A0.80-μm-diameter oil droplet is observed between two parallel electrodes spaced 11 mm apart. the droplet hangs motionless if the upper electrode is 17.8 v more positive than the lower electrode. the density of the oil is 885kg/m3. part a what is the droplet's mass? express your answer to two significant figures and include the appropriate units. m m = nothing nothing request answer part b what is the droplet's charge? express your answer to two significant figures and include the appropriate units. q q = nothing nothing request answer part c does the droplet have a surplus or a deficit of electrons? how many? does the droplet have a surplus or a deficit of electrons? how many? deficit 9 electrons surplus 9 electrons surplus 16 electrons deficit 7 electrons

A)

The radius of the oil droplet is half of its diameter:

Assuming the droplet is spherical, its volume is given by

The density of the droplet is

Therefore, the mass of the droplet is equal to the product between volume and density:

B)

The potential difference across the electrodes is

and the distance between the plates is

So the electric field between the electrodes is

The droplet hangs motionless between the electrodes if the electric force on it is equal to the weight of the droplet:

So, from this equation, we can find the charge of the droplet:

C) Surplus of 9 electrons

The droplet is hanging near the upper electrode, which is positive: since unlike charges attract each other, the droplet must be negatively charged. So the real charge on the droplet is

we can think this charge has made of N excess electrons, so the net charge is given by

where

is the charge of each electron

Re-arranging the equation for N, we find:

so, a surplus of 9 electrons.