each sphere have to be an integer multiple of e, the charge of a single electron?

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