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Droplet's motion in electric field and gravitational field?

  1. Jun 15, 2009 #1

    A drop falling in air moves with terminal velocity (uniform velocity) when drag force balances its weight.
    But when charged droplet moves in electric field (In Millikan's Oil drop Method when there is vaccume between two plates) it is suspended between two plates when electric force balances gravitational force.

    Why this droplet does not suspends in air while both forces gravitational and drag are equal?
    why it moves on with terminal velocity?

    Why charged particle does not moves on in electric field when both forces (i.e electric and gravitational are equal)?

    Please Answer...
  2. jcsd
  3. Jun 15, 2009 #2


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    Science Advisor
    Gold Member

    Drag is dependent upon the speed of the drop. When the drop is initially released, it has zero velocity and thus zero drag but it will experience the gravitational force. Only when the drop has been accelerated to its terminal velocity will the drag cancel out the gravitational force. But according to Newton's laws, no acceleration will occur if there are no net forces. So there is no force that will decelerate the drop to a standstill. On the other hand, the force from the electric field is always acting on the drop as it is dependent upon the drop's net charge, not its velocity. Thus, we can always increase the electric field to not just match the gravitational force but also to bring the drop to a standstill if it does start falling.
  4. Jun 15, 2009 #3


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    Homework Helper

    Simple answer to #1: if something is not moving, there can be no drag force :wink: In order for the drag force to balance the gravitational force, the droplet has to be moving downward with some speed.

    And I guess the same reasoning goes for #2: even if the droplet (or charged particle) is suspended, not moving anywhere, it still feels the electric force and the gravitational force. So there's no need for it to be moving to have its forces balanced.
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