Question: In a modern version of Millikan's famous experiment, a charged radioactive sphere of mass 1ng (=?kg) is suspended motionless between two parallel conducting plates spaced by 1.0mm.
The apparatus (plates and sphere) is in vacuum. Initially, a potential difference of 15.3kV is required to counteract the gravitational force.
a. Determine the magnitude of the following forces:
i. Gravitational force.
ii. Electric force.
iii. Viscous force.
b. What is the charge on the sphere?
c. By radioactive decay, the sphere periodically emits a charged particle, and the voltage required to balance the gravitational force changes suddenly. You observe the following:
i. The sphere is balanced by a voltage of 15.3kV for a short time, whereupon.
ii. the sphere begins to fall, and the voltage must be increased to 30.6kV in order once again to levitate the sphere, and then.
iii. the sphere falls and is unaffected by the magnitude or sign of the potential difference applied to the plates. Explain these observations and determine the charge of the particle emitted in each radioactive decay.