Robert Millikan received a Nobel Prize for determining the charge on the electro

Question

Robert Millikan received a Nobel Prize for determining the charge on the electron. To do this he set up a potential difference between two horizontal parallel metal plates. He then sprayed drops of oil between the plates and adjusted the potential difference until the drops of a certain size remained suspended at rest between the plates as shown. Suppose that when the potential difference between the plates is adjusted until the electric field is 10.000N/C downward, a certain drop with a mass of 3.27times10 "kg remains suspended. What is the magnitude of the charge on this drop? The electric field is downward, but the electric force is upward. Explain why. If the distance between the plates is 0.01m, what is the potential difference between the plates? The oil in the drop slowly evaporates while the drop is being observed, but the charge on the drop remains the same. Indicate whether the drop remains at rest, moves upward, or moves downward. Explain briefly. Oil Drop

Explanation / Answer

a)

Force due to electric field = force due to gravitational force

qE=mg

=>q = mg/E =(3.27*10-16)(9.81)/10000

q=3.2*10-19 C

b)

The Electric field is oriented in the direction of positive test charge .So Electric field is moving downward .However a negative charge would experience a force upward .So the drop must be negatively charged to a experience a force upward.

c)

Potential difference between the plates

V=Ed =10000*0.01

V=100 Volts

d)

Drop moves upward since the force due to gravity decreases ,theipward electric force remains constant.So this unbalance would cause the drop to accelerate upward

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