Millikan Oil Drop: Balancing Forces with Changing Mass

Steps:

• Electric force upward (qE) balances gravitational force downward (mg) for stationary droplet.
• Evaporation reduces mass m, decreasing mg while qE stays constant.
• Net force now upward (qE > mg), so droplet accelerates upward.
• To stop acceleration, reduce qE by increasing plate separation d (E = V/d decreases).

Why D is correct:

• Upward acceleration occurs as reduced mg < qE; increasing d lowers E (from E = V/d), reducing qE to rebalance with smaller mg.

Why the others are wrong:

• A: Acceleration is upward, not downward; decreasing d would increase E and worsen upward acceleration.
• B: Acceleration is upward, not downward; increasing d correctly reduces E but direction is wrong.
• C: Acceleration is upward, but decreasing d increases E, amplifying upward force.

Final answer: D