Oil drop prob.

[Physicsisfun2005]

another fun Ap phyiscs problem..short as usual:

In one run of the oil drop experiment a 20 mg drop was suspended between parallel plates 1.5 cm apart when the potential difference was 300 V. Compute the number of excess electronic charges on the drop.

is this just saying "compute the oil drop's charge" ?....if so why do i need to know the mass in this problem? .....i think i can compute it w/o that info.

[Tide]

Yes, they are asking for the drop's charge and you need the mass because you must balance gravity and the electrical force.

[Physicsisfun2005]

ahhh ic.....thanx