An oil drop is stationary when what downward force exactly equals the upward electrical force on the drop?

• Correct Answer: gravitational
• radiation
• magnetic
• newtonian

Explanation: The charge on the plates is adjustable. By measuring the terminal velocity of the oil drops with the electric field off, Millikan could determine the mass of the drops. Millikan and his graduate assistant were able to determine the force of the field on the drops when it was turned on by comparing the velocity of the drops with the field on to their velocity with the field off. This is easily determined when the oil drop is stationary; namely, when the downward gravitational force exactly equals the upward electrical force on the drop.

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