Question
A spherical rain drop of radius $1.0 \mathrm{mm}$ has a charge of $+2.0 \mathrm{nC}$. The electric field in the vicinity is $2.0 \mathrm{kN} / \mathrm{C}$ downward. The terminal speed of an identical but uncharged drop is $6.5 \mathrm{m} / \mathrm{s}$. The drag force is related to the drop's speed by $F_{\mathrm{d}}=b v^{2}$ (turbulent drag rather than viscous drag). Calculate the terminal speed of the charged rain drop.
Step 1
We know that the drag force $F_d$ is equal to $b v^2$, where $v$ is the terminal speed of the uncharged drop. We also know that the weight of the drop $mg$ is balanced by the drag force at terminal speed, so we can write $mg = b v^2$. Show more…
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A rain drop of radius $1.5 \mathrm{~mm}$, experiences a drag force $F=\left(2 \times 10^{-5} \mathrm{v}\right) \mathrm{N}$, while falling through air from a height $2 \mathrm{~km}$, with a velocity $\bar{v} .$ The terminal velocity of the rain drop will be nearly (use $g=10 \mathrm{~ms}^{-2}$ ) (a) $200 \mathrm{~ms}^{-1}$ (b) $80 \mathrm{~ms}^{-1}$ (c) $7 \mathrm{~ms}^{-1}$ (d) $3 \mathrm{~ms}^{-1}$
Properties of Liquids
Round 1
Raindrops acquire an electric charge as they fall. Suppose a 2.5-mm-diameter drop has a charge of +18 pC. If the strength of the earth's electric field is 100 N/C, how does the magnitude of the electric force on the droplet compare to the weight force?
(a) If a spherical raindrop of radius 0.650 $\mathrm{mm}$ carries a charge of $-1.20 \mathrm{pC}$ uniformly distributed over its volume, what is the potential at its surface? (Take the potential to be zero at an infinite distance from the raindrop. (b) Two identical raindrops, each with radius and charge specified in part (a) collide and merge into one larger raindrop. What is the radius of this larger drop, and what is the potential at its surface, if its charge is uniformly distributed over its volume?
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