# PHYSICS

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In the early 1900s, Robert Millikan used small charged droplets of oil, suspended in an electric field, to make the first quantitative measurements of the electron’s charge. A 0.67-um-diameter droplet of oil, having a charge of +e, is suspended in midair between two horizontal plates of a parallel-plate capacitor. The upward electric force on the droplet is exactly balanced by the downward force of gravity. The oil has a density of 860 kg/m^3, and the capacitor plates are 4.0 mm apart.

What must the potential difference between the plates be to hold the droplet in equilibrium?

• PHYSICS - ,

Actually, in the Millikan experiment, there can be one or several charges per drop. What was observed was that the charges were a small multiple of e.

In your case, go along with the assumption that the charge is +e and equate the electric force
e V/d to the particle's weight. d is the plate separation. Solve for V.

Ignore the small effect of buoyancy, the error will be less than 1%. Millikan had to take it into account