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A droplet grows from 1 to 2 micrometres in a certain time by condensation. Roughly how much longer is needed to reach 4 micrometres?

  • A.about the same time again
  • B.about half as long
  • C.about four times as long
  • D.about twice as long

Answer: C

Correct answer: about four times as long

Condensational growth obeys \( r\,\mathrm{d}r/\mathrm{d}t = \) constant, so the square of the radius grows linearly with time. Going from 1 to 2 micrometres costs \( 4-1=3 \) units of \( r^2 \); going from 2 to 4 costs \( 16-4=12 \), four times as much. That slowing is the central fact of the process and the reason condensation alone cannot make rain.

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